Nicotinic acetylcholine receptors in the mesolimbic pathway: primary role of ventral tegmental area alpha6beta2* receptors in mediating systemic nicotine effects on dopamine release, locomotion, and reinforcement.

alpha6* nicotinic acetylcholine receptors (nAChRs) are highly and selectively expressed by mesostriatal dopamine (DA) neurons. These neurons are thought to mediate several behavioral effects of nicotine, including locomotion, habit learning, and reinforcement. Yet the functional role of alpha6* nAChRs in midbrain DA neurons is mostly unknown. The aim of this study was to determine the composition and in vivo functional role of alpha6* nAChR in mesolimbic DA neurons of male rats. Immunoprecipitation and immunopurification techniques coupled with cell-specific lesions showed that the composition of alpha6* nAChR in the mesostriatal system is heterogeneous, with (non-alpha4)alpha6beta2* being predominant in the mesolimbic pathway and alpha4alpha6beta2* in the nigrostriatal pathway. We verified whether alpha6* receptors mediate the systemic effects of nicotine on the mesolimbic DA pathway by perfusing the selective antagonists alpha-conotoxin MII (CntxMII) (alpha3/alpha6beta2* selective) or alpha-conotoxin PIA (CntxPIA) (alpha6beta2* selective) into ventral tegmental area (VTA). The intra-VTA perfusion of CntxMII or CntxPIA markedly decreased systemic nicotine-elicited DA release in the nucleus accumbens and habituated locomotion; the intra-VTA perfusion of CntxMII also decreased the rate of nicotine infusion in the maintenance phase of nicotine, but not of food, self-administration. Overall, the results of these experiments show that the alpha6beta2* nAChRs expressed in the VTA are necessary for the effects of systemic nicotine on DA neuron activity and DA-dependent behaviors such as locomotion and reinforcement, and suggest that alpha6beta2*-selective compounds capable of crossing the blood-brain barrier may affect the addictive properties of nicotine and therefore be useful in the treatment of tobacco dependence.

Localized delivery of fibroblast growth factor-2 and brain-derived neurotrophic factor reduces spontaneous seizures in an epilepsy model.

A loss of neurons is observed in the hippocampus of many patients with epilepsies of temporal lobe origin. It has been hypothesized that damage limitation or repair, for example using neurotrophic factors (NTFs), may prevent the transformation of a normal tissue into epileptic (epileptogenesis). Here, we used viral vectors to locally supplement two NTFs, fibroblast growth factor-2 (FGF-2) and brain-derived neurotrophic factor (BDNF), when epileptogenic damage was already in place. These vectors were first characterized in vitro, where they increased proliferation of neural progenitors and favored their differentiation into neurons, and they were then tested in a model of status epilepticus-induced neurodegeneration and epileptogenesis. When injected in a lesioned hippocampus, FGF-2/BDNF expressing vectors increased neuronogenesis, embanked neuronal damage, and reduced epileptogenesis. It is concluded that reduction of damage reduces epileptogenesis and that supplementing specific NTFs in lesion areas represents a new approach to the therapy of neuronal damage and of its consequences.

A role for leukocyte-endothelial adhesion mechanisms in epilepsy.

The mechanisms involved in the pathogenesis of epilepsy, a chronic neurological disorder that affects approximately one percent of the world population, are not well understood. Using a mouse model of epilepsy, we show that seizures induce elevated expression of vascular cell adhesion molecules and enhanced leukocyte rolling and arrest in brain vessels mediated by the leukocyte mucin P-selectin glycoprotein ligand-1 (PSGL-1, encoded by Selplg) and leukocyte integrins alpha(4)beta(1) and alpha(L)beta(2). Inhibition of leukocyte-vascular interactions, either with blocking antibodies or by genetically interfering with PSGL-1 function in mice, markedly reduced seizures. Treatment with blocking antibodies after acute seizures prevented the development of epilepsy. Neutrophil depletion also inhibited acute seizure induction and chronic spontaneous recurrent seizures. Blood-brain barrier (BBB) leakage, which is known to enhance neuronal excitability, was induced by acute seizure activity but was prevented by blockade of leukocyte-vascular adhesion, suggesting a pathogenetic link between leukocyte-vascular interactions, BBB damage and seizure generation. Consistent with the potential leukocyte involvement in epilepsy in humans, leukocytes were more abundant in brains of individuals with epilepsy than in controls. Our results suggest leukocyte-endothelial interaction as a potential target for the prevention and treatment of epilepsy.

Differential effects of nicotinic antagonists perfused into the nucleus accumbens or the ventral tegmental area on cocaine-induced dopamine release in the nucleus accumbens of mice.

RATIONALE: The mesolimbic dopamine (DA) system is considered a principal site for nicotine-cocaine interactions. OBJECTIVES AND METHODS: The aim of this paper is to study the effects of local perfusions (through the microdialysis cannula) of nicotinic acetylcholine receptor (nAChR) antagonists in the ventral tegmental area (VTA, where mesolimbic DA cell bodies are located) or nucleus accumbens (nAc, where mesolimbic DA nerve terminals project) on cocaine-elicited increase in DA levels in the nAc of mice using intracerebral microdialysis. RESULTS: Intra-nAc perfusion of mecamylamine (a nonselective central nicotinic antagonist) or coperfusion of methyllycaconitine (MLA, 10 nM) and dihydro-beta-erythroidine (DHbetaE, 10-100 muM) decreased cocaine-elicited increase in DA perfusate levels. In contrast, intra-nAc perfusion of MLA alone (a relatively selective antagonist of alpha7 subunit-containing nAChRs) increased, while DHbetaE (a relatively selective antagonist of heteromeric nAChR subtypes) did not alter, cocaine-elicited increase in DA perfusate levels. Intra-VTA perfusion of MLA (100 nM) or DHbetaE (100 micro M) significantly increased the cocaine-elicited increase of DA levels in the nAc or VTA, whereas DHbetaE and MLA coperfusion or mecamylamine perfusion had no significant effect. CONCLUSIONS: These results show that intra-nAc and intra-VTA perfusion of nAChR antagonists differentially affect cocaine-elicited increase in DA levels in a region and subtype-specific manner. This suggests that multiple cholinergic/nicotinic pathways influence the effects of cocaine on mesolimbic DA neurons in complex, and sometimes opposing, patterns.

Inhibition of both alpha7* and beta2* nicotinic acetylcholine receptors is necessary to prevent development of sensitization to cocaine-elicited increases in extracellular dopamine levels in the ventral striatum.

RATIONALE: Several studies have suggested that nicotine treatment can modulate the behavioral and neurochemical responses to other psychostimulants, such as cocaine. OBJECTIVES: The current study examined the hypothesis that nicotinic acetylcholine receptor (nAChR) blockade influences the ability of cocaine to elicit increases in extracellular dopamine levels. MATERIALS AND METHODS: Pharmacological studies using nicotinic antagonists as well as genetic inactivation of beta2* nAChRs were used to determine the effect of nAChR blockade on dopamine levels in ventral striatum elicited by acute or repeated administrations of cocaine in mice. RESULTS: Administration of mecamylamine (a general nicotinic antagonist that is not highly selective for individual nAChR subtypes) or co-administration of methyllycaconitine (a more selective antagonist of alpha7* nAChRs) with dihydro-beta-erythroidine (a more selective antagonist of beta2* nAChRs and other heteromeric nAChR subtypes) prevented sensitization of cocaine-elicited increases in extracellular DA levels in the ventral striatum in wild-type mice. In contrast, neither of the more specific antagonists alone was effective in preventing sensitization. Finally, methyllycaconitine administration prevents sensitization in beta2-/- mice but not in beta2+/+ or wild-type mice. CONCLUSIONS: These data indicate that inhibition of both alpha7* and beta2* nAChRs is necessary to prevent development of sensitization of cocaine-elicited increases in extracellular dopamine levels in the ventral striatum of mice.

Dopamine responsiveness to drugs of abuse: A shell-core investigation in the nucleus accumbens of the mouse.

The existence of subterritories within the nucleus accumbens has now been widely supported by histochemical, neurochemical, electrophysiological, as well as morphological and ultrastructural studies and suggest specific afferent and efferent systems involved in different behavioral aspects. Microdialysis studies in the rat have consistently shown that most drugs of abuse increase extracellular dopamine levels preferentially in the shell subregion of the nucleus accumbens. The study of the relative roles of NAc subregions may considerably help our understanding of the neurobiological basis of drug addiction. Accordingly, the aim of the present work was to extend the outcome of rat studies to the mouse species. Five major drugs of abuse were systemically and acutely administered to mice with a microdialysis probe implanted in either the shell or the core. A statistical comparison was performed on data transformed as percentage values of baseline dopamine vs. logarithmic values with baseline dopamine as a covariate. Results show a significant increase in dopamine levels in both the shell and core subregions following cocaine, amphetamine, nicotine, ethanol, and morphine treatments. A difference between shell and core after cocaine, nicotine, and morphine was evident when data were analyzed as percent values of baseline. However, such a shell-core dichotomy became no longer significant when ANOVA was applied on the statistically more appropriate logarithmic transformation of data with baseline as a covariate. The significant baseline differences among groups of mice (dopamine levels in the shell significantly lower compared with dopamine levels in the core) may have compromised, at least in part, the statistical procedure usually applied in microdialysis studies. These findings suggest that a careful evaluation of the data is required when subtle changes in extracellular levels of DA are measured.

Effects of antidepressant drugs and GR 205171, an neurokinin-1 (NK1) receptor antagonist, on the response in the forced swim test and on monoamine extracellular levels in the frontal cortex of the mouse.

We tested fluoxetine, bupropion and GR 205171, a selective neurokinin-1 receptor antagonist on forced swimming test (FST) response and on levels of monoamines in frontal cortex of CD1 mice by microdialysis techniques. All drugs decreased immobility time. Fluoxetine augmented all monoamines, bupropion enhanced catecholamines, and GR 205171 was totally ineffective. Results suggest that FST response may not be related to levels of monoamines in the mouse frontal cortex.