Effects of cortical stimulation on auditory-responsive thalamic neurones in anaesthetized guinea pigs

In the present study, we investigated neuronal responses to acoustic stimuli and cortical stimulation in the medial geniculate body (MGB) through in vivo intracellular recordings in anaesthetized guinea pigs. Of the 54 neurones examined with acoustic stimuli, 36 showed excitatory postsynaptic potential (EPSP) responses and 19 showed inhibitory postsynaptic potential (IPSP) responses to acoustic stimuli. Of the 36 EPSP neurones examined with corticofugal modulation, 29 received corticofugal depolarization, 3 corticofugal inhibition, and 4 showed no effect. Of the 19 IPSP neurones, 17 received corticofugal inhibition and 2 were not affected. The mean amplitude of the EPSPs evoked by acoustic stimuli was similar to that evoked by the electrical cortical stimulation (9.19 +/- 5.55 mV versus 9.22 +/- 5.16 mV). There was a significant correlation between the parameters of the EPSPs evoked by an acoustic stimulus and those evoked by cortical stimulation. The mean amplitude of the IPSP evoked by electrical cortical stimulation was significantly greater than that evoked by acoustic stimuli (11.6 +/- 3.8 mV versus 9.1 +/- 3.7 ms, P < 0.05). Seven auditory EPSP and 7 IPSP neurones were examined with corticofugal modulation and labelled with Neurobiotin. Of the 7 EPSP neurones, 5 showed excitatory responses to cortical stimulation and 2 demonstrated no effects. Four of the 5 neurones that received corticofugal depolarization were located in the lemniscal MGB and 1 in the non-lemniscal MGB; of the remaining 2, 1 was located in the lemniscal and the other in the non-lemniscal MGB. Of the 7 IPSP neurones, 1 received an excitatory corticofugal input followed by an inhibitory input and 4 received only an inhibitory corticofugal input, while the remainder demonstrated no corticofugal effects. All 7 neurones were located in the non-lemniscal MGB. The result that both ascending and descending inputs caused similarly shaped EPSPs reflects a neuronal endogenous characteristic irrespective of the physical locations of the synapses. The IPSP responses to both acoustic stimuli and electrical cortical stimulation are likely to be caused by feedback from the thalamic reticular nucleus.