IN-VITRO CLASSICAL-CONDITIONING OF ABDUCENS NERVE DISCHARGE IN TURTLES

In vitro classical conditioning of abducens nerve activity was perform ed using an isolated turtle brainstem-cerebellum preparation by direct stimulation of the cranial nerves. Using a delayed training procedure , the in vitro preparation was presented with paired stimuli consistin g of a 1 sec train stimulus applied to the auditory nerve (CS), which immediately preceded a single shock US applied to the trigeminal nerve . Conditioned and unconditioned responses were recorded in the ipsilat eral abducens nerve. Acquisition exhibited a positive slope of conditi oned responding in 60% of the preparations. Application of unpaired st imuli consisting of CS-alone, alternate CS and US, or backward conditi oning failed to result in conditioning, or resulted in extinction of C Rs. Latencies of CR onset were timed such that they occurred midway th rough the CS. Activity-dependent uptake of the dye sulforhodamine was used to examine the spatial distribution of neurons labeled during con ditioning. These data showed label in the cerebellum and red nucleus d uring conditioning whereas these regions failed to label during uncond itioned responses. Furthermore, the principal abducens nucleus labeled heavily during conditioning. These findings suggest the feasibility o f examining classical conditioning in a vertebrate in vitro brainstem- cerebellum preparation. It is postulated that the abducens nerve CR re presents a behavioral correlate of a blink-related eye movement. Multi ple sites of conditioning are hypothesized, including the cerebellorub ral circuitry and brainstem pathways that activate the principal abduc ens nucleus.