Two types of interneuron in the dorsal lateral geniculate nucleus of the rat: A combined NADPH diaphorase histochemical and GABA immunocytochemical study

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Abstract
The rationale for this study was to provide a comprehensive light microscopical description of the morphology of diaphorase‐reactive neurons and neuropil elements in the dorsal lateral geniculate nucleus (dLGN) of the rat. An additional objective was to quantitatively assess whether a subpopulation of the diaphorase‐reactive neurons, previously shown to be GABA‐immunoreactive, constitute a distinct type of local‐circuit neuron in the rat dLGN. Diaphorase activity was localised in a population of predominantly bipolar fusiform neurons. These cells were weak to moderately stained and possessed the morphological features of intrinsic inhibitory neurons, previously called class B neurons in the rat dLGN. Quantitative estimates indicated that the diaphorase‐reactive neurons constituted approximately 10% of the total neuron composition of the dLGN. The majority (about 83%) of the diaphorase‐reactive cells were located in the lateral half of the nucleus. In addition, a dense plexus of diaphorase‐reactive varicose fibres was found throughout the dLGN lying between the oriented fibre bundles coursing dorsoventrally through the LGN. Diaphorase‐reactive punctae were found to be closely associated with the somata and proximal dendritic segments of nonreactive neurons and also with the stained proximal dendritic segments of diaphorase‐reactive dLGN neurons. The source of the diaphorasereactive varicose fibres in the dLGN was unknown. Evidence suggests, however, that they are of extrinsic origin. The GABA‐immunoreactive nature of the diaphorase neurons in the dLGN was demonstrated by colocalising GABA immunoreactivity within the somata of diaphorase‐reactive cells. The majority (> 90%) of diaphorase‐reactive dLGN neurons were GABA‐immunopositive. Also present was a distinct population of GABA‐immunopositive neurons that were not diaphorase‐reactive. In this study, cells that were solely GABA‐immunopositive have been called class B1 neurons, while cells that were both diaphorase‐reactive and GABA‐immunoreactive have been called class B2 neurons. Size‐frequency distributions of somatic profile areas established that the two populations of GABA‐immunoreactive neuron were significantly different. Class B1 neurons constituted 57%, with class B2 cells representing 43% of all GABA‐immunostained neurons in the rat dLGN. The characteristic morphological features, neurochemical identity and frequency of the diaphorase‐reactive neurons in the rat dLGN indicate that they represent a subpopulation of inhibitory interneurons with the ability to affect intrinsic dLGN operations and thalamocortical interactions using the neuromodulator nitric oxide.