The Journal of Neuroscience, June 12, 2013 • 33(24):10021–10026 • 10021 Brief Communications Squid Have Nociceptors That Display Widespread Long-Term Sensitization and Spontaneous Activity after Bodily Injury Robyn J. Crook,1 Roger T. Hanlon,2 and Edgar T. Walters1 1Department of Integrative Biology and Pharmacology, University of Texas Medical School at Houston, Houston, Texas 77030, and 2Program in Sensory Physiology and Behavior, Marine Biological Laboratory, Woods Hole, Massachusetts 02543 Bodily injury in mammals often produces persistent pain that is driven at least in part by long-lasting sensitization and spontaneous activity (SA) in peripheral branches of primary nociceptors near sites of injury. While nociceptors have been described in lower verte- bratesandinvertebrates,outsideofmammalsthereislimitedevidenceforperipheralsensitizationofprimaryafferentneurons,andthere arenoreportsofpersistentSAbeinginducedinprimaryafferentsbynoxiousstimulation.Cephalopodmolluscsarethemostneurallyand behaviorally complex invertebrates, with brains rivaling those of some vertebrates in size and complexity. This has fostered the opinion that cephalopods may experience pain, leading some governments to include cephalopods under animal welfare laws. It is not known, however, if cephalopods possess nociceptors, or whether their somatic sensory neurons exhibit nociceptive sensitization. We demon- strate that squid possess nociceptors that selectively encode noxious mechanical but not heat stimuli, and that show long-lasting periph- eral sensitization to mechanical stimuli after minor injury to the body. As in mammals, injury in squid can cause persistent SA in peripheral afferents. Unlike mammals, the afferent sensitization and SA are almost as prominent on the contralateral side of the body as they are near an injury. Thus, while squid exhibit peripheral alterations in afferent neurons similar to those that drive persistent pain in mammals, robust changes far from sites of injury in squid suggest that persistently enhanced afferent activity provides much less information about the location of an injury in cephalopods than it does in mammals. Introduction pain, but outside of mammals very little is known about nocice- Almost all animals exhibit defensive responses to noxious stimuli ptor sensitization (Walters and Moroz, 2009). Sensitization of that cause incipient or immediate tissue injury (Kavaliers, 1988; peripheral branches of nociceptors has not been found in noci- Walters, 1994). Primary nociceptors that encode noxious stimuli ceptors of lower vertebrates (Ashley et al., 2007) and has been have been reported in several invertebrate and vertebrate species demonstrated in only two invertebrates, a leech (Pastor et al., (Smith and Lewin, 2009), but most research has focused on 1996) and a gastropod mollusc, Aplysia (Billy and Walters, 1989; mammalian nociceptors because of their critical roles in normal Illich and Walters, 1997); less direct evidence for nociceptor sen- and abnormal pain states (Basbaum et al., 2009). A distinctive sitization also exists for Drosophila (Babcock et al., 2009). SA in property of mammalian nociceptors that plays a major role in nociceptors persisting for days or longer after injury or inflam- long-lasting pain is nociceptive sensitization, often expressed af- mation has only been described in mammals (Djouhri et al., ter injury or inflammation as increased sensitivity of peripheral 2006; Walters, 2012). branches and sometimes by persistent generation of spontaneous Cephalopods have the largest and most complex brains activity (SA) (Gold and Gebhart, 2010; Walters, 2012). Compar- among the invertebrates, encouraging the opinion that cephalo- ison to similar phenomena in nonmammalian species may pro- pods may experience pain, and leading some governments to vide insight into the evolution of mechanisms that can promote include cephalopods under animal welfare laws (Harvey-Clark, 2011). However, almost nothing is known about their nocicep- Received Feb. 11, 2013; revised May 3, 2013; accepted May 8, 2013. tive systems. Here, we show that a cephalopod—the squid Author contributions: R.J.C., R.T.H., and E.T.W. designed research; R.J.C. and E.T.W. performed research; R.J.C. Doryteuthis (Loligo) pealeii—possesses nociceptors that exhibit and E.T.W. analyzed data; R.J.C., R.T.H., and E.T.W. wrote the paper. nociceptive alterations with interesting similarities and differ- This work was supported by NSF Grants IOS-1146987 to E.T.W. and IOS-1145478 to R.T.H., and the Baxter ences to those reported in other invertebrates and mammals. Pharmaceuticals Fellowship and Bang Summer Research Fellowship from the Marine Biological Laboratory to R.J.C. We thank Katharine Dickson and the staff of the Marine Resources Center at the Marine Biological Laboratory for assistance with animal care. Materials and Methods The authors declare no competing financial interests. Correspondence should be addressed to either Robyn J. Crook or Edgar T. Walters, Department of Integrative Animal subjects, welfare considerations, and anesthesia. Male and female Biology and Pharmacology, University of Texas Health Science Center at Houston, 6431 Fannin Street, Houston, TX adult squids were collected near Woods Hole, MA and housed in groups 77030, E-mail: [email protected] or [email protected]. of 2–8 for up to 2 weeks in running seawater (17Ϫ21°C) and fed small DOI:10.1523/JNEUROSCI.0646-13.2013 fish (Fundulus spp.) daily. General guidelines for vertebrate animal wel- Copyright © 2013 the authors 0270-6474/13/3310021-06$15.00/0 fare were followed (Crook and Walters, 2011; Crook et al., 2011). Ani- 10022 • J. Neurosci., June 12, 2013 • 33(24):10021–10026 Crook et al. • Squid Nociceptors Sensitize after Injury Figure 1. Squid have mechanosensitive nociceptors in their fins. A, Innervation of the fin by the fin nerve, which provides the only direct communication with the brain. B, Fin nerve branches visualized by injection of air into the main trunk of the fin nerve. C, Excised fin-nerve preparation showing the recording electrode, crush injury site, and medial (M) and caudal (C) test sites on the fin, proximal and distal to the crush site, respectively. D, Extracellularly recorded activity evoked by pressing a glass probe (1.5 mm diameter) with progressively increasing force against the fin. Arrowsindicatethefirstappearanceofidentifiedunits.E,ActivityatthesamesiteevokedbyapplicationofvonFreyfilamentshavingprogressivelygreaterbendingforces(indicated).Coloredarrows indicate initial spikes of the same units shown in D. mals in poor health were killed by prolonged immersion in isotonic Results MgCl2 solution. Sample sizes for studies of in vivo injury were limited to Squid have nociceptors those needed to demonstrate large effects. Experimental injury to the fin The excised squid fin (Fig. 1A) receives extensive innervation (Fig. 1) was small relative to the size of the animal and compared with naturally occurring, survivable injuries observed by us. Injured animals ex- from the CNS (Fig. 1B), and afferent activity from neurons in- hibited normal feeding behavior and unchanged mortality rates. Before fin nervating the entire fin can be recorded via a suction electrode on the cut end of the fin nerve (Fig. 1C). Pressing a smooth probe excision, animals were anesthetized by immersion in isotonic MgCl2 solu- tion (Mooney et al., 2010) until chromatophores relaxed and respiratory against the excised fin with steadily increasing force (Fig. 1D) movements ceased, then killed by decapitation and decerebration. usually produced a progressive recruitment of single units (in- Injury and noxious stimulation. In both the excised fin-nerve prepara- dicated by spikes with constant amplitude and waveform) and tion and in vivo, four firm crushes (ϳ2 s each) were delivered with an increase in firing rate. The same pattern was observed when serrated forceps (2 mm width) to the site in Figure 1C. Crushes extended test stimuli were delivered via a series of four von Frey fila- 10–15 mm into the fin. Control fins received light touch from the same ments (Fig. 1E) that deliver relatively constant forces (0.2, 4, forceps. In some excised fins, several 0.5 ml injections of isotonic MgCl 2 10, or 100 g) during bending. At the illustrated test site (Figs. solution containing Fast Green dye were made subcutaneously and in- tramuscularly through a 23 gauge needle at the site to be crushed. Visu- 1D,E), units with smaller spike amplitudes happened to be alization both of the dye and relaxation of cutaneous chromatophores activated by weaker stimuli than units with larger amplitudes, demonstrated that the solution spread Ͻ1 cm from the injury site during but no significant differences were found in amplitudes of the postinjury test period. Peripheral injection of isotonic MgCl2 into spikes evoked in single units having low and high mechanical molluscs produces local blockade of action potentials (Crook and Wal- thresholds (n ϭ 16 preparations). ters, 2011). Fin mechanoafferents exhibited many different spike ampli- Testing and extracellular recording. Afferent spike activity was sampled tudes, but most test sites had units with medium to large spikes at 20 kHz via a suction electrode on the fin nerve central to the excised fin (10–80 ␮V under our recording conditions; Fig. 1D,E). Test (Kier et al., 1985). This is the only nerve connecting the fin to the CNS stimulation of these sites always evoked responses from multiple (Young, 1976). Test stimuli were delivered