Multiple single units and population responses during inhibitory gating of hippocampal auditory response in freely-moving rats

Paired clicks were presented to awake, freely-moving rats to examine neuronal activity associated with inhibitory gating of responses to repeated auditory stimuli. The rats had bundles of eight microwires implanted into each of four different brain areas: CA3 region of the hippocampus, medial septal nucleus, brainstem reticular nucleus, and the auditory cortex. Single- unit recordings from each wire were made while the local auditory-evoked potential was also recorded. The response to a conditioning stimulus was compared to the response to a test stimulus delivered 500 ms later: the ratio of the test response to the conditioning response provided a measure of inhibitory gating. Auditory-evoked potentials were recorded at all sites. Overall, brainstem reticular nucleus neurons showed the greatest gating of local auditory-evoked potentials, while the auditory cortex showed the least. However, except for the auditory cortex, both gating and non-gating of the evoked response were recorded at various times in all brain regions. Gating of the hippocampal response was significantly correlated with gating in the medial septal nucleus and brainstem reticular nucleus, but not the auditory cortex. Single-unit neuron firing in response to the clicks was most pronounced in the brainstem reticular nucleus and the medial septal nucleus, while relatively few neurons responded in the CA3 region of the hippocampus and the auditory cortex. Taken together, these data support the hypothesis that inhibitory gating of the auditory-evoked response originates in the non- lemniscal pathway and not in cortical areas of the rat brain.