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Parvalbumin is expressed in a reciprocal circuit linking the medial geniculate body and auditory neocortex in the rabbit

Recent studies of the rabbit auditory forebrain have shown that antibodies directed against the calcium‐binding protein parvalbumin (PV) specifically demarcate auditory neocortex and the ventral division of the medial geniculate body (MGV). The auditory cortex is characterized by two PV‐ immunoreactive bands: dense terminal‐like labeling within layer III/IV and a prominent band of PV+ somata in the upper half of layer VI. In some cases, there are distinct patches of PV immunoreactivity within layers III/IV of auditory cortex that appear similar to the patchy termination of thalamocortical axons labeled by the injection of anterograde tracers into MGV. The presence of PV+ patches in III/IV, PV+ somata in layer VI, and the high density of PV+ neurons and terminals in the MGV suggest the existence of a reciprocal PV+ circuit linking primary auditory cortex (AI) and the MGV. In the present study, double‐labeling experiments in adult rabbits were carried out to provide evidence for this circuit. Focal injections of the tracers biocytin or biotinylated dextran amine (BDA) into the MGV labeled thalamocortical afferent patches within layer III/IV and retrogradely labeled corticothalamic neurons in layer VIa of the ipsilateral auditory cortex. Adjacent sections stained with antibodies against PV revealed terminal‐like PV‐immunoreactive patches in III/IV and PV+ somata in VIa that were in register with those labeled by BDA injections into the MGV. Serial section reconstruction of BDA‐labeled corticothalamic neurons in VIa revealed pyramidal cells with tangentially oriented basal dendrites and sparsely branched apical dendrites that ascended to layer I. Fluorescent double‐labeling studies demonstrated that a subpopulation of corticothalamic neurons also express PV. PV‐negative corticothalamic neurons were also found. Discrete injections of BDA into auditory cortex labeled bands of neurons in the ipsilateral MGV, whose orientation paralleled the fibrodendritic laminae characteristic of this subdivision. Retrograde double‐labeling experiments showed that most MGV relay neurons also express PV. Small numbers of PV‐negative relay neurons were also found. These studies provide evidence for the existence of multiple, chemically coded pathways linking primary auditory cortex and the MGV. J. Comp. Neurol. 400:349–362, 1998. © 1998 Wiley‐Liss, Inc.