Modulation of the thalamocortical projections on different layers of auditory cortex in guinea pigs

The goal of this study was to understand the spontaneous neuronal activities and acoustic responses of neurons in the auditory cortex (AC), and the modulation of different divisions of the medial geniculate body (MGB) on different layers of the AC, through in vivo intracellular and/or extracellular recordings in adult guinea pigs.;Thirty-one of forty intracellular recorded neurons (77.50%) and one hundred and thirty of one hundred and forty nine extracellular recorded units (87.25%) showed excitatory responses to a noise burst stimulus. Some neurons in upper sublayer I and lower sublayer II responded to acoustic stimuli first. Then layers IV, III, and VI responded, while finally layers I, II and V responded. AC neurons showed spontaneous discharges, with occasional oscillatory activity.;Electrical stimulation of different MGm areas had different effects on layer I neurons which may be involved in the maintenance of the state of layer I neuronal activity. Electrical stimulation of MGv induced long delayed oscillations or short delayed AP, followed by long lasting oscillations on layer II neurons. Similar long delayed oscillation was also observed on layer III neurons following electrical stimulation of MGv. The sequence of thalamic stimulation induced oscillations was the same as that of acoustic response, from bottom to upper layers. The MGm stimulation induced EPSP/AP followed by rhythmic oscillation on layer III neurons had similar latencies to their acoustic responses while layer II neurons had shorter delayed response. The difference may be related to the activation of different projections from MGm to cortical layer III maybe via layer I. Spontaneous neuronal activity in layers I to III were always inhibited by the electrical stimulation of MGm. GABAergic neurons were likely involved in this process. However, spontaneous activity of some neurons in layers IV and V could be changed from tonic discharging to rhythmic oscillation by the electrical stimulation of MGm.;The pathway from the MGm could regulate the general arousal of the AC. A fast feedforward modulation of the upper layers of the AC through the MGm pathway might enable the preparation of the AC to receive auditory information forwarded from the MGv.