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Axolemmal and Septal Conduction in the Impedance of the Earthworm Medial Giant Nerve Fiber

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Axolemmal and Septal Conduction in the Impedance of the Earthworm Medial Giant Nerve Fiber 692 Biophysical Journal Volume 67 August 1994 692-695 Axolemmal and Septal Conduction in the Impedance of the Earthworm Medial Giant Nerve Fiber Todd L. Krause,*t Harvey M. FRshman,t and George D. Bittner* -Depar of Zoology, Instiuhe for Neurscienc, College of PhaFracy, Univesty of Texas, Austin, Texas 78712, and tDeparment of Physioly & Biogysics, Uniersity of Texas, Mecical Branch, Galveston, Texas 7755-0641 USA ABSTRACT Ionic conducto in Fe axolemmal and sepa miem gbranes of the mecial giant fiber (MGF) of the earthworm (EW) Lumbricus terretris was asessed by impedance specrcopyin the frequency range 2.5-1000 Hz. Impedance loi in the complex plane were descrbed by two semfi-crcular arcs, one at a owr characterstic frequency (100 Hz) and the oer at a higherfrequency (500Hz). The bwerfrequency arc had a chord r e of 53 kil and was not fected by membrane potential changes or ion channel blockers [tetrodotoxin (TTX), 3,4-diaminine (3,4-DAP), 4-aminopyridine (4-AP), and tetraethyl- ammonium (TEA)]. The higherfrequency arc had a chord resistance of 274 kIl at resting potenal, was voltage-dependent, and was affected bythe addition ofTTX, 3,4-DAP, 4-AP, and TEAto the physiological EW salines. When all four blockers were added to the bathing solto, the imnpdance locus was desrbed by two volag-inrpend arcs. Cosidering the effects of these and other (i.e., Cd and Ni) ion channel blockers, we conclude that 1) the higher frequency ocus reflects condutio by voltage- sensitive ion diannels in the axokemmal Imebrane, which contains at least four ion channels selctive for sodium, calium, and potassium (delayed recifierand calcium-dependent), and 2) the owerfrequency locus reflect voltage-insensitive channels in the septal membrane, which separates adcet MGFs. INTRODUCTION The earthworm medial giant fiber (MGF) has been charac- MATERIALS AND METHODS terized morphologically as having septa that partition adja- cent MGFs (Coggeshall, 1966). Several studies in MGFs have analyzed the conduction of gap junctions in the septal The ventral nerve cod (VNC) of an adult earthworm contain one medial membrane (Brink and Barr, 1977; Brink et al., 1988; Brink giant fiber (MGF) and two lateral git fibes 15- 20an in length. Each and Fan, 1989) but have only alluded to the presence of MGF is comprised of many (150-200) individual axons about 60-100 pm in diameter and 1-2 mm in length that are connected in series at their ends sodium and potassium channels in the axolemmal membrane by low resistance gap juncons in m brane p ing struetures caLed (Brink and Barr, 1977). "septa" (Bullocket al, 1965; Coggeshafl, 1966; Brink and Barr, 1977). Eac Studies in our laboratories have focused on understanding MGF is atached to its cel body by a lng, thin (p ably high resistance) cellular responses of the MGF to injury, such as axonal seal- proces (Mulloney, 1970; Gunther, 1975). To obtain a VNC in vitro, we first anesthetized sexually mature earth- ing (Krause et al., 1994) and regeneration (Birse and Bittner, worcs in 0.2% (w/w) chlortone. Then, the VNC was dissected f the 1976; 1981; Lyckman and Bittner, 1992). We used imped- animal (see Krause et aL, 1994) and pinned out in a Sylgard coated dish with ance spectrscopy (rapid acquisition of complex im dance a physiological earthworm salie (EWS) ctaining 40.1 mM NaC, 10 mM [Z(jf)] data and subsequent analysis and model fitting in the Na2SO2, 25 mM Na-acetate, 0.5 K2S04, 3.0 mM CaCl, 0.5 mM MgCI2, 1.25 mM Tris, 1.4 mM HEPES, and 50-100 pM carbachol of pH 7.4 and frequency domain (FLshman, 1992)) to quantify electrical 180 mOsm. changes in axolemmal integrity caused by injury and recov- ery from injury (Krause et al., 1994). In the course of these studies, we quantified the effect of secific ion-channel Impedance blockers on Z(jf) and identified the types of voltage- sensitive axolemmal ion channels that dominate the The tips of two glass eectrolyte-filled (3 M Ka) microeectrodes were imped- placed ntraellularly withm 40 pm of each other for "driving-poit" im- ance of the earthworm MGF. Further, we identified conduc- pedance measurements. Micrekxlrodes were placed between nerve roots tion in septa of MGFs from a voltage- and channel blocker- where septa often exist in the MGF. Therfore, electrode-to-septum distance insensitive feature in Z(jf). (0.5-1 mm) was less than one-third of the space constant of the preparaton (2.5-35 mm). Current was applied through one electrde, and membrane voltage was recorded with the other electrode. hmpedance determinatin were made by impedance spectrscopy (see Fishman, 1992). Briefly, a synthesized waveform composed of the sum of 400 sinusoids, from 2.5 to 100OHz m minements of 25Hz, was applied repetitively to the preparation as a "snall signal" acurent (10 nA) to elicit Raeeid for publica 30 March 1994 and in final form 27 May 1994. linear responses. The syntesized, periodic signal was synchronized with a sampled-data acquisition system and fast Fourier computation to preserve Address to Dr. reprint requests Harvey Fishman, Department of Physiology the phase relatioships between response and driving sinusoids and to enable & of Build- Biophysics, University Texas, Medical Branch, Basic Science rapid collction of data (400 ms for a single impedance fimcton of fre- Galveston, TX 77555-0641. TeL: 409-772-1826; Fax: ing, 409-772-3381; quency). The measued driving current, thrugh the preparatiOn was E-mail: hn eutmb.edu. i(tQ) Fourier tansformed to the frequency (J) domain to obtain the complex o 1994 by the Biophysical Society current, I(jf), expressed in a data array and stored as real and iuaginary 0006-3495/94/0692A04 $2.00 [i = (-I)- parts at each of the 400 discrete frquencies of the sinusoids Krause et al. Krue.Irrnce of Earthworm Meial Giant Fiber 63 in the i(t) waveform The MGF voltage response, tQ), to i(t) was subse- amnpydin [3,4,-DAP, 1 mM, 4-aminridie [4-AP; 1 mM] and tet- quently Fourier-tansformed to give the complex vohge response, V(jf), raethyl im [TEA; 20 mM] were used to block delayed rectifier po- and stored as another data array of real and imaginay values at the same tassium channels 20 mM TEA was also used to block Ca-activated discrete fequancies at whid t were made for i(t4 The com - posium channels plex impedance, Z(jf), at ach of the 400 disarete fequencies of the si- nusoids that d (t) sthen computed from the stored data arrays as the complex ratio o(jf[I(jf) RESULTS The complex impednce was expressed in rectangula form as Pharml gical on 4f) of the MGA Z(jf) = R(f) +jX(f) In the absence of ion channel the of where the realpan, R(J), and iuagnay part, X(f), reflect the dissipative any blockers, sign (resistive) and energy sorae (capacitive) p es ofthe MGF condung the imaginary part [X(f)] of Z(jf) (Eq. 1) was positive system, respctively. Z(jf) dinati were plotted as X(f) vs. R(f) from 2.5 to 25 Hz for a 20-mV depolaion from rest (see (cmplex plane plots) at each of the 400 frequncies of the sinusoidal cxn- data points above abscissa for -50 mV locus in Fig. 2A). ponents in i(t For each Z(jf) detennation, pwarmetesnin an pedance AnX(f) >0 represents an inductive-like reactance caused by model (see next section) were varied systematically until a ium mean and and sodium con- square enro between the real and maginayparts ofZ(jf) and those ofthe time- voltage- dependent potassium modelwere obtained (see Fishman and Lipicky, 1991) The impedae locus ductances in the axolemmal membrane (Poussart et al., of the model with se pameter vae consed e best fit to Z(jf), 1977), as fit reported in the squid giant axon by Cole and which are repire by a solid line in each Z(jf) plot. Baker (1941). This inductive-like reactance remained In eveiy MGF measured, three Z(jf) d were made, one at the resting m brn potential (typicaly -70 mV) and one each at ±20 mV fiom the resting potential by applying a direct acent beginning 400 ms before the data acquiston intervaL 60 A=% EWS c ,.~. -70mV pl. I I "..W U. - I Impedance model OW% !:I - ~ ~~~~~~~~bS "X , A series impedance model [Z(jf) = Z,(jf) + Z(jf) + Z,(jf)J was fitted x -6D- to Z(jf) data obtained from each MGF. In this simple model (Fig1), Z, is * -5knB. the ohmic xcess resistance (Le, axoplasmic resistance and soutioD resis- tance to the bath grmd electode) R,. Z.(jf) represents the voltage- S. independent impedance of the septum, and Z((jf) rpesents the voltage- EWS+TTX+4AP+3,4DAP and time-dependent impedane of the axolemma (Cole, 1972) Values for . ..-7mV each ofthe electical elements in Fig 1 were obained from a fit ofthe series fiLLsg | | | - r ^ | | . .. impedance model to Z(jf) measmred duing a 20-mV hyperpoarizatio of nV \.P' the MGF from rest, a memb potential at which volage-dependent ion conduction should be n gible. Zjf) data were then obtained at resting >t=-~~~~% potental and at a depolarization of 20 mV from rest These data were fitted with the series impedance model holding the values for R, and Z, cnstant and equal to the values obtainedin the hyperpolaized state, whereasZ was -18OD aBowed to vary. C. 60. C EEWS+TTX+4AP+3,40AP+TEA Pharm colgial i nof ion canels -4 Known blocks of several common ion channel types (for potasum chan- ;f I .
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