Selective administration of nicotine into catecholaminergic regions of rat brainstem stimulates adrenocorticotropin secretion

Nicotine e-cigarette vapor inhalation and self-administration in a rodent model: Sex- and nicotine delivery-specific effects on metabolism and behavior

E-cigarettes, which deliver vaporized nicotine, have dramatically risen in popularity in recent years, despite many unanswered questions about safety, efficacy in reducing dependence, and overall impact on public health. Other factors, such as sex, also play an important role in determining behavioral and neurochemical responses to drugs of abuse. In these studies, we sought to develop a protocol for vaporized e-cigarette nicotine self-administration in rats, as a foundation to better understand the differing effects of nicotine exposure routes on behavior and physiological function. We report a novel method that elicits robust nicotine vapor self-administration in male and female rats. Our findings indicate that 5-mg/ml nicotine vape solution provides a high level of consistency in lever-pressing behavior for both males and females. Moreover, in male rats, we find that such e-cigarette nicotine vapor induces similar blood levels of nicotine's main metabolite, cotinine, as that found with intravenous nicotine self-administration. Therefore, the breathing pattern during vapor exposure in males leads to similar levels of titrated nicotine intake as with intravenous nicotine self-administration. Interestingly, a differential effect was found in the females, in which the same conditions of vapor exposure led to decreased cotinine levels with vapor compared to intravenous self-administration. Finally, differences in nicotine-mediated locomotion provide further support of the physiological effects of e-cigarette vapor inhalation. Taken together, our findings reveal important sex differences in nicotine intake based on the route of exposure, and we further establish a protocol for nicotine vapor self-administration in rats.

Selective decrease of cholinergic signaling from pedunculopontine and laterodorsal tegmental nuclei has little impact on cognition but markedly increases susceptibility to stress

The pedunculopontine tegmental nucleus (PPT) and laterodorsal tegmental nucleus (LDT) are heterogeneous brainstem structures that contain cholinergic, glutamatergic, and GABAergic neurons. PPT/LDT neurons are suggested to modulate both cognitive and noncognitive functions, yet the extent to which acetylcholine (ACh) signaling from the PPT/LDT is necessary for normal behavior remains uncertain. We addressed this issue by using a mouse model in which PPT/LDT cholinergic signaling is highly decreased by selective deletion of the vesicular ACh transporter (VAChT) gene. This approach interferes exclusively with ACh signaling, leaving signaling by other neurotransmitters from PPT/LDT cholinergic neurons intact and sparing other cells. VAChT mutants were examined on different PPT/LDT-associated cognitive domains. Interestingly, VAChT mutants showed no attentional deficits and only minor cognitive flexibility impairments while presenting large deficiencies in both spatial and cued Morris water maze (MWM) tasks. Conversely, working spatial memory determined with the Y-maze and spatial memory measured with the Barnes maze were not affected, suggesting that deficits in MWM were unrelated to spatial memory abnormalities. Supporting this interpretation, VAChT mutants exhibited alterations in anxiety-like behavior and increased corticosterone levels after exposure to the MWM, suggesting altered stress response. Thus, PPT/LDT VAChT-mutant mice present little cognitive impairment per se, yet they exhibit increased susceptibility to stress, which may lead to performance deficits in more stressful conditions.-Janickova, H., Kljakic, O., Rosborough, K., Raulic, S., Matovic, S., Gros, R., Saksida, L. M., Bussey, T. J., Inoue, W., Prado, V. F., Prado, M. A. M. Selective decrease of cholinergic signaling from pedunculopontine and laterodorsal tegmental nuclei has little impact on cognition but markedly increases susceptibility to stress.

Effects of acute and chronic nicotine on catecholamine neurons of the nucleus of the solitary tract

Nicotine is an addictive drug that has broad effects throughout the brain. One site of action is the nucleus of the solitary tract (NTS), where nicotine initiates a stress response and modulates cardiovascular and gastric function through nicotinic acetylcholine receptors (nAChRs). Catecholamine (CA) neurons in the NTS influence stress and gastric and cardiovascular reflexes, making them potential mediators of nicotine's effects; however nicotine's effect on these neurons is unknown. Here, we determined nicotine's actions on NTS-CA neurons by use of patch-clamp techniques in brain slices from transgenic mice expressing enhanced green fluorescent protein driven by the tyrosine hydroxylase promoter (TH-EGFP). Picospritzing nicotine both induced a direct inward current and increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in NTS-CA neurons, effects blocked by nonselective nAChR antagonists TMPH and MLA. The increase in sEPSC frequency was mimicked by nAChRα7 agonist AR-R17779 and blocked by nAChRα7 antagonist MG624. AR-R17779 also increased the firing of TH-EGFP neurons, an effect dependent on glutamate inputs, as it was blocked by the glutamate antagonist NBQX. In contrast, the nicotine-induced current was mimicked by nAChRα4β2 agonist RJR2403 and blocked by nAChRα4β2 antagonist DHβE. RJR2403 also increased the firing rate of TH-EGFP neurons independently of glutamate. Finally, both somatodendritic and sEPSC nicotine responses from NTS-CA neurons were larger in nicotine-dependent mice that had under gone spontaneous nicotine withdrawal. These results demonstrate that 1) nicotine activates NTS-CA neurons both directly, by inducing a direct current, and indirectly, by increasing glutamate inputs, and 2) NTS-CA nicotine responsiveness is altered during nicotine withdrawal.

Projection target-specific action of nicotine in the caudal nucleus of the solitary tract

The brainstem nucleus of the solitary tract (NTS) is the key integrating relay in the central processing of sensory information from the thoracic and from most subdiaphragmatic viscera. Modulation of neuronal excitability and synaptic activity in the NTS by nicotinic agents can have potent effects on vital physiological functions, such as feeding, digestion, respiration, and blood circulation. Caudal NTS neurons demonstrate considerable heterogeneity in projection targets, synaptic properties, and expression of nicotinic acetylcholine receptors (nAChRs). However, despite its heterogeneity, the caudal NTS may contain discrete subsets of neurons with unique projection target-specific properties. To test this hypothesis, we used in vivo fluorescent tracing and ex vivo patch-clamp electrophysiology to evaluate responsiveness to nicotine of anatomically identified caudal NTS neurons that project to the hypothalamic paraventricular nucleus (PVN) and the brainstem caudal ventrolateral medulla (CVLM). The results of this study demonstrate that responsiveness to nicotine correlates with where the neurons project. Specifically, PVN-projecting caudal NTS neurons respond to nicotine only presynaptically (i.e., via activation of presynaptic nAChRs and potentiation of synaptic release of glutamate), suggesting indirect, glutamate-dependent effects of nicotine on the PVN-projecting NTS circuitry. By contrast, CVLM-projecting caudal NTS neurons exhibit only limited presynaptic, but dominant somatodendritic, responsiveness to nicotine, suggesting that the effects of nicotine on the CVLM-projecting NTS circuitry are direct and largely glutamate independent. Understanding the relationships among function-specific brainstem/hypothalamic neuronal networks, nuclei, and individual neurons could help develop therapies targeting identifiable neuronal circuits to offset impaired autonomic homeostasis.

Elevated corticosterone in the dorsal hindbrain increases plasma norepinephrine and neuropeptide Y, and recruits a vasopressin response to stress

Repeated stress and chronically elevated glucocorticoids cause exaggerated cardiovascular responses to novel stress, elevations in baseline blood pressure, and increased risk for cardiovascular disease. We hypothesized that elevated corticosterone (Cort) within the dorsal hindbrain (DHB) would: 1) enhance arterial pressure and neuroendocrine responses to novel and repeated restraint stress, 2) increase c-Fos expression in regions of the brain involved in sympathetic stimulation during stress, and 3) recruit a vasopressin-mediated blood pressure response to acute stress. Small pellets made of 10% Cort were implanted on the surface of the DHB in male Sprague-Dawley rats. Blood pressure was measured by radiotelemetry. Cort concentration was increased in the DHB in Cort-treated compared with Sham-treated rats (60 ± 15 vs. 14 ± 2 ng Cort/g of tissue, P < 0.05). DHB Cort significantly increased the integrated arterial pressure response to 60 min of restraint stress on days 6, 13, and 14 following pellet implantation (e.g., 731 ± 170 vs. 1,204 ± 68 mmHg/60 min in Sham- vs. Cort-treated rats, day 6, P < 0.05). Cort also increased baseline blood pressure by day 15 (99 ± 2 vs. 108 ± 3 mmHg for Sham- vs. Cort-treated rats, P < 0.05) and elevated baseline plasma norepinephrine and neuropeptide Y concentrations. Cort significantly enhanced stress-induced c-Fos expression in vasopressin-expressing neurons in the paraventricular nucleus of the hypothalamus, and blockade of peripheral vasopressin V1 receptors attenuated the effect of DHB Cort to enhance the blood pressure response to restraint. These data indicate that glucocorticoids act within the DHB to produce some of the adverse cardiovascular consequences of chronic stress, in part, by a peripheral vasopressin-dependent mechanism.

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.

Hormones, nicotine, and cocaine: clinical studies

Nicotine and cocaine each stimulate hypothalamic-pituitary-adrenal and -gonadal axis hormones, and there is increasing evidence that the hormonal milieu may modulate the abuse-related effects of these drugs. This review summarizes some clinical studies of the acute effects of cigarette smoking or IV cocaine on plasma drug and hormone levels and subjective effects ratings. The temporal covariance between these dependent measures was assessed with a rapid (2 min) sampling procedure in nicotine-dependent volunteers or current cocaine users. Cigarette smoking and IV cocaine each stimulated a rapid increase in LH and ACTH, followed by gradual increases in cortisol and DHEA. Positive subjective effects ratings increased immediately after initiation of cigarette smoking or IV cocaine administration. However, in contrast to cocaine's sustained positive effects (<20 min), ratings of "high" and "rush" began to decrease within one or two puffs of a high-nicotine cigarette while nicotine levels were increasing. Peak nicotine levels increased progressively after each of three successive cigarettes smoked at 60 min intervals, but the magnitude of the subjective effects ratings and peak ACTH and cortisol levels diminished. Only DHEA increased consistently after successive cigarettes. The possible influence of neuroactive hormones on nicotine dependence and cocaine abuse and the implications for treatment of these addictive disorders are discussed.

Nicotine: alcohol reward interactions

It is well established that the continued intake of drugs of abuse is reinforcing-that is repeated consumption increases preference. This has been shown in some studies to extend to other drugs of abuse; use of one increases preference for another. In particular, the present review deals with the interaction of nicotine and alcohol as it has been shown that smoking is a risk factor for alcoholism and alcohol use is a risk factor to become a smoker. The review discusses changes in the brain caused by chronic nicotine and chronic alcohol intake to approach the possible mechanisms by which one drug increases the preference for another. Chronic nicotine administration was shown to affect nicotine receptors in the brain, affecting not only receptor levels and distribution, but also receptor subunit composition, thus affecting affinity to nicotine. Other receptor systems are also affected among others catecholamine, glutamate, GABA levels and opiate and cannabinoid receptors. In addition to receptor systems and transmitters, there are endocrine, metabolic and neuropeptide changes as well induced by nicotine. Similarly chronic alcohol intake results in changes in the brain, in multiple receptors, transmitters and peptides as discussed in this overview and also illustrated in the tables. The changes are sex and age-dependent-some changes in males are different from those in females and in general adolescents are more sensitive to drug effects than adults. Although nicotine and alcohol interact-not all the changes induced by the combined intake of both are additive-some are opposing. These opposing effects include those on locomotion, acetylcholine metabolism, nicotine binding, opiate peptides, glutamate transporters and endocannabinoid content among others. The two compounds lower the negative withdrawal symptoms of each other which may contribute to the increase in preference, but the mechanism by which preference increases-most likely consists of multiple components that are not clear at the present time. As the details of induced changes of nicotine and alcohol differ, it is likely that the mechanisms of increasing nicotine preference may not be identical to that of increasing alcohol preference. Stimulation of preference of yet other drugs may again be different -representing one aspect of drug specificity of reward mechanisms.

Chronic blockade of hindbrain glucocorticoid receptors reduces blood pressure responses to novel stress and attenuates adaptation to repeated stress

Exogenous glucocorticoids act within the hindbrain to enhance the arterial pressure response to acute novel stress. Here we tested the hypothesis that endogenous glucocorticoids act at hindbrain glucocorticoid receptors (GR) to augment cardiovascular responses to restraint stress in a model of stress hyperreactivity, the borderline hypertensive rat (BHR). A 3- to 4-mg pellet of the GR antagonist mifepristone (Mif) was implanted over the dorsal hindbrain (DHB) in Wistar-Kyoto (WKY) and BHRs. Control pellets consisted of either sham DHB or subcutaneous Mif pellets. Rats were either subjected to repeated restraint stress (chronic stress) or only handled (acute stress) for 3-4 wk, then all rats were stressed on the final day of the experiment. BHR showed limited adaptation of the arterial pressure response to restraint, and DHB Mif significantly (P Collapse

Key Words Collapse

MESH Headings

• Adaptation, Physiological/physiology
• Animals
• Blood Pressure/physiology
• Body Weight/physiology
• Corticosterone/blood
• Disease Models, Animal
• Heart Rate/physiology
• Hormone Antagonists/pharmacology
• Hypertension/metabolism
• Hypertension/physiopathology
• Male
• Mifepristone/pharmacology
• Rats
• Rats, Inbred WKY
• Rats, Sprague-Dawley
• Receptors, Glucocorticoid/antagonists & inhibitors
• Receptors, Glucocorticoid/drug effects
• Receptors, Glucocorticoid/metabolism
• Rhombencephalon/metabolism
• Stress, Physiological/physiology

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Grants

• R01-HL-076807 NHLBI NIH HHS

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Affiliation(s)

Andrea G Bechtold Department of Medical Pharmacology, School of Medicine, University of California Davis, Davis, California, USA
Gina Patel
Guenther Hochhaus
Deborah A Scheuer

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Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Effects of low- and high-nicotine cigarette smoking on mood states and the HPA axis in men

The acute effects of smoking a low- or high-nicotine cigarette on hypothalamic-pituitary-adrenal (HPA) hormones, subjective responses, and cardiovascular measures were studied in 20 healthy men who met American Psychiatric Association Diagnostic and Statistical Manual IV criteria for nicotine dependence. Within four puffs (or 2 min) after cigarette smoking began, plasma nicotine levels and heart rate increased significantly (P<0.01), and peak ratings of 'high' and 'rush' on a Visual Analogue Scale were reported. Reports of 'high', 'rush', and 'liking' and reduction of 'craving' were significantly greater after smoking a high-nicotine cigarette than a low-nicotine cigarette (P<0.05). Peak plasma nicotine levels after high-nicotine cigarette smoking (23.9+/-2.6 ng/ml) were significantly greater than after low-nicotine cigarette smoking (3.63+/-0.59 ng/ml) (P<0.001). After smoking a low-nicotine cigarette, adrenocorticotropin hormone (ACTH), cortisol, dehydroepiandrosterone (DHEA), and epinephrine did not change significantly from baseline. After high-nicotine cigarette smoking began, plasma ACTH levels increased significantly above baseline within 12 min and reached peak levels of 21.88+/-5.34 pmol/l within 20 min. ACTH increases were significantly correlated with increases in plasma nicotine (r=0.85; P<0.0001), DHEA (r=0.66; P=0.002), and epinephrine (r=0.86; P<0.0001). Cortisol and DHEA increased significantly within 20 min (P<0.05) and reached peak levels of 424+/-48 and 21.13+/-2.55 ng/ml within 60 and 30 min, respectively. Thus cigarette smoking produced nicotine dose-related effects on HPA hormones and subjective and cardiovascular measures. These data suggest that activation of the HPA axis may contribute to the abuse-related effects of cigarette smoking.

Extrahypothalamic corticotropin-releasing hormone mediates (-)-nicotine-induced elevation of plasma corticosterone in rats

(-)-Nicotine activates the hypothalamic-pituitary-adrenal axis via an activation of the brainstem catecholaminergic neurons in rats. The present study was undertaken to clarify the mechanisms involved in the (-)-nicotine-induced activation of brainstem catecholaminergic neurons in anesthetized rats. Physostigmine (a cholinesterase inhibitor) (0.31 and 0.77 micromol/animal, i.p.) dose-dependently elevated plasma corticosterone in the presence of scopolamine (a muscarinic receptor antagonist) (2.3 micromol/animal, i.p.). (-)-Nicotine (250 and 500 nmol/animal, i.c.v.) dose-dependently elevated plasma corticosterone with concomitant noradrenaline release in the hypothalamic paraventricular nucleus. The (-)-nicotine (500 nmol/animal, i.c.v.)-induced elevation of corticosterone was abolished by phentolamine (an alpha-adrenoceptor antagonist) (0.66 micromol/animal, i.c.v.), and attenuated by (+/-)-sotalol (a beta-adrenoceptor antagonist) (0.97 micromol/animal, i.c.v.). The (-)-nicotine-induced increases of plasma corticosterone and hypothalamic noradrenaline release were abolished either by hexamethonium (a nicotinic acetylcholine receptor antagonist) (1.8 micromol/animal, i.c.v.), CP-154,526 (butyl-ethyl-[2,5-dimethyl-7-(2,4,6-trimethylphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]amine) (a selective CRF-1 receptor antagonist) (1.3 micromol/animal, i.c.v.) or indomethacin (a cyclooxygenase inhibitor) (1.2 micromol/animal, i.c.v.). These results suggest that (-)-nicotine elevates plasma corticosterone by CRF-1 receptor- and prostaglandin-mediated noradrenaline release in the paraventricular nucleus in rats.

Norepinephrine secretion in the hypothalamic paraventricular nucleus of rats during unlimited access to self-administered nicotine: An in vivo microdialysis study

Norepinephrine (NE) secretion within the hypothalamic paraventricular nucleus (PVN) is pivotal to endocrine and behavioral responses. Activation of NE afferents to PVN also is necessary for the hypothalamo-pituitary-adrenal axis response to passively administered nicotine. The mode of drug delivery is a critical determinant of the dynamics of neurotransmitter secretion, yet the PVN NE response to nicotine self-administration (SA) is unknown. Herein, rats housed in operant chambers had unlimited 23 hr access to self-administered nicotine. In vivo microdialysis of PVN NE was performed, collecting consecutive 7 min samples over 9 hr sessions during three phases of nicotine SA: acquisition (day 1); early maintenance, once stable rates of SA were achieved (day 9.2 +/- 0.6); later maintenance (day 18.6 +/- 0.8). On d1, nicotine animals had an increased percentage of SA episodes (SAEs) in which NE levels were elevated (80 vs 30% with saline; p < 0.01). By early maintenance, a fourfold increase in such episodes was observed in nicotine animals (p < 0.01), and the overall NE level was greater (1.30 +/- 0.24 vs 0.63 +/- 0.07 pg/10 microl in saline; p < 0.05); NE increased during the first, but not the last, SAE. The pattern was similar during later maintenance, although NE responsiveness declined (overall NE level, 0.96 +/- 0.19 in nicotine vs 0.52 +/- 0.08 pg/10 microl in saline; p < 0.05). Therefore, nicotine SAEs were associated with sustained increases in NE secretion during all three phases of SA. However, the reduced NE responsiveness observed both within the dialysis session in each phase and by later versus early maintenance is consistent with progression of partial daily desensitization of PVN NE secretion to nicotine SA. Therefore, in rats chronically self-administering nicotine, the drug stimulates sustained PVN NE secretion that may alter neuroendocrine and behavioral responses mediated by the PVN. Compared with studies of chronic human smokers, our nicotine SA model may reflect the CNS noradrenergic responses that occur during human cigarette smoking.

Regulation of feeding-associated peptides and receptors by nicotine

Although numerous epidemiological studies have provided convincing evidence for the inverse association between tobacco smoking and body weight, the molecular mechanisms underlying this relationship are not well-understood. Nicotine, as a potent secretagogue, could be expected to influence the levels and expression of many classes of neurotransmitters, as well as of cell-membrane constituents linked to neurotransmission, including signal transducers and related effectors. A potentially major group of candidate molecules that could be involved in feeding-related actions of nicotine are the numerous neuropeptides and peptide hormones shown in the past two decades to regulate food intake and energy expenditure. These could include neuropeptide Y (NPY), orexins, leptins, and uncoupling proteins (UCPs). Some of these peptides were already shown to respond to nicotine treatment in terms of regulation of levels and of activity at the level of cell-membrane receptors. The primary objective of this review is to summarize our current understanding of the regulatory effects of nicotine on the food intake and energy expenditure as related to the expression levels of leptin, NPY, orexin, uncoupling proteins, and of NPY and orexin receptors.

Nicotinic activation of CRH neurons in extrahypothalamic regions of the rat brain

Nicotine is known to have multiple effects on neuroendocrine, autonomic, and behavioral responses. Its neuroendocrine effect on the stress-responsive hormone, ACTH, depends on central pathways that act on corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN). Other CRH neurons throughout the brain also are involved in coordinating aspects of the stress response, but very little is known about the effect of nicotine on CRH neurons in extrahypothalamic regions that are involved in the autonomic and behavioral responses to stress. The current study sought to determine the extent of nicotinic activation of extrahypothalamic CRH neurons, since these neurons may be involved in mediating the central effects of nicotine. Freely moving rats were pretreated with a low dose of colchicine, infused with nicotine (0.045 mg/kg/30 s or 0.135 mg/kg/90 s, i.v.), and cardiac perfused 1 h later. Double-label immunocytochemistry identified the activated (positive for cFos protein) CRH neurons in limbic structures (bed nucleus of the stria terminalis [BNST] and central nucleus of the amygdala [CNA]), the dorsal raphe (DR), and Barrington's nucleus (BN); comparisons were made to the PVN. In all of these areas, nicotine activated CRH neurons in a dose-dependent manner, showing differential sensitivity and efficacy with respect to region. CNA CRH neurons were most responsive and were maximally stimulated by the low dose of nicotine (62% of CRH neurons were cFos+, compared to 10-27% of the CRH population in other regions, including the PVN). Although the BNST also was activated by the low dose, only the non-CRH+ neurons were involved; in contrast, 41% of the BNST CRH neurons responded to the higher dose. Nicotinic activation of DR neurons was dose-dependent, with 22% of the CRH neurons activated by the high dose. Few BN neurons were activated by the low dose of nicotine, but 26% of the CRH population responded to the higher dose. These results indicate that the effect(s) of nicotine on the brain may be mediated, in part, by the selective activation of specific extrahypothalamic regions containing CRH neurons that also are involved in autonomic and behavioral responses to stress. The large fraction of CRH neurons responding to the low dose of nicotine in the CNA suggests that this limbic region may be particularly important in mediating these CNS effects of nicotine.

The locus coeruleus noradrenergic system in the rat brain studied by dual-probe microdialysis

A dual-probe microdialysis technique was applied to the locus coeruleus (LC) and prefrontal cortex (PFC) of the brain of conscious rats. One probe was implanted close to the LC and was used to apply receptor-specific compounds by retrograde microdialysis. The effects of the LC infusions were recorded by a sampling noradrenaline by a second probe that was implanted in the ipsilateral prefrontal cortex. Infusion of sodium channel blocker tetrodotoxin (1 microM; 90 min) into the LC decreased extracellular noradrenaline in the PFC to approximately 20% of control values. Infusion of alpha2-adrenoceptor agonist clonidine (100 microM, infused during 15 or 45 min) near to the LC, decreased extracellular noradrenaline in the PFC to 35 and 20% of controls, respectively. These results indicate that > 80% of the extracellular levels of noradrenaline in the PFC is derived from LC innervation, and confirms the importance of alpha2-autoreceptors on noradrenergic neurons in the LC. Infusion of the cholinergic receptor agonist, carbachol (100 microM, 45 min) near to the LC increased extracellular noradrenaline in the PFC to approximately 150% of controls. Infusions of the excitatory amino-acid agonists NMDA and kainate into the LC caused marked increases in extracellular noradrenaline in the PFC to 240 and 200% of controls, respectively. The experiments with clonidine, carbachol, NMDA and kainate were repeated in anesthetized rats. Clonidine and carbachol were similarly effective as in conscious animals but the effects of NMDA and kainate on extracellular noradrenaline in the PFC were clearly suppressed: 145 and 130% of controls, respectively. These results suggest that increased arousal or behavioural activation might have contributed to the increases in extracellular noradrenaline that was seen after infusion of the glutamate agonists. These results also provide evidence for localization of cholinergic-, NMDA-, non-NMDA-receptor on noradrenergic neurons in the LC. Finally it is concluded that dual-probe microdialysis is a useful method to further investigate the pharmacology of LC-noradrenergic neurons. Carbachol and clonidine are suitable tools for a rapid and reversible stimulation or inhibition, respectively, of noradrenergic LC neurons.

Nicotine activates NPY and catecholaminergic neurons in brainstem regions involved in ACTH secretion

Nicotine rapidly and potently stimulates ACTH secretion via a centrally mediated mechanism. The purpose of the current study was to identify the phenotype of nicotine-sensitive neurons in brainstem catecholaminergic regions previously shown to be responsive to nicotine. Immunocytochemical double-labeling was used to detect c-Fos expression in neurons positive for activin, galanin, or neuropeptide Y (NPY), in comparison to those containing tyrosine hydroxylase (TH, catecholaminergic biosynthetic enzyme). These neuropeptides were chosen because (1) each is located in nicotine-sensitive brainstem regions, (2) neurons containing each of these peptides project to the hypothalamic paraventricular nucleus, and (3) each has been shown to affect ACTH secretion. Freely moving, adult, male rats received an intravenous (i.v.) infusion of saline or nicotine (0.045 mg/kg over 30 s or 0.135 mg/kg over 90 s) and were cardiac perfused 60 min thereafter. Nicotine significantly increased c-Fos expression in a dose-dependent manner in the brainstem regions examined. In nucleus tractus solitarius (NTS)-A2 and NTS-C2, both NPY+ and TH+ neurons responded to the lower dose of nicotine, whereas the activin and galanin neurons in these regions were unresponsive to either dose of nicotine. In contrast, the higher dose of nicotine was required to activate NPY+ neurons in the A1 region and both NPY+ and galanin+ neurons in the locus coeruleus; the C1 region was unresponsive to nicotine. Since plasma ACTH is elevated by the low dose of nicotine and only NTS neurons are activated by this dose, NPY projections from the NTS are likely to contribute to nicotine-stimulated ACTH secretion, in addition to the previously described catecholaminergic neurons.

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.

Nicotine and stress: effect on sex hormones and lipid profile in female rats

This work aimed to study the changes in sex hormones and lipid profile in adult female albino rats subjected to treatment with nicotine (N), immobilization stress (S), or their combinations (N+S). These treatments were applied either for one day (T1) or daily for 10 days (T10), after which rats in the estrus stage were used for the determination of plasma corticosterone (CS), serum sex hormones as progesterone (P), estrogen (E), FSH, LH and serum lipid profile including total cholesterol (TC), HDL-C, LDL-C, triacylglycerol (TG) and non esterified fatty acids (NEFA). It was clear that either N or S raised plasma CS and serum P levels in both the treatment regimens and that N+S induced a higher level of these hormones compared to each treatment alone. Serum E level was only elevated during T10 regimen only. An increase in serum LH level was only observed after a single exposure to either N or S, however their combination abolished the stimulatory effect induced by each treatment alone. Serum FSH was not altered by exposure to either N or S alone in both regimens, but in the T10 regimen their combination significantly lowered FSH level. Regarding the effect on serum lipid profile, serum TC was increased in all T10 regimen groups. LDL-C was increased by N+S treatment in both regimens, however no change in HDL-C level was observed in all groups. Serum NEFA was increased in all the treated groups during T10 regimen, while in the T1 regimen NEFA level was only elevated by the combination N+S. Serum TG was insignificantly altered in all the treated groups. The observed changes in the lipid pattern were attributed to the alterations occurred in CS and female sex hormones that caused by N, S or their combinations.