Antennular projections to the midbrain of the spiny lobster. III. Central arborizations of motoneurons

The central organization of antennular motoneurons in the brain of the spiny lobster, Panulirus argus, was analyzed by combining biocytin backfills with serial reconstructions of the antennular nerves and the brain. Eighty-nine to 99 antennular motoneurons occur in each hemibrain. The somata of the motoneurons are distributed in a consistent pattern in two complex soma clusters, the ventral paired mediolateral cluster of the deutocerebrum and the dorsal unpaired median cluster of the tritocerebrum. The motoneurons arborize ipsilaterally in the lateral and median antennular neuropils and the tegumentary neuropil. The backfills indicate a minimum of five morphological types of motoneurons with different arborization patterns. The innervation pattern of the motoneurons, together with previously reported innervation patterns of antennular sensory afferents, suggest that the lateral antennular neuropil is a lower motor center driving local antennular reflexes in response to chemical and mechanical stimulation of the antennule, whereas the median antennular neuropil is a lower motor center for equilibrium responses. © 1993 Wiley-Liss, Inc.     

Early stages in the development of spinal motor neurons.

In order to identify early events in the differentiation of motor neurons, the expression of several developmentally regulated, neuronal molecules was investigated by immunohistochemistry on consecutive sections of cervical spinal cord. Motor neurons are among the first neurons to be born and to differentiate within the embryonic rat spinal cord. They undergo their terminal mitosis on embryonic days 10 and 11 (E10-11) and acquire detectable levels of the transmitter synthesizing enzyme, choline acetyltransferase, by E11.5. Staining with antibodies to the 68 kD neurofilament protein revealed motor neurons extending processes out the ventral root as early as E10.5. Monoclonal antibodies to two different epitopes on the cell adhesive molecule, NCAM, bound to myotomes on E10.5, and began to recognize ventral horn neurons by E11. Two other markers of developing neurons, the growth-associated protein, GAP-43, and the surface glycoprotein, TAG-1, were clearly detected on young motor neurons by E11.5. Thus, during the 36 hours following the final mitosis of their precursors, motor neurons acquire cytoskeletal, enzymatic, and cell surface components that distinguish them from other developing cells within the spinal cord. Not all of the newly acquired molecules continue to be expressed by motor neurons. Immunoreactivity for TAG-1 was lost by E12.5, followed by a gradual reduction of immunoreactivity for GAP-43 and the highly polysialylated form of NCAM. By E15, only antibodies to choline acetyltransferase (Phelps et al., J. Comp. Neurol. 307:1-10, 1990), and to neurofilaments, selectively stained motor neurons within the embryonic spinal cord. The transient presence of GAP-43, TAG-1, and the embryonic form of NCAM coincides with a period of vigorous axonal growth and declines when motor neurons reach their targets. This report describes the temporal sequence of early stages in the differentiation of the rodent motor neuronal phenotype. Some of these changes may be related to interactions with their synaptic partners.     

Localization of nitric oxide synthase in the central complex and surrounding midbrain neuropils of the locust Schistocerca gregaria

Nitric oxide (NO), generated enzymatically by NO synthase (NOS), acts as an important signaling molecule in the nervous systems of vertebrates and invertebrates. In insects, NO has been implicated in development and in various aspects of sensory processing. To understand better the contribution of NO signaling to higher level brain functions, we analyzed the distribution of NOS in the midbrain of a model insect species, the locust Schistocerca gregaria, by using NADPH diaphorase (NADPHd) histochemistry after methanol/formalin fixation; results were validated by NOS immunohistochemistry. NADPHd yielded much higher sensitivity and resolution, but otherwise the two techniques resulted in corresponding labeling patterns throughout the brain, except for intense immunostaining but only weak NADPHd staining in median neurosecretory cells. About 470 neuronal cell bodies in the locust midbrain were NADPHd-positive positive, and nearly all major neuropil centers contained dense, sharply stained arborizations. We report several novel types of NOS-expressing neurons, including small ocellar interneurons and antennal sensory neurons that bypass the antennal lobe. Highly prominent labeling occurred in the central complex, a brain area involved in sky-compass orientation, and was analyzed in detail. Innervation by NOS-expressing fibers was most notable in the central body upper and lower divisions, the lateral accessory lobes, and the noduli. About 170 NADPHd-positive neurons contributed to this innervation, including five classes of tangential neuron, two systems of pontine neuron, and a system of columnar neurons. The results provide new insights into the neurochemical architecture of the central complex and suggest a prominent role for NO signaling in this brain area.     

Processing of olfactory information at three neuronal levels in the spiny lobster

Odor quality coding was analyzed at three neuronal levels, receptor cell and two levels of chemosensory interneurons, in the olfactory system of the spiny lobster Panulirus argus. Responses to three of the most stimulatory compounds for this animal—taurine, glutamate and betaine—were recorded at each level in order to compare basic neuronal response properties, single cell and population response spectra, and across-neuron patterns. Mean response specificity increased for cells at each successive neuronal level. The increase in breadth of tuning between receptor cells and low-order interneurons was paralleled by an increase in interstimulus across-neuron correlations. However, in high-order interneuronns, there was relative decline in across-neuron correlations, indicating that the more broadly-tuned high-order interneurons are better able to discriminate between any two compounds than are the more narrowly-tuned low-order interneurons. Although stimulus quality appears to be coded by interneurons as an across-fiber pattern, the fact that some low-order and high-order interneurons retained the narrow specificity of receptor cells suggests that labeled lines may have an important funtion in coding throughout the olfactory pathway of thespiny lobster.     

The topographic organization of hypothalamic and brain stem projections to the hippocampus

Direct projections primarily ipsilateral to hippocampus from medial septal, diagonal band, supramammillary, submammillothalamic, locus coeruleus, and dorsal and medianus raphe nuclei were demonstrated. The locus coeruleus projects primarily through the cingulum and fornix superior to the dorsal posterior hippocampus, with its terminal fields in the stratum lacunosum moleculare of the subiculum and areas CA 1-CA 2 of the dorsal posterior hippocampus. LC projections to the granular layer of the dentate hilus were not found. Raphe nuclei project through the cingulum, fornix superior, and primarily the fimbria, to the dorsal and ventral posterior hippocampus, with their terminal fields in the stratum lacunosum moleculare of the dorsal posterior subicular region, stratum radiatum of CA 1-CA 3 in the dorsal hippocampus, and the stratum polymorph of the dentate gyrus, primarily in its superficial part. Raphe projections to the anterior hippocampal rudiment were found. However, no projection was found to the subiculum of the ventral posterior hippocampus, nor to stratum oriens. Hypothalamic nuclei project through the fornix superior and the fimbria, mainly to the dorsal posterior hippocampus with abundant terminal fibers in the depth of the dentate hilus. Smaller cells in these hypothalamic nuclei appear projecting to the ventral hippocampus. The number of neurons in the entorhinal area, the diagonal band, and the hypothalamic nuclei projecting to the hippocampus suggests these groups as the main sources of the extrinsic hippocampal afferents. In addition, they may also serve as relay stations for inputs from more caudal nuclei, and the topographic organization of their terminal fields as described herein may have important functional implications.     

Origin of mammalian thalamocortical projections. I. Telencephalic projections of the medial geniculate body in the opossum (Didelphis virginiana)

Telencephalic projections from the medial geniculate nucleus (MG) in opossum were traced with tritiated leucine autoradiography and by horseradish peroxidase and fluorescent dye retrograde labeling techniques. The results show that the opossum's MG contains two separate populations of neurons-one in the anterior two-thirds of MG projecting to auditory neocortex, the other occupying the entire caudal one-third of MG and projecting mostly to lateral amygdala and putamen. Because the subcortical projection of the MG in opossum is larger than that seen in any other mammal to date, it is reminiscent of the subcortical projections of the MG in reptiles and birds. Furthermore, when the subcortical projections of the MG in reptiles and opossums are compared with similar subcortical projections of the MG in rats, cats, and monkeys, the proportion of the MG neurons projecting to subcortical structures is seen to be inversely related to the recency of each animal's common ancestry with primates. The possibility that the subcortical projection of the MG in mammals is homologous with that seen in reptiles or birds implies that it might be a dwindling vestige of the projection present in the common ancestry of reptiles and mammals.     

Identification of putative neuroblasts at the base of adult neurogenesis in the olfactory midbrain of the spiny lobster, Panulirus argus

Continuous neurogenesis persists during adulthood in the olfactory midbrain of decapod crustaceans, including spiny lobsters, Panulirus argus. This encompasses generation of projection and local interneurons, whose somata are in the lateral soma cluster (LC) and medial soma cluster (MC), respectively. Both neuronal types originate from immediate precursors labeled by a single injection of BrdU and located in a small proliferation zone within each cluster. The aim of this study was to identify neuroblasts as a source of the dividing cells by multiple injections of BrdU over 2 days. All animals receiving multiple injections had one or a few 'extra' BrdU-positive nuclei near the proliferation zones, and these nuclei were significantly larger than nuclei of neurons or BrdU-positive cells in the proliferation zones. Since the defining morphological feature of neuroblasts in preadult neurogenesis in arthropods is being larger than their progeny, these large extra BrdU-positive nuclei represent "putative adult neuroblasts." Multiple BrdU-injections revealed a clump of small cells enclosing the putative adult neuroblasts in LC and MC, and these cells shared morphological characteristics with newly identified putative glial cells in the soma clusters and perivascular cells in the walls of arterioles. These results on P. argus suggest that adult neurogenesis is based on one adult neuroblast per soma cluster, adult neurogenesis appears to be a continuation of embryonic and larval neurogenesis, and the newly identified clumps of cells surrounding the putative adult neuroblasts might provide them with specific microenvironments necessary for their unusual lifelong proliferative and self-renewal capacity.     

Organization of subcortical pathways for sensory projections to the limbic cortex. I. Subcortical projections to the medial limbic cortex in the rat

Subcortical afferent projections to the medial limbic cortex were examined in the rat by the use of retrograde axonal transport of horseradish peroxidase. Small iontophoretic injections of horseradish peroxidase were placed at various locations within the dorsal and ventral cingulate areas, the dorsal agranular and ventral granular divisions of the retrosplenial cortex and the presubiculum. Somata of afferent neurons in the thalamus and basal forebrain were identified by retrograde labeling. Each of the anterior thalamic nuclei was found to project to several limbic cortical areas, although not with equal density. The anterior dorsal nucleus projects primarily to the presubiculum and ventral retrosplenial cortex; the anterior ventral nucleus projects to the retrosplenial cortex and the presubiculum with apparently similar densities; and the anterior medial nucleus projects primarily to the cingulate areas. The projections from the lateral dorsal nucleus to these limbic cortical areas are organized in a loose topographic fashion. The projection to the presubiculum originates in the most dorsal portion of the lateral dorsal nucleus. The projection to the ventral retrosplenial cortex originates in rostral and medial portions of the nucleus, whereas afferents to the dorsal retrosplenial cortex originate in caudal portions of the lateral dorsal nucleus. The projection to the cingulate originates in the ventral portion of the lateral dorsal nucleus. Other projections from the thalamus originate in the intralaminar and midline nuclei, including the central lateral, central dorsal, central medial, paracentral, reuniens, and paraventricular nuclei, and the ventral medial and ventral anterior nuclei. In addition, projections to the medial limbic cortex from the basal forebrain originate in cells of the nucleus of the diagonal band. Projections to the presubiculum also originate in the medial septum. These results are discussed in regard to convergence of sensory and nonsensory information projecting to the limbic cortex and the types of visual and other sensory information that may be relayed to the limbic cortex by these projections.     

Cytoarchitecture and ultrastructure of neural stem cell niches and neurogenic complexes maintaining adult neurogenesis in the olfactory midbrain of spiny lobsters, Panulirus argus

New interneurons are continuously generated in small proliferation zones within neuronal somata clusters in the olfactory deutocerebrum of adult decapod crustaceans. Each proliferation zone is connected to a clump of cells containing one neural stem cell (i.e., adult neuroblast), thus forming a "neurogenic complex." Here we provide a detailed analysis of the cytoarchitecture of neurogenic complexes in adult spiny lobsters, Panulirus argus, based on transmission electron microscopy and labeling with cell-type-selective markers. The clump of cells is composed of unique bipolar clump-forming cells that collectively completely envelop the adult neuroblast and are themselves ensheathed by a layer of processes of multipolar cell body glia. An arteriole is attached to the clump of cells, but dye perfusion experiments show that hemolymph has no access to the interior of the clump of cells. Thus, the clump of cells fulfills morphological criteria of a protective stem cell niche, with clump-forming cells constituting the adult neuroblast's microenvironment together with the cell body glia processes separating it from other tissue components. Bromodeoxyuridine pulse-chase experiments with short survival times suggest that adult neuroblasts are not quiescent but rather cycle actively during daytime. We propose a cell lineage model in which an asymmetrically dividing adult neuroblast repopulates the pool of neuronal progenitor cells in the associated proliferation zone. In conclusion, as in mammalian brains, adult neurogenesis in crustacean brains is fueled by neural stem cells that are maintained by stem cell niches that preserve elements of the embryonic microenvironment and contain glial and vascular elements.     

Organization of cortical and subcortical projections to medial prefrontal cortex in the cat

We have analyzed the cortical and subcortical afferent connections of the medial prefrontal cortex (MPF) in the cat with the specific aim of characterizing subregional variations of afferent connectivity. Thirteen tracer deposits were placed at restricted loci within a cortical district extending from the proreal to the subgenual gyrus. The distribution throughout the forebrain of retrogradely labeled neurons was then analyzed. Within the thalamus, retrogradely labeled neurons were most numerous in the mediodorsal nucleus and in the ventral complex. The projection from each region exhibited continuous topography such that more medial thalamic neurons were labeled by tracer from more ventral and posterior cortical deposits. Marked retrograde labeling without any sign of topographic order occurred in a narrow medioventral sector of the lateroposterior nucleus. Several additional thalamic nuclei contained small numbers of labeled neurons. In a subset of nuclei closely affiliated with the limbic system (the parataenial, paraventricular, reuniens, and basal ventromedial nuclei), retrograde labeling occurred exclusively after deposits at extremely ventral and posterior cortical sites. Within the amygdala, retrogradely labeled neurons occupied the anterior basomedial nucleus, the posterior basolateral nucleus, and a narrow strip of the lateral nucleus immediately adjoining the basolateral nucleus. The number of labeled neurons was greater after more ventral deposits. Very ventral deposits resulted in extensive labeling of the cortical amygdala. Within the cerebral cortex, the distribution of labeled neurons depended on the location of the tracer deposit. Comparatively dorsal deposits produced prominent retrograde transport to the anterior and posterior cingulate areas, to the agranular insula, and to lateral prefrontal cortex. Comparatively ventral deposits gave rise to prominent labeling of the hippocampal subiculum, various parahippocampal areas, and prepiriform cortex. On the basis of afferent connections, it is possible to divide the cat's medial prefrontal cortex into an infralimbic component, MPFil, marked by strong afferents from prepiriform cortex and the cortical amygdala, and a dorsal component, MPFd, without afferents from these structures. Further, within MPFd, it is possible to define an axis, running from ventral and posterior to dorsal and anterior levels, along which limbic afferents gradually become weaker and projections from cortical association areas gradually become stronger.     

Cells of origin of spinothalamic tract projections to the medial and lateral thalamus in the cat

The double fluorescent retrograde labeling method was used to examine the distribution of spinothalamic tract (STT) cells that project to the medial and lateral thalamus in the cat. Injections of one fluorescent tracer (Fast Blue or Diamidino Yellow) were made throughout the lateral thalamus and injections of the other tracer were made in the medial thalamus at sites extrapolated from recording track coordinates. Survival times were successively extended (up to 5 weeks) in order to maximize labeling in both the cervical and lumbosacral spinal cord. On average, over 2,000 labeled contralateral STT cells were counted in serial sections from segments C5-7 and L5-S2. Numerical variability of the order of a factor of two was attributable to inherent differences between individual animals. The total number of cells labeled with fluorescent tracers was comparable to the number labeled with horseradish peroxidase in control cases, although there were significant differences between the laminar distributions of labeling produced by the two methods. Injections made anterior to the thalamus to control for labeling due to leakage or passing fibers did not produce substantial spinal labeling. The laminar distribution of fluorescent dye-labeled STT cells was consistent; about half (47%) were located in lamina I, 8% were in lamina V, 5% in lamina VI, 20% in lamina VII, and 20% in lamina VIII. The proportions of STT cells in laminae I and V were higher in cervical segments (57% and 12%, respectively) than in lumbosacral segments (38% and 6%). The dominant contribution of lamina I cells to the STT thus revealed by the fluorescent tracers is striking. The proportions of STT cells labeled from the medial and the lateral thalamus varied with segmental and laminar location and with injection placement. The majority (62%) of STT cells in most cases projected only to the medial thalamus, 25% projected only to the lateral thalamus, and 13% projected to both. The STT cell populations in laminae I, VII, and VIII each displayed this common projection pattern. In contrast, cells in laminae V and VI projected predominantly to the lateral thalamus. Twice as many STT cells in lamina I (19%) projected to both the medial and the lateral thalamus as from other laminae. A greater proportion of laminae V-VIII STT cells in segments L5-6 projected to the lateral thalamus, and in S1-2, more projected to the medial thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Retrograde HRP demonstration of afferent projections to the midbrain and nest calls in the ring dove

Midbrain control of vocalization was evaluated in the ring dove by determining the major afferent inputs with retrograde tract tracing technique. Horseradish peroxidase was infused into various portions of the nucleus intercollicularis, an estrogen concentrating area, which disrupts nest calls when lesioned and induces the vocalization when stimulated by estrogen. Most labelled cell bodies were found in the archistriatum, including a region homologous to the mammalian amygdala.     

The efferent projections of the periaqueductal gray in the rat: A Phaseolus vulgaris-leucoagglutinin study. I. Ascending projections

This study has examined the ascending projections of the periaqueductal gray in the rat. Injections of Phaseolus vulgaris-leucoagglutinin were placed in the dorsolateral or ventrolateral subregions, at rostral or caudal sites. From either region, fibers ascended via two bundles. The periventricular bundle ascended in the periaqueductal and periventricular gray matter. At the posterior commissure level, this bundle divided into a dorsal component that terminated in the intralaminar and midline thalamic nuclei, and a ventral component that supplied the hypothalamus. The ventral bundle formed in the deep mesencephalic reticular formation and supplied the ventral tegmental area, substantia nigra pars compacta, and the retrorubral field. The remaining fibers were incorporated into the medial forebrain bundle. These supplied the lateral hypothalamus and forebrain structures, including the preoptic area, the nuclei of the diagonal band, and the lateral division of the bed nucleus of the stria terminalis. The dorsolateral subregion preferentially innervated the centrolateral and paraventricular thalamic nuclei and the anterior hypothalamic area. The ventrolateral subregion preferentially innervated the parafascicular and central medial thalamic nuclei, the lateral hypothalamic area, and the lateral division of the bed nucleus of the stria terminalis. Although the dorsolateral and ventrolateral subregions gave rise to differential projections, the projections from both the rostral and caudal parts of either subregion were similar. This suggests that the dorsolateral and ventrolateral subregions are organized into longitudinal columns that extend throughout the length of the periaqueductal gray. These columns may correspond to those demonstrated in recent physiological studies. © 1995 Willy-Liss, Inc.     

Comparison of vestibular and abducens internuclear projections to the medial rectus subdivision of the oculomotor nucleus in the monkey

Comparisons were made of projections from the vestibular nuclei (VN) and abducens internuclear neurons (AIN) to cell group A of the medial rectus subdivision (MRS) of the oculomotor nuclear complex. Cell group A, the major component of the MRS, receives projections only from the ipsilateral VN and the contralateral AIN. Neither ipsilateral vestibular projections to cell group A, arising from the medial vestibular nucleus, nor projections from MVN to the opposite abducens nucleus, match the massive projection of AIN to the MRS.     

Retinal synapses of the cat medial interlaminar nucleus and ventral lateral geniculate nucleus differ in size and synaptic organization

The retinal terminals of the medial interlaminar nucleus (MIN) and ventral lateral geniculate nucleus (VLG) have been examined quantitatively to determine if there are morphological differences in their synaptic ultrastructure which reflect their distinctive physiologies. The cross-sectional area and density (number per unit area) of synaptic contact zones with conventional and presynaptic dendrites (F2 profiles) were measured for each retinal terminal. The densities of F2 presynaptic dendrites and F1 flattened vesicle axon terminals were also measured. Retinal terminals in MIN were often large (mean size= 2.7 μm² area) and had a high density of synaptic contacts (0.14 per μm surface area) with conventional dendrites, presynaptic dendrites, and dendritic spines. A high density of F2 presynaptic dendrites (0.08 per μm² area) was found in MIN. F1 axon terminals were also found frequently (0.04 per μm²). MIN retinal terminals were often organized in glomeruli like those of the dorsal lateral geniculate nucleus. The retinal terminals in VLG were almost always small (mean size= 0.94 μm² area), although they also had a high density of synaptic contacts (0.17 per μm surface area). They frequently synapsed on small dendrites and dendritic spines and less frequently on large dendrites. Unlike MIN, retinal terminals in VLG rarely contacted F2 presynaptic dendrites which were much less frequent in VLG (0.01 per μm² area). Like MIN, VLG contained numerous F1 axon terminals (0.06 per μm² area). No typical retinal glomeruli were found in VLG. These results show that MIN, which contains many Y cells, has a population of large retinal terminals and many F2 presynaptic dendrites. VLG, which apparently has only W cells, contains only small retinal terminals and has fewer F2 presynaptic dendrites. Both have a high density of F1 flat vesicle axon terminals.     

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): I. Normal topography

The topography of retinal projections to the superior colliculus and dorsal lateral geniculate nucleus of a wallaby, the tammar (Macropus eugenii), was investigated by an anatomical method. Small laser lesions were made in the retinas of experimental animals, and the remaining retinal projections were visualized by means of horseradish-peroxidase histochemistry. The position of each lesion was correlated with the position of the filling defects in the terminal label. The whole of the retina projects to the contralateral superior colliculus. The nasal retina is represented caudally, and the temporal retina rostrally. The ventral retina is represented medially, and the dorsal retina laterally. There is a projection to the ipsilateral superior colliculus, but it is patchy and its topography could not be determined by this method. The retinotopic map in the contralateral dorsal lateral geniculate nucleus has the nasal retina represented rostrally and the temporal retina caudally in the nucleus. The dorsal retina is represented ventrally, and the ventral retina is represented dorsally. It appears that the whole of the retina projects contralaterally, and in addition the temporal retina projects ipsilaterally. The maps of visual space through the two eyes were shown to be in topographic register in the binocular region by making a deposit of HRP in the visual cortex. This resulted in a column of retrogradely labeled cells in the nucleus. This column crossed the laminae, which are innervated by the ipsilateral and contralateral eye at right angles.     

Afferent projections related to attack sites in the ventral midbrain tegmentum of the cat

Quiet attack was elicited by electrical stimulation of the ventrolateral midbrain tegmentum of the cat. The paw was not used other than to position or hold the rat during the bite. Bites were directed toward the head and neck region and were not accompanied by autonomic responses other than pupillary dilation and sometimes slight piloerection on the back.Horseradish peroxidase was deposited at the attack sites. Cells labeled with the peroxidase reaction product were located in gyrus proreus, gyrus genualis, nucleus accumbens, bed nucleus of the stria terminalis, bed nucleus of the anterior commissure, nucleus of the diagonal band, substantia innominata, anterior amygdaloid area, ventromedial hypothalamic area, paraventricular nucleus, perifornical hypothalamic area, lateral hypothalamic area, dorsal hypothalamic area, fields of Forel, midbrain reticular formation, superior colliculus, ventral central grey, lateral central gray, locus coeruleus, parabrachial nuclei, nucleus of the lateral lemniscus, oral pontine reticular nucleus, and the dorsal raphe. Other regions were less prominently labeled. Previous studies have shown most of these sites to have some involvement in attack.     

Histochemical and pharmacological analysis of catecholaminergic projections to the perifornical hypothalamus in relation to feeding inhibition

Three techniques, namely, midbrain lesions, fluorescence histochemistry, and brain cannulation, were used in combination to analyze noradrenergic projections to the paraventricular nucleus of the hypothalamus (PVN) and their function in stimulating feeding behavior. The convergence of evidence indicates that the dorsal component of the central tegmental tract (CTT), which ascends through the dorsal pons and then projects through the medial tegmental radiations (TR) into the ventral tegmentum just dorsal to the medial lemniscus, contains the crucial noradrenergic axons which innervate the PVN and mediate noradrenergic stimulation of feeding behavior. The primary evidence for this conclusion is that dorsal tegmental electrolytic or 6-OHDA lesions which damaged specifically these fibers invariably caused: (1) a reduction of catecholamine varicosities within the PVN (most notably, fine and moderate-size, rounded varicosities within the parvocellular area); (2) a strong reduction or loss of the feeding response elicited by PVN injection of the presynaptically-acting drugs tranylcypromine and desipramine; and (3) a potentiation of the same response produced by injected norepinephrine. These pharmacological and neurochemical changes in the PVN were reduced in magnitude if the dorsal CTT and medial TR fivers received only partial damage, and these changes did not occur at all if the lesion fell immediately dorsal to these fibers without damaging them. Specific lesions in the ventral tegmentum, which also failed to damage the dorsal CTT and TR axons but instead damaged the ventral component of the CTT, not only failed to disrupt the action of the antidepressant agents but actually potentiated their effectiveness in the PVN. Ventromedial lesions, however, which severed the rostroventral extension of the dorsal CTT and medial TR fibers, had the same behavioral consequences as had the dorsal lesions which damaged this projection at a more dorsocaudal level. Finally, damage to other catecholamine projections had little effect on PVN function in stimulating eating.     

Identification of medial preoptic neurons that concentrate estradiol and project to the midbrain in the rat

Fluorescent dye retrograde tracing was combined with steroid hormone autoradiography to study the midbrain projections of the estrogen-concentrating neurons in the preoptic region of the rat brain. Microinjections of the dyes DAPI, true blue, or a mixture of DAPI and primuline were made into the ventral tegmental area and into the midbrain central gray of ovariectomized, adrenalectomized 2-3-month-old female rats; 3 or 4 days later these animals were injected with [3H]estradiol; the brains were then processed for autoradiography. After exposures of from 3 to 12 months, the autoradiograms were developed and examined for reduced silver grains under cell nuclei (indicating binding of [3H]estradiol) and retrogradely transported fluorescent dye in the cytoplasm (indicating an efferent projection to the midbrain). Numerous [3H]estradiol-concentrating neurons in the medial preoptic region were found to send their axons to the medial midbrain. The largest numbers of estrogen target neurons that were afferent to the ventral tegmental area and to the midbrain central gray were found in the medial preoptic nucleus, in the surrounding medial preoptic area, and in the ventral bed nucleus of the stria terminalis. Double-labeled neurons were also identified in the preoptic suprachiasmatic area, in the lateral preoptic area, and in the rostral anterior hypothalamic area. Thus, a subset of the gonadal steroid target cells of the preoptic region have long projections to the medial midbrain, and a subset of the medial preoptic neurons that project to the ventral tegmental area and to the midbrain central gray concentrate estrogen. Behaviors (for example, maternal behavior, male copulatory behavior, and wheel-running) that are regulated by estrogen action in the medial preoptic region may be controlled by the direct estrogen-sensitive pathway to the medial midbrain revealed in this study.