Anatomy, physiology, and synaptic responses of rat layer V auditory cortical cells and effects of intracellular GABA(A) blockade.

The varied extracortical targets of layer V make it an important site for cortical processing and output, which may be regulated by differences in the pyramidal neurons found there. Two populations of projection neurons, regular spiking (RS) and intrinsic bursting (IB), have been identified in layer V of some sensory cortices, and differences in their inhibitory inputs have been indirectly demonstrated. In this report, IB and RS cells were identified in rat auditory cortical slices, and differences in thalamocortical inhibition reaching RS and IB cells were demonstrated directly using intracellular GABA(A) blockers. Thalamocortical synaptic input to RS cells was always a combination of excitation and both GABA(A) and GABA(B) inhibition. Stimulation seldom triggered a suprathreshold response. IB cell synaptic responses were mostly excitatory, and stimulation usually triggered action potentials. This apparent difference was confirmed directly using intracellular chloride channel blockers. Before intracellular diffusion, synaptic responses were stable and similar to control conditions. Subsequently, GABA(A) was blocked, revealing a cell's total excitatory input. On GABA(A) blockade, RS cells responded to synaptic stimulation with large, suprathreshold excitatory events, indicating that excitation, while always present in these cells, is masked by GABA(A). In IB cells that had visible GABA(A) input, it often masked an excitatory postsynaptic potential (EPSP) that could lead to additional suprathreshold events. These findings indicate that IB cells receive less GABA(A)-mediated inhibitory input and are able to spike or burst in response to thalamocortical synaptic stimulation far more readily than RS cells. Such differences may have implications for the influence each cell type exerts on its postsynaptic targets.

Auditory cortical projections to the cat inferior colliculus.

The projection from 11 auditory cortical areas onto the subdivisions of the inferior colliculus was studied in adult cats by using two different anterograde tracers to label cortico-collicular (CC) axon terminals. The main results were that: 1) a significant CC projection arose from every field; 2) the principal inferior collicular targets were the dorsal cortex, lateral nucleus, caudal cortex, and intercollicular tegmentum, with only a sparse projection to the central nucleus; 3) the input was usually bilateral, with the ipsilateral side by far the most heavily labeled, and the contralateral projection was a symmetrical subset of the ipsilateral input; 4) the CC system is both divergent and convergent, with single cortical areas projecting to six or more collicular subdivisions, and each auditory midbrain subdivision receiving a convergent projection from two to ten cortical areas; 5) cortical areas devoid of tonotopic organization have topographic projections to collicular target nuclei; 6) the heaviest CC projection terminated in the caudal half of the inferior colliculus; and finally, 7) the relative strength of the cortico-collicular labeling was far less than that of the corresponding corticothalamic projection in the same experiments. The CC system is strategically placed to influence both descending and ascending pathways arising in the inferior colliculus. Nuclei that participate in the premotor system, like the inferior collicular subdivisions that project to the pons, receive substantial corticofugal input. Both the dorsal (pericentral) and the lateral (external) nuclei of the inferior colliculus project to parts of the medial geniculate body whose closest auditory affiliations are with non-tonotopic cortical regions involved in higher order auditory perception. The cortico-collicular system may link brainstem and colliculo-thalamic circuits to coordinate premotor and perceptual aspects of hearing.