Evidence for cholinergic regulation of basal norepinephrine release in the rat olfactory bulb

Role of Brain Endothelin Receptor Type B (ETB) in the Regulation of Tyrosine Hydroxylase in the Olfactory Bulb of DOCA-Salt Hypertensive Rats

BACKGROUND

We previously reported that endothelins (ETs) regulate tyrosine hydroxylase (TH) activity and expression in the olfactory bulb (OB) of normotensive and hypertensive animals. Applying an ET receptor type A (ETA) antagonist to the brain suggested that endogenous ETs bind to ET receptor type B (ETB) to elicit effects.

OBJECTIVE

The aim of the present work was to evaluate the role of central ETB stimulation on the regulation of blood pressure (BP) and the catecholaminergic system in the OB of deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

METHODS

DOCA-salt hypertensive rats were infused for 7 days with cerebrospinal fluid or IRL-1620 (ETB receptor agonist) through a cannula placed in the lateral brain ventricle. Systolic BP (SBP) and heart rate were recorded by plethysmography. The expression of TH and its phosphorylated forms in the OB were determined by immunoblotting, TH activity by a radioenzymatic assay, and TH mRNA by quantitative real-time polymerase chain reaction.

RESULTS

Chronic administration of IRL-1620 decreased SBP in hypertensive rats but not in normotensive animals. Furthermore, the blockade of ETB receptors also decreased TH-mRNA in DOCA-salt rats, but it did not modify TH activity or protein expression.

CONCLUSION

These findings suggest that brain ETs through the activation of ETB receptors contribute to SBP regulation in DOCA-salt hypertension. However, the catecholaminergic system in the OB does not appear to be conclusively involved although mRNA TH was reduced. Present and previous findings suggest that in this salt-sensitive animal model of hypertension, the OB contributes to chronic BP elevation.

The Neuromodulatory Role of the Noradrenergic and Cholinergic Systems and Their Interplay in Cognitive Functions: A Focused Review

The noradrenergic and cholinergic modulation of functionally distinct regions of the brain has become one of the primary organizational principles behind understanding the contribution of each system to the diversity of neural computation in the central nervous system. Decades of work has shown that a diverse family of receptors, stratified across different brain regions, and circuit-specific afferent and efferent projections play a critical role in helping such widespread neuromodulatory systems obtain substantial heterogeneity in neural information processing. This review briefly discusses the anatomical layout of both the noradrenergic and cholinergic systems, as well as the types and distributions of relevant receptors for each system. Previous work characterizing the direct and indirect interaction between these two systems is discussed, especially in the context of higher order cognitive functions such as attention, learning, and the decision-making process. Though a substantial amount of work has been done to characterize the role of each neuromodulator, a cohesive understanding of the region-specific cooperation of these two systems is not yet fully realized. For the field to progress, new experiments will need to be conducted that capitalize on the modular subdivisions of the brain and systematically explore the role of norepinephrine and acetylcholine in each of these subunits and across the full range of receptors expressed in different cell types in these regions.

Behavioral effects in adult zebrafish after developmental exposure to carbaryl

Chemical exposure during the early life stages of development may have long lasting effects on organisms that are rarely studied. The present work intended to evaluate the effect of embryonic exposure to the pesticide carbaryl on adult fish behavior. Zebrafish (Danio rerio) embryos were exposed, for 4 days, to sublethal concentrations of carbaryl (0.01, 0.1 and 1.0 mg/L) plus a control and then kept in standard cultivation conditions until adulthood. A battery of behavioral tests was then performed to assess anxiety-like behavior (locomotor activity, thigmotaxis and novel tank diving test), social behavior, and feeding. Developmental exposure of zebrafish to sublethal concentrations of carbaryl produced important behavioral alterations in the adulthood. Main effects included decreased locomotion/hypoactivity (increase in slow movements and decrease of medium and rapid movements), especially in the light periods. Moreover, spatial pattern also changed: while during dark periods control fish increased activity in the outer zone of the tank, this was not observed in exposed fish. Overall, this demonstrated the importance of life stage exposure, clearly demonstrating long lasting effects of a (chemical) stress event at embryonic stages. This data supports the need of considering this scenario in environmental risk evaluations. Further work should focus on the mechanistic effects of developmental disruption responsible for the effects observed.

Effect of basal forebrain stimulation on extracellular acetylcholine release and blood flow in the olfactory bulb

The olfactory bulb receives cholinergic basal forebrain input, as does the neocortex; however, the in vivo physiological functions regarding the release of extracellular acetylcholine and regulation of regional blood flow in the olfactory bulb are unclear. We used in vivo microdialysis to measure the extracellular acetylcholine levels in the olfactory bulb of urethane-anesthetized rats. Focal chemical stimulation by microinjection of L-glutamate into the horizontal limb of the diagonal band of Broca (HDB) in the basal forebrain, which is the main source of cholinergic input to the olfactory bulb, increased extracellular acetylcholine release in the ipsilateral olfactory bulb. When the regional cerebral blood flow was measured using laser speckle contrast imaging, the focal chemical stimulation of the HDB did not significantly alter the blood flow in the olfactory bulb, while increases were observed in the neocortex. Our results suggest a functional difference between the olfactory bulb and neocortex regarding cerebral blood flow regulation through the release of acetylcholine by cholinergic basal forebrain input.

Dichotomous Distribution of Putative Cholinergic Interneurons in Mouse Accessory Olfactory Bulb

Sensory information processing in the olfactory bulb (OB) relies on diverse populations of bulbar interneurons. In rodents, the accessory OB (AOB) is divided into two bulbar regions, the anterior (aAOB) and posterior (pAOB), which differ substantially in their circuitry connections and associated behaviors. We previously identified and characterized a large number of morphologically diverse cholinergic interneurons in the main OB (MOB) using transgenic mice to visualize the cell bodies of choline acetyltransferase (ChAT-expressing neurons and immunolabeling (Krosnowski et al., 2012)). However, whether there are cholinergic neurons in the AOB is controversial and there is no detailed characterization of such neurons. Using the same line of ChAT^{(bacterial artificial chromosome, BAC)}-enhanced green fluorescent protein (eGFP) transgenic mice, we investigated cholinergic neurons in the AOB. We found significant differences in the number and location of GFP-expressing (GFP+), putative cholinergic interneurons between the aAOB and pAOB. The highest numbers of GFP+ interneurons were found in the aAOB glomerular layer (aGL) and pAOB mitral/tufted cell layer (pMCL). We also noted a high density of GFP+ interneurons encircling the border region of the pMCL. Interestingly, a small subset of glomeruli in the middle of the GL receives strong MCL GFP+ nerve processes. These local putative cholinergic-innervated glomeruli are situated just outside the aGL, setting the boundary between the pGL and aGL. Many but not all GFP+ neurons in the AOB were weakly labeled with antibodies against ChAT and vesicular acetylcholine transporter (VAChT). We further determined if these GFP+ interneurons differ from other previously characterized interneuron populations in the AOB and found that AOB GFP+ interneurons express neither GABAergic nor dopaminergic markers and most also do not express the glutamatergic marker. Similar to the cholinergic interneurons of the MOB, some AOB GFP+ interneurons express the calcium binding protein, calbindin-D28K. Moreover, exposure to either a male intruder or soiled bedding from a mating cage leads to an increase in the number of c-Fos-expressing MCL GFP+ neurons. Taken together, our data reveal a population of largely unidentified putative cholinergic neurons in the AOB. Their dichotomous distribution in the aAOB and pAOB suggests region-specific cholinergic involvement in olfactory information processing.

Anesthetic regimes modulate the temporal dynamics of local field potential in the mouse olfactory bulb

Anesthetized preparations have been widely used to study odor-induced temporal dynamics in the olfactory bulb. Although numerous recent data of single-cell recording or imaging in the olfactory bulb have employed ketamine cocktails, their effects on networks activities are still poorly understood, and odor-induced oscillations of the local field potential have not been characterized under these anesthetics. Our study aimed at describing the impact of two ketamine cocktails on oscillations and comparing them to awake condition. Anesthesia was induced by injection of a cocktail of ketamine, an antagonist of the N-methyl-d-aspartate receptors, combined with one agonist of α2-adrenergic receptors, xylazine (low affinity) or medetomidine (high affinity). Spontaneous and odor-induced activities were examined in anesthetized and awake conditions, in the same mice chronically implanted with an electrode in the main olfactory bulb. The overall dynamic pattern of oscillations under the two ketamine cocktails resembles that of the awake state. Ongoing activity is characterized by gamma bursts (>60 Hz) locked on respiration and beta (15-40 Hz) power increases during odor stimulation. However, anesthesia decreases local field potential power and leads to a strong frequency shift of gamma oscillations from 60-90 Hz to 100-130 Hz. We conclude that similarities between oscillations in anesthetized and awake states make cocktails of ketamine with one α2-agonist suitable for the recordings of local field potential to study processing in the early stages of the olfactory system.

The basal forebrain modulates spontaneous activity of principal cells in the main olfactory bulb of anesthetized mice

Spontaneous activity is an important characteristic of the principal cells in the main olfactory bulb (MOB) for encoding odor information, which is modulated by the basal forebrain. Cholinergic activation has been reported to inhibit all major neuron types in the MOB. In this study, the effect of diagonal band (NDB) stimulation on mitral/tufted (M/T) cell spontaneous activity was examined in anesthetized mice. NDB stimulation increased spontaneous activity in 66 MOB neurons which lasted for 2–35 s before returning to the baseline level. The majority of the effected units showed a decrease of interspike intervals (ISI) at a range of 8–25 ms. Fifty-two percent of NDB stimulation responsive units showed intrinsic rhythmical bursting, which was enhanced temporarily by NDB stimulation, whereas the remaining non-rhythmic units were capable of synchronized bursting. The effect was attenuated by scopolamine in 21 of 27 units tested. Only four NDB units were inhibited by NDB stimulation, an inhibition that lasted less than 10 s. The NDB stimulation responsive neurons appeared to be M/T cells. Our findings demonstrate an NDB excitation effect on M/T neurons that mostly requires muscarinic receptor activation, and is likely due to non-selectivity of electrical stimulation. This suggests that cholinergic and a diverse group of non-cholinergic neurons in the basal forebrain co-ordinately modulate the dynamics of M/T cell spontaneous activity, which is fundamental for odor representation and attentional perception.

Olfactory bulb monoamine concentrations vary with time of day

The olfactory bulb (OB) has been recently identified as a circadian oscillator capable of operating independently of the master circadian pacemaker, the suprachiasmatic nuclei of the hypothalamus. OB oscillations manifest as rhythms in clock genes, electrical activity, and odor sensitivity. Dopamine, norepinephrine, and serotonin have been shown to modulate olfactory information processing by the OB and may be part of the mechanism that underlies diurnal changes in olfactory sensitivity. Rhythmic release of these neurotransmitters could generate OB rhythms in electrical activity and olfactory sensitivity. We hypothesized that these monoamines were rhythmically released in the OB. To test our hypotheses, we examined monoamine levels in the OB, over the course of a day, by high-performance liquid chromatography coupled to electrochemical detection. We observed that dopamine and its metabolite, 3-4-dihydroxyphenylacetic acid, rhythmically fluctuate over the day. In contrast, norepinephrine is arrhythmic. Serotonin and its metabolite hydroxyindoleacetic acid appear to rhythmically fluctuate. Each of these monoamines has been shown to alter OB circuit behavior and influence odor processing. Rhythmic release of serotonin may be a mechanism by which the suprachiasmatic nuclei communicate, indirectly, with the OB.

Diverse populations of intrinsic cholinergic interneurons in the mouse olfactory bulb

Cholinergic activities affect olfactory bulb (OB) information processing and associated learning and memory. However, the presence of intrinsic cholinergic interneurons in the OB remains controversial. As a result, morphological and functional properties of these cells are largely undetermined. We characterized cholinergic interneurons using transgenic mice that selectively mark choline acetyltransferase (ChAT)-expressing cells and immunolabeling. We found a significant number of intrinsic cholinergic interneurons in the OB. These interneurons reside primarily in the glomerular layer (GL) and external plexiform layer (EPL) and exhibit diverse distribution patterns of nerve processes, indicating functional heterogeneity. Further, we found these neurons express ChAT and vesicular acetylcholine transporter (VAChT), but do not immunoreact to glutamatergic, GABAergic or dopaminergic markers and are distinct from calretinin-expressing interneurons. Interestingly, the cholinergic population partially overlaps with the calbindin D28K-expressing interneuron population, revealing the neurotransmitter identity of this sub-population. Additionally, we quantitatively determined the density of VAChT labeled cholinergic nerve fibers in various layers of the OB, as well as the intensity of VAChT immunoreactivity within the GL, suggesting primary sites of cholinergic actions. Taken together, our results provide clear evidence showing the presence of a significant number of cholinergic interneurons and that these morphologically and distributionally diverse interneurons make up complex local cholinergic networks in the OB. Thus, our results suggest that olfactory information processing is modulated by dual cholinergic systems of local interneuron networks and centrifugal projections.

α7-Nicotinic acetylcholine receptor: role in early odor learning preference in mice

Recently, we have shown that mice with decreased expression of α7-nicotinic acetylcholine receptors (α7) in the olfactory bulb were associated with a deficit in odor discrimination compared to wild-type mice. However, it is unknown if mice with decreased α7-receptor expression also show a deficit in early odor learning preference (ELP), an enhanced behavioral response to odors with attractive value observed in rats. In this study, we modified ELP methods performed in rats and implemented similar conditions in mice. From post-natal days 5–18, wild-type mice were stroked simultaneously with an odor presentation (conditioned odor) for 90 s daily. Control mice were only stroked, exposed to odor, or neither. On the day of testing (P21), mice that were stroked in concert with a conditioned odor significantly investigated the conditioned odor compared to a novel odor, as observed similarly in rats. However, mice with a decrease in α7-receptor expression that were stroked during a conditioned odor did not show a behavioral response to that odorant. These results suggest that decreased α7-receptor expression has a role in associative learning, olfactory preference, and/or sensory processing deficits.

Nonlinear effects of noradrenergic modulation of olfactory bulb function in adult rodents

The mammalian main olfactory bulb receives a significant noradrenergic input from the locus coeruleus. Norepinephrine (NE) is involved in acquisition of conditioned odor preferences in neonatal animals, in some species-specific odor-dependent behaviors, and in adult odor perception. We provide a detailed review of the functional role of NE in adult rodent main olfactory bulb function. We include cellular, synaptic, network, and behavioral data and use computational simulations to tie these different types of data together.

Adrenergic receptor-mediated disinhibition of mitral cells triggers long-term enhancement of synchronized oscillations in the olfactory bulb

Norepinephrine (NE) is widely implicated in various forms of associative olfactory learning in rodents, including early learning preference in neonates. Here we used patch-clamp recordings in rat olfactory bulb slices to assess cellular actions of NE, examining both acute, short-term effects of NE as well as the relationship between these acute effects and long-term cellular changes that could underlie learning. Our focus for long-term effects was on synchronized gamma frequency (30-70 Hz) oscillations, shown in prior studies to be enhanced for up to an hour after brief exposure of a bulb slice to NE and neuronal stimulation. In terms of acute effects, we found that a dominant action of NE was to reduce inhibitory GABAergic transmission from granule cells (GCs) to output mitral cells (MCs). This disinhibition was also induced by clonidine, an agonist specific for alpha(2) adrenergic receptors (ARs). Acute NE-induced disinhibition of MCs appeared to be linked to long-term enhancement of gamma oscillations, based, first, on the fact that clonidine, but not agonists specific for other AR subtypes, mimicked NE's long-term actions. In addition, the alpha(2) AR-specific antagonist yohimbine blocked the long-term enhancement of the oscillations due to NE. Last, brief exposure of the slice to the GABA(A) receptor antagonist gabazine, to block inhibitory synapses directly, also induced the long-term changes. Acute disinhibition is a plausible permissive effect of NE leading to olfactory learning, because, when combined with exposure to a specific odor, it should lead to neuron-specific increases in intracellular calcium of the type generally associated with long-term synaptic modifications.

Noradrenergic regulation of GABAergic inhibition of main olfactory bulb mitral cells varies as a function of concentration and receptor subtype

The main olfactory bulb (MOB) receives a rich noradrenergic innervation from the pontine nucleus locus coeruleus (LC). Previous studies indicate that norepinephrine (NE) modulates the strength of GABAergic inhibition in MOB. However, the nature of this modulation and the NE receptors involved remain controversial. The goal of this study was to investigate the role of NE receptor subtypes in modulating the GABAergic inhibition of mitral cells using patch-clamp electrophysiology in rat MOB slices. NE concentration dependently and bi-directionally modulated GABA(A) receptor-mediated spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) recorded in mitral cells. Low doses of NE suppressed sIPSCs and mIPSCs because of activation of alpha2 receptors. Intermediate concentrations of NE increased sIPSCs and mIPSCs primarily because of activation of alpha1 receptors. In contrast, activation of beta receptors increased sIPSCs but not mIPSCs. These results indicate that NE release regulates the strength of GABAergic inhibition of mitral cells depending on the NE receptor subtype activated. Functionally, the differing affinity of noradrenergic receptor subtypes seems to allow for dynamic modulation of GABAergic inhibition in MOB as function of the extracellular NE concentration, which in turn, is regulated by behavioral state.

Noradrenergic induction of odor-specific neural habituation and olfactory memories

For many mammals, individual recognition of conspecifics relies on olfactory cues. Certain individual recognition memories are thought to be stored when conspecific odor cues coincide with surges of noradrenaline (NA) triggered by intensely arousing social events. Such familiar stimuli elicit reduced behavioral responses, a change likely related to NA-dependent plasticity in the olfactory bulb (OB). In addition to its role in these ethological memories, NA signaling in the OB appears to be relevant for the discrimination of more arbitrary odorants as well. Nonetheless, no NA-gated mechanism of long-term plasticity in the OB has ever been directly observed in vivo. Here, we report that NA release from locus ceruleus (LC), when coupled to odor presentation, acts locally in the main OB to cause a specific long-lasting suppression of responses to paired odors. These effects were observed for both food odors and urine, an important social recognition cue. Moreover, in subsequent behavioral tests, mice exhibited habituation to paired urine stimuli, suggesting that this LC-mediated olfactory neural plasticity, induced under anesthesia, can store an individual recognition memory that is observable after recovery.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

Differential effects of unilateral olfactory deprivation on noradrenergic and cholinergic systems in the main olfactory bulb of the rat

The lack of environmental olfactory stimulation produced by sensory deprivation causes significant changes in the deprived olfactory bulb. Olfactory transmission in the main olfactory bulb (MOB) is strongly modulated by centrifugal systems. The present report examines the effects of unilateral deprivation on the noradrenergic and cholinergic centrifugal systems innervating the MOB. The morphology, distribution, and density of positive axons were studied in the MOBs of control and deprived rats, using dopamine-beta-hydroxylase (DBH)-immunohistochemistry and acetylcholinesterase (AChE) histochemistry in serial sections. Catecholamine content was compared among the different groups of MOBs (control, contralateral, and ipsilateral to the deprivation) using high-performance liquid chromatography analysis. Sensory deprivation revealed that the noradrenergic system developed adaptive plastic changes after olfactory deprivation, including important modifications in its fiber density and distribution, while no differences in cholinergic innervation were observed under the same conditions. The noradrenergic system underwent an important alteration in the glomerular layer, in which some glomeruli showed a dense noradrenergic innervation that was not detected in control animals. The DBH-positive glomeruli with the highest noradrenergic fiber density were compared with AChE-stained sections and it was observed that the strongly noradrenergic-innervated glomeruli were always atypical glomeruli (characterized by their strong degree of cholinergic innervation). In addition to the morphological findings, our biochemical data revealed that olfactory deprivation caused a decrease in the content of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid in the ipsilateral MOB in comparison to the contralateral and control MOBs, together with an increase in noradrenaline levels in both the ipsilateral and contralateral MOBs. Our results show that regulation of the noradrenergic centrifugal system in the MOB depends on environmental olfactory stimulation and that it is highly reactive to sensory deprivation. By contrast, the cholinergic system is fairly stable and does not exhibit clear changes after the loss of sensory inputs.

Modulation of nicotine-induced attentional enhancement in rats by adrenoceptor antagonists

RATIONALE

Understanding the neuropharmacological mechanisms mediating attentional enhancement by nicotine would help a targeted search for nicotinic compounds with retained therapeutic but reduced unwanted side-effects. Previous studies suggested that the dopamine-releasing effects of nicotine may not be of primary importance for its attention-enhancing properties.

OBJECTIVES

The present study examined the role of noradrenergic neurotransmission for the effects of nicotine on different response indices of an attentional paradigm.

METHODS

The effects of systemic injections of the alpha(1)-adrenoceptor antagonist prazosin that also displays significant affinity at alpha(2B) and alpha(2C)-adrenoceptors and the beta-adrenoceptor antagonist propranolol were tested in both the presence and absence of nicotine in rats trained in a version of the five-choice serial reaction time task.

RESULTS

Nicotine generally enhanced the accuracy of signal detection, reduced omission errors and shortened response latencies. At the largest doses tested, both prazosin (1 mg/kg) and propranolol (10 mg/kg) impaired performance. For propranolol, these effects depended on the rate of target signal presentation. The two compounds differentially modulated the effects of nicotine. Propranolol (6 mg/kg and 10 mg/kg) but not prazosin reduced its effects on omission errors and accuracy. By contrast, prazosin (0.5 mg/kg) reversed the nicotine-induced reductions in response latency.

CONCLUSIONS

The data provide the first evidence that beta-adrenoceptors are involved in mediating the effects of nicotine on signal detection, while activation of alpha-adrenoceptors may contribute to its effects on response speed. This is a further indication that, from among nicotine's wide range of neuropharmacological effects, specific facets can be dissociated that are responsible for its attention-enhancing properties.

Acetylcholine and olfactory perceptual learning

Olfactory perceptual learning is a relatively long-term, learned increase in perceptual acuity, and has been described in both humans and animals. Data from recent electrophysiological studies have indicated that olfactory perceptual learning may be correlated with changes in odorant receptive fields of neurons in the olfactory bulb and piriform cortex. These changes include enhanced representation of the molecular features of familiar odors by mitral cells in the olfactory bulb, and synthetic coding of multiple coincident odorant features into odor objects by cortical neurons. In this paper, data are reviewed that show the critical role of acetylcholine (Ach) in olfactory system function and plasticity, and cholinergic modulation of olfactory perceptual learning at both the behavioral and cortical level.

Developmental regulation of nicotinic acetylcholine receptor-mediated [3H]norepinephrine release from rat cerebellum

Presynaptic modulation of synaptic transmission is the primary function of central nicotinic acetylcholine receptors (nAChRs) in developing and adult brain. nAChR activation regulates release of various neurotransmitters, including norepinephrine (NA). Given evidence that NA may serve a critical functional role in cerebellar development, we have undertaken studies to determine whether nAChRs modulate NA release in developing cerebellum. In vitro experiments using cerebellar slices examined the effects of nAChR stimulation on release of radiolabeled NA ([3H]NA). Our data indicate the presence of functional nAChRs on NA terminals in immature cerebellum and subsequent developmental regulation of receptor properties. During postnatal week one, the maximally effective dose of nicotine released 35.0 +/- 1.2% of cerebellar [3H]NA stores. There was a subsequent decline in maximal nicotine-stimulated NA release until postnatal day 30, when Emax values were statistically indistinguishable from adult. Although the efficacy of nicotine changed substantially throughout development, EC50 values did not differ significantly (EC50 = 4.4-12.0 micro m). Pharmacological analysis indicated that this developmental shift in maximum nicotine effect reflects a change in the properties of the nAChRs. These data support recent findings of a possible functional role of nAChRs in regulating cerebellar ontogeny, and provides further support for the role of NA as a neurotrophic factor during development.

Effects of acetyl-L-carnitine on regional cerebral glucose metabolism in awake rats

The time-course and relation to dose of regional metabolic rates for glucose (rCMRglc) were measured in awake adult Fischer-344 rats after administration of acetyl-L-carnitine (ALCAR), an agent that modulates neuronal energy processes and neurotransmitter synthesis. rCMRglc were determined with the quantitative [(14)C]2-deoxy-D-glucose technique in 50 brain regions at 10, 30 and 60 min after i.v. administration of ALCAR 500 mg/kg and at 30 min after ALCAR 250 and 700 mg/kg or coadministration of acetate (500 mg/kg) and carnitine (500 mg/kg). ALCAR resulted in significant rCMRglc increases that were maximal by 30 min; by that time, ALCAR 250 produced small, non-significant increase in rCMRglc (no region affected, mean increase 13%) and ALCAR 500 and 750 similar, larger increases in rCMRglc (eight and 11 brain regions affected, mean increases 21 and 22%, respectively). In contrast with ALCAR, carnitine plus acetate did not alter significantly rCMRglc in any brain regions, suggesting that acetate metabolism and carrier are not involved in ALCAR pharmacological activities. ALCAR increased rCMRglc more markedly in subcortical cholinergic (i.e. diagonal band, preoptic magnocellular area) and non-cholinergic (i.e. locus coeruleus, median raphe) nuclei and, to a lesser degree, in limbic and sensorimotor cortical areas. The topographic distribution of rCMRglc increases induced by ALCAR differs from those produced by cholinergic muscarinic agonists and suggest a preferential activation of nicotinic receptors.

Nisoxetine infusion into the olfactory bulb enhances the capacity for male rats to identify conspecifics

In the present report, the norepinephrine uptake inhibitor nisoxetine as well as a cocktail of nisoxetine and the alpha-adrenergic receptor antagonist phentolamine were infused unilaterally into the olfactory bulb during microdialysis to assess their effects upon the capacity of male rats to identify conspecifics. A social discrimination test was conducted while simultaneously measuring olfactory bulb norepinephrine output in the dialysate before, during, and after behavioral testing. Nisoxetine significantly increased norepinephrine levels in the olfactory bulb compared with the Ringer's solution control group. Following such increases in olfactory bulb norepinephrine, identification responses were enhanced compared with that observed in the Ringer's control. In the presence of phentolamine, nisoxetine elevated olfactory bulb norepinephrine to levels similar to that obtained in the nisoxetine alone group, however, investigatory responses directed to the conspecifics indicated an absence of identification capacity similar to that observed in the Ringer's control group. These results reveal a direct link between norepinephrine transmission in the olfactory bulb and enhanced identification via its activation of postsynaptic alpha-adrenergic receptors. These results also show that inhibition of norepinephrine uptake may represent an important mechanism involved with the enhancement of social identification and suggest a possible novel effect for the antidepressant nisoxetine.

Nicotine-induced enhancement of glutamatergic and GABAergic synaptic transmission in the mouse amygdala

Presynaptic nicotinic acetylcholine receptors (nAChRs) are thought to mediate some of the cognitive and behavioral effects of nicotine. The olfactory projection to the amygdala, and intra-amygdaloid projections, are limbic relays involved in behavioral reinforcement, a property influenced by nicotine. Co-cultures consisting of murine olfactory bulb (OB) explants and dispersed amygdala neurons were developed to reconstruct this pathway in vitro. Whole cell patch-clamp recordings were obtained from amygdala neurons contacted by OB explant neurites, and spontaneous and evoked synaptic currents were characterized. The majority of the 108 innervated amygdala neurons exhibited glutamatergic spontaneous postsynaptic currents (PSCs), 20% exhibited GABAergic spontaneous PSCs, and 17% exhibited both. Direct extracellular stimulation of OB explants elicited glutamatergic synaptic currents in amygdala neurons. Antibodies to nAChR subunits co-localized with an antibody to synapsin I, a presynaptic marker, along OB explant processes, consistent with the targeting of nAChR protein to presynaptic sites of the mitral cell projections. Hence, we examined the role of presynaptic nAChRs in modulating synaptic transmission in the OB-amygdala co-cultures. Focal application of 500 nM to 1 microM nicotine for 5-60 s markedly increased the frequency of spontaneous PSCs, without a change in the amplitude, in 39% of neurons that exhibited glutamatergic spontaneous PSCs (average peak fold increase = 125.2 +/- 33.3). Nicotine also enhanced evoked glutamatergic currents elicited by direct stimulation of OB explant fibers. Nicotine increased the frequency of spontaneous PSCs, without a change in the amplitude, in 35% of neurons that exhibited GABAergic spontaneous PSCs (average peak fold increase = 63.9 +/- 34.3). Thus activation of presynaptic nAChRs can modulate glutamatergic as well as GABAergic synaptic transmission in the amygdala. These results suggest that behaviors mediated by olfactory projections may be modulated by presynaptic nAChRs in the amygdala, where integration of olfactory and pheromonal input is thought to occur.

Bilateral olfactory deprivation reveals a selective noradrenergic regulatory input to the olfactory bulb

Unilateral olfactory deprivation in the rat induces changes in the catecholaminergic system of the olfactory bulb. Nevertheless, evidence suggests that unilateral deprivation does not fully prevent stimulation of the deprived bulb. The present report analyses the response of the catecholaminergic system of the olfactory bulb in fully deprived rats obtained by bilateral naris occlusion. The complete deprivation produces more rapid and dramatic changes in both the intrinsic and extrinsic catecholaminergic systems of the olfactory bulb. Intrinsic responses involve a rapid decrease in dopamine-containing cells to about 25% of controls, correlated with a decreased Fos expression in juxtaglomerular cells of all olfactory glomeruli, with the only exception of those of the atypical glomeruli which maintain unaltered expression of both markers. In parallel with these events, there is a progressive increase in the density of extrinsic noradrenergic axons arising from neurons in the locus coeruleus, which shows, in parallel, a progressive increase in Fos expression. This model demonstrates plastic changes in the catecholaminergic system of the olfactory bulb forming a valid morphological substrate for lowering thresholds in the processing of olfactory information. In addition to this generalized response, there is another one, directed to a specific subset of olfactory glomeruli (atypical glomeruli) involved in the processing of odor pheromone-like cues related to behavioral responses, that could be responsible for keeping active this reduced and selected group of glomeruli carrying crucial olfactory information. These results indicate the existence of adaptive changes in the catecholaminergic system of the olfactory bulb as a response to the lack of afferent peripheral stimulation. These changes involve dopamine- and noradrenaline-immunoreactive elements, in a strategy presumably directed at maintaining to the highest possible level the ability to detect olfactory signals.

A genetic rat model of cholinergic hypersensitivity: implications for chemical intolerance, chronic fatigue, and asthma

The fact that only some individuals exposed to environmental chemicals develop chemical intolerance raises the possibility that genetic factors could be contributing factors. The present communication summarizes evidence from a genetic animal model of cholinergic supersensitivity that suggests that an abnormal cholinergic system could be one predisposing genetic factor. The Flinders Sensitive Line (FSL) rats were established by selective breeding for increased responses to an organophosphate. It was subsequently found that these FSL rats were also more sensitive to direct-acting muscarinic agonists and had elevated muscarinic receptors compared to the selectively bred parallel group, the Flinders Resistant Line (FRL) rats, or randomly bred control rats. Increased sensitivity to cholinergic agents has also been observed in several human populations, including individuals suffering from chemical intolerance. Indeed, the FSL rats exhibit certain behavioral characteristics such as abnormal sleep, activity, and appetite that are similar to those reported in these human populations. In addition, the FSL rats have been reported to exhibit increased sensitivity to a variety of other chemical agents. Peripheral tissues, such as intestinal and airway smooth muscle, appear to be more sensitive to both cholinergic agonists and an antigen, ovalbumin. Hypothermia, a centrally mediated response, is more pronounced in the FSL rats after nicotine and alcohol, as well as agents that are selective for the dopaminergic and serotonergic systems. In some cases, the increased sensitivity has been detected in the absence of any changes in the receptors with which the drugs interact (dopamine receptors), while receptor changes have been seen in other cases (nicotine receptors). Therefore, there may be multiple mechanisms underlying the multiple chemical sensitivity-chemical intolerance of the FSL rats. An elucidation of these mechanisms may provide useful clues to those involved in chemical intolerance in humans.

Association of an odor with activation of olfactory bulb noradrenergic beta-receptors or locus coeruleus stimulation is sufficient to produce learned approach responses to that odor in neonatal rats

These experiments examined the sufficiency of pairing an odor with either intrabulbar activation of noradrenergic beta-receptors or pharmacological stimulation of the locus coeruleus to support learned odor preferences in Postnatal Day 6-7 rat pups. The results showed that pups exposed to odor paired with beta-receptor activation limited to the olfactory bulb (isoproterenol, 50 microM) displayed a conditioned approach response on subsequent exposure to that odor. Furthermore, putative stimulation of the locus coeruleus (2 microM idazoxan or 2 mM acetylcholine) paired with odor produced a subsequent preference for that odor. The effects of locus coeruleus stimulation could be blocked by a pretraining injection of the beta-receptor antagonist propranolol (20 mg/kg). Together these results suggest that convergence of odor input with norepinephrine release from the locus coeruleus terminals within the olfactory bulb is sufficient to support olfactory learning.

Norepinephrine increases rat mitral cell excitatory responses to weak olfactory nerve input via alpha-1 receptors in vitro

A rat olfactory bulb in vitro slice preparation was used to investigate the actions of norepinephrine on spontaneous and afferent (olfactory nerve) evoked activity of mitral cells. Single olfactory nerve shocks elicited a characteristic mitral cell response consisting of distinct, early and late spiking components separated by a brief inhibitory epoch. Bath-applied norepinephrine (1 microM) increased the early spiking component elicited by perithreshold (79% increase, P<0.02), but not by suprathreshold (3% decrease, P>0.05), intensity olfactory nerve shocks. The facilitatory effect of norepinephrine was due to a reduction in the incidence of response failures to perithreshold intensity shocks. Norepinephrine also decreased the inhibitory epoch separating the early and late spiking components by 44% (P<0.05). By contrast, norepinephrine had no consistent effect on the spontaneous discharge rate of the mitral cells. The effects of norepinephrine were mimicked by the al receptor agonist phenylephrine (1 microM, P<0.001). Both norepinephrine and phenylephrine modulation of mitral cell responses were blocked by the al adrenergic antagonist WB-4101 (1 microM). These findings are consistent with observations that the main olfactory bulb exhibits the highest density of alpha1 receptors in the brain. The alpha2 receptor agonist clonidine (100 nM) and the beta receptor agonist isoproterenol (1 microM) had inconsistent effects on mitral cell spontaneous and olfactory nerve-evoked activity. These results indicate that norepinephrine increases mitral cell excitatory responses to weak but not strong olfactory nerve inputs in vitro via activation of al receptors. This is consistent with recent findings in vivo that synaptically released norepinephrine preferentially increases mitral cell excitatory responses to weak olfactory nerve inputs. Taken together, these results suggest that the release of norepinephrine in the olfactory bulb may increase the sensitivity of mitral cells to weak odors. Olfactory cues evoke norepinephrine release in the main olfactory bulb, and norepinephrine plays important roles in early olfactory learning and reproductive/maternal behaviors. By increasing mitral cell responses to olfactory nerve input, norepinephrine may play a critical role in modulating olfactory function, including formation and/or recall of specific olfactory memories.