GABAA-receptor heterogeneity in the adult rat brain: differential regional and cellular distribution of seven major subunits

GABAA-receptors display an extensive structural heterogeneity based on the differential assembly of a family of at least 15 subunits (alpha 1-6, beta 1-3, gamma 1-3, delta, rho 1-2) into distinct heteromeric receptor complexes. The subunit composition of receptor subtypes is expected to determine their physiological properties and pharmacological profiles, thereby contributing to flexibility in signal transduction and allosteric modulation. In heterologous expression systems, functional receptors require a combination of alpha-, beta-, and gamma-subunit variants, the gamma 2-subunit being essential to convey a classical benzodiazepine site to the receptor. The subunit composition and stoichiometry of native GABAA-receptor subtypes remain unknown. The aim of this study was to identify immunohistochemically the main subunit combinations expressed in the adult rat brain and to allocate them to identified neurons. The regional and cellular distribution of seven major subunits (alpha 1, alpha 2, alpha 3, alpha 5, beta 2,3, gamma 2, delta) was visualized by immunoperoxidase staining with subunit-specific antibodies (the beta 2- and beta 3-subunits were covisualized with the monoclonal antibody bd-17). Putative receptor subtypes were identified on the basis of colocalization of subunits within individual neurons, as analyzed by confocal laser microscopy in double- and triple-immunofluorescence staining experiments. The results reveal an extraordinary heterogeneity in the distribution of GABAA-receptor subunits, as evidenced by abrupt changes in immunoreactivity along well-defined cytoarchitectonic boundaries and by pronounced differences in the cellular distribution of subunits among various types of neurons. Thus, functionally and morphologically diverse neurons were characterized by a distinct GABAA-receptor subunit repertoire. The multiple staining experiments identified 12 subunit combinations in defined neurons. The most prevalent combination was the triplet alpha 1/beta 2,3/gamma 2, detected in numerous cell types throughout the brain. An additional subunit (alpha 2, alpha 3, or delta) sometimes was associated with this triplet, pointing to the existence of receptors containing four subunits. The triplets alpha 2/beta 2,3/gamma 2, alpha 3/beta 2,3/gamma 2, and alpha 5/beta 2,3/gamma 2 were also identified in discrete cell populations. The prevalence of these seven combinations suggest that they represent major GABAA-receptor subtypes. Five combinations also apparently lacked the beta 2,3-subunits, including one devoid of gamma 2-subunit (alpha 1/alpha 2/gamma 2, alpha 2/gamma 2, alpha 3/gamma 2, alpha 2/alpha 3/gamma 2, alpha 2/alpha 5/delta).(ABSTRACT TRUNCATED AT 400 WORDS)

Information coding in the olfactory system: evidence for a stereotyped and highly organized epitope map in the olfactory bulb

In the mammalian olfactory system, information from approximately 1000 different odorant receptor types is organized in the nose into four spatial zones. Each zone is a mosaic of randomly distributed neurons expressing different receptor types. In these studies, we have obtained evidence that information highly distributed in the nose is transformed in the olfactory bulb of the brain into a highly organized spatial map. We find that specific odorant receptor gene probes hybridize in situ to small, and distinct, subsets of olfactory bulb glomeruli. The spatial and numerical characteristics of the patterns of hybridization that we observe with different receptor probes indicate that, in the olfactory bulb, olfactory information undergoes a remarkable organization into a fine, and perhaps stereotyped, spatial map. In our view, this map is in essence an epitope map, whose approximately 1000 distinct components are used in a multitude of different combinations to discriminate a vast array of different odors.

A family of metabotropic glutamate receptors

Three cDNA clones, mGluR2, mGluR3, and mGluR4, were isolated from a rat brain cDNA library by cross-hybridization with the cDNA for a metabotropic glutamate receptor (mGluR1). The cloned receptors show considerable sequence similarity with mGluR1 and possess a large extracellular domain preceding the seven putative membrane-spanning segments. mGluR2 is expressed in some particular neuronal cells different from those expressing mGluR1 and mediates an efficient inhibition of forskolin-stimulated cAMP formation in cDNA-transfected cells. The mGluRs thus form a novel family of G protein-coupled receptors that differ in their signal transduction and expression patterns.

Diversity revealed by a novel family of cadherins expressed in neurons at a synaptic complex

In mammals, neurons are highly differentiated and play distinctive functions even in the same brain region. We found a novel cadherin-related neuronal receptor (Cnr) gene family by studying Fyn-binding activity in mouse brain. CNR1 protein is located in the synaptic junction and forms a complex with Fyn. Sequence analysis of eight Cnr products of approximately 20 genes indicates that these comprise a novel cadherin family of the cadherin superfamily. The expression patterns of each member of this novel family were grossly similar to each other but restricted to subpopulations of neurons of the same type. The diversity of the Cnr family genes suggests that there are molecular mechanisms that govern highly differentiated neural networks in the mammalian CNS.

Immunohistochemical localization of substance P receptor in the central nervous system of the adult rat

In an attempt to reveal the function sites of substance P (SP) in the central nervous system (CNS), the distribution of SP receptor (SPR) was immunocytochemically investigated in adult rat and compared with that of SP-positive fibers. SPR-like immunoreactivity (LI) was mostly localized to neuronal cell bodies and dendrites. Neurons with intense SPR-LI were distributed densely in the cortical amygdaloid nucleus, hilus of the dentate gyrus, locus ceruleus, rostral half of the ambiguus nucleus, and intermediolateral nucleus of the thoracic cord; moderately in the caudatoputamen, nucleus accumbens, olfactory tubercle, median, pontine, and magnus raphe nuclei, laminae I and III of the caudal subnucleus of the spinal trigeminal nucleus, and lamina I of the spinal cord; and sparsely in the cerebral cortex, basal nucleus of Meynert, claustrum, gigantocellular reticular nucleus, and lobules IX and X of the cerebellar vermis. Neurons with weak to moderate SPR-LI were distributed more widely throughout the CNS. The regional patterns of distribution of SPR-LI were not necessarily the same as those of SP-positive fibers. The entopedunucular nucleus, substantia nigra, and lateral part of the interpeduncular nucleus showed intense SP-LI but displayed almost no SPR-LI. Conversely, the hilus of the dentate gyrus, anterodorsal thalamic nucleus, central nucleus of the inferior colliculus, and dorsal tegmental nucleus showed intense to moderate SPR-LI but contained few axons with SP-LI. These findings confirmed the presence of the "mismatch" problem between SP and SPR localizations. However, the distribution of SPR-LI was quite consistent with that of the SP-binding activity, which has been studied via autoradiography. This indicates that the sites of SPR-LI revealed in the present study represent most, if not all, sites of SP-binding activity.

Regeneration into the spinal cord of transected dorsal root axons is promoted by ensheathing glia transplants

The permissivity of adult olfactory bulb to the ingrowth of olfactory axons could be due to the unique properties of ensheathing glia. To test whether these glial cells could be used to promote axonal regeneration in a spontaneously nonregenerating system, we transplanted suspensions of pure ensheathing cells into a rhizotomized spinal cord segment. Ensheathing cells were purified away from other cell types by immunoaffinity, using anti-p75 nerve growth factor receptor. After laminectomy at the lower thoracic level, the spinal cord was exposed and one dorsal root (T10) was completely transected at the cord entry point. The root stump was microsurgically anastomosed to the cord and a suspension of ensheathing cells was transplanted in the spinal cord at the dorsal root entry zone. Three weeks after transplantation, numerous regenerating dorsal root axons were observed reentering the spinal cord. Ingrowth of dorsal root axons was observed using DiI and antibodies against calcitonin gene-related peptide and growth-associated protein. Primary sensory afferents invaded laminae 1, 2, and 3, grew through laminae 4 and 5, and reached the dorsal grey commissure and lamina 4 of the contralateral side. We did not observe regenerating axons within the ipsilateral ventral horn and dorsal column. Transplanted ensheathing cells reached the same laminae as axons. Neither ensheathing cells nor regenerating axons invaded those laminae they did not innervate under normal circumstances. In conclusion, the regeneration of injured dorsal root axons into the adult spinal cord was possible after ensheathing glia transplantation. The use of ensheathing cells as stimulators of axonal growth might be generalized to other central nervous system injuries.

Immunohistochemical localization of mu-opioid receptors in the central nervous system of the rat

Of the three major types of opioid receptors ( mu, delta, kappa) in the nervous system, mu-opioid receptor shows the highest affinity for morphine that exerts powerful effects on nociceptive, autonomic, and psychological functions. So far, at least two isoforms of mu-opioid receptors have been cloned from rat brain. The present study attempted to examine immunohistochemically the distribution of mu-opioid receptors in the rat central nervous system with two kinds of antibodies to recently cloned mu-opioid receptors (MOR1 and MOR1B). One antibody recognized a specific site for MOR1, and the other bound to a common site for MOR1 and MOR1B. Intense MOR1-like immunoreactivity (LI) was seen in the 'patch' areas and subcallosal streak in the striatum, medial habenular nucleus, medial terminal nucleus of the accessory optic tract, interpeduncular nucleus, median raphe nucleus, parabrachial nuclei, locus coeruleus, ambiguous nucleus, nucleus of the solitary tract, and laminae I and II of the medullary and spinal dorsal horns. Many other regions, including the cerebral cortex, amygdala, thalamus, and hypothalamus, also contained many neuronal elements with MOR1-LI. The distribution pattern of the immunoreactivity revealed with the antibody to the common site for MOR1 and MOR1B (MOR1/1B-LI) was almost the same as that of MOR1-LI. Both MOR1-LI and MOR1/1B-LI were primarily located in neuronal cell bodies and dendrites. However, the immunoreactivities were observed in the accessory optic tract, fasciculus retroflexus, solitary tract, and primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns. The presynaptic location of MOR1-LI and MOR1/1B-LI was confirmed by lesion experiments: Enucleation, placing a lesion in the medial habenular nucleus, removal of the nodose ganglion, or dorsal rhizotomy resulted in a clear reduction of the immunoreactivities, respectively, in the nuclei of the accessory optic tract, some subnuclei of the interpeduncular nucleus, nucleus of the solitary tract, or laminae I and II of the spinal dorsal horn. The results indicate that the mu-opioid receptors are widely distributed in the brain and spinal cord, mainly postsynaptically and occasionally presynaptically. Opioids, including morphine, may inhibit the excitation of neurons via the postsynaptic mu-opioid receptors, and also suppress the release of neurotransmitters and/or neuromodulators from axon terminals through the presynaptic mu-opioid receptors.

CRF2 alpha and CRF2 beta receptor mRNAs are differentially distributed between the rat central nervous system and peripheral tissues

We have recently described the cloning and characterization of a novel corticotropin-releasing factor receptor subtype (CRF2) from rat brain that exists in two alternatively spliced forms, CRF2 alpha and CRF2 beta. These forms differ in their N-terminal coding sequence which results in the production of two distinct receptors of 411 and 431 amino acids, respectively. To assess whether these two forms might represent distinct targets for CRF action, RNase protection and in situ hybridization studies were performed using specific N-terminal cRNA probes. The results showed a differential distribution of the mRNAs for these two receptor forms in the rat. The mRNA for CRF2 alpha is found almost exclusively in the brain, particularly in the hypothalamus, lateral septum, and olfactory bulb, whereas the mRNA for CRF2 beta appears to be both in the brain and in the periphery, with the greatest abundance in the heart and skeletal muscle. Thus, the data suggest that these alternatively spliced forms of the CRF2 receptor may represent functionally distinct CRF receptors. In addition, it highlights the importance of probe specificity for in situ hybridization studies.

Distribution of brain-derived neurotrophic factor in rats and its changes with development in the brain

A newly established, sensitive, two-site enzyme-immunoassay system for brain-derived neurotrophic factor (BDNF) is described. Using this system, we investigated the tissue distribution of BDNF and developmental changes in tissue levels of BDNF in rats. The minimal limit of detection of the assay was 3 pg/0.2 ml of assay mixture. BDNF was successfully solubilized from tissues in the presence of guanidine hydrochloride but not in any of the other buffers examined. In the rat brain at 1 month of age, the highest level of BDNF was detected in the hippocampus (5.41 ng/g of wet weight), followed by the hypothalamus (4.23 ng/g) and the septum (1.68 ng/g). In other regions, levels of BDNF ranged between 0.9 and 1.7 ng/g. The level of BDNF in the posterior lobes of the cerebellum from rats at 30 days of age was slightly higher than that in the anterior lobes. The concentration of BDNF increased in all regions of the brain with postnatal development. In peripheral tissues, BDNF was found at very low concentrations (0.65 ng/g in the spleen, 0.21 ng/g in the thymus, and 0.06 ng/g in the liver). The subfractionation of the hippocampal homogenate indicated that approximately 50% of BDNF was contained in the crude nuclear fraction. Immunoblots of BDNF-immunoreactive proteins extracted from the hippocampus, hypothalamus, and cerebellum contained doublet bands of protein of approximately 14 kDa, a value close to the molecular mass of recombinant human BDNF. Immunocytochemical investigations showed that, in the hippocampus, BDNF was localized in the nucleus of the granule cells in the dentate gyrus and of the cells in the pyramidal cell layer. The frequency of cells that were stained in the dentate gyrus was greater than that of cells in the pyramidal cell layer.

Subclasses of vomeronasal receptor neurons: differential expression of G proteins (Gi alpha 2 and G(o alpha)) and segregated projections to the accessory olfactory bulb

Differential expression of G proteins (Gi alpha 2 and G(o alpha) and the separate central projections of Gi alpha 2- and G(o alpha)-immunoreactive (ir) vomeronasal receptor neurons were investigated in the mouse and rat using immunocytochemical methods. In the vomeronasal organ (VNO), receptor neurons with their cell bodies located in the middle layer (middle 1/3) of the vomeronasal sensory epithelium express Gi alpha 2. Axons of these Gi alpha 2-ir neurons can be followed from VNO to the anterior part, but not the posterior part, of the nerve-glomerular (N-GL) layer of the accessory olfactory bulb (AOB). Another population of receptor neurons, which are located in the deep layer (basal 1/3) of the vomeronasal sensory epithelium, express G(o alpha), and axons of the G(o alpha)-ir neurons can be traced to the posterior part, but not the anterior part, of the N-GL layers of the AOB. The axons of the two subclasses of receptor neurons are intermingled near the VNO and become segregated as they enter the AOB. Removal of the AOB results in retrograde degeneration of both Gi alpha 2-ir and G(o alpha)-ir receptor neurons in the VNO. These results suggest that at least two subclasses of receptor neurons exist in the VNO: the Gi alpha 2-ir neurons in the middle layer and the G(o alpha)-ir neurons in the deep layer of the VNO. The Gi alpha 2-ir neurons in the middle layer of the VNO project to the anterior part of the AOB, while the G(o alpha)-ir neurons in the deep layer of the VNO project to the posterior half of the AOB. These results are similar to our previous observations in the gray short-tailed opossum, suggesting that the existence of at least two subclasses of receptor neurons in the vomeronasal epithelium with differential projections to the AOB is a conserved feature among mammals.

Cellular localization of vasopressin V1a receptor messenger ribonucleic acid in adult male rat brain, pineal, and brain vasculature

Vasopressin V1a receptor (V1aR) transcripts were localized in brain, pineal, and superficial brain vascular tissues of adult male rats using hybridization histochemistry and an [35S]riboprobe complementary to the messenger ribonucleic acid (mRNA) encoding the fifth to the midseventh transmembrane regions of the receptor. V1aR mRNA was extensively distributed throughout brain and was expressed in 1) superficial cells of the granule cell layers of the main olfactory bulb, hippocampal dentate gyrus, and cerebellum; 2) numerous anatomically distinct brain nuclei; 3) isolated cells dispersed throughout the central nervous system; 4) cells of the choroid plexus, occasional blood vessels in the olfactory bulb and interpeduncular nucleus, and extraparenchymal intracranial vasculature; and 5) some white matter structures. Numerous cells expressing V1aR transcripts were found in forebrain structures, including primary olfactory (piriform) cortex, the anterior and posterior olfactory nuclei; dorsal, intermediate, and ventral lateral septal nuclei; the septo-fimbrial nucleus and accumbens nucleus; and numerous hypothalamic regions with the most intense hypothalamic labeling in the arcuate, stigmoid, suprachiasmatic, and periventricular nuclei and the lateral hypothalamic area. Cells expressing V1aR transcripts were ubiquitous throughout the midbrain, pontine, and medullary regions. A lower intensity signal was found in cells of the parvocellular paraventricular and anteroventral nucleus of the thalamus, circumventricular organs including the pineal, and the subfornical organ. V1aR transcripts were not generally detected in parenchymal vasculature, but could be found over large blood vessels in the interpeduncular nucleus and medial olfactory bulb; transcripts were commonly detected in perivascular brain cells. V1aR mRNA was abundantly expressed by choroid plexus, endothelial cells of midline blood vessels between the main olfactory bulbs, and superficial vascular tissue on all brain surfaces. These data confirm the presence of the vascular/hepatic-type V1aR gene in brain tissue and document an extensive expression. The distribution of V1aR mRNA suggests that there are at least two types of vasopressin-responsive cells in brain: one type exemplified by lateral septal ara neurons innervated by classical axodendritic/somatic synaptic vasopressinergic terminals and a second, perivascular/vascular type that would facilitate humoral vasopressinergic signaling in the brain.

Treatment with estrogen and progesterone affects relative levels of brain-derived neurotrophic factor mRNA and protein in different regions of the adult rat brain

Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) were used to examine the effects of acute estrogen and progesterone replacement on relative levels of brain-derived neurotrophic factor (BDNF) mRNA and protein in different regions of the adult rat brain. Adult ovariectomized animals were killed 53 h after receiving estrogen (E53), 53 h after receiving estrogen and 5 h after receiving progesterone (E53P), or 72 h after receiving estrogen and 24 h after receiving progesterone (E72P). Ovariectomized controls were killed 53 and 72 h after receiving vehicle. Tissues from the hippocampus, pyriform cortex, olfactory bulbs, septum, and nucleus basalis/ventral pallidum were dissected. Tissues from the right hemisphere were processed for quantitative RT-PCR analysis of BDNF mRNA, and tissues from the left hemisphere were processed for the detection and quantification of BDNF protein by ELISA. The results demonstrate significant increases in BDNF mRNA in the pyriform cortex of E53- and E53P-treated animals, as well as an increase in BDNF protein in the pyriform cortex of E72P-treated animals, relative to controls. Significant increases in BDNF mRNA were likewise detected in the hippocampus of E53- and E72P-treated animals, but were accompanied by a significant decrease in BDNF protein in the hippocampus of E53P- and E72P-treated animals relative to controls. No significant changes in BDNF mRNA or protein were detected in the olfactory bulbs, frontal cortex, or nucleus basalis/ventral pallidum following hormone treatment; however, an increase in BDNF protein was detected in the septum of E53-treated animals. This may indicate an increase in the retrograde transport of BDNF from the hippocampus to the septum, which could help account for the decrease in BDNF protein detected in the hippocampus following hormone treatment. These findings demonstrate that hormone replacement significantly affects relative levels of BDNF mRNA and protein within specific regions of the brain. These effects may, in turn, contribute to the effects of estrogen replacement on hippocampal connectivity and cognitive processes that have recently been reported.

Expression of neurturin, GDNF, and their receptors in the adult mouse CNS

Neurturin (NTN) and glial cell line-derived neurotrophic factor (GDNF) are the first two members of the GDNF family (GF) of neurotrophic factors. These two proteins are potent survival factors for several populations of central and peripheral neurons in mature and developing rodents. The receptor for these factors is a multicomponent complex that includes the RET (rearranged during transfection) tyrosine kinase receptor and one of two glycosyl phosphatidylinositol (GPI)-linked ligand-binding components called GDNF family receptor alphas (GFRalpha-1 and GFRalpha-2). We have used in situ hybridization to study the mRNA expression of NTN, GDNF, RET, GFRalpha-1, and GFRalpha-2 in the central nervous system (CNS) of adult mice. GF receptors are expressed in several areas in which neuronal populations known to respond to NTN and GDNF are located, including the ventral horn of the spinal cord and the compacta region of the substantia nigra. In addition, we have demonstrated receptor expression in other areas of the brain including the thalamus and hypothalamus. Neurons in these areas express GF receptors, and therefore, may respond to NTN or GDNF. NTN and GDNF are expressed in targets of neurons that express GF receptors. The pattern of GF factor and receptor expression in the adult brain suggests a role for these factors in maintaining neuronal circuits in the mature CNS.

A novel member of the netrin family, beta-netrin, shares homology with the beta chain of laminin: identification, expression, and functional characterization

The netrins are a family of laminin-related molecules. Here, we characterize a new member of the family, beta-netrin. beta-Netrin is homologous to the NH(2) terminus of laminin chain short arms; it contains a laminin-like domain VI and 3.5 laminin EGF repeats and a netrin C domain. Unlike other netrins, this new netrin is more related to the laminin beta chains, thus, its name beta-netrin. An initial analysis of the tissue distribution revealed that kidney, heart, ovary, retina, and the olfactory bulb were tissues of high expression. We have expressed the molecule in a eukaryotic cell expression system and made antibodies to the expressed product. Both in situ hybridization and immunohistochemistry were used to describe the cellular source of beta-netrin and where beta-netrin is deposited. beta-Netrin is a basement membrane component; it is present in the basement membranes of the vasculature, kidney, and ovaries. In addition, beta-netrin is expressed in a limited set of fiber tracts within the brain, including the lateral olfactory tract and the vomeronasal nerve. Functional studies were performed and show that beta-netrin promotes neurite elongation from olfactory bulb explants. Together, these data suggest that beta-netrin is important in neural, kidney, and vascular development.

Sex comparison of neuronal Fos immunoreactivity in the rat vomeronasal projection circuit after chemosensory stimulation

In rodents, reproductively relevant pheromonal cues are detected by receptors in the vomeronasal organ, which in turn transmit this information centrally via the accessory olfactory bulb, the medial nucleus of the amygdala, the posterior medial bed nucleus of the stria terminalis and the medial preoptic area. In the rat, more neurons are present in males than in females at virtually every relay in this vomeronasal projection circuit. Using Fos immunoreactivity as a marker of neuronal activation, we compared the ability of pheromonal cues derived from the urine and feces of estrous or anestrous female rats to activate neurons in this vomeronasal projection circuit in sexually experienced, gonadectomized male and female rats which were chronically treated in adulthood with a high dose of testosterone propionate (5 mg/kg). When compared with rats killed after 2 h of exposure to clean bedding, male and female subjects exposed for 2 h to bedding from estrous females had similar and significant increments in the number of Fos-immunoreactive neurons at each level of the vomeronasal projection circuit, including the granular layer of the accessory olfactory bulb, the posterior dorsal portion of the medial amygdaloid nucleus, the posterior medial portion of the bed nucleus of the stria terminalis and the medial preoptic area. Exposure to bedding from anestrous females stimulated similar and significant increments in Fos immunoreactivity in most of these same brain regions. Chemosensory stimulation failed to augment Fos immunoreactivity in neurons located in the ventrolateral subregion of the ventromedial nucleus of the hypothalamus or in the midbrain central tegmental field, sites at which mating has previously been shown to augment Fos immunoreactivity in both sexes. Finally, chemosensory stimulation augmented Fos immunoreactivity in the nucleus accumbens shell and core, two regions receiving dopaminergic afferents which have been implicated in sexual reward. On two occasions all subjects were given simultaneous access to bowls containing bedding from estrous versus anestrous females. Both males and females spent significantly more time investigating the estrous bedding, although the total time spent investigating either type of bedding was significantly greater in males. The results suggest that the previously established sexual dimorphism in the morphology of the rat's vomeronasal projection circuit is not reflected in the functional responsiveness of neurons in this circuit to chemosensory cues emitted by female conspecifics.

Representation of Natural Stimuli in the Rodent Main Olfactory Bulb

Natural odorants are complex mixtures of diverse chemical compounds. Monomolecular odorants are represented in the main olfactory bulb by distinct spatial patterns of activated glomeruli. However, it remains unclear how individual compounds contribute to population representations of natural stimuli, which appear to be unexpectedly sparse. We combined gas chromatography and intrinsic signal imaging to visualize glomerular responses to natural stimuli and their fractionated components. While whole stimuli activated up to 20 visible glomeruli, each fractionated component activated only one or few glomeruli, and most glomeruli were activated by only one component. Thus, responses to complex mixtures reflected activation by multiple components, with each contributing only a small part of the overall representation. We conclude that the population response to a complex stimulus is largely the sum of the responses to its individual components, and activation of an individual glomerulus independently signals the presence of a specific component.

Brain insulin receptor causes activity-dependent current suppression in the olfactory bulb through multiple phosphorylation of Kv1.3

Insulin and insulin receptor (IR) kinase are found in abundance in discrete brain regions yet insulin signaling in the CNS is not understood. Because it is known that the highest brain insulin-binding affinities, insulin-receptor density, and IR kinase activity are localized to the olfactory bulb, we sought to explore the downstream substrates for IR kinase in this region of the brain to better elucidate the function of insulin signaling in the CNS. First, we demonstrate that IR is postnatally and developmentally expressed in specific lamina of the highly plastic olfactory bulb (OB). ELISA testing confirms that insulin is present in the developing and adult OB. Plasma insulin levels are elevated above that found in the OB, which perhaps suggests a differential insulin pool. Olfactory bulb insulin levels appear not to be static, however, but are elevated as much as 15-fold after a 72-h fasting period. Bath application of insulin to cultured OB neurons acutely induces outward current suppression as studied by the use of traditional whole-cell and single-channel patch-clamp recording techniques. Modulation of OB neurons is restricted to current magnitude; IR kinase activation does not modulate current kinetics of inactivation or deactivation. Transient transfection of human embryonic kidney cells with cloned Kv1.3 ion channel, which carries a large proportion of the outward current in these neurons, revealed that current suppression was the result of multiple tyrosine phosphorylation of Kv1.3 channel. Y to F single-point mutations in the channel or deletion of the kinase domain in IR blocks insulin-induced modulation and phosphorylation of Kv1.3. Neuromodulation of Kv1.3 current in OB neurons is activity dependent and is eliminated after 20 days of odor/sensory deprivation induced by unilateral naris occlusion at postnatal day 1. IR kinase but not Kv1.3 expression is downregulated in the OB ipsilateral to the occlusion, as demonstrated in cryosections of right (control) and left (sensory-deprived) OB immunolabeled with antibodies directed against these proteins, respectively. Collectively, these data support the hypothesis that the hormone insulin acts as a multiply functioning molecule in the brain: IR signaling in the CNS could act as a traditional growth factor during development, be altered during energy metabolism, and simultaneously function to modulate electrical activity via phosphorylation of voltage-gated ion channels.

The distribution of collapsin-1 mRNA in the developing chick nervous system

Collapsin-1 is a secreted glycoprotein that inhibits the extension of specific growth cones in vitro. It has been hypothesized to serve as a repulsive guidance cue for extending growth cones in vivo. Here we report the distribution of collapsin-1 message as demonstrated by in situ hybridization using digoxigenin-labeled RNA probes in wholemounts and tissue sections. In the early chick brain collapsin-1 is expressed in specific regions of the retina, the olfactory bulb, and the diencephalon. In the hindbrain collapsin-1 is first expressed in rhombomere 5 and later in bilaterally symmetric rostrocaudal stripes. Collapsin-1 is expressed in high levels in the ventral horn of the spinal cord and in ventricular stripes that extend rostrally to the hindbrain. In the periphery, collapsin-1 is expressed in the dermamyotome and in ectoderm and epidermis. Based on collapsin's expression patterns we tested axons extending from explants of ventral spinal cord and olfactory bulb for sensitivity to collapsin and show that the former are sensitive to collapsin whereas the latter are not. The distribution of collapsin mRNA is consistent with it playing a role in the organization of sensory axonal projections within the spinal cord and skin.

Differences in salmon GnRH and chicken GnRH-II contents in discrete brain areas of male and female rainbow trout according to age and stage of maturity

We have developed sensitive and specific radioimmunoassays (RIA) for salmon gonadotropin-releasing hormone (sGnRH) and chicken GnRH-II (cGnRH-II). Synthetic sGnRH and cGnRH-II(2-10) were conjugated to bovine serum albumin and injected into rabbits to raise specific antisera. The antiserum against sGnRH showed cross-reactivities of 1.58 and 0.08% for cGnRH-II and lamprey GnRH, respectively. The antiserum against cGnRH-II showed cross-reactivities of 0.05 and 0.01% for sGnRH and lamprey GnRH, respectively. Both antisera were observed not to cross-react with mammalian GnRH and cGnRH-I or other peptide hormones. Synthetic sGnRH and cGnRH-II were iodinated using the chloramine-T method. The iodinated GnRH was purified by HPLC using a reverse-phase C18 column. The RIA system was developed as a double antibody method. Brain extracts of rainbow trout showed displacement curves which were parallel to the sGnRH and cGnRH-II standards in each RIA. HPLC analysis followed by RIA has revealed that rainbow trout brain contains two types of GnRH: sGnRH and cGnRH-II. Total sGnRH content in the brain was about three-fold higher than that of cGnRH-II. In the olfactory bulbs, telencephalon, optic tectum-thalamus, hypothalamus, and pituitary, sGnRH content (per region) was higher than cGnRH-II content, whereas cerebellum and medulla oblongata contained much more cGnRH-II than sGnRH. sGnRH content in the optic tectum-thalamus and pituitary was the highest in 1-year-old immature fish and 3-year-old mature fish, respectively. Medulla oblongata showed the highest cGnRH-II content in all groups. sGnRH concentrations (per milligram of protein) were high in the pituitary and intermediate in the olfactory bulbs, hypothalamus, and telencephalon. In all groups, the cGnRH-II concentration was high in the medulla oblongata, whereas the concentration in the olfactory bulbs and pituitary gland was below the detectable limit in most individuals.

Neurotrophic factors in the primary olfactory pathway

The number of identified growth factors continues to increase rapidly with many being implicated in the development of the nervous system, although for most of them the autocrine and paracrine pathways of cellular regulation still remain to be elucidated. The primary olfactory pathway, consisting of the olfactory epithelium and olfactory bulb, is presented here as a very useful model for the analysis of growth factor function. Review of the available literature suggests that a large proportion of neuroactive growth factors and their receptors are present in the olfactory epithelium or olfactory bulb. Furthermore, the primary olfactory pathway is one of the most plastic in the nervous system with neurogenesis continuing to contribute new sensory neurones in the olfactory epithelium and new interneurones in the olfactory bulb throughout adult life. The rich diversity of growth factors and their receptors in the olfactory system indicates that it will be useful in elucidating how these molecules regulate the formation of the nervous system. The olfactory epithelium in particular is proving useful as a model for the actions of growth factors in directing the neuronal lineage from stem cell to mature neurone.

The type 3 serotonin receptor is expressed in a subpopulation of GABAergic neurons in the rat neocortex and hippocampus

We used in situ hybridization and immunocytochemistry to investigate the presence of GABA in neurons that express the type 3 serotonin receptor (5-HT3R). Quantitative analysis indicated that more than 90% of 5-HT3R expressing cells are GABAergic in the neocortex and hippocampus. The co-existence of 5-HT3R and GABA in cortical and hippocampal neurons indicates that serotonin, via 5-HT3R, can affect GABA release and suggests the participation of 5-HT3R in the inhibitory regulation of forebrain neurons.

beta-amyloid deposition and neurofibrillary tangle formation in the olfactory bulb in ageing and Alzheimer's disease

Impaired olfaction, hyposmia or anosmia are part of the clinical phenotype in neurodegenerative disorders including Alzheimer's disease (AD). It has been proposed that the most severely affected areas are interconnected with the central olfactory system in contrast to the relative sparing of other sensory areas which lack olfactory connections. The pathology of the first synaptic relay in the olfactory pathway, the olfactory bulb (OB), has been studied in AD, but the results have been inconsistent. In order to define more fully the pathology of the OB, we analysed 15 AD and 15 control cases, using amyloid and tau immunohistochemistry on serial sections. This study demonstrates for the first time that all layers of the OB are severely affected in AD and in normal ageing. The principal effector cells of the OB, the mitral cells, developed neurofibrillary tangles (NFTs) both in AD and in controls. All the cases, with the exception of two of the controls, contained NFTs. Amyloid immunoreactivity was detected in diffuse, primitive, classical and compact deposits in AD, while five control cases contained mainly diffuse deposits. We did not find a correlation between amyloid deposition and NFT formation. Among the control cases, two contained neither amyloid nor NFTs, eight had NFTs but no amyloid and only five had both NFTs and amyloid. All the AD cases had NFT and amyloid deposition. Our data suggest that the earlier pathology in the OB is NFT formation and more than ten NFTs/section is compatible with 93.3% diagnostic accuracy for AD.

Olfactory receptor proteins in axonal processes of chemosensory neurons

Olfactory receptors are supposed to act not only as molecular sensors for odorants but also as cell recognition molecules guiding the axons of olfactory neurons to their appropriate glomerulus in the olfactory bulb. This concept implies that olfactory receptor proteins are located in sensory cilia and in the axons. To approach this critical issue, antibodies were generated against two peptides, one derived from olfactory receptor mOR256-17, one derived from the "mOR37" subfamily. By means of immunohistochemistry and double-labeling studies using transgenic mouse lines as well as Western blot analyses, it was demonstrated that the newly generated antibodies specifically recognized the receptor proteins. To scrutinize the hypothesis that olfactory receptor proteins may also be present in the axonal processes and the nerve terminals, serial sections through the olfactory bulb were probed with the antibodies. Two glomeruli in each bulb were stained by anti-mOR256-17, one positioned in the medial, one in the lateral hemisphere. Fiber bundles approaching the glomeruli through the outer nerve layer also displayed intense immunofluorescence. A similar picture emerged for the antibody anti-mOR37, a small number of glomeruli in the ventral domain of the bulb was stained. On serial sections through the olfactory bulb of mOR37-transgenic mouse lines, double-labeling experiments demonstrated that distinct immunoreactive glomeruli corresponded to glomeruli that were targeted by neurons expressing a particular member of the mOR37 receptor subfamily. These data indicate that olfactory receptor (OR) proteins are indeed present in the axonal processes and nerve terminals of olfactory sensory neurons, thus supporting the notion that ORs may participate in the molecular processes underlying the fasciculation and targeting of olfactory axons.

Molecular cloning and expression of a cDNA encoding a receptor for pituitary adenylate cyclase activating polypeptide (PACAP)

We have cloned and sequenced a novel cDNA (RPR7) encoding a receptor for pituitary adenylate cyclase activating polypeptide (PACAP). RPR7 was identified by PCR of rat pituitary cDNA, and full-length clones were isolated from a rat olfactory bulb cDNA library. When expressed in COS cells, RPR7 was functionally coupled to increases in intracellular cyclic adenosine monophosphate (cAMP) in response to stimulation by PACAP-38, PACAP-27, vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI). The order of potency of these ligands was PACAP-38-PACAP-27 > VIP > PHI, suggesting that the receptor corresponds to the pharmacologically characterised PACAP Type I receptor.