GAD-immunoreactive neural elements in the basilar pontine nuclei and nucleus reticularis tegmenti pontis of the rat. I. Light microscopic studies

Summary Immunocytochemical procedures employing the unlabeled antibody enzyme (PAP) method were used to visualize those neuronal elements in the basilar pontine nuclei (BPN) and nucleus reticularis tegmenti pontis (NRTP) of the rat which contain glutamic acid decarboxylase (GAD), a key enzyme involved in the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). Neuronal somata, axons, and axon terminals in the BPN and NRTP exhibited GAD-immunoreactivity. Both thick and thin varieties of labeled axons and terminals were distributed in varying densities throughout the BPN and NRTP. The greatest accumulation of labeled terminals was noted in the ventrolateral and lateral border regions of the BPN while a slightly less dense aggregation was observed along the ventral, ventromedial and midline regions of the pontine gray. Labeled fibers and terminals were also observed in the dorsal and ventral peduncular regions as well as central portions of the lateral, ventral, and medial pontine areas. Axonal and terminal labeling was present throughout the NRTP but no focal increases in density similar to those in the BPN were apparent. No obvious GAD-labeled fiber bundles could be observed to enter the BPN or NRTP. However, small fascicles of labeled axons were seen to course ventrally around the dorsolateral aspect of the cerebral peduncle to reach lateral pontine areas while other labeled axons descended through clefts in the mid-portion of the cerebral peduncle or passed through the medial lemniscus and around the medial portion of the cerebral peduncle to enter the pontine gray. The number of GAD-positive neuronal somata in the BPN and NRTP is quite sparse in comparison to that of neighboring brainstem regions such as the ventral nucleus of the lateral lemniscus and the pontine reticular formation. The majority of labeled neuronal somata were less than 20 m in diameter although some, particularly in NRTP, were as large as 28 m. A variety of morphologies was represented in the population of labeled somata ranging from spheroidal to spindle-shaped or polygonal. GAD labeled somata were distributed throughout the BPN and NRTP but were most numerous in the rostral medial pontine region and the lateral, ventral, and medial border areas of the BPN, adjacent to the fibers of the brachium pontis. From these observations it is proposed that the population of GAD positive cells in the BPN and NRTP gives rise to at least a portion of the labeled axon terminals while other GABA-ergic neurons extrinsic to the pontine gray serve as a source of the remainder of the labeled axons and terminals.