Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body

Presently little is known about what basic synaptic and cellular mechanisms are employed by thalamocortical neurons in the two main divisions of the a uditory thalamus to elicit their distinct responses to sound. Using intrace llular recording and labeling methods, we characterized anatomic features, membrane properties, and synaptic inputs of thalamocortical neurons in the dorsal (MGD) and ventral (MGV) divisions in brain slices of rat medial geni culate body. Quantitative analysis of dendritic morphology demonstrated tha t tufted neurons in both divisions had shorter dendrites, smaller dendritic tree areas, more profuse branching, and a greater dendritic polarization c ompared with stellate neurons, which were only found in MGD. Tufted neuron dendritic polarization was not as strong or consistent as earlier Golgi stu dies suggested. MGV and MGD cells had similar intrinsic properties except f or an increased prevalence of a depolarizing sag potential in MGV neurons. The sag was the only intrinsic property correlated with cell morphology,oy, seen only in tufted neurons in either division. Many MGV and MGD neurons r eceived excitatory and inhibitory inferior colliculus CIC inputs (designate d IN/EX or EX/IN depending on excitation/inhibition sequence). However, a s ignificant number only received excitatory inputs (EX/O) and a few only inh ibitory IN/O. Both MGV and MGD cells displayed similar proportions of respo nse combinations, but suprathreshold EX/O responses only were observed in t ufted neurons. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) had multiple distinguishable amplitude levels implying convergence. Excitatory inputs activated alpha-amino-3-hydroxy-5-methyl-4-isoxazoleprop ionic acid (AMPA) and N-methyl-D-aspartate NMDA) receptors the relative con tributions of which were variable. For IN/EX cells with suprathreshold inpu ts, first-spike timing was independent of membrane potential unlike that of EX/O cells. Stimulation of corticothalamic (CT) and thalamic reticular nuc leus (TRN) axons evoked a GABA(A) IPSP, EPSP, GABA(B) IPSP sequence in most neurons with both morphologies in both divisions. TRN IPSPs and CT EPSPs w ere graded in amplitude, again suggesting convergence. CT inputs activated AMPA and NMDA receptors. The NMDA component of both IC and CT inputs had an unusual voltage dependence with a detectable DL-2-amino-5-phosphonovaleric acid-sensitive component even below -70 mV. First-spike latencies of CT ev oked action potentials were sensitive to membrane potential regardless of w hether the TRN IPSP was present. Overall, our in vitro data indicate that r eported regional differences in the in vivo responses of MGV and MGD cells to auditory stimuli are not well correlated with major differences in intri nsic membrane features or synaptic responses between cell types.