Evidence For Direct Projections Research Articles

Evidence for direct projections from the basal nucleus of the amygdala to retrosplenial cortex in the Macaque monkey

The role of the primate retrosplenial cortex (RSC) in memory processing and spatial navigation has been well established. Recently, processing emotionally salient information has been attributed to the RSC as well. Little anatomical data, however, exist linking the RSC with known emotional processing centers within the brain. The amygdala has been implicated as a substrate for modulating memory for emotionally salient events; yet no study to date has demonstrated that this area has a direct connection in the primate brain. With modern retrograde tracer injections into the RSC and adjacent cortical areas of the monkey (Macaca fascicularis), we demonstrate that there are efferent projections from the basal nucleus of the amygdala to the RSC and area 31. These projections offer anatomical data supporting the hypothesis that the RSC might receive emotionally salient input directly from the amygdala and suggest a role for the RSC as a node within a neural system potentially capable of integrating emotional information for use in memory or other cognitive processes.

Organization of cortical afferent input to orbitofrontal areas in the rhesus monkey

Odorant signal processing takes place in a diverse group of primary olfactory areas which receive direct input from the olfactory bulb. Orbitofrontal cortices participate in olfactory functions, but the pathways through which they receive olfactory or other input have not been clearly defined. The retrograde tracers horseradish peroxidase and fluorescent dyes were injected in orbital cortices to study their afferent cortical connections. Labeled neurons in primary olfactory areas (prepiriform cortex, anterior olfactory nucleus and olfactory tubercle) were directed mainly to a posterior orbitofrontal region and to a lesser extent the neighboring caudal part of area 13. There was no evidence of direct projections from primary olfactory areas to the rostral parts of area 13, or to areas 12 or 11. Most labeled neurons in primary olfactory areas were directed to agranular cortices, fewer projected to dysgranular areas, and there was no evidence that any reached granular cortices. The areas which received the most robust olfactory projections showed the lowest degree of laminar organization among prefrontal cortices. Early processing in the olfactory system thus takes place in areas which differ sharply on structural grounds from “early” eulaminate post-Rolandic sensory cortices. In addition to olfactory cortical projections, numerous labeled neurons in transitional (limbic) cortices were directed to orbital areas, and fewer but still substantial numbers of afferent neurons were found in eulaminate cortices. Unlike post-Rolandic unimodal sensory areas, which seem to be committed to the processing of input from one sensory modality via sequential and/or parallel pathways, caudal orbital areas received highly distributed input from primary olfactory areas, and in addition, from gustatory, visual, auditory and somatosensory areas. The structural and connectional features of olfactory recipient orbital cortices thus differ markedly from those observed in other sensory association areas and suggest a mode of processing adapted early in cortical evolution.

Evidence for direct projections from the nucleus of the solitary tract onto medullary adrenaline cells.

The anatomic relationships between projections from the intermediate portion of the nucleus of the solitary tract (NTS) and phenylethanolamine N-methyl transferaselike-immunoreactive (PNMT-LI) cells in the medulla oblongata were analyzed in Sprague-Dawley rats by using two-color immunoperoxidase staining combined with the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). PHA-L was iontophoretically deposited in the intermediate region of the NTS and the subjacent dorsal motor nucleus of the vagus (DMX) at the level of the middle of the area postrema. Black-stained PHA-L-immunoreactive (PHA-LI) fibers and terminals were present throughout the length of the ipsilateral NTS, where many were seen in close apposition to amber-stained PNMT-LI cells belonging to the C2 cell group. Less profuse PHA-LI projections were present in the contralateral NTS, especially at the level of the PHA-L deposit. Bundles of smooth fibers emerged from the lateral edge of the DMX and could be followed to the ventrolateral surface of the medulla; these were likely vagal efferent fibers. PHA-LI terminal arborizations were prominent in the region of the nucleus ambiguus (NA) and amongst PNMT-LI cells in the lateral C1 cell group ventral to NA. Multiple sites of contiguity between PHA-LI varicose fibers or boutons and PNMT-LI cells in C1 were observed frequently and probably represent sites of functional connection.