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.

The anxiolytic-like effect of 5-HT1B receptor ligands in rats: a possible mechanism of action.

We have examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg kg(-1)), and the influence of flumazenil (Ro 15-1788, 10 mg kg(-1)), a benzodiazepine receptor antagonist, on the anxiolytic-like activity of CP 94253 (5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine), a 5-HT1B receptor agonist, SB 216641 (N-[3-[3-(dimethylamino)ethoxy]-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide), a 5-HT1B receptor antagonist, and GR 127935 (N-[4-methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-l, l'-biphenyl-4-carboxamide), a 5-HT1B/1D receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. CP 94253 (2.5 mg kg(-1)), SB 216641 (2.5 mg kg(-1)), GR 127935 (10 mg kg(-1)) and diazepam (5 mg kg(-1)) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test in vehicle- and p-CA-pretreated rats. Flumazenil did not change the anxiolytic-like effect of CP 94253 (2.5 mg kg(-1)), but wholly blocked the anxiolytic-like effects of SB 216641 (2.5 mg kg(-1)), GR 127935 (10 mg kg(-1)) and diazepam (5 mg kg(-1)). p-CA and flumazenil alone were inactive in the conflict drinking test. The results suggested that the anxiolytic-like effect of the 5-HT1B receptor ligands CP 94253, SB 216641 and GR 127935 was possibly linked to the postsynaptic 5-HT1B receptors or/and 5-HT1B heteroreceptors. The results suggested also that benzodiazepine receptors were indirectly involved in the effects of SB 216641 and GR 127935 (but not of CP 94253), which might have been due to a possible interaction between the 5-HT and the GABA/benzodiazepine systems.

Involvement of presynaptic 5-HT(1A) and benzodiazepine receptors in the anticonflict activity of 5-HT(1A) receptor antagonists.

In the present paper, we examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg/kg), and the influence of flumazenil (Ro 15-1788, 10 mg/kg), a benzodiazepine receptor antagonist, on the anticonflict activity of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY 100635) and trans-1-(2-methoxy-phenyl)-4-[4-succinimidocyclohexyl]piperazine (MP 349), pre- and postsynaptic 5-HT(1A) receptor antagonists, and 1-(2-methoxyphenyl)-4-(4-succinimidobutyl)piperazine (MM 77), a postsynaptic 5-HT(1A) receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. WAY 100635 (0.5-1 mg/kg), MP 349 (0.25-0.5 mg/kg), MM 77 (0.03-0.25 mg/kg) and diazepam (2.5-5 mg/kg) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test. In p-chloroamphetamine-pretreated rats, neither WAY 100635 (1 mg/kg) nor MP 349 (0.25 mg/kg) induced an anticonflict effect, whereas MM 77 (0.06 mg/kg) did produce it. Flumazenil fully blocked the anticonflict effects of WAY 100635 (1 mg/kg) and diazepam (5 mg/kg), and it partly but significantly reduced the anticonflict effects of MP 349 (0.25 mg/kg) and MM 77 (0.06 mg/kg). p-Chloroamphetamine and flumazenil alone were inactive in the conflict drinking test. The present results suggest that, first, the anticonflict effect of the 5-HT(1A) receptor antagonists, WAY 100635 and MP 349, but not MM 77, is linked to the presynaptically located 5-HT(1A) receptors, and second, benzodiazepine receptors are indirectly involved in such effects of WAY 100635, MP 349 and MM 77, due maybe to a possible interaction between the 5-HT and the gamma-aminobutyric acid (GABA)/benzodiazepine systems.

Enhanced inhibitory avoidance training protects against the amnesic effect of p-chloroamphetamine.

The contribution of acetylcholine (ACh) to memory processing is well documented, but it has been proposed that it is not necessary for memory consolidation after an enhanced learning experience. It has been suggested that serotonin (5-HT) interacts with ACh during memory consolidation, although the nature of this interaction is unknown in the case of strong learning. As an initial approach to the study of these interactions, we determined whether training of inhibitory avoidance using relatively high aversive stimulation protects against the typical retention deficits produced by pre-training administration of the 5-HT releaser p-chloroamphetamine (PCA). Rats were trained after intraperitoneal administration of PCA or isotonic saline, using 2.0, 2.5, 3.0 or 3.5 mA and retention of the task was measured 24 h later. A significant amnesic state was observed only in the PCA groups that had been trained with the two lower intensities. These results indicate that 5-HT systems behave similarly to ACh systems, in the sense that the amnesic effect produced by interference with their physiological activity may be cancelled when animals are submitted to an intense learning situation.

Antisense oligonucleotide inhibition of tryptophan hydroxylase activity in mouse brain.

To examine in vivo effectiveness of antisense oligonucleotides against tryptophan hydroxylase (TPH) mRNA, adult male swiss-NIH mice were implanted with in-dwelling cannula into the 4th ventricle and after recovery infused with either antisense oligonucleotide to TPH, scrambled control oligo or saline vehicle for four consecutive days. An additional group of animals bearing cannula were injected a single time i.p. with the TPH inhibitor para-chlorophenylalanine (PCPA; 300 mg/kg). All animals were sacrificed on the afternoon of the 4th day of treatment. TPH activity was measured by enzymatic assay and HPLC quantification of end-product synthesis. There was a significant decrease (> 50%) in TPH activity in both the PCPA-treated and antisense-oligo infused animals compared to either scrambled-oligo or saline-infused subjects (ANOVA; P < 0.05). There was no difference between saline and scrambled oligo-infusion. In a separate group of animals treated in the same way, behavioral tests were conducted on the afternoon of the 4th day. Two tests of anxiety, the hole-board apparatus and the elevated plus-maze, indicated some significant effects of PCPA treatment and/or antisense oligo-infusion but confounding effects due to alterations in locomotion could not be ruled out. However, tests on a rotorod apparatus indicated that antisense oligo-infused animals retained good balance and coordination in that their performance significantly improved on the second test, as did that of scrambled-oligo infused animals. In contrast, PCPA-treated animals did not improve, suggesting that locomotor performance had been impaired. These data support the notion that antisense oligo blockade may offer advantages over pharmacological manipulations of enzyme activity.