Modulation of dopamine release by the nicotinic agonist epibatidine in the frontal cortex and the nucleus accumbens of naive and chronic nicotine treated rats

The metabotropic glutamate 2/3 receptor agonist LY379268 blocked nicotine-induced increases in nucleus accumbens shell dopamine only in the presence of a nicotine-associated context in rats

The metabotropic glutamate 2/3 (mGlu2/3) receptor agonist LY379268 ([-]-2-oxa-4-aminobicyclo [3.1.0] hexane-4,6-dicarboxylate) attenuates both nicotine self-administration and cue-induced nicotine seeking in rats. In this study, the effects of LY379268 (1 mg/kg) or saline pretreatment on nicotine-induced increases in nucleus accumbens (NAcc) shell dopamine were evaluated using in vivo microdialysis under different experimental conditions: (i) nicotine (0.4 mg/kg, base) was experimenter-administered subcutaneously to nicotine-naïve rats; (ii) nicotine was experimenter-administered either subcutaneously (0.4 mg/kg) or by a single experimenter-administered infusion (0.06 mg/kg, base) in rats with a history of nicotine self-administration (nicotine experienced) in the absence of a nicotine-associated context (ie, context and cues associated with nicotine self-administration); (iii) nicotine (0.06 mg/kg) was self-administered or experimenter-administered in nicotine-experienced rats in the presence of a nicotine-associated context. In saline-pretreated nicotine-naïve and nicotine-experienced rats, nicotine increased NAcc shell dopamine regardless of the context used for testing. Interestingly, LY379268 pretreatment blocked nicotine-induced increases in NAcc shell dopamine in nicotine-experienced rats only when tested in the presence of a nicotine-associated context. LY379268 did not block nicotine-induced increases in NAcc shell dopamine in nicotine-naïve or -experienced rats tested in the absence of a nicotine-associated context. These intriguing findings suggest that activation of mGlu2/3 receptors negatively modulates the combined effects of nicotine and nicotine-associated contexts/cues on NAcc dopamine. Thus, these data highlight a critical role for mGlu2/3 receptors in context/cue-induced drug-seeking behavior and suggest a neurochemical mechanism by which mGlu2/3 receptor agonists may promote smoking cessation by preventing relapse induced by the combination of nicotine and nicotine-associated contexts and cues.

Drug discrimination in methamphetamine-trained rats: effects of cholinergic nicotinic compounds

Accumulating evidence suggests that acetylcholine nicotinic systems may contribute importantly to the abuse-related effects of d-methamphetamine (d-MA). The present study was conducted to compare the effects of indirect dopamine (DA) agonists (d-amphetamine, d-MA, and l-methamphetamine), full [(-)-nicotine, anabaseine, (+)-epibatidine, (-)-epibatidine, isoarecolone] and partial (varenicline) nicotinic agonists, and other cholinergic compounds (mecamylamine, dihydro-β-erythroidine hydrobromide, methyllycaconitine, atropine, scopolamine, rivastigmine, and donepezil) in rats trained to discriminate 0.3 mg/kg i.p. d-MA from saline. All indirect DA agonists fully substituted for d-MA in a dose-related manner. Among nicotinic agonists, only (-)-nicotine fully substituted for d-MA in a dose-dependent manner, whereas all other nicotinic agonists and, to a limited extent, muscarinic antagonists produced partial d-MA-like responding. Other cholinergic compounds failed to produce d-MA-like discriminative stimulus effects. In drug interaction studies, varenicline served to dose-dependently attenuate the d-MA-like effects of (-)-nicotine, whereas mecamylamine, but not varenicline, reduced the discriminative stimulus effects of the training dose of d-MA. Differences between (-)-nicotine and other nicotinic agonists may be related to their ability to activate the DA system. These results provide further evidence that nicotinic mechanisms may be useful neurochemical targets for the development of therapeutics for the management of monoaminergic stimulant abuse and addiction.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.

Impaired spatial working memory and altered choline acetyltransferase (CHAT) immunoreactivity and nicotinic receptor binding in rats exposed to intermittent hypoxia during sleep

Exposure to intermittent hypoxia (IH), such as occurs in sleep-disordered breathing (SDB), is associated with cognitive impairment, neurodegeneration, oxidative stress, and inflammatory responses within rodent brain regions such as the basal forebrain. In this region, damage to cholinergic neurons correlates with working memory deficits in a number of neurodegenerative disorders, suggesting that degeneration of cholinergic systems may also contribute to the working memory impairments observed after IH exposures. We therefore examined basal forebrain choline acetyltransferase (CHAT) immunohistochemistry, nicotinic receptor binding in the prefrontal cortex (PFC), and working memory, in male rats tested on a delayed matching to place (DMP) task in the water maze following exposure to either room air (RA) or intermittent hypoxia (IH; alternating 90s epochs of 21% and 10% O(2) during sleep). IH-treated animals displayed impaired working memory with respect to controls, along with significant reductions in CHAT-stained neurons in the medial septal nucleus, in both the vertical and horizontal limbs of the diagonal band, and the substantia inominata after 14 days of IH exposure. In addition, increases in nicotinic binding and receptor affinity in the PFC were observed after 14 days of IH exposure. Thus, a loss of cholinergic neuronal phenotype in the basal forebrain may contribute to the cognitive impairments associated with CIH exposure. However, compensatory mechanisms may also be activated in other brain regions, and may provide potential therapeutic targets for the cognitive impairments associated with SDB.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidine's stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinson's disease and schizophrenia.

The effects of endomorphins and diprotin A on striatal dopamine release induced by electrical stimulation—An in vitro superfusion study in rats

The endomorphins (EM1: Tyr-Pro-Trp-Phe-NH2, and EM2: Tyr-Pro-Phe-Phe-NH2) are recently discovered endogenous ligands for mu-opioid receptors (MORs) with role of neurotransmitters or neuromodulators in mammals. Cessation of their physiological action may be effected through rapid enzymatic degradation by the dipeptidyl-peptidase IV (DPPIV) found in the brain synaptic membranes. An in vitro superfusion system was utilized to investigate the actions of EM1, EM2 and specific DPPIV inhibitor diprotin A on the striatal release of dopamine (DA) induced by electrical stimulation in rats. The involvement of the different MORs (MOR1 and MOR2) in this process was studied by pretreatment with MOR antagonists beta-funaltrexamine (a MOR1 and MOR2 antagonist) and naloxonazine (a MOR1 antagonist). EM1 significantly increased the tritium-labelled dopamine DA release induced by electrical stimulation. EM2 was effective only when the slices were pretreated with diprotin A. beta-Funaltrexamine antagonized the stimulatory effects of both EM1 and EM2. The administration of naloxonazine did not appreciably influence the action of EM1, but blocked the action of EM2, at least when the slices were pretreated with diprotin A. These data suggest that both EM1 and EM2 increase DA release from the striatum and, though diprotin A does not affect the action of EM1, it inhibits the enzymatic degradation of EM2. The DA-stimulating action induced by EM1 seems to be mediated by MOR2, while that evoked by EM2 appears to be transmitted by MOR1.

Dopamine release in human neocortical slices: Characterization of inhibitory autoreceptors and of nicotinic acetylcholine receptor-evoked release

The autoinhibitory control of electrically evoked release of [3H]-dopamine and the properties of that induced by nicotinic receptor (nAChR) stimulation were studied in slices of the human neocortex. In both models [3H]-dopamine release was action potential-induced and exocytotic. The selective dopamine D2 receptor agonist (-)-quinpirole reduced electrically evoked release of [3H]-dopamine, yielding IC50 and I(max) values of 23 nM and 76%, respectively. Also, the effects of several other subtype-selective dopamine receptor ligands confirmed that the terminal dopamine autoreceptor belongs to the D2 subtype. The autoinhibitory feedback control was slightly operative under stimulation conditions of 90 pulses and 3 Hz, with a biophase concentration of endogenous dopamine of 3.6 nM, and was enhanced under blockade of dopamine reuptake. [3H]-dopamine release evoked in an identical manner in mouse neocortical slices was not inhibited by (-)-quinpirole, suggesting the absence of dopamine autoreceptors in this tissue and underlining an important species difference. Also, nAChR stimulation-induced release of [3H]-dopamine revealed a species difference: [3H]-dopamine release was evoked in human, but not in rat neocortical slices. The nAChRs inducing [3H]-dopamine release most probably belong to the alpha3/beta2subtype, according to the potencies and efficacies of subtype-selective nAChR ligands. Part of these receptors may be located on glutamatergic neurons.

Nicotine and epibatidine alter differently nomifensine-elevated dopamine output in the rat dorsal and ventral striatum

We studied the effects of nicotine and epibatidine given in combination with dopamine uptake inhibitor, nomifensine, on striatal extracellular dopamine and its metabolites by using brain microdialysis in freely moving rats. Nomifensine (3 mg/kg) elevated extracellular dopamine in the caudate-putamen, and clearly more in the nucleus accumbens. In the caudate-putamen, nicotine (0.5 mg/kg) and epibatidine (0.6 microg/kg but not 3.0 microg/kg) enhanced nomifensine's effect on dopamine. The effect of nomifensine on accumbal dopamine was enhanced by nicotine, but inhibited by epibatidine at 0.6 microg/kg. The larger dose of epibatidine had no effect. Thus, the effects of the smaller epibatidine dose (0.6 microg/kg) on the dopamine output in the caudate-putamen but not in the accumbens resemble those of nicotine 0.5 mg/kg. Discrepancies in the effects of epibatidine and nicotine are most probably due to differences in their affinities to nicotinic receptor subtypes regulating dopamine release. Further, different responses to low concentrations of epibatidine between the brain areas suggest that there are differences in the nicotinic regulation of nigrostriatal and mesolimbic dopaminergic pathways.

Microdialysis—theoretical background and recent implementation in applied life-sciences

In the past decade microdialysis has become a method of choice in the study of unbound tissue concentrations of both endogenous and exogenous substances. Microdialysis has been shown to offer information about substances directly at the site of action while being well tolerable and safe. The large variety of its field of application has been demonstrated. However, a few challenges have to be met to make this method generally applicable in routine applications. This review will provide an overview over theoretical aspects that have to be considered during the implementation of microdialysis. Moreover, a comparison between microdialysis and other tissue sampling techniques will demonstrate advantages and limitations of the methods mentioned. Subsequently, it will present a critical synopsis of a variety of scientific/biomedical applications of this method with emphasis on the most recent literature, focussing on target tissues while giving examples of substances examined. It is concluded that microdialysis will be of great value in future investigations of pharmacokinetics, pharmacodynamics and in monitoring of disease status and progression.