Expression of neuronal nitric oxide synthase/NADPH-diaphorase during olfactory deafferentation and regeneration

Postsynaptic activity of inhibitory neurons evokes hemodynamic fMRI responses

Functional MRI responses are localized to the synaptic sites of evoked inhibitory neurons, but it is unknown whether, or by what mechanisms, these neurons initiate functional hyperemia. Here, the neuronal origins of these hemodynamic responses were investigated by fMRI or local field potential and blood flow measurements during topical application of pharmacological agents when GABAergic granule cells in the rat olfactory bulb were synaptically targeted. First, to examine if postsynaptic activation of these inhibitory neurons was required for neurovascular coupling, we applied an NMDA receptor antagonist during cerebral blood volume-weighted fMRI acquisition and found that responses below the drug application site (up to ~1.5 mm) significantly decreased within ~30 min. Similarly, large decreases in granule cell postsynaptic activities and blood flow responses were observed when AMPA or NMDA receptor antagonists were applied. Second, inhibition of nitric oxide synthase preferentially decreased the initial, fast component of the blood flow response, while inhibitors of astrocyte-specific glutamate transporters and vasoactive intestinal peptide receptors did not decrease blood flow responses. Third, inhibition of GABA release with a presynaptic GABAB receptor agonist caused less reduction of neuronal and blood flow responses compared to the postsynaptic glutamate receptor antagonists. In conclusion, local hyperemia by synaptically-evoked inhibitory neurons was primarily driven by their postsynaptic activities, possibly through NMDA receptor-dependent calcium signaling that was not wholly dependent on nitric oxide.

Pax6 is essential for the maintenance and multi-lineage differentiation of neural stem cells, and for neuronal incorporation into the adult olfactory bulb

The paired type homeobox 6 (Pax6) transcription factor (TF) regulates multiple aspects of neural stem cell (NSC) and neuron development in the embryonic central nervous system. However, less is known about the role of Pax6 in the maintenance and differentiation of adult NSCs and in adult neurogenesis. Using the +/Sey(Dey) mouse, we have analyzed how Pax6 heterozygosis influences the self-renewal and proliferation of adult olfactory bulb stem cells (aOBSCs). In addition, we assessed its influence on neural differentiation, neuronal incorporation, and cell death in the adult OB, both in vivo and in vitro. Our results indicate that the Pax6 mutation alters Nestin(+)-cell proliferation in vivo, as well as self-renewal, proliferation, and survival of aOBSCs in vitro although a subpopulation of +/Sey(Dey) progenitors is able to expand partially similar to wild-type progenitors. This mutation also impairs aOBSC differentiation into neurons and oligodendrocytes, whereas it increases cell death while preserving astrocyte survival and differentiation. Furthermore, Pax6 heterozygosis causes a reduction in the variety of neurochemical interneuron subtypes generated from aOBSCs in vitro and in the incorporation of newly generated neurons into the OB in vivo. Our findings support an important role of Pax6 in the maintenance of aOBSCs by regulating cell death, self-renewal, and cell fate, as well as in neuronal incorporation into the adult OB. They also suggest that deregulation of the cell cycle machinery and TF expression in aOBSCs which are deficient in Pax6 may be at the origin of the phenotypes observed in this adult NSC population.

Transplanted human adipose tissue-derived stem cells engraft and induce regeneration in mice olfactory neuroepithelium in response to dichlobenil subministration

We used immunodeficient mice, whose dorsomedial olfactory region was permanently damaged by dichlobenil inoculation, to test the neuroregenerative properties of transplanted human adipose tissue-derived stem cells after 30 and 60 days. Analysis of polymerase chain reaction bands revealed that stem cells preferentially engrafted in the lesioned olfactory epithelium compared with undamaged mucosa of untreated transplanted mice. Although basal cell proliferation in untransplanted lesioned mice did not give rise to neuronal cells in the olfactory mucosa, we observed clusters of differentiating olfactory cells in transplanted mice. After 30 days, and even more at 60 days, epithelial thickness was partially recovered to normal values, as also the immunohistochemical properties. Functional reactivity to odorant stimulation was also confirmed through electro-olfactogram recording in the dorsomedial epithelium. Furthermore, we demonstrated that engrafted stem cells fused with mouse cells in the olfactory organ, even if heterokaryons detected were too rare to hypothesize they directly repopulated the lesioned epithelium. The data reported prove that the migrating transplanted stem cells were able to induce a neuroregenerative process in a specific lesioned sensory area, enforcing the perspective that they could become an available tool for stem cell therapy.

Possible effects of nitric oxide synthases on odor-induced behavioral changes in mice

Although exposure to olfactory stimulants can induce neurobehavioral changes, there is a lack of data regarding specific effects on neurotransduction, especially nitric oxide (NO)-mediated neurotransduction. We investigated the relationship between altered behavioral performance and changes in the expression of nitric oxide synthase (NOS), nNOS, iNOS, and eNOS, in 2-methylbutyric (2MB) acid-exposed mice. Mice were exposed to 2MB acid by inhalation and olfactory preference/avoidance and motor coordination were measured. Additionally, we examined NOS expression in the olfactory bulb of the mouse brain. Mice exposed to 2MB acid showed significant changes in olfactory preference and rotarod activity as compared with controls. Although there was no change in nNOS and iNOS expressions in the olfactory bulb of 2MB acid-exposed mice, eNOS expression increased significantly in the olfactory bulb of 9.0M 2MB acid-exposed mice. These data indicate that altered eNOS expression in the olfactory bulb may contribute to 2MB acid-induced behavioral changes.

Intranasal exposure to manganese disrupts neurotransmitter release from glutamatergic synapses in the central nervous system in vivo

Chronic exposure to aerosolized manganese induces a neurological disorder that includes extrapyramidal motor symptoms and cognitive impairment. Inhaled manganese can bypass the blood-brain barrier and reach the central nervous system by transport down the olfactory nerve to the brain's olfactory bulb. However, the mechanism by which Mn disrupts neural function remains unclear. Here we used optical imaging techniques to visualize exocytosis in olfactory nerve terminals in vivo in the mouse olfactory bulb. Acute Mn exposure via intranasal instillation of 2-200 μg MnCl(2) solution caused a dose-dependent reduction in odorant-evoked neurotransmitter release, with significant effects at as little as 2 μg MnCl(2) and a 90% reduction compared to vehicle controls with a 200 μg exposure. This reduction was also observed in response to direct electrical stimulation of the olfactory nerve layer in the olfactory bulb, demonstrating that Mn's action is occurring centrally, not peripherally. This is the first direct evidence that Mn intoxication can disrupt neurotransmitter release, and is consistent with previous work suggesting that chronic Mn exposure limits amphetamine-induced dopamine increases in the basal ganglia despite normal levels of dopamine synthesis (Guilarte et al., J Neurochem 2008). The commonality of Mn's action between glutamatergic neurons in the olfactory bulb and dopaminergic neurons in the basal ganglia suggests that a disruption of neurotransmitter release may be a general consequence wherever Mn accumulates in the brain and could underlie its pleiotropic effects.

Antenatal insults modify newborn olfactory function by nitric oxide produced from neuronal nitric oxide synthase

Newborn feeding, maternal, bonding, growth and wellbeing depend upon intact odor recognition in the early postnatal period. Antenatal stress may affect postnatal odor recognition. We investigated the exact role of a neurotransmitter, nitric oxide (NO), in newborn olfactory function. We hypothesized that olfactory neuron activity depended on NO generated by neuronal NO synthase (NOS). Utilizing in vivo functional manganese enhanced MRI (MEMRI) in a rabbit model of cerebral palsy we had shown previously that in utero hypoxia-ischemia (H-I) at E22 (70% gestation) resulted in impaired postnatal response to odorants and poor feeding. With the same antenatal insult, we manipulated NO levels in the olfactory neuron in postnatal day 1 (P1) kits by administration of intranasal NO donors or a highly selective nNOS inhibitor. Olfactory function was quantitatively measured by the response to amyl acetate stimulation by MEMRI. The relevance of nNOS to normal olfactory development was confirmed by the increase of nNOS gene expression from fetal ages to P1 in olfactory epithelium and bulbs. In control kits, nNOS inhibition decreased NO production in the olfactory system and increased MEMRI slope enhancement. In H-I kits the MEMRI slope did not increase, implicating modification of endogenous NO-mediated olfactory function by the antenatal insult. NO donors as a source of exogenous NO did not significantly change function in either group. In conclusion, olfactory epithelium nNOS in newborn rabbits probably modulates olfactory signal transduction. Antenatal H-I injury remote from delivery may affect early functional development of the olfactory system by decreasing NO-dependent signal transduction.

Types of cholecystokinin-containing periglomerular cells in the mouse olfactory bulb

The periglomerular cells (PG) of the olfactory bulb (OB) are involved in the primary processing and the refinement of sensory information from the olfactory epithelium. The neurochemical composition of these neurons has been studied in depth in many species, and over the last decades such studies have focused mainly on the rat. The increasing use of genetic models for research into olfactory function demands a profound characterization of the mouse olfactory bulb, including the chemical composition of bulbar interneurons. Regarding both their connectivity with the olfactory nerve and their neurochemical fate, recently, two different types of PG have been identified in the mouse. In the present report, we analyze both the synaptology and the chemical composition of specific PG populations in the murine olfactory bulb, in particular, those containing the neuropeptide cholecystokinin. Our results demonstrate the existence in the mouse of non-GABAergic PG and that these establish synaptic contacts with the olfactory nerve within the glomeruli. Based on previous classifications, we propose that this population would constitute a new subtype of type 1 mouse PG. In addition, we demonstrate the partial coexistence of cholecystokinin with the calcium-binding proteins neurocalcin and parvalbumin. All these findings add further data to our knowledge of the synaptology and neurochemistry of mouse PG. The differences observed from other rodents reflect the neurochemical heterogeneity of PG in the mammalian OB.

Adult brain nitrergic activity after concomitant prenatal exposure to ethanol and methyl mercury

Pregnant rats were exposed to ethanol (EtOH) and/or methyl mercury (MeHg) during fetal brain development. Nitrergic activity was quantified by densitometric measurement of formazan deposits in the hippocampus, cerebellum and striatum of two-month-old offspring following histochemical assay for NADPH-diaphorase (NADPH-d) activity. Compared to control subjects, an increase in nitrergic activity was found in the molecular layer of dentate gyrus and in the lacunosum molecular and stratum radiatum of CA1 (cornus amoni 1) in the EtOH+MeHg group, whereas a single administration of EtOH increased the activity in all striatal segments. The cerebellum seems to be less sensitive at this time-point to intoxication, and presented an increase only at the molecular layer of EtOH-exposed animals when compared to the MeHg and EtOH+MeHg groups (ANOVA, one-way followed by Tukey's test, p<0.05 or p<0.01). Taken together, results suggest that developmental exposure to EtOH and MeHg, singularly or in combination, alters nitrergic activity in adult rat in different ways depending on the region and layer of the central nervous system (CNS), and that these alterations might be related to different local metabolic properties.

Chemical characterization of Pax6-immunoreactive periglomerular neurons in the mouse olfactory bulb

The Pax6 transcription factor is a key element along brain development in both the visual and olfactory systems. The involvement of Pax6 in neural fate is well documented in the visual system, whereas in the olfactory system, and in particular in the olfactory bulb (OB), its expression during adulthood has only begun to be elucidated. In the OB, the modulation of primary sensory information is first performed by periglomerular cells (PG). A considerable body of information has unveiled the neurochemical heterogeneity of these neurons. Thus it is well known that Pax6 coexists with dopaminergic/GABAergic mouse PG. However, the presence of this transcription factor in other mouse PG subpopulations has not been studied. Here, we analyzed whether Pax6 is expressed in PG containing the calcium-binding proteins neurocalcin and parvalbumin, and the neuropeptide cholecystokinin. Our results show that Pax6 is not expressed by these PG subpopulations, suggesting that it is mainly restricted to GABAergic PG populations. These findings provide new data in the chemical characterization of mouse Pax6-positive PG.

Hyperpolarization-activated and cyclic nucleotide-gated channels are differentially expressed in juxtaglomerular cells in the olfactory bulb of mice

In the olfactory bulb, input from olfactory receptor neurons is processed by neuronal networks before it is relayed to higher brain regions. In many neurons, hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels generate and control oscillations of the membrane potential. Oscillations also appear crucial for information processing in the olfactory bulb. Four channel isoforms exist (HCN1-HCN4) that can form homo- or heteromers. Here, we describe the expression pattern of HCN isoforms in the olfactory bulb of mice by using a novel and comprehensive set of antibodies against all four isoforms. HCN isoforms are abundantly expressed in the olfactory bulb. HCN channels can be detected in most cell populations identified by commonly used marker antibodies. The combination of staining with marker and HCN antibodies has revealed at least 17 different staining patterns in juxtaglomerular cells. Furthermore, HCN isoforms give rise to an unexpected wealth of co-expression patterns but are rarely expressed in the same combination and at the same level in two given cell populations. Therefore, heteromeric HCN channels may exist in several cell populations in vivo. Our results suggest that HCN channels play an important role in olfactory information processing. The staining patterns are consistent with the possibility that both homomeric and heteromeric HCN channels are involved in oscillations of the membrane potential of juxtaglomerular cells.

Olfactory bulb interneurons releasing NO exhibit the Reelin receptor ApoEr2 and part of those targeted by NO express Reelin

Nitric oxide (NO) and Reelin both modulate neuronal plasticity in developing and mature synaptic networks. We recently showed a loss of neuronal nitric oxide synthase (nNOS) protein in the olfactory bulb of reeler mutants and advanced the hypothesis that the Reelin and NO signalling pathways may influence each other. We now studied the distribution of NO sensitive guanylyl cyclase (NOsGC), Reelin and its receptor Apolipoprotein E2 (ApoEr2) in the olfactory bulb by multiple fluorescence labelling and tested whether nNOS and ApoEr2 colocalize in this area. We also essayed the protein content of NOsGC in the reeler olfactory bulb and tested whether there are any changes in nNOS and NOsGC protein in other reeler brain areas. Olfactory bulb interneurons expressing ApoEr2 and nNOS are only few in the glomerular layer but represent the large majority of granule cell layer interneurons. Conversely, NOsGC interneurons are rare in the granule cell layer and abundant as periglomerular cells. Reelin containing periglomerular cells almost entirely belong to the NOsGC subset. These data further support the hypothesis of a reciprocal signalling between Reelin/NOsGC and ApoEr2/nNOS expressing neurons to affect olfactory bulb activity. We also show that a significant rise in NOsGC content accompanies the decrease of nNOS protein in the reeler olfactory bulb. The same reciprocal changes present in the cortex/striatum and the hippocampus of reeler mice. Thus, the influence that the deficit of extracellular Reelin seems to exert on nNOS and its receptor is not limited to the olfactory bulb but is a general feature of the reeler brain.

Exercise and food ad libitum reduce the impact of early in life nutritional inbalances on nitrergic activity of hippocampus and striatum

Nutritional imbalances were produced by varying litter size pups per dam: 3 (small), 6 (medium), and 12 (large). On the 21st day, 4 subjects of each litter, were sacrificed and the remaining were grouped, 2 per cage, with or without running wheels, with food and water ad libitum. Adult subjects were tested in water maze, their brains processed for NADPH-diaphorase histochemistry and quantified by densitometry. No differences were detected in water maze. At 21st day, S and L compared with M presented reduced NADPH-d in the stratum molecular of dentate gyrus (DG), stratum lacunosum of CA1 and in all CA3 layers but not in the striatum. On the 58th day, actvity remained low in S and L in CA3 and striatum and L in CA1 and DG. Voluntary exercise increased NADPH-d in DG, CA1, CA3, and striatum in S, and in the stratum lacunosum of CA1 and CA3 in L.

Changes in cell migration and survival in the olfactory bulb of the pcd/pcd mouse

Postnatally, the Purkinje cell degeneration mutant mice lose the main projecting neurons of the main olfactory bulb (OB): mitral cells (MC). In adult animals, progenitor cells from the rostral migratory stream (RMS) differentiate into bulbar interneurons that modulate MC activity. In the present work, we studied changes in proliferation, tangential migration, radial migration patterns, and the survival of these newly generated neurons in this neurodegeneration animal model. The animals were injected with bromodeoxyuridine 2 weeks or 2 months before killing in order to label neuroblast incorporation into the OB and to analyze the survival of these cells after differentiation, respectively. Both the organization and cellular composition of the RMS and the differentiation of the newly generated neurons in the OB were studied using specific markers of glial cells, neuroblasts, and mature neurons. No changes were observed in the cell proliferation rate nor in their tangential migration through the RMS, indicating that migrating neuroblasts are only weakly responsive to the alteration in their target region, the OB. However, the absence of MC does elicit differences in the final destination of the newly generated interneurons. Moreover, the loss of MC also produces changes in the survival of the newly generated interneurons, in accordance with the dramatic decrease in the number of synaptic targets available.

GABAergic phenotype of periglomerular cells in the rodent olfactory bulb

Periglomerular (PG) cells in the rodent olfactory bulb are heterogeneous anatomically and neurochemically. Here we investigated whether major classes of PG cells use gamma-aminobutyric acid (GABA) as a neurotransmitter. In addition to three known subtypes of PG cells expressing tyrosine hydroxylase (TH), calbindin D-28k (CB), and calretinin (CR), we identified a novel PG cell population containing the GABAA receptor alpha5 subunit. Consistent with previous studies in the rat, we found that TH-positive cells were also labeled with antibodies against GABA, whereas PG cells expressing CB or the alpha5 subunit were GABA-negative. Using GAD67-GFP knockin mice, we found that all PG cell subtypes expressed GAD67-GFP. Calretinin labeled the major fraction (44%) of green fluorescent protein (GFP)-positive cells, followed by TH (16%), CB (14%), and the alpha5 subunit (13%). There was no overlap between these neuronal populations, which accounted for approximately 85% of GAD67-GFP-positive cells. We then demonstrated that PG cells labeled for TH, CB, or CR established dendrodendritic synapses expressing glutamic acid decarboxylase (GAD) or the vesicular inhibitory amino acid transporter, VGAT, irrespective of their immunoreactivity for GABA. In addition, CB-, CR-, and TH-positive dendrites were apposed to GABAA receptor clusters containing the alpha1 or alpha3 subunits, which are found in mitral and tufted cells, and the alpha2 subunit, which is expressed by PG cells. Together, these findings indicate that all major subtypes of PG cells are GABAergic. In addition, they show that PG cells provide GABAergic input to the dendrites of principal neurons and are interconnected with other GABAergic interneurons, which most likely are other PG cells.

Nitric oxide synthase protein levels, not the mRNA, are downregulated in olfactory bulb interneurons of reeler mice

Homozygous mutations in the Reelin gene result in severe disruption of brain development. The histogenesis of layered regions, like the neocortex, hippocampus and the cerebellum, is most notably affected in mouse reeler mutants and similar traits are also present in mice lacking molecular components of the Reelin signalling pathway. Moreover, there is evidence for an additional role of Reelin in sustaining synaptic plasticity in adult networks. Nitric oxide is an important gaseous messenger that can modulate neuronal plasticity both in developing and mature synaptic networks and has been shown to facilitate synaptic changes in the hippocampus, cerebellum and olfactory bulb. We studied the distribution and content of neuronal nitric oxide synthase in the olfactory bulbs of reeler and wildtype mice. Immunocytochemistry reveals that Reelin and neuronal nitric oxide synthase containing interneurons are two distinct, non overlapping cell populations of the olfactory bulb. We show by in situ hybridization that both nitrergic and Reelin expressing cells represent only a subset of olfactory bulb GABAergic neurons. Immunoblots show that neuronal nitric oxide synthase protein content is decreased by two thirds in reeler mice causing a detectable loss of immunolabelled cells throughout the olfactory bulb of this strain. However, neuronal nitric oxide synthase mRNA levels, essayed by quantitative real-time RT-PCR, are unaffected in the reeler olfactory bulb. Thus, disruption of the Reelin signalling pathway may modify the turnover of neuronal nitric oxide synthase in the olfactory bulb and possibly affects nitric oxide functions in reeler mice.

Heterogeneity of nitric oxide synthase-containing neurons in the mouse main olfactory bulb

The distribution and structural features of nitric oxide [corrected] synthase (NOS) containing intrinsic neurons were studied in the mouse main olfactory bulb (MOB). NOS positive neurons were heterogeneous, including some subpopulations of periglomerular cells, granule cells, interneurons in the external plexiform layer, superficial and deep short-axon cells and stellate cells. NOS positive periglomerular cells were frequently calretinin immunoreactive and, although rarely, calbindin positive. Importantly, some middle and external tufted cells were also confirmed to be NOS positive, some of which were also cholecystokinin (CCK) positive. Retrograde tracer experiments showed that some NOS positive tufted cells, which were also CCK positive, constitute the intrabulbar association system and the projection system to the olfactory tubercle. In addition, another particular subpopulation of NOS positive neurons with no or little CCK immunoreactivity appeared to project to areas covering the dorsal endopiriform nucleus, claustrum and insular cortex. Furthermore, diverse types of neurons other than mitral/tufted cells were also suggested to be projection neurons of the MOB. The present study revealed the diversity of NOS positive neurons in the mouse MOB and further revealed that they were different from those reported previously in the rat MOB in structural and chemical properties.

NADPH-diaphorase histochemical changes in the hippocampus, cerebellum and striatum are correlated with different modalities of exercise and watermaze performances

Nitric oxide is involved in memory and motor learning. We investigated possible influences of exercise on spatial memory and NADPH-diaphorase (NADPH-d) histochemical activity in the hippocampus, striatum and cerebellum. Fifteen albino Swiss mice between the 22nd and 55th post-natal days were exercised in the following modalities: voluntary (V), acrobatic (A), acrobatic/voluntary (AV) and forced (F) and compared to inactive group (I). After the exercise period, all subjects were tested in the water maze for 3 days. Animal brains were processed for NADPH-d histochemistry. Densitometry of the neuropil of the hippocampus, striatum and cerebellum and morphometric analysis of NADPHd+ type I neurons of the striatum were done. Exercise groups presented higher levels of NADPH-d activity in the molecular and polymorphic layers of dentate gyrus and lacunosum molecular layer of CA1. The A group presented higher NADPH-d activity in the cerebellar granular layer than all other groups. Branching points and dendritic segment densities of NADPH-d type I neurons were higher in V, A and AV than in F and I groups. Exercise groups revealed best performances on water maze tests. Thus, different modalities of exercise increases in different proportions for the nitrergic activity in the hippocampus, striatum and cerebellum, and these changes seem to be beneficial to spatial memory.

Regulation of intracellular cyclic GMP levels in olfactory sensory neurons

Cyclic AMP is the primary second messenger mediating odorant signal transduction in mammals. A number of studies indicate that cyclic GMP is also involved in a variety of other olfactory signal transduction processes, including adaptation, neuronal development, and long-term cellular responses in the setting of odorant stimulation. However, the mechanisms that control the production and degradation of cGMP in olfactory sensory neurons (OSNs) remain unclear. Here, we investigate these mechanisms using primary cultures of OSNs. We demonstrate that odorants increase cGMP levels in intact OSNs in vitro. Different from the rapid and transient cAMP responses to odorants, the cGMP elevation is both delayed and sustained. Inhibition of soluble guanylyl cyclase and heme oxygenase blocks these odorant-induced cGMP increases, whereas inhibition of cGMP PDEs (phosphodiesterases) increases this response. cGMP PDE activity is increased by odorant stimulation, and is sensitive to both ambient calcium and cAMP concentrations. Calcium stimulates cGMP PDE activity, whereas cAMP and protein kinase A appears to inhibit it. These data demonstrate a mechanism by which odorant stimulation may regulate cGMP levels through the modulation of cAMP and calcium level in OSNs. Such interactions between odorants and second messenger systems may be important to the integration of immediate and long-term responses in the setting odorant stimulation.

Functional properties of dopaminergic neurones in the mouse olfactory bulb

The olfactory bulb of mammals contains a large population of dopaminergic interneurones within the glomerular layer. Dopamine has been shown both in vivo and in vitro to modulate several aspects of olfactory information processing, but the functional properties of dopaminergic neurones have never been described due to the inability to recognize these cells in living preparations. To overcome this difficulty, we used a transgenic mouse strain harbouring an eGFP (enhanced green fluorescent protein) reporter construct under the promoter of tyrosine hydroxylase, the rate-limiting enzyme for cathecolamine synthesis. As a result, we were able to identify dopaminergic neurones (TH-GFP cells) in living preparations and, for the first time, we could study the functional properties of such neurones in the olfactory bulb, in both slices and dissociated cells. The most prominent feature of these cells was the autorhythmicity. In these cells we identified five main voltage-dependent conductances: the two having largest amplitude were a fast transient Na(+) current and a delayed rectifier K(+) current. In addition, we observed three smaller inward currents, sustained by Na(+) ions (persistent type) and by Ca(2)(+) ions (LVA and HVA). Using pharmacological tools and ion substitution methods we showed that the pacemaking process is supported by the interplay of the persistent Na(+) current and of a T-type Ca(2)(+) current. We carried out a complete kinetical analysis of the five conductances present in these cells, and developed a Hodgkin-Huxley model of TH-GFP cells, capable of reproducing accurately the properties of living cells, including autorhytmicity, and allowing a precise understanding of the process.

The localization of neuronal nitric oxide synthase may influence its role in neuronal precursor proliferation and synaptic maintenance

Neuronal nitric oxide synthase (nNOS) is implicated in some developmental processes, including neuronal survival, differentiation, and precursor proliferation. To define the roles of nNOS in neuronal development, we utilized the olfactory system as a model. We hypothesized that the role of nNOS may be influenced by its localization. nNOS expression was developmentally regulated in the olfactory system. During early postnatal development, nNOS was expressed in developing neurons in the olfactory epithelium (OE), while in the adult its expression was restricted to periglomerular (PG) cells in the olfactory bulb (OB). At postnatal week 1 (P1W), loss of nNOS due to targeted gene deletion resulted in a decrease in immature neurons in the OE due to decreased proliferation of neuronal precursors. While the pool of neuronal precursors and neurogenesis normalized in the nNOS null mouse by P6W, there was an overgrowth of mitral or tufted cells dendrites and a decreased number of active synapses in the OB. Cyclic GMP (cGMP) immunostaining was reduced in the OE and in the glomeruli of the OB at early postnatal and adult ages, respectively. Our results suggest that nNOS appears necessary for neurogenesis in the OE during early postnatal development and for glomerular organization in the OB in the adult. Thus, the location of nNOS, either within cell bodies or perisynaptically, may influence its developmental role.

Neuropil and neuronal changes in hippocampal NADPH-diaphorase histochemistry in the ME7 model of murine prion disease

Nitric oxide (NO) has been implicated in neurotoxicity and cerebral blood flow changes in chronic neurodegeneration, but its activity in the mammalian prion diseases has not been studied in detail. Nicotine adenine dinucleotide phosphate (NADPH)-diaphorase (NADPH-d) histochemistry is a simple and robust histochemical procedure that allows localization of the tissue distribution of NO synthases. The aim of the present study is to assess whether NADPH-d histochemical activity is altered in the hippocampus in the ME7 model of prion disease in C57BL/6J mice. At early and late stages after the initiation of the disease we assessed features of the NADPH-d positive cells and the neuropil histochemical activity in CA1 and dentate gyrus using densitometric analysis. In C57BL/6J mice 13 weeks postinjection of the prion agent ME7, when behavioural changes first become apparent, neuropil NADPH-d histochemical staining increases, whereas at late stages it decreases dramatically. Both type I and type II NADPH-d positive cells were found to survive throughout the hippocampal formation into the late stages of the disease, but diaphorase activity was reduced in dendritic branches and abnormal varicosities were present in both dendritic and axonal processes of NADPH-d positive type I cells. The pathophysiological implications of the results remain to be investigated but both blood flow alteration and NO neurotoxicity may be features of the disease.

Organization of the main olfactory bulbs of some mammals: Musk shrews, moles, hedgehogs, tree shrews, bats, mice, and rats

We immunohistochemically examined the organization of the main olfactory bulbs (MOBs) in seven mammalian species, including moles, hedgehogs, tree shrews, bats, and mice as well as laboratory musk shrews and rats. We focused our investigation on two points: 1) whether nidi, particular spheroidal synaptic regions subjacent to glomeruli, which we previously reported for the laboratory musk shrew MOBs, are also present in other animals and 2) whether the compartmental organization of glomeruli and two types of periglomerular cells we proposed for the rat MOBs are general in other animals. The general laminar pattern was similar among these seven species, but discrete nidi and the nidal layer were recognized only in two insectivores, namely, the mole and laboratory musk shrew. Olfactory marker protein-immunoreactive (OMP-IR) axons extended beyond the limits of the glomerular layer (GL) into the superficial region of the external plexiform layer (EPL) or the nidal layer in the laboratory musk shrew, mole, hedgehog, and tree shrew but not in bat, mouse, and rat. We observed, in nidi and the nidal layer in the mole and laboratory musk shrew MOBs, only a few OMP-IR axons. In the hedgehog, another insectivore, OMP-IR processes extending from the glomeruli were scattered and intermingled with calbindin D28k-IR cells at the border between the GL and the EPL. In the superficial region of the EPL of the tree shrew MOBs, there were a small number of tiny glomerulus-like spheroidal structures where OMP-IR axons protruding from glomeruli were intermingled with dendritic branches of surrounding calbindin D28k-IR cells. Furthermore, we recognized the compartmental organization of glomeruli and two types of periglomerular cells in the MOBs of all of the mammals we examined. These structural features are therefore considered to be common and important organizational principles of the MOBs.

Effects of chronic nicotine administration on nitric oxide synthase expression and activity in rat brain

Although there is substantial evidence concerning the influence of nicotine on nitric oxide (NO) synthesis in the vascular system, there are fewer studies concerning the central nervous system. Although NO metabolites (nitrates/nitrites) increase in several rat brain regions after chronic injection of nicotine, the cellular origin of this rise in NO levels is not known. The aim of the present work was to assess the effects of repetitive nicotine administration on nitric oxide synthase (NOS) expression and activity in male and female rat brains. To determine levels of nitrate/nitrite, the Griess reaction was carried out in tissue micropunched from the frontal cortex, striatum, and accumbens of both male and female rats untreated (naïve) or injected with saline or nicotine (0.4 mg/kg for 15 days). In parallel, coronal sections of fixed brains from equally treated animals were immunostained for neuronal NOS or histochemically labelled for NADPH-diaphorase activity. Nicotine treatment increased NO metabolites significantly in all brain regions compared with naïve or saline-treated rats. By contrast, analysis of the planimetric counting of NOS/NADPH-diaphorase-positive neurons failed to demonstrate any significant effect of the nicotine treatment. A significant decrease was observed with both techniques employed in saline-injected female rats compared with naïve animals, suggesting a stress response. The mismatch between the biochemical and the histological data after chronic nicotine treatment is discussed. The up-regulation of NO sources other than neurons is proposed.

Volumetric and horseradish peroxidase tracing analysis of rat olfactory bulb following reversible olfactory nerve lesions

Olfactory receptor neurons can regenerate from basal stem cells. Receptor neuron lesion causes degenerative changes in the olfactory bulb followed by regeneration as new olfactory receptor axons innervate the olfactory bulb. To our knowledge, parametric analyses of morphometric changes in the olfactory bulb during degeneration and regeneration do not exist except in abstract form. To better characterize olfactory bulb response, we performed morphometric analysis in rats following reversible olfactory nerve lesion with diethyldithiocarbamate. We also performed anterograde tracing of the olfactory nerve with wheatgerm agglutinin linked to horseradish peroxidase. Results of morphometry and tracing were complementary. The glomerular layer and external plexiform layer showed shrinkage of 45 and 26%, respectively, at 9 days. No significant shrinkage occurred in any other layer. Individual glomeruli shrank by 40-50% at 3 and 9 days following lesion. These data show that degenerative changes occur both in the glomeruli and transneuronally in the external plexiform layer. Olfactory nerve regeneration (identified by WGA-HRP transport) paralleled volumetric recovery. Recovery occurred first in ventral and lateral glomeruli between 9 and 16 days followed by recovery in medial and dorsal glomeruli. These data indicate substantial transynaptic degeneration in the olfactory bulb and a heretofore unrecognized gradient in olfactory nerve regeneration that can be used to systematically study recovery of a cortical structure.