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N Vitro Classical Turtles Conditioning of Abducens Nerve Discharge In The Journal of Neuroscience, July 1995, E(7): 5036-5048 /n vitro Classical Conditioning of Abducens Nerve Discharge in Turtles Joyce Keifer, Kim E. Armstrong$ and James C. Houk Department of Physiology, Northwestern University Medical School, Chicago, Illinois 60611 In vitro classical conditioning of abducens nerve activity in responseto a cornea1airpuff. When a previously neutral au- was performed using an isolated turtle brainstem-cerebel- ditory conditioned stimulusis paired with the airpuff, following lum preparation by direct stimulation of the cranial nerves. a number of paired stimulus presentations,the eyelid closesin Using a delayed training procedure, the in vitro preparation responseto the auditory stimulus. Previous studieshave shown was presented with paired stimuli consisting of a 1 set that the neural pathways important in conditioned and uncon- train stimulus applied to the auditory nerve (CS), which im- ditioned blink responsesare located in the brainstem and cere- mediately preceded a single shock US applied to the tri- bellum. However, the cellular mechanismsunderlying this form geminal nerve. Conditioned and unconditioned responses of motor learning remain elusive. were recorded in the ipsilateral abducens nerve. Acquisi- In vitro model systemsof associative learning have been es- tion exhibited a positive slope of conditioned responding tablished in invertebrate speciessuch as the marine mollusks in 60% of the preparations. Application of unpaired stimuli Aplysia (Lukowiak and Sahley, 1981), Limax (Chang and Gel- consisting of CS-alone, alternate CS and US, or backward perin, 1980), Hermissenda(Farley and Alkon, 1987), and Pleu- conditioning failed to result in conditioning, or resulted in robrunchaea (Mpitsos and Davis, 1973). These preparations extinction of CFis. Latencies of CR onset were timed such have greatly facilitated our understandingof the neural mecha- that they occurred midway through the CS. Activity-depen- nismsof learning and memory. Similar in vitro model systems dent uptake of the dye sulforhodamine was used to ex- of associativelearning in vertebrates has previously been ham- amine the spatial distribution of neurons labeled during pered by the technical limitations of maintaining the viability of conditioning. These data showed label in the cerebellum large portions of intact neural tissue in isolation. This problem and red nucleus during conditioning whereas these was overcome by the development of an in vitro brainstem- regions failed to label during unconditioned responses. cerebellumpreparation from the turtle which takes advantageof Furthermore, the principal abducens nucleus labeled this species’extraordinary resistanceto anoxia (Keifer and Houk heavily during conditioning. These findings suggest the 1991). Extensive neural connectionsare preserved in this prep- feasibility of examining classical conditioning in a verte- aration which allow the activation of sensorypathways by direct brate in vifro brainstem-cerebellum preparation. It is pos- electrical stimulation of the cranial nerves rather than by more tulated that the abducens nerve CR represents a behavioral natural stimuli such as an airpuff or tone. correlate of a blink-related eye movement. Multiple sites of Recently, an in vitro model of the turtle eye-blink reflex was conditioning are hypothesized, including the cerebelloru- developed (Keifer, 1993a).Neural dischargecharacteristic of the bral circuitry and brainstem pathways that activate the eye-blink reflex was evoked by a single shock stimulus applied principal abducens nucleus. to the trigeminal nerve, which contains afferents from the face [Key words: classical conditioning, in vitro, abducens and eye, while recording from the ipsilateral abducensnerve, nerve, turtle, positive feedback, sulforhodamine] which contains efferent fibers that project to the eye muscles. This abducens nerve discharge was postulated to represent a Conditioned responselearning is a simple form of associative neural correlate of the eye-blink reflex becauseit corresponded learning. A promising model to study the neural mechanisms closely with extraocular EMGs recorded during eyeball retrac- underlying associative learning is the classically conditioned tion in a reducedpreparation, and correspondedwell with EMGs eye-blink reflex (Thompson 1986; Glickstein and Yeo, 1990; obtained during airpuff-evoked blinks in alert turtles. Hence, the Bloedel et al., 1991). In behaving mammals,this responsecon- abducensnerve responseobtained in vitro appearsto be a neural sists of retraction of the globe into the orbit which allows the model of the behavioral eye-blink reflex. nictitating membrane/eyelidto sweep passively acrossthe eye Behaving turtles have previously been conditioned using a variety of classical and instrumental training paradigms (An- drews, 1915; Poliakov, 1930; Boycott and Guillery, 1962; Spi- Received Oct. 27, 1994; revised Feb. 17, 1995; accepted Feb. 21, 1995. gel, 1963; Pert and Bitterman, 1969; Graf and Tighe, 1971; This study was supported by NSF Grant BNS-9109572 to J.K. J.C.H. was Manton et al., 1972). A classically conditioned head withdrawal supported by POl-NS-17489. We thank Dr. John Disterhoft for helpful com- responseto a tone was reported (Poliakov, 1930). Using a more ments on the manuscript. Correspondence should be addressed to Dr. Joyce Keifer, Depart complex discrimination paradigm, Andrews (1915) conditioned ment of Anatomy and Structural Biology, University of South Dakota, School turtles to withdraw from a feeding station at the sound of a of Medicine, Vermillion, SD 57069. “Present address: Beckman Institute, University of Illinois, 405 North Ma- whistle and to approach at the sound of a bell. Brightnessand thews Avenue, Urbana, IL 61801. olfactory discrimination learning has also been achieved (Boy- Copyright 0 1995 Society for Neuroscience 0270-6474/95/155036-13$05.00/O cott and Guillery, 1962; Spigel, 1963). The Journal of Neuroscience, July 1995, 15(7) 5037 In this study, the in vitro brainstem-cerebellumpreparation pulses) applied to the posterior root of the eighth nerve (CS) which was presentedwith paired stimuli consisting of a train stimulus immediately preceded a single shock to the trigeminal nerve (US), and both stimuli terminated simultaneously. In experiments using the rabbit, applied to the posterior eighth nerve (CS), which contains pri- CS-US intervals were typically between 250-500 msec (Gormezano et marily auditory fibers, and immediately precededa single shock al.. 1962: McCormick and Thomvson. 1984: Disterhoft et al.. 1985: stimulusapplied to the trigeminal nerve (US). The results show Ye; et ai., 1985), although intervals ranging ‘up to 1 set have’proven that this pairing procedure results in a classically conditioned adequate for conditioning (Schneiderman and Gormezano, 1964). Due to the cooler temperature of the turtle preparation (22-24”(Z) and pre- abducensnerve responsegenerated entirely in vitro. Application sumed slower conduction time as compared to the rabbit, a 1 set CS- of unpaired stimuli fails to produce acquisition, or results in US interval was chosen for the present study instead of a shorter CS extinction of conditioned responses,suggesting that the close train. A shorter CS-US interval has not yet been tested. Furthermore, temporal associationof the CS and US is required to generate the tone CS in behavioral experiments using mammals generally ranged the conditioned activity. This study representsa first step in es- between 800 Hz to 1 kHz, or consisted of white noise (Schneiderman and Gormezano, 1964; McCormick and Thompson, 1984; Yeo et al., tablishing this in vitro preparation as a model system in which 1985). Auditory nerve fibers of the turtle are most sensitive to stimuli to examine associativelearning in a vertebrate. Using this mod- between 100-500 Hz at room temperature (Patterson, 1966; Art and el, cellular and pharmacologicalstudies of the neuronal mech- Fettiplace, 1984) and therefore a CS of 100 Hz was used. Intertrial anismsunderlying this form of motor learning will compliment intervals in the present study were 30 set, the average interval typically those studiesin progressusing whole animal preparations. used in behavioral studies, and were not varied to reduce the complexity of the stimuli in this initial study. Fifty paired stimuli comprised a single A preliminary report appeared in abstract form (Keifer and pairing session and each session was followed by a 30 min rest period Houk, 1990). in which there was no stimulation. The rest period was introduced be- cause studies using the rabbit model have varied widely in the number Materials and Methods of pairing sessions (or blocks of paired stimuli) presented to the animal and these were given on a daily basis. Since it was not practical to In vitro turtle brainstem-cerebellum. Preparation of the in vitro brain- apply blocks of paired stimuli on a daily basis in vitro, blocks were stem-cerebellum has been described in -detail elsewhere (Keifer and separated by a 30 min rest period in which there was no stimulation. Houk 1991). Brieflv. freshwatervond turtles(Chrvsemvs Dicta, n = 53) Application of this stimulus protocol was continued until a conditioned of 4-6 inch carapace length w&e anesthetized by hgpdthermia (0°C’; response (CR) was acquired, which occurred most often by the second Marcus, 1981). Following decapitation, the brain and cervical segments or third pairing session (71% of preparations showing acquisition). of the spinal cord were
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