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These findings indicate that IB cells receive less GABAA-mediated inhibitory input and …

Biology Articles » Anatomy & Physiology » Anatomy, Physiology, and Synaptic Responses of Rat Layer V Auditory Cortical Cells and Effects of Intracellular GABAA Blockade » Introduction

- Anatomy, Physiology, and Synaptic Responses of Rat Layer V Auditory Cortical Cells and Effects of Intracellular GABAA Blockade

Auditory cortex is the last in a series of structures dedicated to the interpretation of auditory input. Many subcorticalauditory nuclei have specialized circuits or synapses that haveno correlates in the other sensory systems. For instance, themedial nucleus of the trapezoid body (MNTB) contains large calycealsynapses specialized for rapid, precise synaptic transmission(Trussell 1997ref-arrow.gif). Recent evidence (Smith and Populin 1999ref-arrow.gif) suggeststhat the thalamic input layers of cat auditory cortex may differfrom those of visual and somatosensory cortices. It is possiblethat rat auditory cortex also contains unique circuits and cellsspecialized for auditory information processing. Alternatively,auditory cortex may process stimuli using circuits similar tothose found elsewhere in cortex. It is therefore important tokeep these possible specializations in mind and study auditorycortex both in terms of its possible auditory functions and asa part of cerebralcortex.

Sensory cortex influences its targets through a topographically organized descending system originating in layers V and VI,and recent work has begun to elucidate the possible physiologicalroles of this system (Guillery 1995ref-arrow.gif; Miller 1996ref-arrow.gif; Sherman andGuillery 1996ref-arrow.gif). Layer V is of particular interest because itscells form part of the projection to the thalamus and the entiretyof the cortical projection to subthalamic nuclei. In addition,layer V, with cells from all cortical layers (barring layer I),participates in callosal and ipsilateral corticocortical circuits.Layer V has several anatomic and physiological pyramidal celltypes and a variety of interneurons (Kawaguchi 1993ref-arrow.gif; Kawaguchiand Kubota 1996ref-arrow.gif). One pyramidal cell type, the intrinsically bursting(IB) cell, is found only in layer V and the deepest region oflayer IV in the rat (Connors et al. 1982ref-arrow.gif, 1988ref-arrow.gif; McCormick et al.1985ref-arrow.gif, somatosensory cortex; Kasper et al. 1994aref-arrow.gif, visual cortex).The IB cell produces high-frequency bursts of action potentialsand is distinguished by its morphology, which is different fromthat of regular spiking (RS) cells, which also populate layerV (Chagnac-Amitai et al. 1990ref-arrow.gif; Kasper et al. 1994aref-arrow.gif-cref-arrow.gif). It is possiblethat the diverse targets of layer V necessitate a variety of anatomicand physiological response types, but the role of these differenttypes within layer V and their effects on postsynaptic targetsare only beginning to be understood (Guillery 1995ref-arrow.gif; Miller 1996ref-arrow.gif;Sherman and Guillery 1996ref-arrow.gif).

In this study we characterized the physiological and anatomic properties of single cells in layer V of primary auditory cortex,their synaptic inputs, and how their responses to these inputsmight modulate their cortical and subcortical targets. This reportuses three techniques to approach these issues. First, by examiningascending synaptic inputs to layer V cells, latencies, patterns,and degrees of excitation and inhibition can be identified. Second,intracellular blockers of GABAA allow confirmation of earlierwork, which studied the role of inhibition indirectly (Chagnac-Amitaiand Connors 1989ref-arrow.gif; Nicoll et al. 1996ref-arrow.gif), as well as further studyof the strength of the inhibitory input and its ability to shapethe thalamocortical synaptic responses of layer V cells. Third,anatomic results can be correlated to physiological data to givea clearer picture of auditory and more general cortical circuitry.Part of this work was published previously in abstract form (Heftiand Smith 1996ref-arrow.gif, 1999ref-arrow.gif).

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