Distribution of [3H]quinuclidinyl benzilate, [3H]nicotine, and [125I]alpha-bungarotoxin binding sites in the nucleus tractus solitarii of the cat

Tobacco addiction and the dysregulation of brain stress systems

Tobacco is a highly addictive drug and is one of the most widely abused drugs in the world. The first part of this review explores the role of stressors and stress-associated psychiatric disorders in the initiation of smoking, the maintenance of smoking, and relapse after a period of abstinence. The reviewed studies indicate that stressors facilitate the initiation of smoking, decrease the motivation to quit, and increase the risk for relapse. Furthermore, people with depression or an anxiety disorder are more likely to smoke than people without these disorders. The second part of this review describes animal studies that investigated the role of brain stress systems in nicotine addiction. These studies indicate that corticotropin-releasing factor, Neuropeptide Y, the hypocretins, and norepinephrine play a pivotal role in nicotine addiction. In conclusion, the reviewed studies indicate that smoking briefly decreases subjective stress levels but also leads to a further dysregulation of brain stress systems. Drugs that decrease the activity of brain stress systems may diminish nicotine withdrawal and improve smoking cessation rates.

Heterogeneity of nicotinic acetylcholine receptor expression in the caudal nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is the principal integrating relay in the processing of visceral sensory and gustatory information. In the present study, patch-clamp electrophysiological experiments were conducted using rat horizontal brainstem sections. Pre-synaptic and somatic/dendritic nicotinic acetylcholine receptors (nAChRs) expressed in neurons of the caudal NTS (cNTS) were found to be randomly distributed between pre-synaptic and somatic/dendritic sites (chi(2)=0.72, df=3, p>0.87, n=200). Pre-synaptic nAChRs were detected by their facilitating effects on glutamatergic neurotransmission of a sub-population of cNTS neurons (categorized as "effect-positive") upon brief picospritzer applications of 0.1-0.5mM nicotine. These effects were resistant to inhibition by 20nM methyllycaconitine (MLA) and 4muM dihydro-beta-erythroidine (DHbetaE), and were replicated by brief picospritzer applications of 0.2-1mM cytisine. Picospritzer applications of 0.2mM RJR-2403, a potent agonist of alpha4beta2 nAChRs, did not facilitate synaptic release of glutamate in effect-positive cNTS neurons. The population of somatic/dendritic nAChRs has been found to be heterogeneous and included nAChRs that were activated by RJR-2403 and/or cytisine, or insensitive to cytisine, or inhibited by MLA. The presented results are consistent with the expression of beta4-containing (i.e., beta4*) nAChRs, likely alpha3beta4*, in pre-synaptic terminals of effect-positive cNTS neurons. Somatic/dendritic nAChRs appear to involve both alpha7 and non-alpha7 subunits. Heterogeneity in the subunit composition of pre-synaptic and somatic/dendritic nAChRs may underlie diverse roles that these receptors play in regulation of behavioral and visceral reflexes, and may reflect specific targeting by endogenous nicotinic agents and nicotine.

Cholinergic modulation of neurons in the gustatory region of the nucleus of the solitary tract

The rostral portion of the nucleus of the solitary tract (rNST) is an obligatory relay for gustatory afferent input on its way to the forebrain. Previous studies have demonstrated excitation of rNTS neurons by glutamate and substance P and inhibition by gamma-aminobutyric acid (GABA) and met-enkephalin (ENK). Despite the existence of cholinergic neurons and putative terminals within the rNTS, there are no data on the effects of acetylcholine (ACh) on rNTS processing. Here, we use patch-clamp recording of rNTS neurons in vitro to examine ACh-mediated responses and voltage-gated conductances in these cells. Results revealed (1) intrinsic voltage-gated inhibition via activation of voltage-gated potassium A-channels (I(A)), found almost exclusively in the medial rNTS, and hyperpolarization-activated potassium/sodium channels (I(h)), found more frequently in the lateral rNST; and (2) ligand-gated inhibition via activation of muscarinic m2 ACh receptors (mAChRs) linked to inward rectifier potassium channels (K(ir)) evenly distributed throughout the rNTS, a mechanism dependent on cholinergic inputs. Muscarinic responses were blocked by AFDX-116, a selective m2 mAChR antagonist, and by BaCl2, an antagonist of K(ir) channels. In addition, many rNTS neurons exhibited excitation via alpha7 and non-alpha7 nicotinic AChRs. Non-alpha7 nAChRs, blocked by 10 microM mecamylamine, occurred more frequently in the lateral rNTS. In contrast, alpha7 nAChRs, blocked by 20 nM methyllcaconitine, were evenly distributed across the nucleus. As previously reported for voltage-activated conductances, none of these currents was related to neuronal morphology. These voltage- and ligand-dependent inhibitory mechanisms would be expected to contribute to the modulation of gustatory processing through the NST.

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST). To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.

Receptor subtypes mediating depressor responses to microinjections of nicotine into medial NTS of the rat

Microinjections (50 nl) of nicotine (0.01-10 microM) into the nucleus of the solitary tract (NTS) of adult, urethan-anesthetized, artificially ventilated, male Wistar rats, elicited decreases in blood pressure and heart rate. Prior microinjections of alpha-bungarotoxin (alpha-BT) and alpha-conotoxin ImI (specific toxins for nicotinic receptors containing alpha7 subunits) elicited a 20-38% reduction in nicotine responses. Similarly, prior microinjections of hexamethonium, mecamylamine, and alpha-conotoxin AuIB (specific blockers or toxin for nicotinic receptors containing alpha3beta4 subunits) elicited a 47-79% reduction in nicotine responses. Nicotine responses were completely blocked by prior sequential microinjections of alpha-BT and mecamylamine into the NTS. Complete blockade of excitatory amino acid receptors (EAARs) in the NTS did not attenuate the responses to nicotine. It was concluded that 1) the predominant type of nicotinic receptor in the NTS contains alpha3beta4 subunits, 2) a smaller proportion contains alpha7 subunits, 3) the presynaptic nicotinic receptors in the NTS do not contribute to nicotine-induced responses, and 4) EAARs in the NTS are not involved in mediating responses to nicotine.

Cholinergic systems in the nucleus of the solitary tract of rats

The pharmacological and physiological properties of excitatory amino acid and ACh systems in the nucleus of the solitary tract (NTS) were studied in slices of rat brain stem by extracellular and intracellular recordings from neurons activated by solitary tract (ST) stimulation. These neurons were characterized as having several long dendrites with multiple varicosities. Synaptic activation of the medial NTS (mNTS) neurons by ST stimulation was mediated by non-N-methyl-D-aspartate (NMDA) glutamate (Glu) receptors, because the excitation was blocked by 6-cyano-7-nitro-quinoxaline-2,3-dione but not by NMDA, nicotinic, or muscarinic antagonists. Identified mNTS neurons were excited by iontophoresis of both Glu and ACh. The most sensitive region of the cell was on the dendrites approximately 100 micrometer from the cell body for both putative neurotransmitters. Nicotinic and/or muscarinic excitatory ACh responses were detected on the mNTS neurons. Our observations suggest that both types of ACh receptors may contribute to the attenuation of the baroreceptor reflex, but the functional correlation of this receptor profile remains to be determined.

Nicotinic acetylcholine receptors in the rat and primate nucleus tractus solitarius and on rat and human inferior vagal (nodose) ganglia: evidence from in vivo microdialysis and [125I]alpha-bungarotoxin autoradiography

The nucleus tractus solitarius is a key brain centre involved in the regulation of numerous autonomic functions. The present study has employed in vitro autoradiography and in vivo microdialysis to investigate the presence and function of nicotinic acetylcholine receptors located in the medial nucleus tractus solitarius of the rat. Autoradiography using [125I]alpha-bungarotoxin (0.5 nM) enabled visualization of binding sites on sections of rat and monkey brainstem. Specific binding was highest in the medial nucleus tractus solitarius. The presence of binding sites was also apparent on sections of rat nodose ganglia/vagus nerve and human inferior vagal ganglia. Subsequent to unilateral ligation of the vagus nerve in the rat, an accumulation of binding sites was visualized adjacent to the ligature. Unilateral nodose ganglionectomy in the rat caused an approximate 97% reduction in [125I]alpha-bungarotoxin binding site density in the medial nucleus tractus solitarius from 814 +/- 183 to 27 +/- 2 d.p.m./mm2. Microdialysis results indicated that local administration of nicotine (1 mM) into the nucleus tractus solitarius of the rat resulted in increases of extracellular L-glutamate of 146 +/- 9% of basal levels. This effect was not reproducible following a second stimulation and was also blocked by prior and co-administration of the nicotinic acetylcholine receptor antagonist mecamylamine (100 microM). Extracellular levels of L-aspartate exhibited a similar pattern although results were not significant. Taken together, these results are supportive of the presence of a population of [125I]alpha-bungarotoxin binding sites located presynaptically with respect to vagal afferent terminals in the medial nucleus tractus solitarius of the rat. It is possible that these binding sites are the site of action of locally administered nicotine on extracellular levels of L-glutamate, the favoured neurotransmitter at primary baroreceptor afferent fibres. These data are discussed in relation to the functional pharmacology of acetylcholine and nicotinic acetylcholine receptors in this region of the brain.

Response of the hypothalamo-pituitary-adrenal axis to nicotine

Nicotine has been shown to be a potent stimulus for the secretion of the stress-responsive hormones, adrenocorticotropin (ACTH) and prolactin. This paper reviews the findings by our laboratory and others that demonstrate the polysynaptic pathways involved in the neuroendocrine responses to systemic nicotine. It will focus primarily on the hypothalamo-pituitary-adrenal (HPA) axis and the effect of nicotine on ACTH secretion, with supplementary information on prolactin secretion, where relevant. Data are presented demonstrating that nicotine acts via a central mechanism to stimulate indirectly the release of ACTH from the anterior pituitary corticotropes. Nicotine does not appear to act directly at the hypothalamic paraventricular nucleus (PVN), the site of the corticotropin-releasing hormone (CRH) neurons crucial to the regulation of ACTH. However, brainstem catecholaminergic regions projecting to the PVN showed a regionally selective and dose-dependent sensitivity to nicotine, particularly the noradrenergic/adrenergic nucleus tractus solitarius (NTS). A reduction in the modulatory effect of these catecholamines (by neurotoxic lesion, synthetic enzyme inhibitors or adrenergic receptor antagonists) resulted in an inhibition of nicotine-stimulated ACTH secretion. In addition, blockade of nicotinic cholinergic receptors (NAchRs) in the brainstem by the antagonist, mecamylamine, resulted in a dose-dependent reduction in norepinephrine (NE) release from terminals in the PVN, and a concomitant reduction in plasma ACTH. The differential sensitivity of these receptors to the nicotinic agonists, cytisine and nicotine, reflects the heterogeneity of the NAchR subtypes involved. The desensitization characteristics of the neuroendocrine responses to both acute and chronic nicotine exposure are indicative of an alteration in these NAchRs.

Effect of antagonism at central nervous system M3 muscarinic receptors on laryngeal chemoresponse

The laryngeal chemoresponse (LCR), comprising laryngeal adductor spasm, central apnea, and subsequent cardiovascular instability, is thought to be a factor in sudden infant death syndrome. A muscarinic subtype receptor, M3, appears to be involved in central respiratory drive and control. Both the duration of the LCR apnea and levels of M3 receptor messenger RNA in the brain stem change according to postnatal age. This study examined the effect of central nervous system antagonism at M3 receptors on the LCR with respect to animal age and dose of antagonist. Ten piglets in each of three age groups (group 1, 5 to 8 days; group 2, 18 to 21 days; and group 3, 40 to 43 days) received a series of four increasing doses of an M3 antagonist (p-fluoro-hexahydro-sila-diphenidol) by intracerebral ventricle injection. The LCR was evoked at baseline and after each dose of antagonist. An effect on susceptible animals (groups 1 and 2) was evident by the second antagonist dose, and persisted for the remainder of the experiment (2 hours). At completion of the experiment, mean apnea duration had decreased in group 1 (61%, p < .05), and group 2 (57%, p < .05), but was unchanged in group 3 (<10%, p not significant). Length of mean baseline apneas correlated directly with degree of apnea shortening. The reduction is not attributable to changes in arterial PO2 or PCO2 or baseline respiratory rate. These results support an age-related influence on the LCR by M3 receptors in younger animals that decreases with maturation.

Neuronal responses to systemic nicotine in the solitary tract nucleus: origin and possible relation with nutritional effects of nicotine

Single-unit activity was recorded extracellularly in the caudal part of the solitary tract nucleus of anesthetized rats. Of 60 recorded neurons, 44 (73.3%) responded to intravenous (I.V.) nicotine. The incidence of response was significantly greater in the cells sensitive to moderate changes in blood glucose level, suggesting that the effects of nicotine on food intake and body weight are partly mediated by the glycemia-sensitive neurons in the caudal nucleus tractus solitarius. Only one-fourth of the neurons affected by I.V. nicotine responded in the same direction to iontophoretic nicotine application, suggesting that sensitivity to systemic nicotine results mainly from an indirect mechanism. Based on the observed effects of nicotinic agonists and antagonists unable to cross the blood-brain barrier, a majority of indirect unit responses to I.V. nicotine might be mediated by peripheral receptors, while the remaining ones might involve central or both central and peripheral receptors.

Adrenocorticotropin response and nicotine-induced norepinephrine secretion in the rat paraventricular nucleus are mediated through brainstem receptors

Nicotine is a potent stimulus for the secretion of ACTH, and norepinephrinergic neurons originating in the brainstem are involved. Prior reports using in vivo microdialysis in alert rats have shown that nicotine, administered i.p. or into the fourth ventricle, stimulated the release of norepinephrine (NE) into the hypothalamic paraventricular nucleus (PVN), the site of neurons containing CRH. In the present studies, rats received an i.v. infusion of nicotine into the jugular vein on alternate days during their active (dark) phase; therefore, direct correlations between the levels of NE microdialyzed from the PVN and plasma ACTH could be made in each animal. Nicotine administered i.v. (0.045-0.135 mg/kg) elicited dose-dependent increases in both NE and ACTH (P < 0.01). A significant correlation was found between nicotine-stimulated NE release in the PVN and ACTH secretion (r = 0.91, P < 0.01). To address whether the site(s) of action of nicotine was on presynaptic receptors on NE terminals in the PVN or on receptors on neurons in brainstem regions accessible from the fourth ventricle, the nicotinic cholinergic antagonist, mecamylamine (0.1-4.8 microg), was microinjected directly into the PVN or into the fourth ventricle before nicotine infusion. Fourth-ventricular administration of mecamylamine (1.6 microg) or higher, before i.v. nicotine (0.09 mg/kg), completely blocked both NE release in the PVN (IC50 = 0.64 microg) and ACTH secretion (IC50 = 0.40 microg) (P < 0.01, compared with vehicle before nicotine), whereas it was ineffective when injected directly into the PVN. The results demonstrate that the nicotinic cholinergic receptors in the brainstem, rather than presynaptic receptors within the PVN itself, mediate nicotine-stimulated PVN NE release and ACTH secretion.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Characterization of muscarinic receptors in the rat nucleus tractus solitarius

Crude membrane preparations from the dorsomedial medulla and whole heart of adult rats were used to characterize muscarinic receptors (mChRs) in the nucleus tractus solitarius (NTS). Isotherm binding assays with [3H]quinuclidinyl benzilate ([3H]QNB) indicated a uniform population of muscarinic cholinergic receptors in both tissues with a Kd of 60.3 pM and a Bmax of 401 fmol/mg protein for NTS membranes while heart membranes had a Kd of 56.6 pM and a Bmax of 193 fmol/mg protein. Competitive binding assays with selective muscarinic antagonists on NTS membranes using 80 pM [3H]QNB showed the following relative Ki values consistent with M2 mChRs: atropine, 1.9 nM; methoctramine, 7.7 nM; 4-diphenylacetoxy-N-methylpiperidine methobromide, 9.4 nM; AF-DX 116, 87.6 nM; and pirenzepine, 657 nM. Similarly, two agonists, carbachol and oxotremorine, showed Ki values of 1.82 microM and 0.379 microM, respectively. All of the competitive binding assays exhibited Hill coefficients close to 1.0 and InPlot analysis showed a single binding site for each agonist and antagonist examined.