Distribution of anterogradely labeled trigeminal and auditory nerve boutons on abducens motor neurons in turtles: implications for in vitro classical conditioning.

A conditioned abducens nerve response is generated in in vitro brainstem preparations from turtles by pairing a weak conditioned stimulus (CS) applied to the auditory nerve that immediately precedes an unconditioned stimulus (US) applied to the trigeminal nerve. Tract-tracing studies showed direct projections from auditory and trigeminal nerves to abducens motor neurons. In light of these findings for convergent CS-US inputs, it is hypothesized that auditory and trigeminal nerve synaptic inputs onto abducens motor neurons are in spatial proximity because the CS is a weak input that may be required to be near the US inputs to have an associative effect, and conditioning occurs only when the CS and US are temporally separated by less than 100 ms. This study examined the spatial relationship of 133 anterogradely labeled synaptic boutons conveying CS or US information on retrogradely labeled abducens motor neurons. The results show that trigeminal and auditory nerve terminal fields occupy primarily the soma and proximal dendrites of abducens motor neurons. Quantitative analysis shows that the majority of labeled boutons (76% and 85% from injections of the trigeminal and auditory nerves, respectively) were apposed to somata or were localized to dendritic segments no more than about 30 microm from the nucleus. There were no quantitative differences between trigeminal and auditory nerve boutons in terms of their localization on dendrites or bouton diameter. Finally, triple labeling experiments demonstrated that individual abducens motor neurons receive inputs from both nerves and that these inputs may be in close spatial proximity to one another. This synaptic arrangement allows for the possibility that in vitro abducens conditioning is generated by coincident CS-US detection mediated by NMDA receptors and may utilize a Hebbian-like plasticity mechanism.