Heterotopic Cortical Afferents to the Medial Prefrontal Cortex in the Rat. A Combined Retrograde and Anterograde Tracer Study

Cortical afferent projections towards the medial prefrontal cortex (mPFC) were investigated with retrograde and anterograde tracer techniques. Heterotopical afferent projections to the medial prefrontal cortex arise in secondary, or higher order, sensory areas, motor areas and paralimbic cortices. On the basis of these projections three subfields can be discriminated within the mPFC. (1) The ventromedial part of mPFC, comprising the pre- and infralimbic areas, receives mainly projections from the perirhinal cortex. (2) The caudal two-thirds of the dorsomedial PFC, comprising frontal area 2 and the dorsal anterior cingulate area, receives projections from the secondary visual areas, the posterior agranular insular area and the retrosplenial areas. (3) The rostral one-third of the dorsomedial PFC is the main recipient of projections from the somatosensory and motor areas and the posterior agranular insular area. The laminar distribution of cells projecting to the mPFC varies considerably in the different cortical areas, just as the laminar distribution of termination of their fibres within the mPFC does. It is concluded that the corticocortical connections corroborate with subcortical connectivity in attributing to the mediodorsal projection cortex of the rat functions which are comparable to those of certain prefrontal, premotor and anterior cingulate areas in the monkey.     

Organization of anterior cingulate and frontal cortical projections to the anterior and laterodorsal thalamic nuclei in the rat

The anterior and laterodorsal thalamic nuclei provide massive projections to the anterior cingulate and frontal cortices in the rat. However, the organization of reciprocal corticothalamic projections has not yet been studied comprehensively. In the present study, we clarified the organization of anterior cingulate and frontal cortical projections to the anterior and laterodorsal thalamic nuclei, using retrograde and anterograde axonal transport methods. The anteromedial nucleus (AM) receives mainly ipsilateral projections from the prelimbic and medial orbital cortices and bilateral projections from the anterior cingulate and secondary motor cortices. The projections from the anterior cingulate cortex are organized such that the rostrocaudal axis of the AM corresponds to the rostrocaudal axis of the cortex, whereas those from the secondary motor cortex are organized such that the rostrocaudal axis of the AM corresponds to the caudorostral axis of the cortex. The ventromedial part of the anteroventral nucleus receives ipsilateral projections from the anterior cingulate cortex and bilateral projections from the secondary motor cortex, in a topographic manner similar to the projections to the AM. The ventromedial part of the laterodorsal nucleus (LD) receives ipsilateral projections from the anterior cingulate and secondary motor cortices. The projections are roughly organized such that more dorsal and ventral regions within the ventromedial LD receive projections preferentially from the anterior cingulate cortex. The difference in anterior cingulate and frontal cortical projections to the anterior and laterodorsal nuclei may suggest that each thalamic nucleus plays a different functional role in spatial memory processing.     

Topography of projections from the medial prefrontal cortex to the amygdala in the rat

The projections from the rat medial prefrontal cortex to the amygdaloid complex were investigated using retrograde transport of fluorescent dyes and anterograde transport of horseradish peroxidase-WGA. The ventral anterior cingulate, prelimbic, infralimbic and medial orbital areas and the taenia tecta were found to project to the amygdaloid complex. The projections from the prelimbic area arose bilaterally. The medial orbital, prelimbic and anterior cingulate areas send convergent projections to the basolateral nucleus. The prelimbic area has additional projections to the posterolateral cortical nucleus and amygdalo-hippocampal area. The infralimbic area does not project to the basolateral nucleus and cortico-amygdaloid projections from this area are focussed on the anterior cortical nucleus and the anterior amygdaloid area. Both prelimbic and infralimbic areas project to an area situated between the central, medial and basomedial nuclei. Based on similar projections, this area appears to be a caudal continuation of the anterior amygdaloid area. The results indicate that the medial prefrontal component of the "basolateral limbic circuit" is restricted to the anterior cingulate and prelimbic areas. No evidence was obtained to support the existence of a medial prefronto-amygdaloid component of the "visceral forebrain".     

Efferent projections of the basolateral amygdala in the opossum, Didelphis virginiana

The autoradiographic anterograde axonal transport technique was used to study efferent projections of the opossum basolateral amygdala. All nuclei of the basolateral amygdala send topographically organized fibers to the bed nucleus of the stria terminalis (BST) via the stria terminalis (ST). Injections into rostrolateral portions of the basal nuclei label fibers that surround the commissural bundle of the ST, cross the midline by passing along the outer aspect of the anterior commissure, and terminate primarily in the contralateral BST, anterior subdivision of the basolateral nucleus (BLa), ventral putamen, and olfactory cortex. Each of the basal nuclei project ipsilaterally to the anterior amygdaloid area, substantia innominata and topographically to the ventral part of the striatum and adjacent olfactory tubercle. The posterior subdivision of the basolateral nucleus (BLp), but not the basomedial nucleus (BM), projects to the ventromedial hypothalamic nucleus. BLa and BLp have projections to the nucleus of the lateral olfactory tract and also send fibers to the central nucleus, as does the lateral nucleus (L). The lateral nucleus also has a strong projection to BM and both nuclei project to the amygdalo-hippocampal area. BLa and BLp send axons to the ventral subiculum and ventral lateral entorhinal area whereas L projects only to the latter area. The lateral nucleus and BLp project to the perirhinal cortex and the posterior agranular insular area. The BLa sends efferents to the anterior agranular insular area. Rostrally this projection is continuous with a projection to the entire frontal cortex located rostral and medial to the orbital sulcus. All of the nuclei of the basolateral amygdala project to areas on the medial wall of the frontal lobe that appear to correspond to the prelimbic and infralimbic areas of other mammals. Despite the great phylogenetic distance separating the opossum from placental mammals, the projections of the opossum basolateral amygdala are very similar to those seen in other mammals. The unique frontal projections of the opossum BLa to the dorsolateral prefrontal cortex appear to be related to the distinctive organization of the mediodorsal thalamic nucleus and prefrontal cortex in this species.     

Prefrontal afferents to the dorsal raphe nucleus in the rat

The prefrontal cortex (PFC) receives strong inputs from monoaminergic cell groups in the brainstem and also sends projections to these nuclei. Recent evidence suggests that the PFC exerts a powerful top-down control over the dorsal raphe nucleus (DR) and that it may be involved in the actions of pharmaceutical drugs and drugs of abuse. In the light of these findings, the precise origin of prefrontal inputs to DR was presently investigated by using the cholera toxin subunit b (CTb) as retrograde tracer. All the injections placed in DR produced retrograde labeling in the medial, orbital, and lateral divisions of the PFC as well as in the medial part of the frontal polar cortex. The labeling was primarily located in layer V. Remarkably, labeling in the medial PFC was denser in its ventral part (infralimbic and ventral prelimbic cortices) than in its dorsal part (dorsal prelimbic, anterior cingulate and medial precentral cortices). After injections in the rostral or caudal DR, the largest number of labeled neurons was observed in the medial PFC, whereas after injections in the mid-rostrocaudal DR, the labeled neurons were more homogeneously distributed in the three main PFC divisions. A cluster of labeled neurons also was observed around the apex of the rostral pole of the accumbens, especially after rostral and mid-rostrocaudal DR injections. Overall, these results confirm the existence of robust prefrontal projections to DR, mainly derived from the ventral part of the medial PFC, and underscore a substantial contribution of the frontal polar cortex.     

Efferent connections of the substantia innominata in the rat

The efferent connections of the substantia innominata (SI) were investigated employing the anterograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The projections of the SI largely reciprocate the afferent connections described by Grove (J. Comp. Neurol. 277:315-346, '88) and thus further distinguish a dorsal and a ventral division in the SI. Efferents from both the dorsal and ventral divisions of the SI descend as far caudal as the ventral tegmental area, substantia nigra, and peripeduncular area, but projections to pontine and medullary structures appear to originate mainly from the dorsal SI. Within the amygdala and hypothalamus, which receive widespread innervation from the SI, the dorsal SI projects preferentially to the lateral part of the bed nucleus of the stria terminalis; the lateral, basolateral, and central nuclei of the amygdala; the lateral preoptic area; paraventricular nucleus of the hypothalamus; and certain parts of the lateral hypothalamus, prominently including the perifornical and caudolateral zones described previously. The ventral SI projects more heavily to the medial part of the bed nucleus of the stria terminalis; the anterior amygdaloid area; a ventromedial amygdaloid region that includes but is not limited to the medial nucleus; the lateral and medial preoptic areas; and the anterior hypothalamus. Modest projections reach the lateral hypothalamus, with at least a slight preference for the medial part of the region, and the ventromedial and arcuate hypothalamic nuclei. Both SI divisions appear to innervate the dorsomedial and posterior hypothalamus and the supramammillary region. In the thalamus, the subparafascicular, gustatory, and midline nuclei receive a light innervation from the SI, which projects more densely to the medial part of the mediodorsal nucleus and the reticular nucleus. Cortical efferents from at least the midrostrocaudal part of the SI are distributed primarily in piriform, infralimbic, prelimbic, anterior cingulate, granular and agranular insular, perirhinal, and entorhinal cortices as well as in the main and accessory olfactory bulbs. The cells of origin for many projections arising from the SI were identified as cholinergic or noncholinergic by combining the retrograde transport of WGA-HRP with histochemical and immunohistochemical procedures to demonstrate acetylcholinesterase activity or choline acetyltransferase immunoreactivity. Most of the descending efferents of the SI appear to arise primarily or exclusively from noncholinergic cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Efferent projections of the infralimbic cortex of the rat.

On the basis of stimulation studies, it has been proposed that the infralimbic cortex (ILC), Brodmann area 25, may serve as an autonomic motor cortex. To explore this hypothesis, we have combined anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheat germ aggutinin conjugated to horseradish peroxidase (WGA-HRP) to determine the efferent projections from the ILC. Axons exit the ILC in one of three efferent pathways. The dorsal pathway ascends through layers III and V to innervate the prelimbic and anterior cingulate cortices. The lateral pathway courses through the nucleus accumbens to innervate the insular cortex, the perirhinal cortex, and parts of the piriform cortex. In addition, some fibers from the lateral pathway enter the corticospinal tract. The ventral pathway is by far the largest and innervates the thalamus (including the paraventricular nucleus of the thalamus, the border zone between the paraventricular and medial dorsal nuclei, and the paratenial, reuniens, ventromedial, parafasicular, and subparafasicular nuclei), the hypothalamus (including the lateral hypothalamic and medial preoptic areas, and the suprachiasmatic, dorsomedial, and supramammillary nuclei), the amygdala (including the central, medial, and basomedial nuclei, and the periamygdaloid cortex) and the bed nucleus of the stria terminalis. The ventral efferent pathway also provides descending projections to autonomic cell groups of the brainstem and spinal cord including the periaqueductal gray matter, the parabrachial nucleus, the nucleus of the solitary tract, the dorsal motor vagal nucleus, the nucleus ambiguus, and the ventrolateral medulla, as well as lamina I and the intermediolateral column of the spinal cord. The ILC has extensive projections to central autonomic nuclei that may subserve a role in modulating visceral responses to emotional stimuli, such as stress.     

Frontal projections to the region of the oculomotor complex in the rat: a retrograde and anterograde HRP study

The retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique were employed to study frontal projections to the perioculomotor region in the rat. Following HRP microinjections or transcannular HRP gel implants into the oculomotor complex (OMC), the majority of retrogradely labeled pyramidal cells were located in lamina V of the dorsomedial frontal shoulder cortex, i.e., medial precentral and anterior cingulate (PrCm/AC) cortices, the proposed frontal eye field (FEF) in the rat. A smaller number of labeled cells were present in the frontal polar cortex, agranular insular (AI), and lateral precentral (PrCl) cortices. Following HRP gel implants into the PrCM/Ac, anterogradely labeled projections were observed to the dorsal medial subthalamic region (nucleus campi Foreli, NCF) and medial accessory nucleus of Bechterew (MAB), and to other subcortical nuclei known to receive inputs from cortical area 8 in the monkey. These results, taken together with previous anatomical and physiological studies, support the conclusion that the PrCm/AC cortex contains the rat FEF. Its homology with the primate FEF is discussed.     

Projections of the medial orbital and ventral orbital cortex in the rat

The medial orbital (MO) and ventral orbital (VO) cortices are prominent divisions of the orbitomedial prefrontal cortex. To our knowledge, no previous report in the rat has comprehensively described the projections of MO and VO. By using the anterograde tracer Phaseolus vulgaris leucoagglutinin and the retrograde tracer Fluoro-Gold, we examined the efferent projections of MO and VO in the rat. Although MO and VO projections overlap, MO distributes more widely throughout the brain, particularly to limbic structures, than does VO. The main cortical targets of MO were the orbital, ventral medial prefrontal (mPFC), agranular insular, piriform, retrosplenial, and parahippocampal cortices. The main subcortical targets of MO were the medial striatum, olfactory tubercle, claustrum, nucleus accumbens, septum, substantia innominata, lateral preoptic area, and diagonal band nuclei of the basal forebrain; central, medial, cortical, and basal nuclei of amygdala; paratenial, mediodorsal, and reuniens nuclei of the thalamus; posterior, supramammillary, and lateral nuclei of the hypothalamus; and periaqueductal gray, ventral tegmental area, substantia nigra, dorsal and median raphe, laterodorsal tegmental, and incertus nuclei of the brainstem. By comparison, VO distributes to some of these same sites, notably to the striatum, but lacks projections to parts of limbic cortex, to nucleus accumbens, and to the amygdala. VO distributes much more strongly, however, than MO to the medial (frontal) agranular, anterior cingulate, sensorimotor, posterior parietal, lateral agranular retrosplenial, and temporal association cortices. The patterns of MO projections are similar to those of the mPFC, whereas the projections of VO overlap with those of the ventrolateral orbital cortex (VLO). This suggests that MO serves functions comparable to those of the mPFC, such as goal-directed behavior, and VO performs functions similar to VLO such as directed attention. MO/VO may also serve as a link between lateral orbital and medial prefrontal cortices.     

Limbic thalamus in rabbit: architecture, projections to cingulate cortex and distribution of muscarinic acetylcholine, GABAA, and opioid receptors.

Nuclei of the thalamus that project to cingulate cortex have been implicated in responses to noxious stimuli, cholinergic and motor functions. The rabbit limbic thalamus may play an important role in these functions, but has not been studied extensively in terms of its cytoarchitecture, the topographical organization of its cortical projections, and differential transmitter regulation of its subnuclei. Therefore, the architecture, projections to cingulate cortex, and radioligand binding were investigated in the anterior, ventral, lateral, and midline nuclei of rabbit thalamus. The anterior nuclei are highly differentiated because both the dorsal and ventral nuclei have parvicellular and magnocellular divisions. Fluorescent dyes were injected into cingulate cortex to evaluate limbic thalamocortical connections. The anterior medial, submedial, and parafascicular nuclei project primarily to anterior cingulate cortex, while they have small or no projections to posterior areas. The ventral anterior and ventral lateral nuclei have a significant projection to dorsal cingulate cortex, including areas 24b and 29d. Projections of the anterior ventral nucleus are topographically organized, since medial parts of the parvicellular division project to rostral area 29, and lateral parts project to caudal area 29. The lateral nuclei and the parvicellular and magnocellular divisions of the anterior dorsal nucleus project with progressively higher densities in the rostrocaudal plane of area 29. Finally, the magnocellular division of the anterior ventral nucleus projects almost exclusively to caudal and ventral area 29, i.e., granular retrosplenial cortex. Ligand binding studies employed coverslip autoradiography and single grain counting techniques. Muscarinic receptor binding was moderate for both pirenzepine and oxotremorine-M in the parvicellular anterior ventral nucleus, while in other nuclei, there was an inverse relationship in the binding for these ligands. Most notably, the anterior dorsal nucleus, which receives no cholinergic input, had very high oxotremorine-M and low pirenzepine binding, while the anterior medial nucleus, which receives a moderate cholinergic input, had the highest pirenzepine binding and very low oxotremorine-M binding. Muscimol binding to GABAA receptors was highest in the anterior ventral nucleus, while it was at moderate levels in the anterior dorsal and lateral nuclei. The binding of Tyr-D-Ala-Gly-MePhe-Gly-ol to mu opioid receptors and 2-D-penicillamine-5-D-penicillamine-enkephalin to delta opioid receptors were both high in the parvicellular and low in the magnocellular divisions of the anterior dorsal nucleus. The magnocellular division of the anterior ventral, the lateral dorsal, and the parafascicular nuclei had high mu opioid binding, while the lateral dorsal and lateral magnocellular nuclei had low levels of delta opioid binding.(ABSTRACT TRUNCATED AT 400 WORDS)     

Intrinsic connections of the rat amygdaloid complex: Projections originating in the central nucleus

Inputs from the amygdaloid and extraamygdaloid areas terminate in various divisions of the central nucleus. To elucidate the interconnections between the different regions of the central nucleus and its connectivity with the other amygdaloid areas, we injected the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L) into the capsular, lateral, intermediate, and medial divisions of the central nucleus in rat. There were a number of labeled terminals near the injection site within each division. The intrinsic connections between the various divisions of the central nucleus were organized topographically and originated primarily in the lateral division, which projected to the capsular and medial divisions. Most of the connections were unidirectional, except in the capsular division, which received a light reciprocal projection from its efferent target, the medial division. The intermediate division did not project to any of the other divisions of the central nucleus. Extrinsic projections from the central nucleus to the other amygdaloid nuclei were meager. Light projections were observed in the parvicellular division of the basal nucleus, the anterior cortical nucleus, the amygdalohippocampal area, and the anterior amygdaloid area. No projections to the contralateral amygdala were found. These data show that the central nucleus has a dense network of topographically organized intradivisional and interdivisional connections that may integrate the intraamygdaloid and extraamygdaloid information entering the different regions of the central nucleus. The sparse reciprocal connections to the other amygdaloid nuclei suggest that the central nucleus does not regulate the other amygdaloid regions but, rather, executes the responses evoked by the other amygdaloid nuclei that innervate the central nucleus. J. Comp. Neurol. 395:53–72, 1998. © 1998 Wiley-Liss, Inc.     

Associational projections of the anterior midline cortex in the rat: intracingulate and retrosplenial connections

Past studies indicate that distinct areas of anterior midline cortex in the rat contribute to diverse functions, such as autonomic nervous system regulation and learning, but the anatomical substrate for these functions has not been fully elucidated. The present study characterizes the associational connections within the midline cortex of the rat by using the anterograde transport of Phaseolus vulgaris leucoagglutinin and Fluororuby. The prelimbic area and the rostral part of the anterior cingulate area (both dorsal and ventral subdivisions) are extensively interconnected with each other. In addition, the caudal half of anterior cingulate cortex has extensive projections to precentral medial cortex and caudally directed projections to retrosplenial cortex. Other cortical areas within anterior midline cortex have relatively limited cortical–cortical projections. The infralimbic, dorsal peduncular, and medial precentral cortices have dense intrinsic projections, but have either very limited or no projections to other areas in the anterior midline cortex. Although it has been suggested that cortical–cortical projections from anterior cingulate cortex and prelimbic cortex to infralimbic cortex may be important for linking learning processes with an autonomic nervous system response, the paucity of direct projections between these areas calls this hypothesis into question. Conversely, the results suggest that the anterior midline cortex contains two regions that are functionally and connectionally distinct.     

Organization of projections from the ventromedial nucleus of the hypothalamus: A Phaseolus vulgaris-Leucoagglutinin study in the rat

The organization of projections from the four parts of the ventromedial nucleus (VMH) and a ventrolaterally adjacent region tentatively identified as the tuberal nucleus (TU) have been analyzed with small injections of the anterograde axonal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Extrinsic and intranuclear projections of each part of the VMH display clear quantitative differences, whereas the overall patterns of outputs are qualitatively similar. Overall, the VMH establishes massive intrahypothalamic terminal fields in other parts of the medial zone, tending to avoid the periventricular and lateral zones. The ventrolateral VMH is more closely related to other parts of the hypothalamus that also express gonadal steroid hormone receptors, including the medial preoptic, tuberal, and ventral premamillary nuclei, whereas other parts of the VMH are more closely related to the anterior hypothalamic and dorsal premammillary nuclei. All parts of the VMH project to the zona incerta (including the A13 region) and parts of the midline thalamus, including the paraventricular and parataenial nuclei and nucleus reuniens. The densest inputs to the septum are to the bed nuclei of the stria terminalis, where the ventrolateral and central VMH innervate the anteroventral and anterodorsal areas and transverse and interfascicular nuclei, whereas the anterior and dorsomedial VMH innervate the latter two. The central, lateral, and medial amygdalar nuclei receive substantial inputs from various parts of the VMH. Other regions of the telencephalon, including the nucleus accurmbens and the piriform-amygdaloid, infralimbic, prelimbic, anterior cingulate, agranular insular, piriform, perirhinal, entorhinal, and postpiriform transition areas, also receive sparse inputs. All parts of the VMH send a massive, topographically organized projection to the periaqueductal gray. Other brainstem terminal fields include the superior colliculus, peripeduncular area, locus coeruleus, Barrington's nucleus, parabrachial nucleus, nucleus of the solitary tract, and the mesencephalic, pontine, gigantocellular, paragigantocellular, and parvicellular reticular nuclei. The projections of the TU are similar to, and a subset of, those from the VMH and are thus not nearly as widespread as those from adjacent parts of the lateral hypothalamic area. Because of these similarities, the TU may eventually come to be viewed most appropriately as the lateral component of the VMH itself. The functional implications of the present findings are discussed in view of evidence that the VMH plays a role in the expression of ingestive, affective, and copulatory behaviors. © 1994 Wiley-Liss, Inc.     

Mamillary body in the rat: topography and synaptology of projections from the subicular complex, prefrontal cortex, and midbrain tegmentum

The retrograde and anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) has been used to trace afferent connections of the rat mamillary body (MB) at the light and electron microscopic levels. Injections of WGA-HRP into different parts of the MB resulted in heavy retrograde labeling in the subicular complex, medial prefrontal cortex, and dorsal and ventral tegmental nuclei. Injections of WGA-HRP into each of these brain regions, respectively, resulted in anterograde labeling with specific distributions and characteristic synaptic organizations in the MB. Projections from the rostrodorsal and caudoventral subiculum terminated in a topographically organized laminar fashion in the medial mamillary nucleus bilaterally, whereas afferent projections from the presubiculum and parasubiculum terminated only in the lateral mamillary nucleus. Labeled axon terminals which originated from the subicular complex were characterized by round vesicles and formed asymmetric synaptic junctions with small-diameter dendrites and dendritic spines in the medial and lateral mamillary nuclei. Projections from the prefrontal cortex originated mainly in the infralimbic area and to a lesser degree in the prelimbic and anterior cingulate areas. Injections of tracer into these brain regions gave rise to dense labeling of axon terminals in the medial mamillary nucleus, pars medianus, and in the anterior dorsomedial portion of the pars medialis. The labeled terminals were characterized by round vesicles and formed asymmetric synaptic junctions with small-diameter dendrites and dendritic spines. Projections from the dorsal tegmental nucleus terminated in the ipsilateral lateral mamillary nucleus, whereas afferent projections from the anterior and posterior subnuclei of the ventral tegmental nucleus terminated topographically in the medial mamillary nucleus. The ventral tegmental nucleus, pars anterior projected to the midline region of the medial nucleus and the dorsolateral and ventromedial subdivisions of the pars posterior projected to medial and lateral parts of the medial nucleus, respectively. In contrast to the synaptic morphology of subicular complex and medial prefrontal cortex axon terminals in the MB, labeled axon terminals in the MB which originated from the midbrain tegmentum were characterized by pleomorphic vesicles and formed symmetric synaptic junctions with neuronal somata and proximal dendrites as well as distal dendrites and dendritic spines.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projections of auditory cortex upon the thalamus and midbrain in the owl monkey.

Two tonotopically organized cortical fields, the primary (AI) and the rostral (R) fields, comprise the core of auditory cortex in the owl monkey. Injections of tritiated proline were made into each of these fields to determine their efferent projections using autoradiographic methods. Both AI and R project to the principal and magnocellular divisions of the medial geniculate body. In addition, R projects to the posterior part of the dorsal division of the medial geniculate. AI sends axons to the dorsomedial region and laminated portion of the central nucleus of the inferior colliculus. Labeling in the central nucleus following AI injections appears as a band of silver grains oriented parallel to isofrequency contours. Axons from R terminate in the dorsomedial region of the central nucleus of the inferior colliculus and in the pericentral and external nuclei of the inferior colliculus. In addition, the rostral field projects to a small area of the medial pulvinar just anterior to the brachium of the superior colliculus.     

Postnatal volumetric development of the prefrontal cortex in the rat

The medial and orbital parts of the prefrontal cortex (PFC) increase in volume during the first weeks of postnatal life. At the end of this period, however, the volumes of both parts of the PFC reach a significantly higher value than in adulthood. Subsequently the volumes decrease until the adult volume is attained. The three subareas of the medial PFC (i.e., the medial precentral area, the dorsal anterior cingulate, and the prelimbic area) reach a maximum volume around day 24, while the two orbital PFC subareas (i.e., the dorsal and ventral agranular insular areas) attain their maximum value around day 30. The differences found in the growth pattern of the five PFC subareas, which are innervated by specific subnuclei of the mediodorsal nucleus of the thalamus, suggest a role of these subnuclei in the PFC development.     

Corticothalamic connections of paralimbic regions in the rhesus monkey

This study addressed the issue of whether paralimbic regions of the cerebral cortex share common thalamic projections. The corticothalamic connections of the paralimbic regions of the orbital frontal, medial prefrontal, cingulate, parahippocampal, and temporal polar cortices were studied with the autoradiographic method in the rhesus monkey. The results revealed that the orbital frontal, medial prefrontal, and temporal polar proisocortices have substantial projections to both the dorsomedial and medial pulvinar nuclei, whereas the anterior cingulate proisocortex (area 24) projects exclusively to the dorsomedial nucleus. These proisocortical areas also have thalamic connections with the intralaminar and midline nuclei. The cortical areas between the proisocortical regions on the one hand and the isocortical areas on the other, that is, the posterior cingulate region (area 23) and the posterior parahippocampal gyrus (areas TF and TH), project predominantly to the dorsal portion of the medial pulvinar nucleus, the anterior nuclear group (AV, AM), and the lateral dorsal (LD) nucleus. Additionally, the posterior cingulate and medial parahippocampal gyri (area TH) have projections to the lateral posterior (LP) nucleus. Thus, it appears that the proisocortical areas, which are characterized by a predominance of infragranular layers and an absence of layer IV, have common thalamic relationships. Likewise, the intermediate paralimbic areas between the proisocortex and isocortical regions, which also have a predominance of infragranular layers but in addition have evidence of a fourth layer, project to the medial pulvinar and to the so-called limbic nuclei, AV, AM, LD, as well as a modality-specific nucleus, LP.     

Septal complex of the telencephalon of the lizard Podarcis hispanica. II. afferent connections

The afferent connections to the septal complex were studied in the lizard Podarcis hispanica (Lacertidae) by means of a combination of retrograde and anterograde tracing. The results of these experiments allow us to classify the septal nuclei into three main divisions. The central septal division (anterior, lateral, dorsolateral, ventrolateral, and medial septal nuclei plus the nucleus of the posterior pallial commissure) receives a massive, topographically organized, cortical projection (medial, dorsal, and ventral areas) and widespread afferents from the tuberomammillary hypothalamus and the basal telencephalon. Moreover, it receives discrete projections from the dorsomedial anterior thalamus, the ventral tegmentum, the midbrain raphe, and the locus coeruleus. The ventromedial septal division (ventromedial septal nucleus) receives a massive projection from the anterior hypothalamus, dense serotonergic innervation, and a faint amygdalohypothalamic projection, but it is devoid of direct cortical input. The midline septal division (nucleus septalis impar and dorsal septal nucleus) receives a nontopographic cortical projection (dorsomedial and dorsal cortices) and afferents from the preoptic hypothalamus, the dorsomedial anterior thalamus, the midbrain central gray, and the reptilian A8 nucleus/substantia nigra. Our results indicate that the cortex provides a physiologically complex, massive input to the septum that terminates over the whole dendritic tree of septal cells. In contrast, most of the ascending afferents make axosomatic contacts by means of pericellular nests. The chemical nature of the main septal afferents and the comparative implications of the available hodological data on the organization of the septal complex of tetrapod vertebrates are discussed. J. Comp. Neurol. 383:489-511, 1997. © 1997 Wiley-Liss, Inc.     

Cingulate cortex projections to the parahippocampal region and hippocampal formation in the rat

In the present study we aimed to determine the topographical and laminar characteristics of cingulate projections to the parahippocampal region and hippocampal formation in the rat, using the anterograde tracers Phaseolus vulgaris-leucoagglutinin and biotinylated dextranamine. The results show that all areas of the cingulate cortex project extensively to the parahippocampal region but not to the hippocampal formation. Rostral cingulate areas (infralimbic-, prelimbic cortices, rostral 1/3 of the dorsal anterior cingulate cortex) primarily project to the perirhinal and lateral entorhinal cortices. Projections from the remaining cingulate areas preferentially target the postrhinal and medial entorhinal cortices as well as the presubiculum and parasubiculum. At a more detailed level the projections show differences in topographical specificities according to their site of origin within the cingulate cortex suggesting the functional contribution of cingulate areas may differ at an individual level. This organization of the cingulate-parahippocampal projections relates to the overall organization of postulated parallel parahippocampal-hippocampal processing streams mediated through the lateral and medial entorhinal cortex respectively. The mid-rostrocaudal part of the dorsal anterior cingulate cortex appears to be connected to both networks as well as to rostral and caudal parts of the cingulate cortex. This region may therefore responsible for integrating information across these specific networks.