The anterograde and retrograde transport of horseradish peroxidase was used to study the anatomical organization of visceral and limbic terminal fields in the insular cortex. Following injections into the ventroposterolateral parvicellular (VPLpc) and ventroposteromedial parvicellular (VPMpc) visceral relay nuclei of the thalamus, dense anterograde and retrograde labeling was present in the posterior granular and dysgranular insular cortices, respectively. The parabrachial nucleus had extensive connections with the posterior dysgranular cortex and to a lesser degree with the anterior dysgranular and granular cortices. In contrast, injections into the medial prefrontal cortex and mediodorsal nucleus of the thalamus resulted in dense anterograde and retrograde labeling primarily in the anterior agranular cortex, whereas injections in the amygdala resulted in axonal labeling in the agranular and dysgranular insular cortices. Injections into the lateral hypothalamic area resulted in dense anterograde and retrograde labeling mainly in the agranular and dysgranular cortices and moderate to light labeling in the granular cortex. Our results indicate that ascending visceral afferents, VPLpc, VPMpc, and parabrachial nuclei, are topographically organized in the granular and dysgranular fields of the insular cortex, whereas the agranular cortex appears to receive highly integrated limbic afferents from the infralimbic cortex and the mediodorsal nucleus of the thalamus. Although these visceral and limbic inputs to the insular cortex are segregated for the most part into different longitudinally oriented strips of cortex, limbic input from the lateral hypothalamic area and the amygdala, which have extensive autonomic as well as limbic connections, are more diffusely distributed over the different regions of the insular cortex. This organization may subserve a role for the insular cortex in integration of autonomic response with ongoing behaviour and emotion.

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)

The present study describes the anatomical organization of projections from functionally defined cell groups of the lateral hypothalamic area. Cardiovascular pressor and depressor sites were identified following microinjection (5-50 nl) of 0.01-1.0 M L-glutamate or D,L-homocysteate into the anesthetized rat. Subsequent injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) or wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made into pressor or depressor sites and their connections with the brainstem and spinal cord were traced. Decreases in blood pressure (10-45 mmHg) and heart rate (20-70 bpm) were elicited from tuberal (LHAt) and posterior (LHAp) regions of the lateral hypothalamic area (LHA). Depressor neurons in the LHAt have descending projections to the central gray, dorsal and median raphe nuclei, pedunculopontine tegmental nucleus, pontine reticular formation, medial and lateral parabrachial nuclei, laterodorsal tegmental region, and medullary reticular formation including the region of the lateral tegmental field, nucleus ambigous, and rostrocaudal ventral lateral medulla. In contrast, descending projections from depressor neurons in the LHAp have dense terminal fields in the rostral, middle, and commissural portions of the nucleus of the solitary tract and the lateral tegmental field as well as the ventrolateral central gray, pedunculopontine tegmental nucleus, and medial and lateral parabrachial nuclei. Both the LHAt and LHAp have light projections to the intermediate region of the cervical and thoracic spinal cord. Increases in blood pressure (10-40 mmHg) and heart rate (20-70 bpm) were elicited almost exclusively from neurons located medial to the LHAt and LHAp in a region surrounding the fornix, termed the perifornical area (PFA). Pressor cells in the PFA have descending projections to the central gray, dorsal and median raphe nuclei, laterodorsal tegmental nucleus, and Barrington's nucleus as well as a light projection to the commissural portion of the nucleus of the solitary tract and the intermediate region of the cervical and thoracic spinal cord. The retrograde labeling observed in the WGA-HRP studies indicates that cells in most terminal fields have reciprocal projections to the pressor and depressor regions of the LHA. The results demonstrate that groups of neurons in the lateral hypothalamus with specific cardiovascular function have differential projections to the brain stem.

Fluorescent double retrograde-tracing studies combined with fluorescent immunostaining for serotonin were carried out to determine the potential patterns of divergence in axonal projections to autonomic and somatic motor sites from medullary raphe and parapyramidal neurons. Injections (20-60 nl) of combinations of fluorescent retrograde tracers (Fast Blue, fluoro-gold, green latex microspheres, Diamidino Yellow) were made into the intermediolateral cell column (IML) of the spinal cord and the brainstem lateral tegmental field or ventral horn of the lumbar spinal cord of male Wistar rats. The animals were perfused after a 7-10-day survival period, and the brains were removed, sectioned (50 microns), and immunostained for serotonin. Following injections of different retrograde-tracer substances into the IML of the thoracic cord and the ventral horn of the lumbar cord, 36% of the neurons with axon collateral projections to the IML and the lumbar ventral horn were serotoninergic. Following injections of different retrograde-tracer substances into the IML and the lateral tegmental field, 26% of the neurons with axon collateral projections to the IML and the lateral tegmental field were serotoninergic. Many of the medullary neurons with projections to the lateral tegmental field and the lumbar cord were located dorsal and lateral to those neurons with projections to the IML. The results indicate that serotoninergic and nonserotoninergic neurons of the midline raphe system and parapyramidal region have axon collateral branches to the IML and the lateral tegmental field or the IML and the lumbar ventral horn. These projection neurons may form the anatomical substrate for the integration of autonomic and somatic motor activity.

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