No abstract

'What' and 'where' visual streams define ventrolateral object and dorsolateral spatial processing domains in the prefrontal cortex of nonhuman primates. We looked for similar streams for auditory-prefrontal connections in rhesus macaques by combining microelectrode recording with anatomical tract-tracing. Injection of multiple tracers into physiologically mapped regions AL, ML and CL of the auditory belt cortex revealed that anterior belt cortex was reciprocally connected with the frontal pole (area 10), rostral principal sulcus (area 46) and ventral prefrontal regions (areas 12 and 45), whereas the caudal belt was mainly connected with the caudal principal sulcus (area 46) and frontal eye fields (area 8a). Thus separate auditory streams originate in caudal and rostral auditory cortex and target spatial and non-spatial domains of the frontal lobe, respectively.

Anterograde tracing methods were used to examine the topographic organization and interrelationship of projections to the neostriatum arising from various areas of association cortex. In contrast to the currently accepted topographic schema, all cortical areas examined project to longitudinal territories that occupy restricted medial-lateral domains of the neostriatum. The posterior parietal and superior arcuate cortices project to dorsolateral portions of the neostriatum; the dorsolateral and dorsomedial frontal cortices project centrally; and the orbitofrontal, anterior cingulate, and superior temporal projections are distributed to ventromedial regions of the caudate nucleus and putamen. In coronal section, cortical terminal fields form a diagonal strip, extending from the dorsal, ventricular border of the caudate nucleus, through the fiber bundles of the internal capsule, to the ventral margin of the putamen. Double labeling studies, in which two cortical areas were injected in the same animal, indicated that convergence of input within neostriatal domains is not governed by reciprocity of corticocortical connectivity. Thus, the interrelationship of projections arising from connectionally linked cortical areas ranged from nearly complete segregation of terminal fields (e.g., from dorsolateral prefrontal and orbital cortices) to extensive overlap of terminal domains (e.g., from frontal and temporal cortices). In the latter case, detailed analysis revealed that frontal and temporal terminals actually were interdigitated rather than intermixed within the zone of overlap. The present findings suggest a new conceptualization of corticostriatal topography in the primate which emphasizes the longitudinal arrangement of cortical terminal domains. Additionally, these findings provide a map for functional parcellation of the neostriatum on the basis of its cortical innervation which may prove useful to understanding normal striatal function, as well as the symptomatology associated with neostriatal injury and disease.

Common efferent projections of the dorsolateral prefrontal cortex and posterior parietal cortex were examined in 3 rhesus monkeys by placing injections of tritiated amino acids and HRP in frontal and parietal cortices, respectively, of the same hemisphere. Terminal labeling originating from both frontal and parietal injection sites was found to be in apposition in 15 ipsilateral cortical areas: the supplementary motor cortex, the dorsal premotor cortex, the ventral premotor cortex, the anterior arcuate cortex (including the frontal eye fields), the orbitofrontal cortex, the anterior and posterior cingulate cortices, the frontoparietal operculum, the insular cortex, the medial parietal cortex, the superior temporal cortex, the parahippocampal gyrus, the presubiculum, the caudomedial lobule, and the medial prestriate cortex. Convergent terminal labeling was observed in the contralateral hemisphere as well, most prominently in the principal sulcal cortex, the superior arcuate cortex, and the superior temporal cortex. In certain common target areas, as for example the cingulate cortices, frontal and parietal efferents terminate in an array of interdigitating columns, an arrangement much like that observed for callosal and associational projections to the principal sulcus (Goldman-Rakic and Schwartz, 1982). In other areas, frontal and parietal terminals exhibit a laminar complementarity: in the depths of the superior temporal sulcus, prefrontal terminals are densely distributed within laminae I, III, and V, whereas parietal terminals occupy mainly laminae IV and VI directly below the prefrontal bands. Subcortical structures also receive apposing or overlapping projections from both prefrontal and parietal cortices. The dorsolateral prefrontal and posterior parietal cortices project to adjacent, longitudinal domains of the neostriatum, as has been described previously (Selemon and Goldman-Rakic, 1985); these projections are also found in close apposition in the claustrum, the amygdala, the caudomedial lobule, and throughout the anterior medial, medial dorsal, lateral dorsal, and medial pulvinar nuclei of the thalamus. In the brain stem, both areas of association cortex project to the intermediate layers of the superior colliculus and to the midline reticular formation of the pons.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased dopamine turnover in the prefrontal cortex impairs spatial working memory performance in rats and monkeys (FG7142/RO15-1788 ABSTRACTThe selective activation of the prefrontal cortical dopamine system by mild stress can be mimicked by anxiogenic g3-carbolines such as FG7142. To investigate the functional relevance of elevated levels of dopamine turnover in the prefrontal cortex, the current study examined the effects of FG7142 on the performance of spatial working memory tasks in the rat and monkey. FG7142 selectively increased prefrontal cortical dopamine turnover in rats and significantly impaired performance on spatial working memory tasks in both rats and monkeys. Spatial discrimination, a task with similar motor and motivational demands (rats), or delayed response performance following zero-second delays (monkeys) was unaffected by FG7142. Further, biochemical analysis in rats revealed a significant positive correlation between dopamine turnover in the prefrontal cortex and cognitive impairment on the delayed alternation task. The cognitive deficits in both rats and monkeys were prevented by pretreatment with the benzodiazepine receptor antagonist, R015-1788, which blocked the increase in dopamine turnover and by the dopamine receptor antagonists, haloperidol, clozapine, and SCH23390. These findings indicate that excessive dopamine activity in the prefrontal cortex is detrimental to cognitive functions mediated by the prefrontal cortex.

Dopamine (DA) D1 receptor compounds were examined in monkeys for effects on the working memory functions of the prefrontal cortex and on the fine motor abilities of the primary motor cortex. The D1 antagonist, SCH23390, the partial D1 agonist, SKF38393, and the full D1 agonist, dihydrexidine, were characterized in young control monkeys, and in aged monkeys with naturally occurring catecholamine depletion. In addition, SKF38393 was tested in young monkeys experimentally depleted of catecholamines with chronic reserpine treatment. Injections of SCH23390 significantly impaired the memory performance of young control monkeys, but did not impair aged monkeys with presumed catecholamine depletion. Conversely, the partial agonist, SKF38393, improved the depleted monkeys (aged or reserpine-treated) but did not improve young control animals. The full agonist, dihydrexidine, did improve memory performance in young control monkeys as well as in a subset of aged monkeys. Consistent with D1 receptor mechanisms, agonist-induced improvements were blocked by SCH23390. Drug effects on memory performance occurred independently of effects on fine motor performance. These results underscore the importance of DA D1 mechanisms in cognitive function, and provide functional evidence of DA system degeneration in aged monkeys. Finally, high doses of D1 agonists impaired memory performance in aged monkeys, suggesting that excessive D1 stimulation may be deleterious to cognitive function.

No abstract

The terminal distribution of thalamic afferents in primate prefrontal cortex has never been examined in any detail. In the present study, WGA-HRP was injected into major subdivisions of the mediodorsal nucleus (MD) in the rhesus monkey in order to determine 1) The areal distribution of MD projections, 2) the layer(s) in which MD afferents terminate, 3) the tangential pattern of the MD axonal terminals, 4) the cells of origin of the reciprocal corticothalamic pathway, and 5) the degree of reciprocity between the corticothalamic and thalamocortical pathways in the different regions of the prefrontal cortex. As expected on the basis of retrograde degeneration and transport studies, injections centered in the magnocellular (MDmc) subnucleus of MD labeled cells and terminals in the ventral and medial prefrontal cortex. Injections involving ventral MDmc labeled the more lateral of these areas (Walker's areas 11 and 12); injections of the dorsal MDmc labeled the ventromedial regions (areas 13 and 14). In contrast, injections involving mainly the lateral, parvicellular (MDpc) moiety labeled cells and terminals in dorsolateral and dorsomedial areas (Walker's 46, 9, and 8B). Area 8A was labeled most prominently when injections included the multiform portion of MD (MDmf) and area 10 had connections with anterior portions of MD. A dorsal-ventral topography for MDpc exists with dorsal MDpc labeling dorsal and dorsomedial prefrontal areas and ventral MDpc labeling dorsolateral prefrontal cortex. Our findings with respect to MD are consistent with a nucleus-to-field organization of its thalamocortical projection system. Outside of the traditional boundaries of prefrontal cortex, lateral MD projections extended to the supplementary motor area (SMA) and the dorsal part of the anterior cingulate (AC) whereas the medial MD projection targeted the ventromedial cingulate cortex and spared SMA. In addition, a few labeled cells and sparse terminals were found in the inferior parietal lobule, the superior temporal sulcus, and the anterior part of the insula after injections that involved the medial part of MD. Labeled terminals were invariably confined to layer IV and adjacent deep layer III. No terminal label was ever observed in layers I, II, superficial III, V, or VI in any part of the cerebral cortex following injections confined to any part of MD.(ABSTRACT TRUNCATED AT 400 WORDS)