Juxtaglomerular Neurons Research Articles

Tyrosine hydroxylase-positive GABAergic juxtaglomerular neurons are the main source of the interglomerular connections in the mouse main olfactory bulb

The interglomerular connections in the mouse olfactory bulb were examined with the retrograde-tracer experiments using Fluorogold. When the injections were restricted to the glomerular layer, we encountered tracer-labeled cells in the glomerular layer and the superficial part of the external plexiform layer, not only near the injection sites but also more than 500 μm distant from the injection sites. Almost of those tracer-labeled neurons distant from the injection sites were large tyrosine hydroxylase-positive juxtaglomerular neurons, some of which were confirmed to have intraglomerular dendrites. Thus, the long interglomerular connections were mainly made by a particular type of dopaminergic-GABAergic juxtaglomerular neurons.

Calcium permeable AMPA receptors and autoreceptors in external tufted cells of rat olfactory bulb

Glomeruli are functional units of the olfactory bulb responsible for early processing of odor information encoded by single olfactory receptor genes. Glomerular neural circuitry includes numerous external tufted (ET) cells whose rhythmic burst firing may mediate synchronization of bulbar activity with the inhalation cycle. Bursting is entrained by glutamatergic input from olfactory nerve terminals, so specific properties of ionotropic glutamate receptors on ET cells are likely to be important determinants of olfactory processing. Particularly intriguing is recent evidence that AMPA receptors of juxta-glomerular neurons may permeate calcium. This could provide a novel pathway for regulating ET cell signaling. We tested the hypothesis that ET cells express functional calcium-permeable AMPA receptors. In rat olfactory bulb slices, excitatory postsynaptic currents (EPSCs) in ET cells were evoked by olfactory nerve shock, and by uncaging glutamate. We found attenuation of AMPA/kainate EPSCs by 1-naphthyl acetyl-spermine (NAS), an open-channel blocker specific for calcium permeable AMPA receptors. Cyclothiazide strongly potentiated EPSCs, indicating a major contribution from AMPA receptors. The current–voltage (I–V) relation of uncaging EPSCs showed weak inward rectification which was lost after >∼10 min of whole-cell dialysis, and was absent in NAS. In kainate-stimulated slices, Co 2+ ions permeated cells of the glomerular layer. Large AMPA EPSCs were accompanied by fluorescence signals in fluo-4 loaded cells, suggesting calcium permeation. Depolarizing pulses evoked slow tail currents with pharmacology consistent with involvement of calcium permeable AMPA autoreceptors. Tail currents were abolished by Cd 2+ and (±)-4-(4-aminophenyl)-2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), and were sensitive to NAS block. Glutamate autoreceptors were confirmed by uncaging intracellular calcium to evoke a large inward current. Our results provide evidence that calcium permeable AMPA receptors reside on ET cells, and are divided into at least two functionally distinct pools: postsynaptic receptors at olfactory nerve synaptic terminals, and autoreceptors sensitive to glutamate released from dendrodendritic synapses.

External tufted cells in the main olfactory bulb form two distinct subpopulations.

The glomeruli of the main olfactory bulb are the first processing station of the olfactory pathway, where complex interactions occur between sensory axons, mitral cells and a variety of juxtaglomerular neurons, including external tufted cells (ETCs). Despite a number of studies characterizing ETCs, little is known about how their morphological and functional properties correspond to each other. Here we determined the active and passive electrical properties of ETCs using in vitro whole-cell recordings, and correlated them with their dendritic arborization patterns. Principal component followed by cluster analysis revealed two distinct subpopulations of ETCs based on their electrophysiological properties. Eight out of 12 measured physiological parameters exhibited significant difference between the two subpopulations, including the membrane time constant, amplitude of spike afterhyperpolarization, variance in the interspike interval distribution and subthreshold resonance. Cluster analysis of the morphological properties of the cells also revealed two subpopulations, the most prominent dissimilarity between the groups being the presence or absence of secondary, basal dendrites. Finally, clustering the cells taking all measured properties into account also indicated the presence of two subpopulations that mapped in an almost perfect one-to-one fashion to both the physiologically and the morphologically derived groups. Our results demonstrate that a number of functional and structural properties of ETCs are highly predictive of one another. However, cells within each subpopulation exhibit pronounced variability, suggesting a large degree of specialization evolved to fulfil specific functional requirements in olfactory information processing.

In vivo imaging of juxtaglomerular neuron turnover in the mouse olfactory bulb

As a consequence of adult neurogenesis, the olfactory bulb (OB) receives a continuous influx of newborn neurons well into adulthood. However, their rates of generation and turnover, the factors controlling their survival, and how newborn neurons intercalate into adult circuits are largely unknown. To visualize the dynamics of adult neurogenesis, we produced a line of transgenic mice expressing GFP in approximately 70% of juxtaglomerular neurons (JGNs), a population that undergoes adult neurogenesis. Using in vivo two-photon microscopy, time-lapse analysis of identified JGN cell bodies revealed a neuronal turnover rate of approximately 3% of this population per month. Although new neurons appeared and older ones disappeared, the overall number of JGNs remained constant. This approach provides a dynamic view of the actual appearance and disappearance of newborn neurons in the vertebrate central nervous system, and provides an experimental substrate for functional analysis of adult neurogenesis.

Olfactory bulb glomeruli: external tufted cells intrinsically burst at theta frequency and are entrained by patterned olfactory input.

Glomeruli, the initial sites of synaptic processing in the olfactory system, contain at least three types of neurons collectively referred to as juxtaglomerular (JG) neurons. The role of JG neurons in odor processing is poorly understood. We investigated the morphology, spontaneous, and sensory-evoked activity of one class of JG neurons, external tufted (ET) cells, using whole-cell patch-clamp and extracellular recordings in rat olfactory bulb slices. ET cells have extensive dendrites that ramify within a single glomerulus or, rarely, in two adjacent glomeruli. All ET neurons exhibit spontaneous rhythmic bursts of action potentials (approximately 1-8 bursts/sec). Bursting is intrinsically generated; bursting persisted and became more regular in the presence of ionotropic glutamate and GABA receptor antagonists. Burst frequency is voltage dependent; frequency increased at membrane potentials depolarized relative to rest and decreased during membrane potential hyperpolarization. Spontaneous bursting persisted in blockers of calcium channels that eliminated low-threshold calcium spikes (LTS) in ET cells. ET cells have a persistent sodium current available at membrane potentials that generate spontaneous bursting. Internal perfusion with a fast sodium channel blocker eliminated spontaneous bursting but did not block the LTS. These results suggest that persistent sodium channels are essential for spontaneous burst generation in ET cells. ET cell bursts were entrained to ON stimuli delivered over the range of theta frequencies. Thus, ET cells appear to be tuned to the frequency of sniffing.

Dopamine Modulates Synaptic Transmission Between Rat Olfactory Bulb Neurons in Culture

The glomerular layer of the olfactory bulb (OB) contains synaptic connections between olfactory sensory neurons and OB neurons as well as connections among OB neurons. A subpopulation of external tufted cells and periglomerular cells (juxtaglomerular neurons) expresses dopamine, and recent reports suggest that dopamine can inhibit olfactory sensory neuron activation of OB neurons. In this study, whole cell electrophysiological and primary culture techniques were employed to characterize the neuromodulatory properties of dopamine on glutamatergic transmission between rat OB mitral/tufted (M/T) cells and interneurons. Immunocytochemical analysis confirmed the expression of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, in a subpopulation of cultured neurons. D2 receptor immunoreactivity was also observed in cultured M/T cells. Dopamine reduced spontaneous excitatory synaptic events recorded in interneurons. Although the D1 receptor agonist SKF38393 and the D2 receptor agonist bromocriptine mesylate mimicked this effect, evoked excitatory postsynaptic potentials (EPSPs) recorded from monosynaptically coupled neuron pairs were attenuated by dopamine and bromocriptine but not by SKF38393. Neither glutamate-evoked currents nor the membrane resistance of the postsynaptic interneuron were affected by dopamine. However, evoked calcium channel currents in the presynaptic M/T cell were diminished during the application of either dopamine or bromocriptine, but not SKF38393. Dopamine suppressed calcium channel currents even after nifedipine blockade of L-type channels, suggesting that inhibition of the dihydropyridine-resistant high-voltage activated calcium channels implicated in transmitter release may mediate dopamine's effects on spontaneous and evoked synaptic transmission. Together, these data suggest that dopamine inhibits excitatory neurotransmission between M/T cells and interneurons via a presynaptic mechanism.

Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals.

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.

Electrophysiology of interneurons in the glomerular layer of the rat olfactory bulb.

In the mammalian olfactory bulb, glomeruli are surrounded by a heterogeneous population of interneurons called juxtaglomerular neurons. As they receive direct input from olfactory receptor neurons and connect with mitral cells, they are involved in the initial stages of olfactory information processing, but little is known about their detailed physiological properties. Using whole cell patch-clamp techniques, we recorded from juxtaglomerular neurons in rat olfactory bulb slices. Based on their response to depolarizing pulses, juxtaglomerular neurons could be divided into two physiological classes: bursting and standard firing. When depolarized, the standard firing neurons exhibited a range of responses: accommodating, nonaccommodating, irregular firing, and delayed to firing patterns of action potentials. Although the firing pattern was not rigorously predictive of a particular neuronal morphology, most short axon cells fired accommodating trains of action potentials, while most delayed to firing cells were external tufted cells. In contrast to the standard firing neurons, bursting neurons produced a calcium-channel-dependent low-threshold spike when depolarized either by current injection or by spontaneous or evoked postsynaptic potentials. Bursting neurons also could oscillate spontaneously. Most bursting cells were either periglomerular cells or external tufted cells. Based on their mode of firing and placement in the bulb circuit, these bursting cells are well situated to drive synchronous oscillations in the olfactory bulb.

Olfactory receptor neurons express D2 dopamine receptors.

The role of the dopamine (DA) in the olfactory bulb (OB) was explored by determining which of the potential target cells express dopamine receptors (DARs). Previously, it was reported that D2-like DAR (D2, D3, and D4 subtypes) radioligand binding is restricted to the outer layers of the OB. The neuronal elements present only in these layers are the axons of the olfactory receptor neurons (ORNs) and the juxtaglomerular (JG) neurons of the glomerular layer. Based on this pattern of D2-like ligand binding, it was suggested that D2-like receptors might be located presynaptically on ORN terminals. The present study was undertaken to investigate this hypothesis. In the outer bulb layers of rats in which the ORNs were destroyed by nasal lavage with ZnSO(4), D2-like radioligand binding was reduced severely. The receptor subtype D2 mRNA, but not D3 mRNA, was detected in adult rat olfactory epithelial tissue. By using in situ hybridization, this D2 mRNA was located preferentially in epithelial layers that contain ORN perikarya. D2 mRNA was eliminated after bulbectomy, a manipulation known to cause retrograde degeneration of the mature ORNs. Taken together, the surgical manipulations indicate that mature ORNs express D2 DARs and are consistent with the hypothesis that functional receptors are translocated to their axons and terminals in the bulb. This suggests that dopamine released from JG interneurons could be capable of presynaptically influencing neurotransmission from the olfactory nerve terminals to OB target cells through the D2 receptor.

Odor-Induced, Activity-Dependent Transneuronal Gene InductionIn Vitro: Mediation by NMDA Receptors

Expression of tyrosine hydroxylase (TH) by juxtaglomerular (JG) neurons of the olfactory bulb (OB) requires innervation of the bulb by olfactory receptor neurons (ORNs). ORN lesion selectively downregulates TH in JG neurons. In reversible odor deprivation, TH expression is downregulated as the naris is closed and then upregulated upon naris reopening. The mechanism or mechanisms regulating this dependence are unknown. TH expression could be regulated by trophic factor release and/or synaptic activity from ORN terminals. We investigated TH expression in cocultures of dissociated postnatal rat OB cells and embryonic olfactory neuroepithelium (OE) slice explants. TH-positive neurons in control dissociated OB cell cultures alone comprise only a small fraction of the total population of cells present in the culture. However, when OE slice explants are cocultured with dispersed OB cells, there is a mean 2.4-fold increase in the number of TH-positive neurons. ORNs in vivo use glutamate as a neurotransmitter. Broad spectrum excitatory amino acid antagonists (kyurenic acid) or selective antagonists of the NMDA receptor (APV) both prevent induction of TH expression in OE-OB cocultures. Furthermore, pulse application of NMDA stimulates TH expression in OB neurons in the absence of OE. In vitro, OB TH neurons express NMDA receptors, suggesting that NMDA stimulation is acting directly on TH neurons. Exposure of OE explants to natural odorants results in upregulation of TH, presumably through increased ORN activity, which could be blocked by APV. These findings indicate that odorant-stimulated glutamate release by ORN terminals regulates TH expression via NMDA receptors on JG dopaminergic neurons.

Differential distribution of inositol 1,4,5-triphosphate receptors in the rat olfactory bulb

The inositol 1,4,5-triphosphate receptor (IP3R) regulates the release of calcium from intracellular stores. In the present study, the distribution of IP3R in the rat main olfactory bulb was determined by immunohistochemistry. Immunofluorescence was used to double label for IP3R and for gamma-aminobutyric acid (GABA) or for projection neurons, which were retrogradely labeled following dextran injection into the lateral olfactory tract (LOT). The expression profile of IP3R changes dramatically during development. In the glomerular layer of adults, many juxtaglomerular neurons are IP3R immunoreactive [IP3R(+)]; the majority of these cells are also GABA immunoreactive [GABA(+)]. Scattered sparsely throughout the external plexiform layer are small numbers of IP3R(+) neurons, a small number of which are LOT-projecting tufted cells. Significant numbers of IP3R(+) neurons are in the granule cell layer; however, most of these cells are GABA(-). The vast majority of mitral cells contain little or no IP3R immunoreactivity. These findings indicate that, in the olfactory bulb of adult rats, IP3R is preferentially localized in specific classes of intrinsic neurons and that it is rarely expressed in projection neurons. In contrast, during the first postnatal week, the receptor is detected almost exclusively in mitral cells. Expression of IP3R in subclasses of intrinsic neurons begins during the second and third weeks, concomitant with a decrease in immunostaining of mitral cells. Adult patterns of IP3R immunostaining are apparent by the fourth postnatal week. These observations raise the possibility that the expression of IP3R in specific classes of neurons during development is activity dependent.

Electron Microscopy of Colocalization of GABA and Tyrosine Hydroxylase Expression in Rat Olfactory Bulb Transplants

Juxtaglomerular (JG) neurons of rat olfactory bulb (OB) are a subset of inhibitory interneurons within the OB, acting via lateral inhibition to modulate the afferent input of the primary olfactory nerve. The JG neurons, composed of periglomerular, external tufted, and short axon cells, have been found to express various neurotransmitters, including γ-amino butyric acid (GABA) and dopamine. A specific set of neurons within the periglomerular population have also been shown to coexpress these neurotransmitters. Deafferentation or functional odor deprivation of the normal OB causes a loss of tyrosine hydroxylase (TH) (the rate limiting enzyme in the dopamine synthesis pathway) expression within the JG cell population, but appears to have no effect on GABA levels. Our laboratory has developed a transplantation model to further study the effects of deafferentation and subsequent reinnervation within this system. Sections from transplant (TX) OBs were reacted for GABA and TH using immunocytochemical localization protocols and studied by electron microscopy. Numerous neuronal populations were found to be either TH or GABA positive in this study, with a specific subpopulation showing colocalization of both. Although the architecture of the TX OB is somewhat disrupted and the TH- and GABA-positive cells were not as uniform in their arrangement as they are in the normal OB, we found that these cells in the TX OB were morphologically similar to the JG cells of normal OB. Positively labeled profiles were also found to receive and form numerous synaptic contacts with both host olfactory nerve axons as well as with the processes of donor neurons. These synaptic contacts were within areas that resemble the glomeruli of normal OB, suggesting that lateral inhibition may occur within the TX OB as it does in the normal. The coexpression of GABA and TH within specific neurons also indicates that a unique population of JG neurons that occur in normal OB are also found within this transplanted system as well.

Tyrosine hydroxylase expression in rat olfactory bulb transplants: an electron microscope study.

Juxtaglomerular (JG) neurons of rat olfactory bulb (OB) have been shown to express tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine synthesis pathway. These JG neurons act as inhibitory dopaminergic interneurons, modulating the incoming signal from the primary olfactory afferents. The JG neurons, comprised of periglomerular, external tufted, and short axon cells, stop expressing TH after lesions of the olfactory nerve or closure of the nares, both of which cause a loss of functional input. Upon reinnervation by a continuously regenerating olfactory nerve, these cells resume their expression of TH. In order to study deafferentation and subsequent reinnervation within this system, our laboratory utilizes a transplantation model. Sections from transplant (TX) OBs are reacted for TH using immunocytochemical localization protocols and studied by light- and especially electron microscopy (EM). Autoradiography of tritiated thymidine-labeled tissue was performed to confirm donor origin of the TX OBs. Although the architecture of the TX OB is somewhat disrupted and the TH-positive cells were not as uniform in their arrangement as they are in the normal OB, we found that the TH cells in the TH OB had a morphology similar to the JG cells observed in normal OB. These TH cells were also found to receive synaptic contacts with host olfactory nerve axons as well as make and receive contacts with the processes of donor neurons. These, synaptic contacts were formed within areas that resemble the glomeruli of normal olfactory bulb, suggesting that the inhibitory synaptic pathway is reestablished within the TX OB. These findings also suggested that host olfactory axons formed a functional contact with the TH cells, possibly inducing them to express this enzyme. This study implies that the TX OB retains a level of plasticity that enables it to recapitulate part of the interneuronal arrangement observed in the normal system.

Directed expression of the growth-associated protein B-50/GAP-43 to olfactory neurons in transgenic mice results in changes in axon morphology and extraglomerular fiber growth

B-50/GAP-43, a neural growth-associated phosphoprotein, is thought to play a role in neuronal plasticity and nerve fiber formation since it is expressed at high levels in developing and regenerating neurons and in growth cones. Using a construct containing the coding sequence of B-50/GAP-43 under the control of regulatory elements of the olfactory marker protein (OMP) gene, transgenic mice were generated to study the effect of directed expression of B-50/GAP-43 in a class of neurons that does not normally express B-50/GAP-43, namely, mature OMP-positive olfactory neurons. Olfactory neurons have a limited lifespan and are replaced throughout adulthood by new neurons that migrate into the upper compartment of the epithelium following their formation from stem cells in the basal portion of this neuroepithelium. Thus, the primary olfactory pathway is exquisitely suited to examine a role of B-50/GAP-43 in neuronal migration, lifespan, and nerve fiber growth. We find that B-50/GAP-43 expression in adult olfactory neurons results in numerous primary olfactory axons with enlarged endings preferentially located at the rim of individual glomeruli. Furthermore, ectopic olfactory nerve fibers in between the juxtaglomerular neurons or in close approximation to blood vessels were frequently observed. This suggests that expression of B-50/GAP-43 in mature olfactory neurons alters their response to signals in the bulb. Other parameters examined, that is, migration and lifespan of olfactory neurons are normal in B-50/GAP-43 transgenic mice. These observations provide direct in vivo evidence for a role of B-50/GAP-43 in nerve fiber formation and in the determination of the morphology of axons.

Expression of tyrosine hydroxylase in the aging, rodent olfactory system

Tyrosine hydroxylase (TH) mRNA, immunoreactivity, and activity were examined as a reflection of dopamine expression in juxtaglomerular neurons intrinsic to the olfactory bulbs of young (6-month-old), middle aged (18-month-old), and aged (25- to 29-month-old) rats and mice. TH expression was maintained at levels observed in young animals in the olfactory bulbs of aged animals from two mouse strains, C57Bl/6JNia and C57Bl/6NNia, and one rat strain, an F1 hybrid between F344 and Brown Norway strains. The parental F344 rat strain exhibited reductions in TH expression of about 20% in 26- to 29-month-old animals as compared to 6- and 18 month-old rats. However, there was significant inter-animal variability. Some aged F344 rats had TH levels that were similar and others had activity levels that were 50% of those in young and middle aged animals. Neither the general condition of the animals nor the presence of adrenal tumors predicted the individuals with reduced TH expression. Olfactory bulb size, estimated from protein content, did not differ between rats and mice of different ages. In addition, expression of olfactory marker protein, a protein found primarily in mature olfactory receptor neurons, also was unchanged indicating the maintenance of afferent innervation. These data suggest that, in contrast to other brain dopamine systems, the expression of the dopamine phenotype is maintained in the aging olfactory bulb.

Expression of the dopaminergic phenotype in the olfactory bulb: neither calcitonin gene-related peptide nor olfactory input is necessary.

In the olfactory bulb, expression of tyrosine hydroxylase (TH) in juxtaglomerular neurons is dependent on innervation by the olfactory nerve. The presence of the neuropeptide calcitonin gene-related peptide (CGRP) within the olfactory nerve has led to the hypothesis that CGRP is responsible for regulation of TH expression in the bulbar neurons. On the other hand, other investigators claim that olfactory receptors never produce CGRP and that functional contact with olfactory axons regulates production of TH by bulbar neurons. Two different experimental procedures were used to test whether either CGRP or contact with the olfactory nerve is essential for production of TH by bulbar neurons in vivo. The peptidergic innervation of the olfactory bulb was eliminated either by neonatal capsaicin treatment, or by stereotaxic, electrolytic lesions of the ophthalmic division of the trigeminal nerve. Both of the treatments leave the olfactory innervation of the bulb intact while eliminating the CGRP-immunoreactive fibers in the olfactory nerve and glomeruli. Subsequent immunocytochemistry reveals a normal complement of bulbar TH-immunoreactive juxtaglomerular neurons in the absence of peptidergic innervation. In order to test whether olfactory nerve input is necessary for expression of TH in vivo, the anlage of the olfactory bulb was removed from embryonic (E16) rat pups and transplanted into the anterior chamber. These ectopic olfactory bulbs, although devoid of olfactory nerve input, contain numerous TH-immunoreactive neurons. Thus olfactory nerve input is not necessary for expression of TH in bulbar neurons.

Development of olfactory marker protein and tyrosine hydroxylase immunoreactivity in the transplanted rat olfactory bulb

Evidence suggests that tyrosine hydroxylase (TH) expression by juxtaglomerular (JG) neurons of the olfactory bulb (OB) is dependent upon input from primary olfactory neurons (ONs), which are identifiable using immunocytochemical localization (ICC-L) methods for olfactory marker protein (OMP). When the input from the continuously regenerating ONs is temporarily removed (either surgically or chemically), JG cells cease TH production until ON contact is reestablished. We are studying this transneuronal regulation using the rat OB in a transplantation (TX) model. Fetal OBs, labeled in utero with tritiated thymidine, were transplanted (TX) into a site vacated by removal of a neonatal host OB. Host animals were sacrificed at varying periods after TX. Alternate sets of frozen sections were then processed for autoradiography or using ICC-L for TH and OMP. As early as 1 week post-TX, OMP-positive fibers and glomerulus-like structures were seen throughout the TX OB. Despite this extensive and rapid OMP reinnervation, TH expression returned very slowly and the number of TH expressing cells never approached control levels. The reduced TH activity in TXs may be due to failure of JG cells to survive or to develop the correct phenotype under TX conditions. Alternatively, input from ON fibers may only be necessary, but not sufficient, for the expression of TH.

Differential effect of functional olfactory bulb deafferentation on tyrosine hydroxylase and glutamic acid decarboxylase messenger RNA levels in rodent juxtaglomerular neurons

Expression of the dopaminergic phenotype in olfactory bulb (OB) juxtaglomerular neurons (constituting a population of periglomerular and external tufted cells) is dependent upon functional innervation by peripheral olfactory receptors. Loss of functional input in rodents, by either peripheral deafferentation or deprivation of odorant access, results in a profound decrease in the expression of juxtaglomerular tyrosine hydroxylase (TH). We have examined the effects of such treatments on the expression of the neurotransmitter biosynthetic enzyme glutamic acid decarboxylase (GAD), which is colocalized with TH in the majority of TH-containing juxtaglomerular neurons. Following either chemically induced OB deafferentation in adult mice or unilateral odor deprivation in neonatal rats, steady-state OB GAD messenger RNA levels remained essentially unchanged as assessed by Northern blot analysis 20-40 days after treatment. These results were confirmed by in situ hybridization analysis, which demonstrated a profound loss of juxtaglomerular TH messenger RNA but no accompanying decrease in regionally colocalized GAD message. Since GAD is found in nearly all dopaminergic OB cells, the preservation of juxtaglomerular GAD message implies that olfactory receptor neurons exert a differential transneuronal regulation of TH and GAD gene transcription.

Localization of tyrosine hydroxylase and olfactory marker protein immunoreactivities in the human and macaque olfactory bulb

These studies utilized specific antisera to examine the distribution and characteristics of tyrosine hydroxylase and olfactory marker protein in the olfactory bulb of the human and macaque monkey. The macaque displayed immunoreactive profiles to both antisera comparable to those described previously for other mammals. Olfactory marker protein antiserum labeled the olfactory nerve layer and glomeruli. Within the glomeruli, labeled processes were interdigitated with unlabeled processes believed to be the postsynaptic dendrites of olfactory bulb neurons. Tyrosine hydroxylase antisera labeled somata surrounding the glomeruli as well as putative dendritic processes with the glomerular neuropil. It appeared that only a subset of juxtaglomerular neurons were immunoreactive. A similar pattern was observed in the human for both antibodies. Fascicles of olfactory marker protein immunoreactive olfactory nerves often coursed long distances into the olfactory bulb prior to arborizing within a glomerulus. The data suggest that olfactory receptor cell axons destined for specific glomeruli fasciculate into bundles prior to reaching the target glomeruli. The immunoreactivity in the human to tyrosine hydroxylase was qualitatively similar to the macaque and other mammals although the number of labeled somata and intraglomerular processes appeared lower. As in the macaque, it appeared that only a subset of juxtaglomerular neurons were labeled.

Carnosine-, calcitonin gene-related peptide- and tyrosine hydroxylase-immunoreactivity in the mouse olfactory bulb following peripheral denervation

We report the effects of olfactory peripheral deafferentation by intranasal irrigation with ZnSO 4 on carnosine and CGRP immunoreactivities in the mouse olfactory system. In the normal rodent olfactory epithelium carnosine immunoreactivity is associated with the olfactory receptor neurons. Conversely, CGRP immunoreactivity appears to be associated with the trigeminal innervation of the nasal cavity. Following lesion the magnitude of carnosine immunoreactivity in the olfactory epithelium is strongly reduced while CGRP immunoreactivity is unaffected. In the olfactory bulb, deafferentation causes a strong reduction of carnosine immunoreactivity in the glomerular layer and, concurrently, of TH immunoreactivity in the juxtaglomerular neurons. CGRP immunoreactive fibers in the olfactory bulb are abundant in the glomerular layer both before and after deafferentation. These data demonstrate that, in the adult mouse, the immunocytochemically detectable levels of CGRP are not altered following lesion and indicate CGRP is not released directly from the olfactory neurons to induce TH production.