Correlation between regional changes in the distributions of GABA-containing neurons and unit response properties in the medial geniculate body of the cat

A consistent change in the distribution of single units as a function of several of their properties of response to clicks, noise and tone bursts was observed along the rostro-caudal axis in the ventral division of the medial geniculate body (vMGB). From posterior to anterior, the proportion of inhibitory response patterns and non-monotonic intensity functions progressively decreased; response latencies were progressively shorter and less dispersed from caudal to rostral. This functional heterogeneity is consistent with the segregation of the thalamocortical interconnections: the anterior part of vMGB projects to the anterior and primary auditory cortical fields, whereas the posterior part of vMGB projects mainly to the posterior auditory cortical field. Changes in the distribution of response properties from posterior to anterior in vMGB were found to be correlated with a progressive decrease of the density of GABA-immunoreactive neurons from caudal to rostral. Since GABAergic neurons in sensory thalamic nuclei are believed to be interneurons, these data suggest that interneurons might contribute to regional changes in the distribution of some response properties along the rostro-caudal axis in vMGB. In particular, the high proportion of inhibitory response patterns and non-monotonic intensity functions in the posterior vMGB might well be related to the high density of GABA-containing neurons caudally, where they exert a strong inhibitory influence on principal cells. Intrinsic neurons might contribute to the segregation of the acoustic information transferred from vMGB to the various auditory cortical fields. In contrast, no clear and systematic change in the distribution of response properties along the rostro-caudal axis was observed in the medial division of the MGB, in which the GABA-immunoreactive neurons were evenly distributed.