Two types of interneurons in the cat visual thalamus are distinguished by morphology, synaptic connections, and nitric oxide synthase content

Abstract

The distribution of the neuronal form of the nitric oxide-synthesizing enzyme, brain nitric oxide synthase (BNOS), was examined in the cat thalamus by using immunocytochemical techniques. BNOS was found in both cells and fibers throughout the visual thalamus. BNOS-stained cells were found consistently in the C laminae of the lateral geniculate nucleus (LGN), the pulvinar nucleus, and the lateral posterior nucleus (LP). In the A laminae of the LGN, variable numbers of BNOS-stained cells also could be detected. BNOS-stained cells were identified as a subset of interneurons because they all stained for glutamic acid decarboxylase (GAD), but not all GAD-stained cells contained BNOS. The average soma area of BNOS-stained cells was slightly greater than the average soma area of GAD-stained cells. BNOS-stained cells display a distinctive dendritic morphology, which is consistent with previous descriptions of class V neurons (Updyke [1979] J. Comp. Neurol. 186:603-619); they have widespread but fairly sparse arbors of thin, somewhat beaded dendrites. BNOS-stained cells participate in a distinct synaptic circuitry. Although many GAD-stained profiles are filled with vesicles and participate in complex synaptic arrangements, known as glomeruli, BNOS-stained dendrites contain small clusters of vesicles and form dendrodendritic contacts in the extraglomerular neuropil. Thus, there appear to be at least two types of gamma-aminobutyric acidergic interneurons in the visual thalamus of the cat. Interneurons that do not contain BNOS (class III morphology) may exert their effects primarily within synaptic glomeruli (Hamos et al. [1985] Nature 317:618-621), whereas interneurons that contain BNOS (class V morphology) contribute primarily to the extraglomerular neuropil.