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

The distribution of the neuronal form of the nitric oxide-synthesizing enzy me, brain nitric oxide synthase (BNOS), was examined in the cat thalamus by using immunocytochemical techniques. BNOS was found in both cells and fibe rs throughout the visual thalamus. BNOS-stained cells were found consistent ly in the C laminae of the lateral geniculate nucleus (LGN), the pulvinar n ucleus, and the lateral. posterior nucleus (LP). In the A laminae of the LG N, variable numbers of BNOS-stained cells also could be detected. BNOS-stai ned cells were identified as a subset of interneurons because they all stai ned for glutamic acid decarboxylase (GAD), but not all GAD-stained cells co ntained BNOS. The average soma area of BNOS-stained cells was slightly grea ter than the average soma area of CAD-stained cells. BNOS-stained cells dis play 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 d endrites. BNOS-stained cells participate in a distinct synaptic circuitry. Although many CAD-stained profiles are filled with vesicles and participate in complex synaptic arrangements, known as glomeruli, BNOS-stained dendrit es contain small clusters of vesicles and form dendrodendritic contacts in the extraglomerular neuropil. Thus, there appear to be at least two types o f gamma-aminobutyric acidergic interneurons in the visual thalamus of the c at. Interneurons that do not contain BNOS (class III morphology) may exert their effects primarily within synaptic glomeruli (Hamos et al. [1985] Natu re 317:618-621), whereas interneurons that contain BNOS (class V morphology ) contribute primarily to the extraglomerular neuropil. J. Comp. Neurol 413 :83-100, 1999. (C) 1999 Wiley-Liss, Inc.