| Neurons within and between the dorsal and ventral subdivisions of the rat medial geniculate body (MGB) have a variety of responses to auditory stimuli that are largely determined by postsynaptic processing of inputs from the inferior colliculus (IC). Little is known about the anatomical and physiological details of these ascending synaptic inputs and their possible roles in shaping the response features to acoustic stimulation. To characterize the factors that govern the acoustic response of MGB neurons, several experimental methods were used. First, intracellular recordings were made from MGB neurons while electrically stimulating IC axons in a rat brain slice preparation to characterize the synaptic features of these inputs. Recorded MGB cells were labeled and studied light microscopically to correlate the synaptic responses with MGB cell location and morphology. Second, retrograde labeling of IC cells that projected to the MGB combined with GABA antibody labeling determined the source and sign (excitatory or inhibitory) of the collicular input. Finally, anterograde labeling of the IC inputs combined with GABA immunocytochemistry at the electron microscopic level determined the size of axon terminals and termination zone on MGB neurons for excitatory and inhibitory IC inputs. For single-shock synaptic stimulation, regardless of location or morphology, MGB neurons could receive excitatory IC inputs and/or inhibitory IC inputs. The short-latency, GABAergic projection from the IC to the MGB is unique among sensory thalamic nuclei and was confirmed physiologically and anatomically. IC epsps that were accompanied by this inhibition often appeared to be a separate population from those that were not and could be distinguished by multiple factors. Ultrastructurally, IC excitatory terminal size and postsynaptic dendrite size could also be segregated into two populations. The results from these studies suggest that the MGB may be organized differently than other thalamic nuclei, since it receives a monosynaptic, feedforward inhibitory input and has two distinct excitatory sensory input responses. Furthermore, while there are some differences between IC anatomy and synaptic physiology for dorsal and ventral division MGB neurons, the response characteristics of the collicular afferents may contribute more to acoustic response variability than differential processing by MGB neurons. |
