Organization of the histaminergic system in the brain of the turtle Chinemys reevesii

To accumulate phylogenetic information on the central histaminergic system, we investigated the histaminergic system in the brain of the Reeves turtle, Chinemys reevesii, using the indirect immunofluorescent method with antiserum against histamine. Histaminergic neuronal cell bodies were found exclusively in the posterior part of the ventral hypothalamus. Histaminergic varicose fibers innervated almost all parts of the turtle brain, but tended to be concentrated in several areas. Very dense innervation was observed in the medial part of the telencephalon, ventrolateral part of the hypothalamus, nucleus habenularis lateralis, and ventromedial part of the tegmentum. Medium density of innervation was seen in the olfactory bulb, nucleus medialis amygdalae, and tectum. Only a few fibers were detected in the lateral part of the telencephalon, dorsal part of the hypothalamus, thalamus, rhombencephalon, and spinal cord. The main ascending fibers were observed in the lateral part of the hypothalamus, sending dense fiber bundles to the cortices dorsomedialis and medialis and nucleus habenularis lateralis. Descending fibers appeared to run in the ventral tegmental area, passing through the dorsal and ventral parts of the midline of the brain stem to the spinal cord. These findings indicate that the general morphological features of the histaminergic system in the turtle brain are similar to those in the mammalian and frog brains.     

Distribution of histaminergic neurons in the brain of the lamprey Lampetra fluviatilis as revealed by histamine-immunohistochemistry

An antiserum against conjugated histamine was used to study the distribution of histaminergic neurons in the CNS of the lamprey Lampetra fluviatilis. Numerous histamine-immunoreactive cell bodies were detected in the dorsal and ventral hypothalamic nuclei and in the adjacent postinfundibular commissural nucleus. Histamine-immunoreactive fibers of high density were present in the ventral hypothalamus, and fibers could also be traced dorsally from the hypothalamus to the corpus striatum and septal nucleus where they appeared to terminate in dense plexuses. Another, smaller group of histamine-immunoreactive perikarya was observed in the border area between mesencephalon and rhombencephalon, near the caudal pole of the mesencephalic reticular nucleus. Sparsely distributed histamine-immunoreactive fibers were present in the ventral mesencephalon. The distribution of histaminergic neurons in cyclostomes, which diverged very early from the main vertebrate line, shows similarities with the corresponding systems in the CNS of amphibians and mammals, which suggests that histaminergic neuronal systems are phylogenetically old and have been conserved during evolution.     

Comparative neuroanatomy of the histaminergic system in the brain of the frog Xenopus laevis

The distribution of the histaminergic neuronal system in the brain of the clawed frog Xenopus laevis was mapped with an antiserum against carbodiimide-fixed histamine and compared to that in mammals. The histamine-immunoreactive cell bodies were located in a small area of the posterolateral hypothalamus, close to the dorsal infundibular nucleus, which contains catecholaminergic and serotonergic neurons. This area may be homologous to the tuberomammillary nucleus in mammals. A thick process extended from each cell between the ependymal cell layer and terminated in the ventricle lumen. The number of histaminergic cell bodies in adult Xenopus brain was relatively low, as compared with the mammalian brain. Preliminary analysis of adjacent sections stained with antisera against GABA or serotonin indicated that the histamine cells were not immunoreactive for these. The pathways and distribution of histaminergic fibers in Xenopus brain showed many similarities to mammals. The densest fiber networks were present in the medial basal forebrain, particularly in the medial amygdala and septum. A distinct cluster of fibers was concentrated around the cell bodies of nucleus accumbens. In most pallial areas, the density was moderate to low. In the primordial piriform cortex and the striatum, very few fibers were seen. In diencephalon, highest fiber densities were found in the anterior and ventral thalamus and posterior and lateral hypothalamus. In hindbrain, the density was highest in the medullary central gray, as in some mammals. The results suggest that the general pattern of the histaminergic system in vertebrate brain is conserved from amphibians to mammals.     

Calcitonin gene-related peptide immunoreactivity in the visceral sensory cortex, thalamus, and related pathways in the rat

It has been proposed that calcitonin gene-related peptide (CGRP) may serve as a major neuromodulator in visceral sensory pathways, but its exact role in the visceral sensory thalamus and cortex has not been determined. We therefore examined the distribution of CGRP-like immunoreactive (CGRPir) innervation of the insular cortex and the parvicellular division of the ventroposterior nucleus of the thalamus (VPpc) in the rat by using immunohistochemistry for CGRP combined with retrograde transport of the fluorescent dye fluoro-gold. Modest numbers of CGRPir fibers were distributed in the dysgranular and agranular insular cortex, but few were observed in the granular insular cortex. The density of CGRPir innervation increased caudally along the rhinal fissue and was considerably greater in the perirhinal cortex. When fluoro-gold was injected into the insular cortex numerous retrogradely labeled neurons were seen in the VPpc, but few of these were CGRPir. Retrogradely labeled CGRPir neurons were, however, seen in the ventral lateral and medial parabrachial (PB) subnuclei. Injection of fluoro-gold into the perirhinal cortex (which is just caudal to the insular cortex along the rhinal fissure) resulted in many retrogradely labeled CGRPir neurons in the posterior thalamic region, including the subparafascicular, the lateral subparafascicular, and the posterior intralaminar nuclei. The VPpc was heavily innervated by CGRPir fibers but contained few CGRPir cell bodies. Injection of fluoro-gold into the VPpc resulted in many retrogradely labeled CGRPir neurons in the external medial PB subnucleus bilaterally, but with a contralateral predominance. Smaller numbers of retrogradely labeled CGRPir neurons were also observed in the ventrolateral PB subnucleus, bilaterally with an ipsilateral predominance. These results suggest that CGRP may be a neuromodulator in the ascending visceral sensory pathways from the PB to the VPpc and the insular cortex, but not between the latter two structures.     

Organization of the histaminergic system in the brain of the teleost, Trachurus trachurus

To accumulate phylogenetic information on the central histaminergic system, we investigated the histaminergic system in the brain of a teleost, the jack mackerel (Trachurus trachurus), using the indirect immunofluorescent method with antiserum against histamine. A small number of histamine-immunoreactive cell bodies were observed in the posterior hypothalamus around the posterior recess. Histamine-immunoreactive fibers innervated the telencephalon, diencephalon, tegmentum, and rostral part of the medulla oblongata. The immunoreactive fibers were very sparse or absent in the olfactory bulb, optic tectum, cerebellum, caudal part of the medulla oblongata, spinal cord, and hypophysis. Ascending fiber bundles were seen in the basal hypothalamus, supplying fiber collaterals to the telencephalon and diencephalon, whereas descending fibers were observed in the midline of the lower brainstem. These findings suggest that the central histaminergic system of the jack mackerel is homologous to those of mammals, reptiles, and amphibians, although poorly developed compared with them. The histamine-immunoreactive neuronal cell bodies found in the border area between the mesencephalon and rhombencephalon of the river lamprey were not detected in the brain of the jack mackerel.     

Direct synaptic connections of axons from superior colliculus with identified thalamo-amygdaloid projection neurons in the rat: Possible substrates of a subcortical visual pathway to the amygdala

The aim of the present study was to identify synaptic contacts from axons originating in the superior colliculus with thalamic neurons projecting to the lateral nucleus of the amygdala. Axons from the superior colliculus were traced with the anterograde tracers Phaseolus vulgaris leucoagglutinin or the biotinylated and fluorescent dextran amine “Miniruby.” Thalamo-amygdaloid projection neurons were identified with the retrograde tracer Fluoro-Gold. Injections of Fluoro-Gold into the lateral nucleus of the amygdala labeled neurons in nuclei of the posterior thalamus which surround the medial geniculate body, viz. the suprageniculate nucleus, the medial division of the medial geniculate body, the posterior intralaminar nucleus, and the peripeduncular nucleus. Anterogradely labeled axons from the superior colliculus terminated in the same regions of the thalamus. Tecto-thalamic axons originating from superficial collicular layers were found predominantly in the suprageniculate nucleus, whereas axons from deep collicular layers were detected in equal density in all thalamic nuclei surrounding the medial geniculate body. Double-labeling experiments revealed an overlap of projection areas in the above-mentioned thalamic nuclei. Electron microscopy of areas of overlap confirmed synaptic contacts of anterogradely labeled presynaptic profiles originating in the superficial layers of the superior colliculus with retrogradely labeled postsynaptic profiles of thalamo-amygdaloid projection neurons. These connections may represent a subcortical pathway for visual information transfer to the amygdala. J. Comp. Neurol. 403:158–170, 1999. © 1999 Wiley-Liss, Inc.     

Histaminergic system in the tree shrew brain

This study mapped the histamine-immunoreactive neuronal system in the brain of the tree shrew (Tupaia belangeri) and compared its structure with that of the rat and guinea pig. The histamine-containing cell bodies lay in the posterior ventral hypothalamus in the tuberomammillary complex, as in the rodents. The morphology of this complex resembled that of the rat. The histaminergic axons projected to nearly all parts of the brain. The main ascending bundle ran ventromedially: the densest innervation was found in the ventral hypothalamus, preoptic area, septum, medial part of nucleus accumbens, and bed nucleus of the stria terminalis. High fiber densities were present in the amygdaloid nuclei and claustrum. Another pathway ran dorsomedially along the periventricular hypothalamus and sent fibers to all parts of the diencephalon. Part of these fibers followed the central gray to the midbrain and spread laterally below the inferior colliculus. Another descending pathway ran through the interfascicular and medial raphe nuclei to meet the pontine central gray. The densest fiber networks were seen in the dorsal tegmental and parabrachial nuclei, and around the locus coeruleus. Also the substantia nigra, interpeduncular and mesencephalic reticular nuclei, colliculi, and vestibular and raphe nuclei received a dense histaminergic innervation. The organization of the fibers in the tree shrew brain resembled more that in the guinea pig than that in the rat. As compared with the guinea pig, more fibers were present, particularly in the globus pallidus, central thalamus, and deep cerebellar nuclei. No fibers were seen in the outer layer of the piriform cortex. In Tupaia, a laminar organization of the fibers was evident in the hippocampus, in contrast to the rodents. Also, a dense periventricular fiber plexus was prominent.     

Biochemical mapping of noradrenergic nerves arising from the rat locus coeruleus

Norepinephrine (NE) concentration was measured in discrete brain regions of the rat following unilateral electrolytic lesions of the locus coeruleus, to determine the distribution of its noradrenergic neurons. Discrete brain nuclei and subdivisions were dissected from frozen sections, and norepinephrine was measured by a sensitive radio-isotopic assay.A significant reduction by 29–63% of control values in norepinephrine content was observed ipsilateral to the lesion in the following areas: all portions of the cerebral cortex examined (entorhinal, hippocampal, cingulate, parietal, and occipital areas), anterior half of the cerebellar cortex, hypothalamic periventricular and paraventricular nuclei, anterior ventral thalamic nucleus, ventral thalamic nucleus, and habenula. It appears that these regions receive unilateral innervation by axons from the locus coeruleus. In 3 regions (the medial geniculate body, inferior colliculus, and posterior half of the cerebellum), NE was reduced bilaterally in a pattern suggesting bilateral innervation from the locus coeruleus. Since no reduction in NE concentration occurred in the medial preoptic nucleus, nucleus interstitialis stria terminalis (ventralis), dorsomedial hypothalamic nucleus, or medial forebrain bundle, axons from noradrenergic neurons in the locus coeruleus do not appear to innervate these regions.The biochemical mapping of noradrenergic nerves from the locus coeruleus is discussed in relation to distribution studies based on the histofluorescence method.     

Projection from the prefrontal cortex to histaminergic cell groups in the posterior hypothalamic region of the rat. Anterograde tracing with Phaseolus vulgaris leucoagglutinin combined with immunocytochemistry of histidine decarboxylase

We investigated the projection from the infralimbic division of the prefrontal cortex (area 25) to histaminergic neurons in the posterior hypothalamic area. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in the prefrontal cortex of rats. Frozen brain sections were subjected to combined PHA-L and histidine decarboxylase (HDC)-peroxidase immunocytochemistry, using nickel-enhanced diaminobenzidine (blue reaction product) to visualize the transported PHA-L, and diaminobenzidine (brown reaction product) to visualize simultaneously the HDC-containing neurons. PHA-L-labeled fibers could be seen coursing in the capsula interna, leaving the telencephalon via the anterior thalamic radiation and the medial forebrain bundle. In the lateral and posterior hypothalamic areas, PHA-L-labeled fibers leave the medial forebrain bundle and traverse the nuclei containing HDC-immunoreactive neurons. Varicosities on the PHA-L-labeled fibers, the majority of which occur en passant, could be observed in close association with the HDC-immunoreactive neurons. The results suggest that the hypothalamic histaminergic neurons receive afferent synaptic input from neurons of the infralimbic division of the prefrontal cortex.     

Postnatal development of primary visual projections in the tammar wallaby (Macropus eugenii)

The time course and pattern of retinal innervation of primary visual areas was traced in pouch-young wallabies. Tritiated proline was injected into one eye of animals ranging in age from 1 to 72 days after birth. These results are compared to the 11 primary visual areas found in the adult wallaby, seven of which receive binocular input while four are monocular. At birth retinal ganglion cell axons have not reached any visual areas. Two to 4 days after birth, all of the axons are crossing to the contralateral optic tract. Nine to 12 days after birth axons begin to invade the contralateral lateral geniculate nucleus, the superior colliculus, and the medial terminal nucleus. Twenty to 21 days after birth, ipsilateral axons invade the lateral geniculate nucleus and superior colliculus. The contralateral projection precedes the ipsilateral projection in all binocular visual areas. By 25 days, ipsilateral and contralateral afferents share common territory in the lateral geniculate nucleus; however, afferents from each eye are initially concentrated in appropriate areas. Between 52 and 72 days, afferents to the dorsal lateral geniculate nucleus are gradually segregated into nine terminal bands. Four are contralateral while five are ipsilateral. By 72 days, the ipsilateral component to the superior colliculus is clustered beneath the contralateral projection a deeper layer. Projections to four monocular visual areas--lateral posterior nucleus, dorsal terminal nucleus, lateral terminal nucleus, and nucleus of the optic tract--are established later than binocular visual areas, except the suprachiasmatic nucleus. The suprachiasmatic nucleus is the last to be bilaterally innervated even though it is situated closest to the optic chiasm. At the light microscope level a mature pattern of visual development is emerging by 72 days, although the eyes do not open until 140 days.     

Collateral projections of single neurons in the posterior thalamic region to both the temporal cortex and the amygdala: a fluorescent retrograde double-labeling study in the rat

It has been reported that the acoustic thalamus of the rat sends projection fibers to both the temporal cortical areas and the lateral amygdaloid nucleus to mediate conditioned emotional responses to an acoustic stimulus. In the present study, fluorescent retrograde double labeling with Fast Blue and Diamidino Yellow has been used in the rat to examine whether single neurons in the posterior thalamic region send axon collaterals to both the temporal cortical areas and lateral amygdaloid nucleus. One of the tracers was injected into the lateral amygdaloid nucleus and the other into the temporal cortical areas close to the rhinal sulcus. Neurons double-labeled with both tracers were found mainly in the posterior intralaminar nucleus and suprageniculate nucleus, and to a lesser extent in the subparafascicular nucleus and medial division of the medial geniculate nucleus. No double-labeled neurons were seen in either the dorsal or ventral division of the medial geniculate nucleus. When one of the tracers was injected into the lateral amygdaloid nucleus and the other into either the dorsal portion of the temporal cortex, the dorsal portion of the entorhinal cortex, or the posterior agranular insular cortex, no double-labeled neurons were found in the posterior thalamic region. The present results indicate that a substantial number of single neurons in the acoustic thalamus project to both the limbic cortical areas and lateral amygdaloid nucleus by way of axon collaterals. These neurons may be implicated in affective and autonomic components of responses to multi-sensory stimuli, including acoustic ones. J. Comp. Neurol. 384:59-70, 1997. © 1997 Wiley-Liss, Inc.     

Immunohistochemical characterization of serotoninergic afferent pathways in the visual system of the rat

Using the immunohistochemical PAP-method occurrence, distribution and terminal aborization of serotoninergic axons were investigated in the visual system of the albino rat. The present study reveals that the innervation varies in quantitative and qualitative manner in the different parts of the visual system. A sparsly serotoninergic innervation was observed in the dorsal lateral geniculate body (d lgb) and in the nucleus lateralis posterior. An intensive serotoninergic innervation was found in the lateral part of the ventral lateral geniculate body (v lgb) and in the area praetectalis. Serotoninergic axons show a lamina specific orientation in the colliculus superior and in the visual cortex. Our findings indicate that serotoninergic axons have an intensive terminal aborization in subcortical visual nuclei, which are more or less connected with oculomotoric functions.     

Organization of cerebral cortical afferent systems in the rat. II. Hypothalamocortical projections

The organization of hypothalamic projections to the cerebral cortex in the rat has been studied using retrograde and anterograde tracer methods. Four separate populations of hypothalamic neurons, which constitute a major source of diffuse cortical innervation, were identified: Tuberal lateral hypothalamic (LHAt) neurons which innervate the cerebral cortex tend to cluster in the perifornical region, in the zona incerta, and along the medial edge of the cerebral peduncle, at levels roughly coextensive with the ventromedial hypothalamic nucleus. Most of these neurons project to the ipsilateral cortex; a small percentage innervate the contralateral cortex, but this varies among cortical terminal fields. The perifornical neurons are organized in a roughly topographic medial-to-lateral relationship with respect to their cortical terminal fields. Field of Forel (FF) neurons, which project primarily to the frontal cortex of the ipsilateral hemisphere, are located just ventral to the medial edge of the medial lemniscus, at the level of the ventromedial basal thalamic nucleus. The more laterally placed neurons innervate the lateral frontal, insular and perirhinal cortex; the more medial neurons, around the mammillothalamic tract, innervate the medial frontopolar, prelimbic, infralimbic, and anterior cingulate cortex. Posterior lateral hypothalamic (LHAp) neurons form a dense cluster spanning the lateral hypothalamus, from the cerebral peduncle to the posterior hypothalamic area at premammillary levels, and extending into the supramammillary nucleus and the adjacent ventral tegmental area. LHAp neurons innervate the entire cerebral cortex, predominantly on the ipsilateral side. Populations of LHAp neurons projecting to different cortical target areas show considerable spatial overlap, but computer plots of the centers of these populations demonstrate a strict topographic relationship with respect to the cerebral cortex. Tuberomammillary (TMN) neurons form a sheet along the ventrolateral surface of the premammillary hypothalamus. About twice as many TMN neurons innervate the ipsilateral, as compared to the contralateral hemisphere; it is not known whether single neurons project to both hemispheres. No topographic organization of the TMN cortical projection is apparent. Injections of different-colored fluorescent dyes into various cortical areas demonstrate that hypothalamic neurons in general have rather restricted cortical terminal fields. Only occasional neurons are found, primarily in LHAt, which are double labeled by injections into different cytoarchitectonic areas.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of connections to and from the visual cortex in the wallaby (Macropus eugenii)

The time course of the development of connections between the visual cortex and the main subcortical visual structures, as well as intrahemispheric and interhemispheric connections, has been studied in the marsupial wallaby (Macropus eugenii) to compare its development with that of placental mammals. Pouch young are born prior to retinal innervation of the primary visual centers and spend a protracted period of development in the pouch, making them ideal for visual, developmental studies. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected into either the presumptive visual cortex or the superior colliculus in young of varying ages. Thalamocortical projections from the dorsal lateral geniculate and lateral posterior nuclei reach the presumptive visual cortex between 12 and 15 days after birth. Descending cortical connections form later. Corticogeniculate axons are first detected in the geniculate and lateral posterior nucleus at 48 days after birth, while corticocollicular axons first reach the superior colliculus at 71 days and, by 81 days, have innervated the superficial layers. Intrahemispheric and interhemispheric connections form even later. By 99 days intrahemispheric axons from area 17 have accumulated in visual association areas but are yet to invade layers III and IV, their major termination zones in adult, while axons projecting back to area 17 have also reached their target area. At this time interhemispheric axons from area 17 have begun to accumulate in the opposite visual cortex, although they have not invaded the cortical layers. By 111 days cortical cells projecting to the opposite visual cortex are first labelled. These have a more widespread distribution in area 17 at 111 and 122 days compared to the adult, where they are confined to the 17/18 border. The results show that the marsupial wallaby has a timetable of similar sequence, but different relative timing, in the formation of cortical connections compared to that of placental mammals. In the first half of the period between conception and eye opening, the timing in the wallaby precedes considerably that in placental mammals. Ascending connections from the thalamus develop relatively earlier in the wallaby but descending collicular connections are delayed until the same relative time that they appear in placental mammals.     

Projection from the prefrontal cortex to histaminergic cell groups in the posterior hypothalamic region of the rat. Anterograde tracing withPhaseolus vulgaris leucoagglutinin combined with immunocytochemistry of histidine decar☐ylase

We investigated the projection from the infralimbic division of the prefrontal cortex (area 25) to histaminergic neurons in the posterior hypothalamic area.Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in the prefrontal cortex of rats. Frozen brain sections were subjected to combined PHA-L and histidine decar☐ylase (HDC)-peroxidase immunocytochemistry, using nickel-enhanced diaminobenzidine (blue reaction product) to visualize the transported PHA-L, and diaminobenzidine (brown reaction product) to visualize simultaneously the HDC-containing neurons. PHA-L-labeled fibers could be seen coursing in the capsula interna, leaving the telencephalon via the anterior thalamic radiation and the medial forebrain bundle. In the lateral and posterior hypothalamic areas, PHA-L-labeled fibers leave the medial forebrain bundle and traverse the nuclei containing HDC-immunoreactive neurons. Varicosities on the PHA-L-labeled fibers, the majority of which occur en passant, could be observed in close association with the HDC-immunoreactive neurons. The results suggest that the hypothalamic histaminergic neurons receive afferent synaptic input from neurons of the infralimbic division of the prefrontal cortex.     

Pattern of retinal projections in the California ground squirrel (Spermophilus beecheyi): anterograde tracing study using cholera toxin

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.     

Development of histamine-immunoreactive neurons in the rat brain

This study was undertaken to reveal the cellular stores of histamine in developing rat brain and to determine the stage of development during which the histamine-immunoreactive neurons can first be detected. Rats from embryonal day 12 to postnatal day 14 were studied. The brains were fixed in 4% 1-ethyl-3(3-dimethylaminopropyl)carbodiimide and standard immunofluorescence technique was used. The first histamine-immunoreactive neurons were seen on embryonic day 13 in the border of mesencephalon and metencephalon. On embryonic day 15 immunoreactive neurons were detected in ventral mesencephalon and rhombencephalon. In caudal, tuberal, and postmammillary caudal magnocellular nuclei histamine-immunoreactive neurons were first detected on embryonic day 20 while those in the hindbrain had disappeared. Histamine-immunoreactive nerve fibers were first detected on embryonic day 15 in rhombencephalon and mesencephalon and in some areas of diencephalon including the mammillary bodies and frontal cortex. On embryonic day 18 the number of immunoreactive nerve fibers in the hindbrain had decreased considerably, but the olfactory bulb, septal and hypothalamic area, and the cerebral cortex showed immunoreaction in fibers. The density of histamine-immunoreactive fiber networks increased until postnatal day 14 when an adultlike pattern of neurons and fibers had developed. Histamine-immunoreactive neurons are present in embryonal CNS and they develop extensive projections to various brain areas.     

Visual system of a naturally microphthalmic mammal: The blind mole rat,Spalax ehrenbergi

Retinal projections and visual thalamo-cortical connections were studied in the subterranean mole rat, belonging to the superspecies Spalax ehrenbergi, by anterograde and retrograde tracing techniques. Quantitative image analysis was used to estimate the relative density and distribution of retinal input to different primary visual nuclei. The visual system of Spalax presents a mosaic of both regressive and progressive morphological features. Following intraocular injections of horseradish peroxidase conjugates, the retina was found to project bilaterally to all visual structures described as receiving retinal afferents in non-fossorial rodents. Structures involved in form analysis and visually guided behaviors are reduced in size by more than 90%, receive a sparse retinal innervation, and are cytoarchitecturally poorly differentiated. The dorsal lateral geniculate nucleus, as defined by cyto- and myelo-architecture, cytochrome oxidase, and acetylcholinesterase distribution as well as by afferent and efferent connections, consists of a narrow sheet 3–5 neurons thick, in the dorsal thalamus. Connections with visual cortex are topographically organized but multiple cortical injections result in widespread and overlapping distributions of geniculate neurons, thus indicating that the cortical map of visual space is imprecise. The superficial layers of the superior colliculus are collapsed to a single layer, and the diffuse ipsilateral distribution of retinal afferents also suggests a lack of precise retinotopic relations. In the pretectum, both the olivary pretectal nucleus and the nucleus of the optic tract could be identified as receiving ipsilateral and contralateral retinal projections. The ventral lateral geniculate nucleus is also bilaterally innervated, but distinct subdivisions of this nucleus or the intergeniculate leaflet could not be distinguished. The retina sends a sparse projection to the dorsal and lateral terminal nuclei of the accessory optic system. The medial terminal nucleus is not present. In contrast to the above, structures of the “non-image forming” visual pathway involved in photoperiodic perception are well developed in Spalax. The suprachiasmatic nucleus receives a bilateral projection from the retina and the absolute size, cytoarchitecture, density, and distribution of retinal afferents in Spalax are comparable with those of other rodents. A relatively hypertrophied retinal projection is observed in the bed nucleus of the stria terminalis. Other regions which receive sparse visual input include the lateral and anterior hypothalamic areas, the retrochiasmatic region, the sub-paraventricular zone, the paraventricular hypothalamic nucleus, the anteroventral and anterodorsal nuclei, the lateral habenula, the mediodorsal nucleus, and the basal telencephalon. These results indicate that the apparently global morphological regression of the visual system conceals a selective expansion of structures related to functions of photoperiodic perception and photo-neuroendocrine regulation. We suggest that the evolution of an atrophied eye and reduced visual system is an adaptively advantageous response to the unique subterranean environment. Factors favoring regression include mechanical aspects, metabolic constraints, and competition between sensory systems. The primary advantage of sensory atrophy is the metabolic economy gained by the reduction of visual structures which do not contribute significantly to the animal's fitness. © 1993 Wiley-Liss, Inc.     

Reciprocal parabrachial-cortical connections in the rat

The projection from the parabrachial nucleus (PB) to the cerbral cortex in the rat was studied in detail using the autoradiographic method for tracing anterograde axonal transport and the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) method for both anterograde and retrograde tracing. PB innervates layers I, V and VI of a continuous sheet of cortex extending from the posterior insular cortex caudally, through the dorsal agranular and the granular anterior insular cortex and on rostrally into the lateral prefrontal cortex. Within the prefrontal area, PB fibers innervate primarily layer V of the ventrolateral cortex caudally, but more rostrally the innervated region includes progressively more dorsal portions of the prefrontal area, until by the frontal pole the entire lateral half of the hemisphere is innervated. This projection originates for the most part in a cluster of neurons in the caudal ventral part of the medial PB subdivision, although a few neurons in the adjacent parts of the PB, the Kolliker-Fuse nucleus and the subcoeruleus region also participate.After injection of WGA-HRP into the PB region, retrogradely labeled neurons were found in layer V of the same cortical areas which receive PB inputs. The importance of this monosynaptic reciprocal brainstem-cortical projection as a possible anatomical substrate for the regulation of cortical arousal is discussed.