Sustained impairment of α2A-adrenergic autoreceptor signaling mediates neurochemical and behavioral sensitization to amphetamine.

BACKGROUND: In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists months after the last administration. It has been shown that behavioral sensitization to amphetamine develops parallel to an increased release of norepinephrine (NE) in the prefrontal cortex (PFC). METHODS: Rats and mice were repeatedly treated with amphetamine (1 or 2 mg/kg intraperitoneally, respectively) to obtain sensitized animals. The NE release in the PFC was measured by microdialysis in freely moving mice (n = 55). Activity of locus coeruleus (LC) noradrenergic neurons was determined in anaesthetized rats (n = 15) by in vivo extracellular electrophysiology. The α2A-adrenergic autoreceptor (α2A-AR) expression was assessed by autoradiography on brain slices, and Gαi proteins expression was measured by western blot analysis of LC punches. RESULTS: In sensitized rats LC neurons had a higher spontaneous firing rate, and clonidine-an α2A-adrenergic agonist-inhibited LC neuronal firing less efficiently than in control animals. Clonidine also induced lower levels of NE release in the PFC of sensitized mice. This desensitization was maintained by a lower density of Gαi1 and Gαi2 proteins in the LC of sensitized mice rather than weaker α2A-AR expression. Behavioral sensitization was facilitated by α2A-AR antagonist, efaroxan, during amphetamine injections and abolished by clonidine treatment. CONCLUSIONS: Our data indicate that noradrenergic inhibitory feedback is impaired for at least 1 month in rats and mice repeatedly treated with amphetamine. This work highlights the key role of noradrenergic autoreceptor signaling in the persistent modifications induced by repeated amphetamine administration.

Chronic stress triggers social aversion via glucocorticoid receptor in dopaminoceptive neurons.

Repeated traumatic events induce long-lasting behavioral changes that are key to organism adaptation and that affect cognitive, emotional, and social behaviors. Rodents subjected to repeated instances of aggression develop enduring social aversion and increased anxiety. Such repeated aggressions trigger a stress response, resulting in glucocorticoid release and activation of the ascending dopamine (DA) system. We bred mice with selective inactivation of the gene encoding the glucocorticoid receptor (GR) along the DA pathway, and exposed them to repeated aggressions. GR in dopaminoceptive but not DA-releasing neurons specifically promoted social aversion as well as dopaminergic neurochemical and electrophysiological neuroadaptations. Anxiety and fear memories remained unaffected. Acute inhibition of the activity of DA-releasing neurons fully restored social interaction in socially defeated wild-type mice. Our data suggest a GR-dependent neuronal dichotomy for the regulation of emotional and social behaviors, and clearly implicate GR as a link between stress resiliency and dopaminergic tone.

Inhibition of monoamine oxidases desensitizes 5-HT1A autoreceptors and allows nicotine to induce a neurochemical and behavioral sensitization.

Although nicotine is generally considered to be the main compound responsible for addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other substances of abuse. We recently showed that a pretreatment with mixed irreversible monoamine oxidases inhibitors (MAOIs), such as tranylcypromine, triggers a locomotor response to nicotine in mice and allows maintenance of behavioral sensitization to nicotine in rats. Moreover, we showed by microdialysis in mice that behavioral sensitization induced by compounds belonging to main groups of drugs of abuse, such as amphetamine, cocaine, morphine, or alcohol, was underlain by sensitization of noradrenergic and serotonergic neurons. Here, this neurochemical sensitization was tested after nicotine, tranylcypromine, or a mixture of both compounds. Data indicate that, whereas neither repeated nicotine nor repeated tranylcypromine alone has any effect by itself, a repeated treatment with a mixture of nicotine and tranylcypromine induces both behavioral sensitization and sensitization of noradrenergic and serotonergic neurons. The development of neurochemical and behavioral sensitizations is blocked by prazosin and SR46349B [(1Z,2E)-1-(2-fluoro-phenyl)-3-(4-hydroxyphenyl)-prop-2-en-one-O-(2-dimethylamino-ethyl)-oxime hemifumarate], two antagonists of alpha1b-adrenergic and 5-HT(2A) receptors, respectively, but not by SCH23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride], a D(1) receptor antagonist. Finally, we found that pretreatments with WAY 100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-hexane carboxamide trihydrochloride], a 5-HT(1A) receptor antagonist, can also induce a behavioral and neurochemical sensitization to repeated nicotine. Complementary experiments with 8-OHDPAT (8-hydroxy-dipropylamino-tetralin), a 5-HT(1A) receptor agonist, and analysis of 5-HT(1A) receptors expression in the dorsal raphe nucleus after a tranylcypromine injection indicate that MAOIs contained in tobacco desensitize 5-HT(1A) autoreceptors to trigger the strong addictive properties of tobacco.

Drugs of abuse specifically sensitize noradrenergic and serotonergic neurons via a non-dopaminergic mechanism.

A challenge in drug dependence is to delineate long-term neurochemical modifications induced by drugs of abuse. Repeated d-amphetamine was recently shown to disrupt a mutual regulatory link between noradrenergic and serotonergic neurons, thus inducing long-term increased responses to d-amphetamine and para-chloroamphetamine, respectively. We show here that such a sensitization of noradrenergic and serotonergic neurons also occurs following repeated treatment with cocaine, morphine, or alcohol, three compounds belonging to main groups of addictive substances. In all cases, this sensitization is prevented by alpha 1b-adrenergic and 5-HT2A receptors blockade, indicating the critical role of these receptors on long-term effects of drugs of abuse. However, repeated treatments with two non-addictive antidepressants, venlafaxine, and clorimipramine, which nevertheless inhibit noradrenergic and serotonergic reuptake, do not induce noradrenergic and serotonergic neurons sensitization. Similarly, this sensitization does not occur following repeated treatments with a specific inhibitor of dopamine (DA) reuptake, GBR12783. Moreover, we show that the effects of SCH23390, a D1 receptor antagonist known to inhibit development of d-amphetamine behavioral sensitization, are due to its 5-HT2C receptor agonist property. SCH23390 blocks amphetamine-induced release of norepinephrine and RS102221, a 5-HT2C antagonist, can reverse this inhibition as well as inhibition of noradrenergic sensitization and development of behavioral sensitization induced by repeated d-amphetamine. We propose that noradrenergic/serotonergic uncoupling is a common neurochemical consequence of repeated consumption of drugs of abuse, unrelated with DA release. Our data also suggest that compounds able to restore the link between noradrenergic and serotonergic modulatory systems could represent important therapeutic targets for investigation.

Paradoxical constitutive behavioral sensitization to amphetamine in mice lacking 5-HT2A receptors.

RATIONALE: Although locomotor response to d-amphetamine is considered as mediated by an increased release of dopamine in the ventral striatum, blockade of either alpha1b-adrenergic or 5-HT2A receptors almost completely inhibits d-amphetamine-induced locomotor response in mice. In agreement with this finding, mice lacking alpha1b-adrenergic receptors hardly respond to d-amphetamine. However, we show here that, paradoxically, mice lacking 5-HT2A receptors (5-HT2A-R KO) exhibit a twofold higher locomotor response to d-amphetamine than wild-type (WT) littermates. OBJECTIVES: To explore why there is a discrepancy between pharmacological and genetic 5-HT2A receptor blockade. MATERIALS AND METHODS: Locomotor response and behavioral sensitization to d-amphetamine were measured in presence of prazosin and/or SR46349B, alpha1b-adrenergic, and 5-HT2A receptor antagonists, respectively. RESULTS: Repeating amphetamine injections still increases 5-HT2A-R KO mice locomotor response to d-amphetamine at a level similar to that of sensitized WT mice. SR46349B (1 mg/kg) has, as expected, no effect in 5-HT2A-R KO mice. One milligrams per kilogram of prazosin completely blocks d-amphetamine-induced locomotor response in 5-HT2A-R KO naïve animals but 3 mg/kg is necessary in sensitized 5-HT2A-R KO mice. CONCLUSIONS: Because naïve 5-HT2A-R KO mice exhibit an increased cortical noradrenergic response to d-amphetamine, our data suggest that repeated d-amphetamine modifies noradrenergic transmission in 5-HT2A-R KO mice. Stimulation of specific 5-HT2A receptors would inhibit noradrenergic neurons. Dramatic decrease in SR46349B efficiency in sensitized WT mice indicates that a disruption of the regulating role of 5-HT2A receptors on noradrenergic transmission occurs during sensitization and thus represents the physiological basis of behavioral sensitization to d-amphetamine.

Elevated dopamine D2High receptors in alpha-1b-adrenoceptor knockout supersensitive mice.

Although amphetamine induces hyperactivity by releasing dopamine (DA), mice that lack alpha1b-adrenoceptors do not release DA in response to amphetamine and do not, therefore, exhibit locomotor supersensitivity to amphetamine. However, such mice reveal hyperlocomotion to p-chloroamphetamine (PCA). Because these alpha1b-adrenoceptor knockout mice have no alterations in the striatal densities of DA D1 or D2 receptors, the basis for any possible dopaminergic contribution to the PCA-induced hyperlocomotion to PCA is unclear. Therefore, because supersensitive animals are generally known to have a higher proportion of DA D2 receptors in the high-affinity state for DA D2(High), we investigated whether there was any change in the alpha1b-adrenoceptor knockout striata in the proportion of DA D2(High) receptors to determine whether there could be a DA-based contribution to the PCA-induced hyperlocomotion. We found that the proportion of D2(High) in the wild type striata was 23 +/- 3.3%, whereas that in the alpha1b-adrenoceptor knockout striata was 52 +/- 2.9%, an increase of 2.3-fold. This elevation agrees with other types of DA-supersensitive animal striata and could assist in eliciting a supersensitive response in these alpha1b-adrenoceptor knockout mice.

Behavioral sensitization to amphetamine results from an uncoupling between noradrenergic and serotonergic neurons.

In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhances this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. We show here that, in naïve animals, noradrenergic and serotonergic systems, besides their behavioral activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes with repeated injections of d-amphetamine; this uncoupling may be responsible for behavioral sensitization and for an increased reactivity of dopaminergic neurons. First, after repeated d-amphetamine injections, a d-amphetamine challenge induces a dramatic increase in cortical extracellular norepinephrine (NE) levels. This increased cortical NE release still occurs after 1 month of withdrawal but is diminished or blocked if sensitization is performed in the presence of prazosin, SR46349B, or both alpha1-adrenergic and 5-HT(2A) receptor antagonists, respectively. A strong correlation between increases in cortical extracellular NE levels and the expression of behavioral sensitization was found. Second, repeated d-amphetamine injections induce an increased reactivity of serotonergic neurons measured by cortical extracellular serotonin (5-HT) levels after the administration of a 5-HT releaser, p-chloroamphetamine. Third, knockout mice for alpha1b-adrenergic (alpha1b-AR KO) or 5-HT(2A) (5-HT(2A)-R KO) receptor, respectively, exhibit a behavioral and biochemical hyperreactivity to the acute injection of p-chloroamphetamine (alpha1b-AR KO; 5-HT levels) and d-amphetamine (5-HT(2A)-R KO; NE levels). Uncoupling between noradrenergic and serotonergic neurons may occur not only in addiction but also during chronic stressful situations, thus facilitating the onset of mental illness.