No abstract

The efferent connections of the cortex projected upon by the mediodorsal thalamic nucleus (MD-projection cortex) have been re-examined autoradiographically in the rat following the microelectrophoretic injection of 3H-proline-leucine into different parts of the medial and sulcal MD-projection cortex. Contrary t o previous negative findings, the present experiments revealed a system of extensive corticocortical projections and indicated that different areas of the MD-projection cortex have distinctive patterns in both their corticocortical and subcortical projections. Thus, cells of Brodmann's area 32 send axons to the retrosplenial cortex, area 29d, the peri-and entorhinal cortices, and the presubiculum. Both supragenual and more posterior regions of area 24 project to the retrosplenial cortex and area 29d, but only the posterior portion projects additionally to the entorhinal area and presubiculum. The cortical targets of axons from the sulcal MD-projection cortex are mainly the anterior part of the piriform cortex and, for the posterolateral part of the sulcal cortex (insular area), the retrosplenial area, lateral entorhinal area, and presubiculum. While the medial and sulcal divisions of the MD-projection cortex project upon one another, the medial-to-sulcal projection is in general denser than its reciprocal.Earlier findings of projections to subcortical structures affiliated with the limbic system such as midline thalamic nuclei, hypothalamus, and paramedian mesencephalic tegmentum are confirmed, and appear to originate primarily in area 32 and the insular part of the sulcal cortex. The corticothalamic projection to MD in general terms reciprocates the organization seen in the thalamocortical projection from the various subnuclei within MD. Previously undocumented projections are demonstrated mainly from area 32 of the medial cortex and the insular part of the sulcal cortex to the lateral and basal amygdaloid nuclei, the medial part of the lateral septa1 nucleus, the nucleus accumbens, and the deep layers of the olfactory tubercle; the medial part of the lateral habenular nucleus receives a projection from areas 32 and 24. Projections to the pretectal area and superior colliculus appear to originate from all parts of the medial MD-projection cortex, but are markedly denser when the posterior part of area 24 is injected. The distribution of this corticotectal projection shows a highly characteristic configuration in which areas of high grain concentration surround areas of lower grain concentration.In the rat, as in all mammals, some cortical area near the frontal pole receives a projection from a prominent thalamic nucleus which is bounded by the internal medullary lamina and later corroborated by independent methods (Beckstead, '76; Krettek and Price, '77b1, the MD-projection cortex of the rat comprises the rostra1 one-quarter or so of the dorsal bank and the stria medullaris-the mediodorsal nu-'

The projections of the nucleus of the solitary tract (NST) were studied by autoradiographic anterograde fiber-tracing and horseradish peroxidase (HRP) retrograde cell-labeling. Tritiated proline and leucine were deposited in electrophysiologically identified regions of NST. Injections of NST at levels caudal to where the vagus enters the nucleus, from which responses were evoked by stimulation of cranial nerves IX and X, revealed topographically organized bilateral projections to, most prominently, the ventrolateral medullary reticular formation which contains neurons of the ambiguus complex, and to the lateral and medial parabrachial nuclei, including a small portion of the medially adjacent central gray substance. Labeled fibers in the ventrolateral reticular formation were present from the nucleus retroambigualis rostralward to the retrofacial nucleus, with the densest concentration located over the nucleus ambiguus proper. The parabrachial projection was confirmed using HRP and shown to originate from cells in the medial subdivision of NST. Due to the problem of fibers en passant, it was not possible to interpret conclusively the cell-labeling seen around the solitary tract after HRP injections made in the region of the nucleus ambiguus. Labeled fibers were also traced from caudal NST to the dorsal motor nucleus of the vagus, but their origin could not be determined with certainty. Other labeled axons, traced to circumscribed parts of the inferior olivary complex and via the contralateral medial lemniscus to VPL of the thalamus, were shown in HRP experiments to originate from the dorsal column nuclei rather than NST. No labeled fibers were traced into the spinal cord, nor were any cells labeled in NST after large HRP deposits in upper cervical segments. Isotope deposits at levels of NST rostral to the entrance of the vagus, from which responses were evoked by rapid stimulation of the tongue, revealed an ipsilateral projection which ascends as a component of the central tegmental tract to the parvicellular part of the ventral posteromedial thalamic nucleus (VPMpc). After small HRP deposits in VPMpc, labeled cells in NST were restricted to the rostral part of the lateral subdivision. No labeled axons were traced from rostral NST to the ambiguus complex or parabrachial area. Injections of 3H-amino acids at intermediate levels of NST resulted in fiber-labeling in VPMpc, the parabrachial area, and the ambiguus complex.

No abstract