Comparison of the fine structure of cortical and collicular terminals in the rat medial geniculate body

Neurons throughout the rat medial geniculate body, including: the dorsal an d ventral divisions, display a variety of responses to auditory stimuli. To investigate possible structural determinants of this variability, measurem ents of axon terminal profile area and postsynaptic dendrite diameter were made on inferior colliculus and corticothalamic terminal profiles in the me dial geniculate body identified by anterograde tracer labeling following in jections into the inferior colliculus or cortex. Over 90% of the synapses o bserved were axodendritic, with few axosomatic synapses. Small (<0.5 <mu>m( 2)) and large (>1.0 mum(2)) collicular profiles were found throughout the m edial geniculate, but were smaller on average in the dorsal division (0.49 +/- 0.49 mum(2)) than in the ventral division (0.70 +/- 0.64 mum(2)). Almos t all corticothalamic profiles were small and ended on small-caliber dendri tes (0.57 +/- 0.25 mum diameter) throughout the medial geniculate. A few ve ry large (>2.0 mum(2)) corticothalamic profiles were found in the dorsal di vision and in the marginal zone of the medial geniculate. GABA immunostaini ng demonstrated the presence of GABAergic profiles arising from cells in th e inferior colliculus. These profiles were compared with GABAergic profiles not labeled with anterograde tracer, which were presumed to be unlabeled i nferior colliculus profiles or thalamic reticular nucleus profiles. The dis tributions of dendritic diameters postsynaptic to collicular, cortical and unlabeled GABAergic profiles were compared with dendritic diameters of intr acellularly labeled medial geniculate neurons from rat brain slices. Our results demonstrate a corticothalamic projection to medial geniculate b ody that is similar to other sensory corticothalamic projections. However, the heterogeneous distributions of excitatory inferior collicular terminal sizes and postsynaptic dendritic diameters, along with the presence of a GA BAergic inferior collicular projection to dendrites in the medial geniculat e body, suggest a colliculogeniculate projection that is more complex than the ascending projections to other sensory thalamic nuclei. These findings may be useful in understanding some of the differences in the response char acteristics of medial geniculate neurons in vivo. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.