Dorsolateral pontine tegmentum and the classically conditioned nictitating membrane response: analysis of CR-related single-unit activity.

Previous investigations have suggested that the dorsolateral pontine tegmentum (DLP) may be part of a system essential for classical conditioning of the nictitating membrane response. The present study examined CR-related firing patterns of extracellularly-recorded single units in the DLP. Differential conditioning, using tonal CSs and periocular electrostimulation as the US, was employed so that firing patterns on CR and non-CR trials could be compared. Cells that exhibited CR-related increases in firing (excitatory cells) were found in reticular formation surrounding the motor trigeminal nucleus (zone h), including the supra-trigeminal region, and in a region dorsal and dorsomedial to the brachium conjunctivum. Cells that exhibited CR-related decreases in firing (inhibitory cells) were observed in subcoeruleus/medial parabrachial regions and dorsal nucleus reticularis pontis oralis and caudalis. A third class of cells (temporal cells) exhibited CR-related shifts in the temporal distribution of spikes; these cells were located in various brain stem regions. The firing of many of the excitatory, inhibitory, and temporal cells preceded the behavioral CR by an amount of time sufficient for causal involvement. In light of evidence indicating that the cerebellum is critically involved in conditioning, the present study suggests that two systems, the cerebellum and the DLP, may be in control of CRs. The relationship of the two systems and their possible roles in conditioning are discussed.