Convergent Designs for Electrogenesis and Electroreception

Convergent designs for electrogenesis and electroreception

Carl D Hopkins

Cornell University, Ithaca, USA

New- and old-world tropical electric fish lack a common electrical ancestor, suggesting that the mechanisms of signal generation and recognition evolved independently in the two groups. Recent research on convergent designs for electrogenesis and electroreception has focused on the structure of electric organs, the neural circuitry controlling the pacemaker driving the electric organ, and the neural circuitry underlying time coding of electric waveforms.

Current Opinion in Neurobiology 1995, 5:769-777

Introduction (Osteoglossomorpha, Teleostei) from Africa and the Gymnotiformes (Ostariophysi, Teleostei) from South Ethologists and neuroethologists have been fascinated America, are distantly related but do not share a common by electric fish and by the evolution of an entirely electrogenic or electroreceptive ancestor 12,121; however, novel electrosensory modality ever since Hans Lissmann’s each of these two groups has a sister group of fish discovery of ‘weak electrogenesis’ in the monnyriform that has arnpullary electroreceptors but no electric fishes of Africa in 1951 [l] and his subsequent discovery organs [4,12]. In spite of the phylogenetic distance, of active electrolocation (the sensing of objects in the both the mormyrifornls and gynmotifornls include environment as distortions in the electric field generated wave- and pulse-discharging species. I3oth groups have by a fish’s own electric organ discharge) [2,3]. We pulse-discharging species that generate complex electric now know that the electric sense is used for electrical organ discharge (EOD) waveforms. They also both have communication [4-6,7”], for passive electrical sensing three types of electroreceptors with distinct functions of prey [8], and for active electrolocation [9,10]. and separate pathways, and both use their electric organs for electrolocation and communication. Electric fish provide a good model system in neuroethology for several reasons: there are many species The parallels between the two groups run even deeper to compare in new- and old--world groups; electric when looking at the cellular basis for behavior. Several behavior is novel and inherently fascinating; the modality new studies [15-191 have explored the mechanisms is convenient to work with physiologically; and there of electrogcnesis, particularly the production of con- are many parallels between the electric sense and plcx EOD waveforms. As an electric discharge is audition Ill]. This review focuses on recent research an electrostatic field and not a propagating wave, that examines convergent designs for electrogenesis and EOD waveforms are unaffected by echo, reverberation, electroreception in new- and old-world fresh-water refraction, reflection or any other phenomenon affecting tropic.11 fish. propagating waves (such as sound) [ 131.

Gymnotiform electric organs Phylolgeny and electrogenesis Dut how does the electric organ generate something more complex than a simple, biphasic, spike-like Key to any comparative neurobiological study is a good discharge that one would expect from the sequential comprehension of the phylogenetic relationship between activation of the caudal and the rostra1 faces of the organisnu in question, and the recent explosive simple electrocytes [14]! Some of the South American growth of cladistics coillbined with new molecular gynmotiforms, such as Gyrtmtrrs rrlmpo, can generate techniques has had a clear ilnpact on the field of a triphasic EOD by firing a subset of electrocytes neuroethology. It is now clear that the two main slightly out of phase with the rest of the population group:; of fresh-water electric fish, the Mornlyriformes [l&19]. Electrocytes near the head have a specialized

Abbreviations AMPA-cc-amino-3-hydroxy--5-methyl-4-isoxazole proprionic acid; BCA nucleus-bulbar command-associated nucleus; ELa--exterolateralis pars anterior: ELL-Glectrosensory lateral line lobe; ELvxterolateralis pars posterior; EOD-electric organ discharge; CABA--y-aminobutyric acid; ICL-inner cellular layer; ITD-interaural time difference; MRN-medullary relay nucleus: NMDA-N-methyl-D-aspartate; PPn-pre-pacemaker nucleus; PPn-c-‘chirp’ part of the diencephalic PPn; PPn-g--gradual frequency shift region of the PPn; SPPn-sub-lemniscal PPn.

0 Current Biology Ltd ISSN 0959-4388 769 770 Neural control

the fish in these four genera produce complex EOD (b) EOD waveforms containing three or even four major peaks waveform (Fig. lb). By contrast, fish in the genera Bra~hyllypoporrrfrs (formerly Hypopotwus) and Microstcrnarchus do not appear to have accessory electric organs, and their EODs are (iii) very simple, being composed of biphasic waveforms. The revised phylogeny based on molecular data shows that these complex accessory electric organs may have had a single common evolutionary origin rather than multiple unrelated origins, as suggested by the traditional (0 phylogeny. ~ (ii) 04

EODs

, ~ -1/ Brachyhypopomus j _ Fig. 1. Complex EOD waveforms generated by a South American electric fish. (a) The gymnotiform Gymnotus carapo generates (b) a complex EOD waveform with four components: (i) an early gradual head negativity, (ii) a strong head-negative peak, (iii) a head-positive peak, and (iv) a head-negative peak. The head-positive peak (iii) results from the synchronous discharge of the posterior faces of the majority of the electrocytes in tubes 2, 3 and 4 of cells in the electric organ, which receive innervation from the anterior or posterior electromotor nerves. The final head negativity (iv) arises from the inward current through the anterior faces of these same electrocytes, which have become depolarized by the passive current flow through them from the posterior face. Early head negativity (i) is caused by inward current through the anterior faces of the electrocytes in tube 1; these are innervated on the anterior side from nerves with a shorter conduction time to the electric organ. These specialized electrocytes generate a local head-negative discharge, which precedes the’main EOD in the tail. Adapted from 1151.

anterior innervation in addition to the typical posterior one, plus a shorter conduction pathway from the pacemaker. This ensures that the rostra1 faces of the electrocytes fire first, making the head negative; this occurs before the electric organ in the tail, which Fig. 2. A recent phylogenetic analysis of the gymnotiform elec .. is caudally innervated, goes head-positive and then tric fish of South America derived from analysis of sequences of mitochondrial DNA. This analysis demonstraies a closk relation- negative again (Fig. 1) [15]. An elegant series of recent ship between the genera Rhamphichthys, Gymnorhamphichthys, papers details the complexity of the electric organs in Steatogenys, and Hy~_‘opygus, whereas previous traditional mor- Gyrrrnot~s carapo and the patterns of innervation and phological studies had placed the last two closer to Brachyhy- motor control [15-191. Other gymnotiforms, such as popomus and Microsternarchus in a separate family (Hypopomi- dae; !ight shading). The revised phylogenetic analysis is consistent Stcatqcnys elegans, Hypopygus lepk4rus and Rhanrphichthys with the common presence of accessory electric organs in Hypopy- achieve multi-phased EODs using electrocytes located in gus, Steatogenys, Cymnorhamphichthys and Rhamphichthys. The accessory electric organs on the underside of the head accessory electric organs found on the underside of the head are re- sponsible for generating the early head-negative phase of the EOD [14], in addition to the main organ in the tail. (indicated by arrows). Adapted from [20**]. Heterogeneous electric organs, accessory electric organs and simple electric organs are more interesting when viewed in the light of phylogeny. Alves-Games et al. [20*] used mitochondrial DNA to construct a Mormyrid electric organs phylogeny for the gymnotiform electric fish of South The 200 species of African mormyrids are well America to re-examine relationships among the pulse known for their EOD diversity [5,21] and for gener- gymnotiforms (Fig. 2). Pulse discharges appear to ating monophasic, biphasic, triphasic, inverted tripha- have arisen independently in the families Gymnotidae sic and even four-phase EOD waveforms (Fig. 3). (Gymwotus) and Electrophoridae (Electrophoms), and Recently, Alves-Games and Hopkins (J Alves-Games, then again in the Hypopomidae/Rhamphichthyidae CD Hopkins, unpublished data) generated a partial families. Especially interesting is the apparent clade phylogeny of the A&can mormyriform electric fishes composed of the genera Gyttrnorhatrrphichfhys, Rhatrr- using mitochondrial DNA, and the phylogeny was phichthys, Hypopyg14s and Stcatqerrys, all of which have used as a framework to re-examine the evolution of accessory electric organs in addition to the more their complex electrocyte morphology (Fig. 4). The typical electric organ located in the long tail. All molecular phylogeny suggests that primitive electric Convergent designs for electrogenesis and electroreception Hopkins 771

(a) Stalkless (b) Non-penetrating (c) Penetrating stalk, (d) Inverted (e) Doubly penetrating stalk, posterior anterior penetrating and non-penetrating innervation innervation stalk, posterior stalk innervation

EODs -- &______;,______+;;o&_____:_

Petrocephalus Brienomyrus Morm yrops Pollimyrus bovei sp. 5 zanclirostris isidori

Fig. 3. The electric organs of the mormyriforms are composed of electrocytes oi varying degrees of complexity, depending on the species. (a) Stalkless electrocytes, found only in Gymnarchus niloticus, are innervated on the posterior side, and only the posterior face fires a spike. The anterior face is deeply convoluted, has a high capacitance, and does not fire a spike. The EOD is a monophasic, head-positive pulse. (b) Non-penetrating stalk electrocytes are found in Petrocephalus, Mormyrus and several other genera, including Brienomyrus; the stalk is innervated on the posterior side. The EOD is biphasic, as the posterior face fires an action potential first, and the anterior face follows. (c) Penetrating stalk electrocytes have anterior innervation, and are widespread in the genera Marcusenius, Gnathonemus, and Brienomyrus. The EOD is always triphasic, although the initial head negativity may be notireable only on expanded gain (thin line in upper trace). The initial head negativity is caused by the inward-directed current in the stalk, which is directed in the posterior direction when the stalk action potential reaches the point of penetration. The large head positivity is generated by the firing oi the posterior iacc, whereas the final head negativity results from the firing of the anterior face. (d) Inverted penetrating stalk electrocytes have EODs that are correspondingly inverted in polarity; they are iound only in the genus Mormyrops. (e) Doubly penetrating and non-penetrating stalk electrocytes are innervated on the posterior side, and are found in the genera Stomatorhinus and Poollimyrus. The EOD in Pollimyrus isidori has an early head positivity that precedes the main biphasic pulse and is thought to arise from the anterior-directed current into the stalk on the posterior side oi rhe electroryte, and the posterior-directed flow on the anterior side. The main biphasic disrharge that follows is probably produced hy rhe posterior iace and then anterior face oi the electrocyte itself. Adapted from [Shl with data irom [X*1. orgasms were stalkless, but then evolved into stalked species specific, EODs mny be used for species and sex and theu penetrating-stalked; however, reversion to recognition (13 Bratton, personal coiiiniuilicatioii). non-penetrating stalks may have occurred several times in various different genera (Fig. 3). Until recently. only one type of catfish, ~Vlalap~cn4nrs cktriws, was known to be elcctrogenic; however, in 19’90, Hagedorn ct al. [25] described very weak electric discharges in several spccics of Syrmdorrrir catfish (family Electric skates Mochokidnc). This ycnr there were additional reports A similar story emerges from R recent investigntiou of of siulilar electric discharges in ccvernl other Afiiicnn the morphology of electrocytes in 63 of the -200 catfish, including tmro nlore species of- Syrr&rrti.\- and known species of electric skates (&joi&i), which oue of Cltni~x. Baron nnd colleagues [26*,27*] report demonstrate R diversity of electrocyte types, including 1(t20 ins duration, 150 pV ci~i~l electric signals from cup-shaped, modified cup-shaped, intermediate-shaped, Clarios catfish during ngouistic cncouuters. The authors and disk-shaped [22*]. Th ese different electrocytcs suggest that these discharge\ arc not mere artiiacts of generate J phylogeny fully cmlsistcnt with phylogcnics muscle activity, but that they sc’rvt‘ as socinl signals, derived iron1 other characteristics [23]. Much remains to which x-c‘ detected by nmpullnry clcctroreccptors. Thex be learned, howcvcr, about the functional significance ut’w studies raise the interestiug pos&ility of a rclativclv of sk.~te electric discharges. Indeed, only a few species uncsplored world of very wcnk (i.e. pV cu-1 instead of have ever been recorded electrically, even though they nlV cm-l), very low frequency (i.e. less than 50 Hz peak appear to have fully functional electric organs in their spectral energy) electric discharges being used iu social tails: in some species, the EOD is important in social internctions nmoug cntfish, which hnvc traditionnlly been com~~lunicatio~~ 1241; in others. where the discharges are viewed ns electrically silent. 772 Neural control

Fig. 4. Partial phylogeny of the mormyriform electric fish from Africa, derived from analysis of mitochondrial DNA sequences, revealing a progressive evolution of complexity of the electrocytes in the electric organ. The osteoglossomorphs, Pantodon and Notopter~s, are inc ludcd for outgroup comparisons. Gyrnnnrchus niloticus is the most antestral mormyriform, with tlosest affinity to P,~nfodon and Notoptcru, JIKI it has a simple electric organ, with no stalk. Petrocephalus is the most primltlve of the Mormyridae from this analysis and the osteology. It\ electric organ is composed of simple, non-penetrating stalk electrocytes and the EOD is correspondingly hiphasic (see Fig. 3). Most of thtx remaining species on the chart have electrocytes with penetrating stalks with corresponding triphasic EODs, except for two of the tlricwmyrur, which have apparently reverted to non-penetrating stalks. Adapted from (J Alves-Comes, CD Hopkins, unpublished data).

Neural control of electric discharges including social signals. Severnl rcccnt papers outline the complex mture of the connections illvolvcd ill thcsc Orle of the most active nrcas of electrogenesis resenrch behaviors (J Alves-Games, CD Hopkin\. uupublisbcd has been the elucidation of central control mechanisms data; [ 10,28”,2c,“,3(t37.n8”,3’)]). for the patterns of electric discharges used in n variety of behavioral functions such as social communication It is known that in the context of social bchnvior. (including threats, retreats, courtship nnd alarm), novelty gyninotiforms and monnyriti,nns nlodulnte the output responses, general alcrtillg responses, and the jnmnling of their electric organs in 3 variety of ways [ 5.0.7”. 101. avoidance response. Much of the work in this nrcn For example, during courtship, mnlc I:‘[ycrrmlrrrrii1. began with the pioneering efforts of the late Wnltcr who normally produce R 3O(t500 Hz steady ~vnvc Heiligenberg to study the neural circuitry behind discharge, generate brief (l(~100 111s) ‘intcrruptlon~‘ of the jnmming avoidance response iI1 the gymlotiform the discharge, which nppenr to stimulntc thr fcn~alc E@wrrrartrricl (reviewed in [ 101). Heiligenberg wanted to approach 2nd ~pam [-Co,4 I]. hth ~lnlec .IIK~ to determine how electrosensory inputs are cm&ted fen&s also make brief ‘interruptions’ or ‘chirps’ .I\ into motor commands to accelerate or decelerate the threats [40,42]. Subordinate lii,qcvrlrrrurrrria clcvntc their nlcdullary pnccmaker; however, work in this field during EOD frequency by less than 20 Hz in ‘long rim’ the past 10 years has led to nn understanding of (lasting for seconds) [40]. Finally, fcmnlr fish \vill 3 far more con~plec list of electromotor behaviors, often genernte slow up-ml&down wnrbling Illodulations Convergent designs for electrogenesis and electroreception Hopkins 773 of their EOD frequency by lO-20Hz, which may nucleus (SPPn) in the midbrain. These pathways were last for seconds or minutes during courtship and first identified by retrograde transport from injections egg laying [41]. The pulse-discharging fish from the of horseradish peroxidase into the pacemaker, and the Bra~/ryllypclpc,,rlf~~ genus generate several displays: brief iunctional sub-divisions were studied by highly localized silences (interruptions) for threat signals; long silences iontophoretic injections of glutnnlate into the pre-motor as alarm signals; partial, or ‘noisy silences accompanied nuclei combined with pharmacological blockers applied by ‘hissing’ background discharges of unknown function; to the pncenlakcr itself 1441. and a rich range of frequency modulations during social The 6st acceleration or ‘chirp’ of the EOD frequency, behavior, including sudden increases in EOD frequency used in the context of threat behavior and male followed by decreases; and ‘chirps’ [35,40,43]. With a courtship in I~rclfllylly~o~~orrlrr~, aud the fast acceleration decade of work including horseradish peroxidase studies, and decrease in anlplitude in the homologous ‘chirp’ of intracellular recording, glutamate iontophoresis, im- E@vrrrrarrrria cau be produced by a depolarization of the munocytochenlistry, pharmacological investigation and relay cells from neurons originating in the ‘chirp’ part electron microscopy, man)- of these display patterns nlny of the diencephalic pre-pacemaker nucleus (PPrl-c). The be described in terms of neural circuitr) synaptic inputs onto relay cells are glutamergic and the receptors are of the AMPA type. The gradual acceleration of the EOD frequency- signaling general arousal and correlating with an increase in motor activity-in Rru~12ylly~~‘I”‘I”‘“lf’L,and the slow long ti-equency rise given in the context of submission and the jamming avoidance response in I~iymrrmrrin, can

Electric be generated by excitatory iuput into the pacemaker organ cells from the gradual frequency shif? region of the PPn (PPn-g) in the diencephalon. Again, with honlo- logous neural pathways, but not necessarily behavioral contexts, PPn-g inputs activate NMDA receptors on Brzchyhypopomus the pacemaker ccl1 dendrites, causing a depolarization of pacenlakers and an increase in the EOD rate [36].

Frequency rises nr~r/ly/tyl,c)E,c)rtll~~ can slow its discharge frequency, and even turn it of’f conlpletely for short or long periods of time, to hide from predators or unknown stimuli, or to signal submission to more dominant fish 13.51. Electric I3ycrrrrrarrrriti shows neither the behavior nor the neural organ pathway for stopping. Kawasaki and Hciligenberg [X.5] found that nr~r/tylrM”‘po”‘rts has a unique inhibitory pathway (contain+, GABA) ti-on1 the PPn-i nucleus to the pacemaker. I~r~lfllyllyl)c’l”)f’tl’2‘cau suddenly shut

Fig. 5. Summary of pacemaker cell control in two species of off its discharge frequency for a brief period without gymnotiforms that illustrates separate pathways for controlling dif- slowing the discharge rate before shutting off, and this ferent electrical behaviors, inc-luding frequency rises and falls in is controlled by descending inputs froul the midbrain the jamming avoidance response (JAK), chirps and Interruptions. SPPn, which terminates on NMDA-type receptors on Although there are similarities between Eigenmnnnia and Bra<-hy- hypopomu\, the GABA system in Br,7rhyhy~opon,us wems to he the relay cells (38**,45”]. A similar pathway causes the absent in Eigenmnnnia. Adapted from [44]. downward frequency shift of the Jamning avoidauce response in Eigcwrrr~~rrrric 134]. In Zi&rrrrr~rrrricl, the gradual deceleration of the EOD frequency in the context of Figure 5 gives schematic diagrams of the neural control the jnniiiiing avoidance rcsponsc derives froiii the more mechanisms for two species of gynmotiforms, and recently discovered Sl’l’n in the nlidbrain, with NMI)A focmes ou the pacemaker nucleus in the medulla, which inputs direct 011 to the relay cell\ in the paceulaker. is composed of two ~~11 types: so-called pacemaker A homologous pathway in I~n~r/~y/~y~~o~~or~~~~~produces cells, which are intrinsic to the uucleus and seem sudden interruptions [X3”]. to generate the rhythm; and relay cells, which were orlglnally thought to pass ou the pacemaker’s rhythm We know much less about the inputs to the pacelllaker - to tble spinal cord, but which are now known to be cells ior the Ah-mu electric fkh, and little i\ known the site of additional nlodulation. With no input fro111 about the control of patterm of electric discharges, higher centers, the pacemaker cell fires at a regular where the inter-pulse interval zequmce\ have been rate, and rlie relay cells and electric organ follow described ethologically as bursts, stops, regularized in a o11e-tc>-one manner. Modulatory inputs derive pulses, scallops, variable pulses. etc. [7”.21,46]. T’hc from two sources, a pre-pacemaker nucleus (1’1’11) in medullary pacemaker of mornlyrids ib composed of a the dienccphnlou and a sub-lcnmiscal pre-paccnlnker conunand nucleus connected directly to a medullnry 774 Neural control

relay nucleus (MRN), which is connected to the spinal for detecting differential phase in Gyrr~rzardrrtr (Fig. 6~) electromotor neurons in the spinal cord in a way similar [5P]. From behavioral studies, we know that temporal to the gymnotiforms. A collateral pathway, described cues are important in recognizing pulse duration, and by Bell ef al. [47], sends neurons from the command mormyrids often have species- and sex-specific EOD nucleus to a lateral bulbar command-associated (RCA) waveforms [13,21,54]. EOD waveforms are encoded nucleus, which in turn projects back to the MRN. But as a pattern of two to three spikes by the Knollenor- inputs into the command nucleus, described briefly by gan receptors, with spike timing determined by the Bell cl a/. [47,48], have not been explored in the same waveform of the stimulus. Different parts of the body detail as those in gymnotiforms. Mormyrids differ from surface respond to one phase of the stimulus or to a 180” gymnotiforms by having a corollary discharge pathway phase-shifted EOD [13]. The Knollenorgan receptor that synchronizes the electrosensory processing in the afferents converge on large diameter spherical cells in hindbrain electrosensory lateral line lobe (ELL). Recent the nucleus of the ELL [SO], where the cells project studies have focused on the collateral pathway to the bilaterally to the midbrain nucleus exterolateralis pars BCA nucleus (the origin of the corollary discharge anterior (ELa). Here, timing information from ditf^ercnt [28”,29”]), which plays an important role in gating parts of the body surf&e is compared. The inputs electrosensory inputs and the plasticity of electrosensory terminate on the two cell types in the ELa: a large cell processing in the ELL. intrinsic to the nucleus, and a small cell projecting from the ELa to the adjacent nucleus, the exterolateralis pars In a recent paper, Kawasaki [4Y] demonstrates that posterior (ELp). Large cells are GABA-ergic, and their the ancestral mormyriform, Gymarch rlilotinrs, the terminals are on the cell bodies of the small cells [ 511. only African electric fish with a wave-like discharge, completely lacks a corollary discharge in the brain, Recently, Amagai cl (II. (S Amagai, personal colll- although it does have a connection between a command munication; S Amagai, MA Friedman, CD Hopkins, nucleus in the medulla and a lateral relay nucleus, which unpublished data; [52]) have recorded from a&rents may well be homologous to the BCA nucleus described arriving in the ELa, from large cells in the ELa, and by Bell rt a/. [47]. Unlike mormyrids, these fish lack a from cells in the ELp that receive inputs from the direct connection between the command nucleus and small cells. It has not yet been possible to record t&m the immediately adjacent MRN. In Gyrrrr~arhrs, nothing the small cells, so it is unknown how they respond to is known of the modulatory inputs into the command waveforms of different duration. Amagai rt a/. (S Amagai, nucleus, nor the mechanism of control of discharge MA Friedman, CD Hopkins, unpublished data) suggest patterns that play a role in conlnlunicatioII. that the presence of GABA in the large cells changes the flmction of the ELa from a delay-line coincidence model to a delay-line blanking model in which throughput to the ELp is inhibited if the duration of the pulse is greater than a given minimum. Thus, there may be direct input Temporal coding and electrosensory circuitry in from afTerents to small cells, which becomes inhibited if the brain preceded by a minimal delay by an input from a large cell f^rom another part of the body surf&e. Amagai ct cl/. Along with the auditory systems of barn owls and (S Amagai, MA Friedman, CD Hopkins, unpublished echolocating bats, the electrosensory systems of electric data) suggest an anatomical arrangement within the ELa fish have been neuroethological favorites for exploring that could map stimulus duration onto position within time coding and time comparison. In a recent review, the nucleus. At the next higher level in the time-coding Carr [ll] compares the mechanism by which the owl pathway, Amagai (S Amagai, personal conlnlullication) gains sensitivity to interaural time differences (ITDs), finds cells that are tuned to specific stimulus durations, and a similar mechanism by which E@wrrraru/ia senses and are thus excellent candidate cells designed to respond regional phase differences in electric stimulus waveforms to specific EOD waveforms used for species recognition. (differential phase). Both systems rely upon neural delays III a very recent study of Gyrrrr~arhs. Kawasaki and (iuo combined with coincidence detectors to produce a [We] found cells in the hindbrain that are tuned to narrowly tuned differential phase or ITD sensitivity, and differential phase, and that probably play a vital role both show extreme specializations for reducing temporal in the phase component of the jamming avoidance jitter and high-speed conduction, up to the point of response. The authors recorded from primary ‘S’-type phase comparison. The time-coding pathway in the electroreceptors, which phase lock to the 400 Hz EC)D gymnotiform &ycnrrranrlia is shown in Figure 6a. stimulus. to show that the primary afS:rents terminate Recent studies of time coding among the African directly on giant cells in the ELL, as well as sending electric fish have demonstrated at least two convergent terminals up into the inner cellular layer (I(:L) of the pathways for decoding temporal information among ELL medial zone. Intracellular fills that cross to the the mormyriforms: one for detecting the duration of opposite side of the ELL show both ipsilateral .md pulse waveforms among mormyrids (Fig. 6b) (S Amagai, contralateral projections from the giant cells into the personal communication; S Amagai, MA Friedman, ICL. It is here that the authors recorded from cells in the CD Hopkins, unpublished data; [50,51,52j) and one ICL that are sensitive to differential phase in the o&r of Convergent designs for electrogenesis and electroreception Hopkins 775

(a) Eigenmannia (b) Brienom yrus (c) Gymnarchus

Delay --__ Delay ,l

Phase A - Phase B 4

------~ Electro- Phase A Phase B Phase A receptor t t T-receptor S-receptor

ufl Electrotonic synapse 0 Giant cell 0 Large cell ---o Inhibitory synapse 0 Spherical cell 0 Small cell

Fig. 6. Time-coding pathways in the gyrnnotiform Eigenrnannia compared with those in the mormyrids, Brienonlvros ancl Gymnxc-bus. (a) In Eigerxmannin, the time-coding elcctroreceptor (T receptor) projects to spherical cells in the ELL. The spherical cell3 send axon, to the midbrain torus layer VI where they terminate on the distal dendrites of small tells, and on the somata oi giant cells. Giant cells send large axons to distant small c-ell targets. Small cells presumably act as coincident detectors, and fire only when the phase oi the stimolu3 in body region A (phase A), differs from the phase of the stimulus in body region B (phase B), by a precise time difference. (b) The Hrierromyrus time-coding pathway bcglns in the periphery wtih the Knollenorgan receptor, and leads to the spherical c-ells in the nucleus of the ELL (nELLI In the hindbrain. These giant cells project bilaterally to the ELa oi the midbrain where they terminate on large and small cells with elcctrotonic synapses. The large cells, whir-h are CABA-ergic, terminate on small cells within the nucleus. Rather than being coin<-idenc-c detectors, the small cells appear to be selectively blanked, depending on the delay oi the inhibitory input iron1 the large cells. (c) The Gyrnnnrchus time- c-(ding pathway resembles the Knollenorgan pathway only remotely. The S-receptor, which is phase-locked to the EOD \timulus, sends large axons to terminate on giant cells in the ELL, but collaterals alsu terminate In the ICL oi the ELL where differential phase-sensitive tells are found. Giant tells also send axons to tonverge on the same tell layer, and although the circuit is unknown, the cells in the ICL appear to be acting; as coincident detectors. Temporal analysis is accomplished entirely in the hindbraln In Cymnarch~. D,lta irorn (S Amagai, personal tomnlun,tJtion; S Amagal, MA Friedman, CD Hopkins, unpublished data; 151 ,52,5$‘*,57]).

tnicrosecolids. AS yet, the inputs to the IC:L cells have Ilot Conclusions been dctcrGted, but it is known that these cells do not rupond to absolute phase diffcrcnccs as do E@rtrrrurrri~I The cotttparntivc neuroethological work on electric fish cells. Instead, they ndapt to steady-state shitis in phase provides an excellent exatiiple of convergent evolution: differences over a range ofcevernl hundred trticroceconds, not sitnply th:tt electric fish evolved electric organs yet cSontirntc to show sensitivity to snull shifts in phncc, tn jcvernl tinies over, but that the mechanistns of generating the order of nCcroseconds, nt?cr a period of ndnptntion. couples dischnrges converge on dif&x-ent solutions to Whnt is striking about the rcsttlts in Gyr,rtr&rttz is the the snnlc probletn. Not only did electric comtnunication t&t th,~t the diKerentin1 phnsc conipnrisons arc nude in evolve sepnmtcl~ in several groups. but there xe the hindbrain, not in the niidbraitt, nnd that there ij J also parallels in the rhythtn control nicchnttisnis in con~~~ti.s\ural pathway ti,r tinte coding itt (Gyrrtrr&ttrx paceniakcr. Not only are time codes important in thnt is not huttd in niornivrids. However, there is Jtttost the independently evolved groups. but the sensory cotnplrte sitnilarity in the cotttputntional algorithtn for processing of temporal cues has converged on similar the .jaliinting avoidance response between Cyrrtrrtirrlrtrs cotnputntional nlgorithtns. These studies deniottstrate xx1 E(qururtrrlrricr. even though the two species evolved that electric fish continue to provide &tile ground electric orgntis nnd electroreccptors indeperidctitl~. for cotiipnmtivc studies, and the evolurionnry process 776 Neural control

continues to provide surprises for the researcher willing 17. Lorenzo D, Velluti JC, Macadar 0. Electrophysiological to do comparative studies. properties of abdominal electrocytes in the weakly electric fish Cymnofus carapo. / Comp Physiol [A] 1988, 162:141-144

18. Lorenzo D, Sierra F, Silva A, Macadar 0: Spinal mechanisms of electric organ discharge synchronization in Gymnotus carapo. / Comp f’hysiol [A] 1990, 167:447-45X. Acknowledgements 19. Macadar 0, Lorenzo D, Vcllut~ JC: Waveform generation of the electric organ discharge in Gymnotus carapo. II. I thank Matthew Friedman and Gamy Harned for help with Electrophysiological properties of single electrocytes. / Cornp the figures and for fruitful discursions during the pnqxw~~tion of Physiol (A/ 1989, 165:353-360.

this review. Supported by grant #MHZ37972 from the Ndtional LO. Alves-Games J, Ort; C, Haygood M, Helllgcnberg W: Institute of Mental Health. . . Phylogenetic analysis of the South American electric fishes (order gymnotiformes) and the evolution of their electrogenic systems: a synthesis based on morphology, electrophysiology, and mitochondrial sequence data. Mel Biol Evol lY?5, 12:298-318. References and recommended reading This paper suggests a new phylogeny ior gymnotlform elettnc tl\he\ based on sequence data from mitochondrial DNA. The sample ~nc lude\ 19 genera and all six iamilies, with six catfish selected ior out-group Papers of particular interest, published within the annual period of comparison. Someofthe results are ronslstent with earlier morphologic al review, have been highlighted as: studies, but this new paper revises relatIonshIps between Eigmrnanni,~ . of special interest and Stemopygus (by separating Sfcmopygus as an independent groupJ, . . of outstanding interest and between Hypopomidae and Rhamphichthyidae.

1. Lissmann HW: Continuous electric signals from the tail of a 21. Hopkins CD: Behavior of mormyridae. In Elecfrorewpfwn. fish, Cymnarchus doticus Cuv. Nature 1951, 167:201-202. Edited by Bullock TH, Heiligenherg WF. New York. John Wiley X Sons; 1986:527-576. 2. Lissmann HW: On the function and evolution of electric organs in fish. J Exp Biol 1958, 35:156-l 91. 22. Jacob BA, McEachran JD, Lyons PL: Electric organs in skates: . variation and phylogenetic significance (Chondrichthyes: 3. Lissmann HW, Machin KE: The mechanisms of object location Rajoidei). 1 Morphol 1994, 221:45-63. in Cymnarchus niloficus and similar fish. / Exp Viol 1958, Combining a phylogenetic analysis of the Rajold skates wth an analy\l\ 35:457-486. oi the morphology of the electric organ shows how cell types 111elec trlc 4. Bullock TH, Heiligenberg W (Eds): Electroreception. In Wiley organs correspond to phylogenetlc groupings. Series In Neurobiology. New York: John Wiley & Sons Inc.; 23. McEachran JD, Mijake T (Edsl: Phylogenetic interrelationships 1986. of skates: a working hypothesis (Chondrichthyes Rajoidei). In 5. Hopkins CD: Neuroethology of electric communication. Annu Advances in the Biology, Ecology, Sysremarics and 9arur ol th(, Rev Neurosci 1988, 11:497-535. Fisheries. NOAA TechnIcal Report. Edited by Pratt HL. (;ruber SH, Tamuchl T (Series editor). 1990, 90:285-X)4. 6. Kramer B: Electrocommunication in teleost fishes: behavior and experiments. In Zoophysiology, vol 29. Berlin: Springer-Verlag; 24. Bratton H, Ayers 1: Observations on the electric organ 1990. discharge of two skate species (Chondrichthyes: Rajidae) and its relationship to behavior. Environ Hiol fishrs lY87. Moller P: Electric fishes: history and behavior. In Fish and 7. 20:241-254. . . Fisheries Series. Edited by Pitcher TJ. London: Chapman & Hall; 1995. 25. Hagedorn M, Womhle M, Finger T: Synodontid catfish: a A broad survey of electric fish behaviors, including early hlstoncal new group of weakly electric fish. Wra~n Behav Evol 1 YYO. references. 35~268-277.

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10. Helligenberg W: Neural nets in electric fish. In Compvtafiona/ are given in bursts oi 5 to 10. Neurosr~ence Series. Edited by Sejnowskl TJ, Poggio TA. 27. Baron VD: African C/arias catfish elicits long-lasting weak Camhridge, Massachusetts: MIT Press; 1991. . electric pulses. Experenria 1994, 50:644-647. 11. Carr C: Processing of temporal information in the brain. Annv The African catilsh C/arias garfepinus produces very weak, very low Kev Neurosri 1993, 16:223-243. irequenry electric discharges when InteractIng in social sltuatlon\

12. Finger TE, Bell CC, Carr CE: Comparisons among electro- 28. Bell C, Van der Emde C: Electric organ corollary discharge receptive teleosts. Why are electrosensory systems so similar? . . pathways in mormyrid fish. II. The medial juxtalobar nucleus. In Elertroreception. Edited by Bullock TH, Helligenherg W / Comp Physml [Ai 1995, 177:463%480. New York: John Wiley & Sons Inc.; 1986:465-481. This paper descrthes an electrophysiologlcal examlnatlon ot a wc ond command pathway In mormynd fish. Lesion\ to the luxt,llobar nut let)\ 13. Hopkins CD: Temporal structure of non-propagated electric show that this pathway mediates the major sensory eifect\ 11, the communication signals. Brain Eehav Eva/ 1986, 28:43%59. mormyromast zones of the ELL, and IS probably important in prowdlng prec 1se tlmlng Information to the lobe. 14. Bennett MVL: Electric organs. In Fish Physiology. Edited by Hoar W, Randall DJ. New York: Academic Pre\\; 29. Bell C, IIunn K, Hall C. Caputl A: Electric organ corollary lY71:347-491. . . discharge pathways in mormyrid fish. I. The mesencephalic command associated nucleus. / Camp Physiol [Ai 1’195, 15. Maradar 0: Motor control of waveform generation in 1771449-462 Cymnotus carapo. 1 Camp Physfol [A] 1993, 173:728-729. Explores the phyw)logy oi the mesencephallc command-aswc bated 16. Caputi A, Macadar 0, Trujillo-Cen6z 0: Waveform generation nucleus In the mormyrld tish and shows that It IS respowhle ior gatlng of the electric organ discharge in Cymnotus carapo. III. Knollenorgan responws 111 the medulla, tor descending exritatlon 01 Analysis of the fish body as an electric source. / Camp granule cells in the mormyrornast zone oi the ELL and lor varlou\ Physiol /A/ 1989, 165.361-370. other senwry functions. This command pathuay does not appear to lx Convergent designs for electrogenesis and electroreception Hopkins 777

Involved in plasticity of electrosensory responses in the mormyromast Using electron microscopy, the authors trace fibers from the pre- zone pacemaker in Hy~.‘o~ornus to the pacemaker, and demonstrate the presence oi CABA- and glutamate-immunoreactive synapses on the Dye JC, Meyer JH: control of the electric organ 30. Central surface oi pacemaker cells. Only glutamate-irnnlunoreactive synapses discharge in weakly electric fish. In E/ertrorereptio/l. Edited are hmd on relay cells. by Bullock TH, Heiligenberg WF. New York: John Wiley & Sons Inc.; lY86:71-102. 46. Crawiord J: Sex recognition by electric cues in a sound- producing mormyrid fish, Pollimyrus isidori. Brain Echav Evol 31. Dye J, Heilrgenberg W, Keller CH, Kawasaki M: Different 1991, 38.20-38. classes of glutamate receptors mediate distinct behaviors in a single brainstem nucleus. Pro<- MU/ Acad Sri USA 1989, 47. Bell CC, Libouban S, Sraho T: Neural pathways related to the 86:8993-8997. electric organ discharge command in mormyrid fish. / Camp 32. Heilrgenberg W, Keller CH, Metrner W, Kawasakr M: Neural 1983, 216:327-338. Structure and function of neurons in the complex of the 48. Grant K, Bell CC, Clausse S, Ravallle M: Morphology and nucleus electrosensorius of the gymnotiform fish, Figenmannia: physiology of the brainstem nuclei controlling the electric detection and processing of electric signals in social organ discharge in mormyrid fish. / Camp Neural 1986, communication. / Comp Physd /A/ 1991, 169:151-164. 245:51 L530. 33. Kawasaki M, Maler L, Kose C, Helligenberg W: Anatomical and functional organization of the prepacemaker nucleus 49. Kawasaki M: The African wave-type electric fish, Gymnarchus in gymnotiform electric fish: the accommodation of two . . niloticus, lacks corollary discharge mechanisms for electrosen- behaviors in one nucleus. / Cornp Neural 1988, 276:113-131. sory gating. / Camp Physiol [A] 1994, 174:133-144. Intracellular recordings tram the pacemaker of Gymnarthus n~lotirus 34. Kawasaki M, Heiligenberg W: Individual prepacemaker with single-cell tills demonstrates that Gymnarc-husi has a pacemaker neurons can modulate the pacemaker cycle of the gymno- or command cell connected indirectly to a relay cell, but that the tiform electric fish, Figenmannia. / Cornp f’hysiol iA/ 198R. pathway runs to a lateral MRN. There is no evidence ot a corollary 162:13-21. discharge system in Cymnarthus.

35. Kawasaki M, Heiligenherg W: Distinct mechanisms of 50. Bell CC, Szabo T: Electroreception in mormyrid fish: modulation in a neuronal oscillator generate different social central anatomy. In E/ectrorecept,on. Edited by Bullock signals in the electric fish Hypopmus. / Camp Physfol /A/ TH, Heiligenberg W. New York: John Wiley & Sons Inc.; 1988. 165:731-741. 1986:375%4’1. 36. Kawasaki M, Heiligenberg W: Different classes of glutamate 51. Mugnaini E, Malet L: Cytology and immunocytochemistry of receptors and CABA mediate distinct modulations of a the nucleus exterolateralis anterior of the mormyrid brain: neuronal oscillator, the medullary pacemaker of a gymnotiform possible role of CABAergic synapses in temporal analysis. Anat electric fish. / Ncurosc-I 1990, 10.3896-3904. Embryo/ 1987, 176:313-336. 37. Rose (;I, Kawasaki M, Helligenberg W: ‘Recognition units’ at the top of a neuronal hierarchy? Prepacemaker neurons 52. Arnagai 5: Time coding in the electrosensory system of in Eigenmannia code the sign of frequency differences mormyrid fish [PhD thesis]. Ithac-a. New York. Cornell University; 1993. unambiguously. / (romp fhysrol [A/ 1988, 162:75Y-772.

38. Spiro JE. Brose N, Hernemann S, Helllgenberg W: Immunolo- 53. Kawasaki M, Cue Y-X: Timing comparison circuitry in the . . calization of NMDA receptors in the central nervous system of . . electrosensory lateral line lobe of an African wave-type weakly electric fish: functional implications for the modulation electric fish, Cymnarchus niloticus. / Ncurosti 1996, in press. of a neuronal oscillator. / Neurocti 1994, 14.6289-6299. Kecordlngs trorn cells in the ICL ot the ELL I” Gymn~rthus show that An artlbodv to NMDA receptor< labels the relay cells in the pace- there are cells sensitive to phase dliierentes on ditferent parts oi the maker-hut not the pacemaker cells themselves--n gyrnnotrtorm tish body suriace. Intracellular recordings demonstrate that afierent fibers The rtxrlt\ cupport the physlologlcat tindlngs with NMDA blockers. converge on the Inputs irom large tells 111the ELL, and that large cells $cnd collateral\ to the opposrte side tri the ELL. Time comparison appears 39. Zupanc GKH. Maler L: Evoked chirping in the weakly electric to take place entirely in the ELL. fish Apleronotus leplorhynchus: a quantitative biophysical analysis. Can / Zoo/ 199 3. 71:.?301-2310. 54. Hopkrnc C-D, Bass AH: Temporal coding of species recognition signals in an electric fish. Scienct~ 1981, 212:85-87. 40 HopkIn\ CD: Electric communication: functions in the social behavior of Eigenmannia virescens. Hehcwour 1974, 55 Hopklns CD: Electric organ discharges and phylogenetic 50 270.-305. . analysis. tn 4th lnrerna~ional Congrex o/ Neurocthobgy. Edited by Burror+s M, Matheson T, NeL%,land 1’. Sc huppe H. Cambridge, 41. Hagcdorn M, Herllgenberg W: <:ourl and spark: electric signals UK: Ceorg Thrern; 1995:418. in the courtship and mating of gymnotoid fish. Anim Rchab A phylogenetic analysis ot the mormyrldae, with an examination oi 1985, 33:25&LhS the pattern ot evolutlor1 ot electric organ morphology in relation to 42. Hophln\ CD: Electric communication in fish. Am Sri 1974, phylogeny 62 4Lh~437. 56. Bass AH: Species differences in electric organs of mormyrids: 43 Hagtsdorn M: The ecology, courtship, and mating of substrates for species-typical electric organ discharge wave- gymnotiform electric fish. in Electrorcr eprror?. Edited by forms. / (-amp Ncurol 1986. 244:313-3 1:). Bullock TH. Helllgenherg H. New York: lohn Wiley R Sons tnc; 1986:497~526. 57. Carr C-, Herligenherg W, Rose (;. A time-comparison circuit in the electric fish midbrain. I Neurorc-r 1986, 10:3LL7-3246. 44. Kawasakr M: Comparative studies on the motor control mechanisms for electric communication in gymnotiform fishes. / C-o!o,npPhysiol [A] 1993, 173:726-728.

45. Kennedy G, Helhgenherg W, Ultrastructural evidence of . . CABA-ergic inhibition and glutamatergic excitation in the pacemaker nucleus of the gymnotiform electric fish, Hypopomus. / (amp Physrol [A] 1994, 174:267-280.