Anxiolytic effects induced by inhibition of the nitric oxide–cGMP pathway in the rat dorsal hippocampus

Rationale Conflicting results have been reported regarding the role of the nitric oxide (NO)–cyclic guanosine monophosphate (cGMP) pathway in the hippocampus on anxiety modulation. Objectives To investigate the effects of intrahippocampal injections of drugs that modify the NO–cGMP pathway in rats submitted to two animal models that are sensitive to anxiolytic drugs, the elevated plus-maze and the Vogel punished licking test. Materials and methods Male Wistar rats with cannulae aimed at the dentate gyrus of the dorsal hippocampus received microinjections of the NO synthase (NOS) inhibitors N ^G-nitro-L-arginine methyl ester (LNAME, 15–300 nmol/0.2 μl), N ^G-nitro-L-arginine (LNOARG, 50–300 nmol/0.2 μl), 7-nitroindazole (7NI, 10–100 nmol/0.2 μl), or the soluble guanylate cyclase inhibitor 1H-oxadiazolo-quinoxalin-1 one (ODQ, 10–100 nmol/0.2 μl), and were submitted to the elevated plus-maze. In a second group, the animals received 7NI, LNAME, or ODQ and were submitted to the Vogel punished licking test. To control for drug-induced changes in locomotor behavior, the animals were submitted to an open arena or to the Rota-rod test. Results All drugs increased the exploration of the open arms of the elevated plus-maze. They also increased the number of punished licks in the Vogel test, indicating an anxiolytic effect. The anxiolytic effect of LNAME was prevented by previous treatment with L-arginine (300 nmol/0.2 μl). Except for the lower dose of LNAME (15 nmol), administration of the NOS inhibitors or ODQ did not change exploratory activity in the open field nor cause any gross locomotor impairment in the Rota-rod test. Conclusion The results suggest that NO plays an anxiogenic role in the dentate gyrus of the dorsal hippocampus.     

Neurobiological mechanisms by which nicotine mediates different types of anxiety

The effects of nicotine administration into the dorsal hippocampus and lateral septum provide further evidence that different neurochemical and neuroanatomical substrates control behaviour in different animal tests. Thus, in the social interaction test (a model of generalised anxiety disorder), bilateral administration of nicotine (1-4 microg) into both regions has anxiogenic effects in test conditions that generate moderate anxiety. The anxiogenic effects are mediated by a nicotine-evoked increase in 5-hydroxytryptamine (5-HT) release and are reversed by co-administration of the 5-HT(1A) receptor antagonist, N-(2-(6-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)-cyclohex -ane carboxamide trichloride (WAY 100,635). On trial 1 in the elevated plus-maze (which models the escape components of panic disorder), nicotine is without effect when administered to the dorsal hippocampus, but has anxiogenic effects after lateral septal administration. On trial 2 in the elevated plus-maze (a model of specific phobia), nicotine (1 microg) has anxiolytic effects when administered to the dorsal hippocampus, but is ineffective (4 and 8 microg) in the lateral septum.     

Acute and chronic effects of nornicotine on locomotor activity in rats: altered response to nicotine

Rationale: Nicotine, a tobacco alkaloid, is known to be important in the acquisition and maintenance of tobacco smoking. Nornicotine, an active nicotine metabolite, stimulates nicotinic receptors and may produce psychomotor effects similar to nicotine. Objective: The present study determined the effects of acute and repeated administration of nornicotine on locomotor activity and compared its effects with those of nicotine. Methods: R(+)-Nornicotine (0.3–10 mg/kg), S(–)-nornicotine (0.3–10 mg/kg), S(–)-nicotine (0.1–1 mg/kg) or saline was administered s.c. to rats acutely or repeatedly (eight injections at 48-h intervals). Activity was recorded for 50 min immediately after each injection. Results: S(–)-Nicotine produced transient hypoactivity, followed by dose-related hyperactivity. Repeated S(–)-nicotine administration resulted in tolerance to the hypoactivity and sensitization to the hyperactivity. Subsequent testing following a saline injection revealed evidence of conditioned hyperactivity. Acute administration of 0.3 mg/kg or 1 mg/kg R(+)- or S(–)-nornicotine produced no effect. Transient hypoactivity was observed at 3 mg/kg and 10 mg/kg R(+)-nornicotine and at 10 mg/kg S(–)-nornicotine. However, rebound hyperactivity was not observed following acute administration of either nornicotine enantiomer, suggesting that nornicotine-induced psychomotor effects differ qualitatively from those of S(–)-nicotine. Repeated R(+)-nornicotine resulted in tolerance to the transient hypoactivity, however hyperactivity was not observed. Repeated S(–)-nornicotine resulted in tolerance to the hypoactivity and the appearance of hyperactivity. Repeated administration of either nornicotine enantiomer resulted in a dose-dependent alteration in response to a 1 mg/kg S(–)-nicotine challenge, suggesting some commonalities in the mechanism of action. Conclusion: Nornicotine likely contributes to the neuropharmacological effects of nicotine and tobacco use. Received: 11 January 1999 / Final version: 25 March 1999     

Corticotropin releasing factor antagonist, α-helical CRF<Subscript>9–41</Subscript>, reverses nicotine-induced conditioned,but not unconditioned,anxiety

Rationale. Unconditioned anxiogenic effects of nicotine have been observed in the social interaction (SI) test 5 min after injection of a low dose and both 5 min and 30 min after injection of a high dose. Conditioned anxiety has also been observed 24 h after testing in the SI with a high dose of nicotine. Objectives. In order to determine whether these three anxiogenic effects shared a common mechanism, we investigated the role of corticotropin releasing factor (CRF). We therefore examined whether the CRF antagonist α-helical CRF_9–41 could block these three anxiogenic effects of nicotine. Methods. To test the unconditioned anxiogenic effects, pairs of male rats were tested in SI 5 min after s.c. vehicle or nicotine (0.1 mg/kg) or 30 min after s.c. vehicle or nicotine (0.45 mg/kg), and 30 min after i.c.v. artificial cerebrospinal fluid (aCSF) or α-helical CRF_9–41. To test conditioned anxiety, rats were exposed to the SI test on day 1, 5 min after vehicle or nicotine (0.1 mg/kg). On day 2, they were re-tested in SI 30 min after i.c.v. aCSF or α-helical CRF_9–41 (5 μg). Results. α-Helical CRF_9–41 did not block the unconditioned anxiogenic effect of either dose of nicotine. Nicotine (0.1 mg/kg, 5 min) elicited a conditioned anxiogenic response that was significantly reversed by α-helical CRF_9–41. The CRF antagonist alone had no effect. Conclusions. CRF is an important mediator of the conditioned anxiety to nicotine, but may not play a role in mediating the acute anxiogenic effects. Electronic Publication     

The role of the dorsal hippocampal serotonergic and cholinergic systems in the modulation of anxiety

A review of the literature suggests that the dorsal hippocampal serotonergic system, and, in particular, the postsynaptic 5-HT(1A) receptor, mediates an anxiogenic response, whereas endogenous dorsal hippocampal cholinergic tone mediates an anxiolytic response. Accordingly, it has been shown that direct dorsal hippocampal administration of the 5-HT(1A) receptor agonist, 8-OH-DPAT, the nicotinic receptor antagonist, mecamylamine, and the M(1) muscarinic receptor antagonist, pirenzepine, all have anxiogenic effects in rats tested in the social interaction test. It is therefore surprising that nicotine also has an anxiogenic effect in this test following dorsal hippocampal administration. However, the anxiogenic effects of mecamylamine and nicotine in the dorsal hippocampus are blocked by coadministration of the 5-HT(1A) receptor antagonist, WAY 100635, suggesting that both of these compounds act by enhancing hippocampal serotonergic transmission, thereby stimulating postsynaptic 5-HT(1A) receptors. This conclusion is supported by the observation that both nicotine and mecamylamine stimulate basal [3H]-5-HT release from dorsal hippocampal slices. A possible mechanism by which nicotinic receptor ligands modulate hippocampal 5-HT release is discussed, and it is proposed that the dorsal hippocampal serotonergic and cholinergic systems are tightly coupled and function antagonistically in the modulation of anxiety, as measured in the social interaction test. These systems are relatively unimportant in controlling behaviour on trial 1 in the plus-maze. On trial 2 in the elevated plus-maze, a model of specific phobia, the endogenous cholinergic system, nicotine, and the M(1) receptor agonist, McN-A-343, all mediate an anxiolytic effect, whereas stimulation of 5-HT(1A) receptors mediates an anxiogenic effect. It is proposed that the hippocampus may predominantly control the avoidance components of phobic anxiety, with other regions, such as the dorsomedial hypothalamus, controlling the escape components.     

Nicotine-induced decreases in VTA electrical self-stimulation thresholds: blockade by haloperidol and mecamylamine but not scopolamine or ondansetron

 The effects of repeated daily injections of (–)-nicotine (+) hydrogen tartrate (mg kg^–1 SC) on electrical self-stimulation of the ventral tegmental area were investigated. Nicotine reduced the frequency required to maintain half-maximal response rates with animals responding in rate-frequency threshold tests. Under these conditions, nicotine induced an increase in the total number of self-stimulation responses per session, but had no statistically significant effects on the maximal response rate. These effects of nicotine were observed by the second day of administration of this drug. Acute injections of the D₂-like dopamine receptor antagonist haloperidol (0.03 mg kg^–1 SC) and of the nicotinic acetylcholine receptor antagonist mecamylamine (1 mg kg^–1 SC) attenuated the effects of nicotine, indicating that the observed effects involve stimulation of D₂-like dopamine receptors as a result of nicotinic receptor activation. The muscarinic acetylcholine receptor antagonist scopolamine (3 mg kg^–1 SC) and the serotonin 5-HT₃ receptor antagonist ondansetron (0.01 and 0.1 mg kg^–1 SC) did not alter the effects of nicotine. The results of this study indicate that repeated daily administration of (–)-nicotine increases the rewarding effects of electrical self-stimulation of the ventral tegmental area. These data are consistent with the proposal that repeated daily injections of nicotine positively effect a mesolimbic dopaminergic substrate of reward. Received: 27 March 1997 / Final version: 20 June 1997     

Cross-tolerance in drug response and differential changes in central nicotinic and N-methyl-d-aspartate receptor binding following chronic treatment with either (+)- or (–)-nicotine

The present study compared the effects of chronic treatment (21 days) with (+)-nicotine and (–)-nicotine on exploratory behaviours and body temperature in rats. Chronic treatment with either (+)- or (–)-nicotine induced tolerance in exploratory behaviours. A cross-tolerance between the two stereoisomers was also observed. Body temperature was differentially affected by chronic (+)- and (–)-nicotine, but cross-tolerance was still observed. In addition, nicotinic acetylcholine receptor (nAChR) and N-methyl-d-aspartate (NMDA) receptor binding in rats treated with (+)- and (–)-nicotine were measured by receptor autoradiography. Chronic treatment with (+)- and (–)-nicotine significantly increased nAChR binding in different brain regions. Furthermore, significant differences in the induction of nAChR binding by chronic (+)- and (–)-nicotine were detected in several brain regions. Chronic treatment with (+)-nicotine significantly increased NMDA receptor binding in the CA1 region of the hippocampus. However, (–)-nicotine had no effect on NMDA receptor binding in any of the regions analysed. These results demonstrate differences between chronic treatment with (+)- and (–)-nicotine in physiological responses such as body temperature and biochemical changes such as changes in nAChRs and NMDA receptor binding, although a cross-tolerance between the two enantiomers in behaviour and body temperature is also observed. Received: 11 May 1999 / Final version: 26 August 1999     

The dorsal raphé nucleus is a crucial structure mediating nicotine's anxiolytic effects and the development of tolerance and withdrawal responses

RATIONALE: Smokers frequently report that they obtain anxiety-reducing (anxiolytic) effects from smoking, and this may be one factor which contributes to nicotine dependence. OBJECTIVE: The aim of this study was to investigate the role of the dorsal raphé nucleus (DRN) in mediating the acute anxiolytic effect of nicotine, the development of tolerance to this effect and the anxiogenic response observed on withdrawal from chronic nicotine. METHODS: The social interaction test of anxiety was used to investigate the effects of a range of doses of (-)-nicotine (2.5-4000 ng) following DRN infusion, and whether co-administration of the specific 5-HT1A receptor antagonist WAY 100635 could antagonise the anxiolytic action of nicotine. We then examined the effects of intra-DRN nicotine (2.5-7 ng) following six daily injections of subcutaneous (s.c.) (-)-nicotine (0.1 mg/kg). Finally, we examined whether s.c. or intra-DRN (-)-nicotine could antagonise the anxiogenic response seen 72 h after the termination of 7 days of nicotine treatment. RESULTS: Acute nicotine administration into the DRN produced dose-related effects: low doses (2.5-10 ng) induced an anxiolytic effect, intermediate doses were behaviourally silent (100-1000 ng), and an anxiogenic effect was seen following administration of a high dose (4 micrograms). The anxiolytic effect of (-)-nicotine (5 ng) was reversed by co-administration of a behaviourally inactive dose of WAY 100635 (200 ng). Following 6 days of treatment with s.c. 0.1 mg/kg per day (-)-nicotine, tolerance developed to its anxiolytic action in the DRN. Rats withdrawn for 72 h following this chronic treatment showed an anxiogenic response which was reversed by (-)-nicotine injected s.c. (0.1 mg/kg) or into the DRN (5 ng). CONCLUSIONS: The present findings therefore suggest that the DRN plays an important role in mediating the acute effects of nicotine on anxiety, as measured in the social interaction test, and that the anxiolytic effect is mediated by activation of somatodendritic 5-HT1A autoreceptors. The DRN is also concerned with mediating the development of tolerance to nicotine's anxiolytic effects and because there is an anxiogenic response 72 h after withdrawal from chronic nicotine, this suggests that an oppositional, compensatory mechanism is mediating the tolerance.     

Tolerance to nicotine's effects in the elevated plus-maze and increased anxiety during withdrawal

In the elevated plus-maze test of anxiety, nicotine (0.1 mg/kg sc; 30 min after injection) had a significant anxiogenic effect, shown by specific decreases in the percentage of time spent on the open arms and in the percentage of open-arm entries. Tolerance developed to this anxiogenic effect after 7 days of nicotine treatment (0.1 mg/kg/day). Five minutes after an acute injection, nicotine (0.1 mg/kg) was ineffective, but after 7 days of treatment a significant anxiolytic effect, shown by specific increases in the percentage of time spent on the open arms and in the percentage of open-arm entries, emerged. After 14 days of nicotine treatment, tolerance developed to this anxiolytic effect. There was a complete dissociation between the effects of nicotine on the measures of anxiety, and on the locomotor activity as measured by closed-arm entries. No changes in closed-arm entries were found after acute administration of nicotine, but rats tested 30 min after their 7th injection made significantly fewer, and those tested 5 min after their 14th injection made significantly more, entries than their respective controls. Rats that were tested after 24 h withdrawal from six daily nicotine injections showed a significant anxiogenic effect. A low dose of nicotine (5 ng) injected into the dorsal hippocampus was without effect in vehicle pretreated rats, but it was able to reverse the anxiogenic effect found after 24 h of withdrawal from 6 days of nicotine treatment.     

Central nicotinic receptor agonists ABT-418, ABT-089, and (–)-nicotine reduce distractibility in adult monkeys

 Increased distractibility is associated with both Alzheimer’s disease and attention deficit disorder. The present study examined the effects of (–)-nicotine and the novel central nicotinic receptor (nAChR) agonists ABT-418 [(S)-3-methyl-2-pyrrolidinyl)isoxazole] and ABT-089 [2-methyl-3-(2-(S)-pyrrolindinylmethoxy)- pyridine dihydrochloride] on the delayed recall accuracy of adult monkeys exposed to distracting stimuli. Unpredictable exposure to a random visual array produced marked decrements in recall accuracy on trials with the shortest delay intervals, reducing the accuracy on these trials by 23.4%. Intramuscular (IM) administration of (–)-nicotine, in doses of 5.4–43.3 nmol/kg, attenuated the effect of the distractor, but did not completely prevent it. Both ABT-418 (2.0–16.2 nmol/kg, IM) and ABT-089 (16.4–32.8 nmol/kg, IM) prevented distractibility, producing increases of 7.5–25.0% in accuracy on trials disrupted by distractor exposure. Further, both compounds also improved accuracy on trials during which distractors were not presented, an effect which was not observed after (–)-nicotine administration. Nicotinic-mediated side effects were not observed following administration of any compound. Thus, nAChR stimulation reduces distractibility in adult monkeys and may, therefore, represent a target for the pharmacologic treatment of disorders associated with susceptibility to distraction. ABT-418 and ABT-089 appear to be particularly useful in this regard, a likely result of their selective agonist activity at nAChRs expressed in the brain. Received: 14 May 1997 / Final version: 19 August 1997