Pindolol binding to 5-HT1A receptors in the human brain confirmed with positron emission tomography

The augmentation effect of (–)pindolol as used in combination with SSRI to treat major depression has been ascribed to blocking of dorsal raphe nucleus cell body 5-HT autoreceptors. In this study, the radioligand [carbonyl-¹¹C]WAY-100635 and positron emission tomography were used to establish whether pindolol at a clinical dose level (10 mg s.o.d.) occupies 5-HT_1A receptors in the human brain in vivo. Three healthy males were recruited and each subject was used as his own control. The 5-HT_1A receptor occupancy was calculated for the frontal and temporal cortex and the raphe nuclei, using and a ratio analysis with the cerebellar cortex as the reference region. Maximal pindolol plasma concentration was reached within 3 h after drug administration. Two hours after pindolol administration, the regional 5-HT_1A receptor occupancy was within the range 7–21% in the three subjects. The study confirms that the 5-HT_1A-receptor may be a clinically significant target for pindolol. Received: 8 March 1999 / Final version: 15 March 1999     

L-701,324, a selective antagonist at the glycine site of the NMDA receptor, counteracts haloperidol-induced muscle rigidity in rats

  Rationale: It has recently been suggested that the overactivity of glutamatergic neurotransmission may contribute to the pathophysiology of Parkinson’s disease. Therefore, a search for new compounds which block glutamatergic receptors and show antiparkinsonian properties in animal models of this disease seems to be justified. Objective: The aim of this study was to determine whether L-701,324 [7-chloro-4-hydroxy-3(3-phenoxy) phenylquinoline-2-(H)-one], a selective and full antagonist at the glycine site of the NMDA receptor, counteracts parkinsonian-like muscle rigidity and catalepsy induced by haloperidol in rats. Methods: The muscle tone was measured as the resistance developed to passive flexion and extension of the hind limb. Electromyographic (EMG) activity was additionally recorded in the gastrocnemius and tibialis anterior muscles. Results: L-701,324 (2.5–40 mg/kg IP) dose-dependently decreased the muscle tone enhanced by haloperidol (1–5 mg/kg IP). Likewise, the haloperidol-enhanced resting EMG activity and the EMG reflex response to passive movements were diminished by lower and almost abolished by higher doses of L-701,324. However, up to a dose of 20 mg/kg IP, L-701,324 did not influence haloperidol (0.5 mg/kg IP)-induced catalepsy. Moreover, L-701,324 (1.25–5 mg/kg IP) given alone or together with haloperidol (0.5–1 mg/kg IP) disturbed rotarod performance. Gross observation of behaviour indicated that rats injected with L-701,324 in doses equal to or higher than 5 mg/kg, alone or in combination with haloperidol, were markedly ataxic, i.e. rats showed signs of disturbed balance and loss of control over their hind limbs. Conclusions: The present study suggests that L-701,324 exhibits a beneficial action in the animal model of parkinsonian rigidity, but not that of parkinsonian akinesia. Nonetheless, this compound is not devoid of motor side-effects. Received: 1 February 1998 / Final version: 20 October 1998     

Mechanism of tolerance development to 2,5-dimethoxy-4-iodoamphetamine in rats: down-regulation of the 5-HT2A, but not 5-HT2C, receptor

  Rationale: Defining the mechanism of tolerance development to hallucinogenic drugs will help to explain their mechanism of action. Objectives: The present study was conducted to determine first, if tolerance develops to the discriminative stimulus (DS) properties of the hallucinogen, 2,5 dimethoxy-4-iodo-amphetamine (DOI) and second, the mechanism mediating tolerance. Methods: Rats were trained to discriminate 0.75 mg/kg DOI from saline on a concurrent VI-30-min schedule of reinforcement with a 15-min time-out for incorrect responses. To evaluate tolerance development, rats were assigned to one of four groups and treated with either chronic saline or chronic DOI. Prior to chronic treatment, two groups were tested for choice behavior following vehicle administration while the remaining two groups were tested following the administration of 0.375 mg/kg DOI. One group from each pre-test condition was injected with either saline or DOI (1 mg/kg) for 8 days. Twenty-four hours after the last chronic injection the pre-test treatments were replicated. Using receptor autoradiography, the density of 5-HT_2A and 5-HT_2C receptors was measured in independent groups of rats that had received identical treatment conditions. Results: Animals receiving chronic DOI showed a 60% decrease in DOI lever responding (from 100% to 40%) when tested on 0.375 mg/kg DOI, while animals receiving chronic saline showed no change in percent choice (100%) on the DOI lever. Significant changes in binding were observed in 5-HT_2A receptors but not 5-HT_2C receptors. The results of tests with antagonists were consistent with the changes in binding. Conclusions: These results suggest that behavioral tolerance to DOI reflects neuroadaptive changes in 5-HT_2A receptors. Received: 17 July 1998 / Final version: 19 January 1999     

Antidepressant effects of nicotine in an animal model of depression

Epidemiological studies indicate a high incidence of cigarette smoking among depressed individuals. Moreover, individuals with a history of depression have a much harder time giving up smoking. It has been postulated that smoking may reflect an attempt at self-medication with nicotine by these individuals. Although some animal and human studies suggest that nicotine may act as an antidepressant, further verification of this hypothesis and involvement of nicotinic cholinergic system in depressive symptoms is required. Flinders Sensitive Line (FSL) rats have been proposed as an animal model of depression. These rats, selectively bred for their hyperresponsiveness to cholinergic stimulation, show an exaggerated immobility in the forced swim test compared to their control Flinders Resistant Line (FRL) rats. Acute or chronic (14 days) administration of nicotine (0.4 mg/kg SC) significantly improved the performance of the FSL but not the FRL rats in the swim test. The effects of nicotine on swim test were dissociable from its effects on locomotor activity. Moreover, the FSL rats had significantly higher [³H]cytisine binding (selective for the α₄β₂ nicotinic receptor subtype) but not [¹²⁵I]alpha-bungarotoxin binding (selective for the α₇ subtype) in the frontal cortex, striatum, midbrain and colliculi compared to FRL rats. These data strongly implicate the involvement of central nicotinic receptors in the depressive characteristics of the FSL rats, and suggest that nicotinic agonists may have therapeutic benefits in depressive disorders. Received: 9 June 1998/Final version: 6 August 1998     

Activation of 5-HT1B receptors in the nucleus accumbens reduces amphetamine-induced enhancement of responding for conditioned reward

Previously, we have demonstrated that 5-hydroxytryptamine (5-HT) injected into the nucleus accumbens attenuates the potentiating effects of d-amphetamine on responding for conditioned reward (CR). The present studies examined the 5-HT receptor involved in this effect by investigating the effects of 5-HT agonists with differing affinities for 5-HT₁ and 5-HT₂ receptors on d-amphetamine-induced potentiation of responding for CR. Rats were trained to associate a light/tone stimulus (subsequently the CR) with water delivery. In a test phase, they were allowed access to a lever delivering the CR, and an inactive (NCR) lever. Responding on the CR lever was greater than responding on the NCR lever, indicating that the light/tone stimulus functioned as a CR. Responding for the CR was selectively potentiated by injections of d-amphetamine (10 μg) into the nucleus accumbens. This effect was reduced by injections into the nucleus accumbens of 5-CT (0.5 and 1 μg), RU24969 (10 μg), CP93,129 (1.25 and 2.5 μg) but not by DOI (10 μg) or 8-OH-DPAT (5 μg). The lower doses of 5-CT and CP93,129 did not reduce baseline responding for CR, or responding for water in a separate group of animals, indicating that the effects of these drugs were behaviourally selective. The higher doses abolished the CR effect, and in the case of 5-CT and RU24969 also reduced responding for water. All of the effective drugs share in common the ability to stimulate 5-HT_1B receptors, albeit with differing selectivities. The effect of CP93,129, the most selective of the 5-HT_1B agonists, to inhibit the response-potentiating effect of d-amphetamine was reversed by the5-HT_1B/1D antagonist GR127935 (3 mg/kg). The results indicate that activation of 5-HT_1B receptors within the nucleus accumbens attenuates the effects of a dopamine-dependent behaviour, and that activation of these receptors can oppose the behavioural effects of elevated mesolimbic dopamine transmission. Received: 22 April 1998/Final version: 28 July 1998     

Aggression heightened by alcohol or social instigation in mice: reduction by the 5-HT1B receptor agonist CP-94,253

Rationale: Models of heightened aggression may be particularly relevant in exploring pharmacological options for the clinical treatment of aggressive and impulsive disorders. Objectives: To investigate and compare the effects of a 5-HT_1B selective agonist, CP-94,253, on aggression that was heightened as a result of 1) social instigation or 2) alcohol treatment. Methods: Male CFW mice were administered 1.0 g/kg EtOH and were subsequently confronted by an intruder in their home cage. In a separate experimental procedure, resident male mice were instigated to aggressive behavior by brief exposure to a provocative stimulus male. To test the hypothesis that activation of the 5-HT_1B receptor subtype would preferentially attenuate heightened aggression, in comparison to the moderate levels of species-typical aggressive behaviors, the selective agonist, CP-94,253 (1.0–30 mg/kg, IP), and antagonists to the 5-HT_1B (GR 127935; 10 mg/kg, IP) and the 5-HT_1A receptor (WAY 100,635; 0.1 mg/kg IP) were used. Results: CP-94,253 suppressed non-heightened aggressive behavior (ED₅₀=7.2 mg/kg ). GR 127935, but not WAY 100,635 shifted the ED₅₀ for CP-94,253 to 14.5 mg/kg. Importantly, the anti-aggressive effects of CP-94,253 were not accompanied by locomotor sedation. Alcohol-heightened and instigation-heightened aggression were suppressed at lower doses than those necessary to suppress non-heightened aggression (ED₅₀=3.8 and 2.7 mg/kg, respectively). Conclusions: The current results support the hypothesis that activation of 5-HT_1B receptors modulates very high levels of aggressive behavior in a pharmacologically and behaviorally specific manner. Received: 5 January 1999 / Final version: 16 April 1999     

5-HT3 receptor over-expression enhances ethanol sensitivity in mice

 Ethanol sensitivity may play a role in the risk of developing alcoholism. The role of 5-HT₃ receptors in sensitivity to ethanol was assessed in mice over-expressing the 5-HT₃ receptor in the forebrain. Sleep time and ED₅₀ for loss of righting reflex (LRR) were used to assess the effect of a high dose of ethanol in transgenic versus non-transgenic mice. The ED₅₀ for ethanol-induced increase in open field activity was used to measure differences in sensitivity to low dose ethanol. The ED₅₀ for ethanol-induced increase in activity was 41% lower in the 5-HT₃ receptor over-expressing transgenic mice compared to non-transgenic mice. However, 5-HT₃ receptor over-expressing mice did not differ from control mice in ethanol metabolism, ED₅₀ for LRR, and ethanol sleep time. Over-expression of 5-HT₃ receptors in mouse forebrain results in an enhanced sensitivity to the stimulating effects of a low dose of ethanol without altering ethanol sedating effects or ethanol metabolism. These data suggest that 5-HT₃ receptors modulate low dose ethanol sensitivity and may explain why, in previous studies, these mice consume less ethanol. Received: 11 December 1998 / Final version: 24 February 1999     

Self-administration of intravenous amphetamine: effect of nucleus accumbens CCKB receptor activation on fixed-ratio responding

Rationale: The mesolimbic dopamine (DA) system is implicated in psychostimulant drug self-administration. The neuropeptide cholecystokinin (CCK) is co-localised with DA and inhibits nucleus accumbens (NAcc) DAergic neurotransmission via CCK_B receptors. Objectives: The present experiment was designed to examine the effects of intra-NAcc CCK_B receptor stimulation on fixed-ratio (FR) amphetamine self-administration. Methods: Wistar rats with intravenous catheters and NAcc cannulae were trained to self-administer amphetamine under a FR3 schedule of reinforcement. Animals performing stable self-administration were microinjected with pentagastrin and assessed during 3-h sessions. Results: Intra-NAcc pentagastrin dose dependently increased amphetamine intake. Conclusions: These results are consistent with the notion that NAcc CCK_B receptor activation attenuates amphetamine reward. Received: 6 May 1999 / Final version: 4 August 1999     

NMDA receptor antagonists enhance 5-HT2 receptor-mediated behavior, head-twitch response, in PCPA-treated mice

Previous work in our laboratory has shown that the N-methyl-D-aspartate (NMDA) receptor antagonists, AP-5, CPP, MK-801, ketamine, dextrorphan and dextromethorphan cause a pronounced enhancement of 5-hydroxytryptamine (5-HT)-induced head-twitch response (HTR) in intact mice, suggesting the involvement of NMDA receptors in the glutamatergic modulation of serotonergic function at the postsynaptic 5-HT2 receptors. The purpose of this study was to extend our previous work on the behavioral interaction between glutamatergic and serotonergic receptors. In the present study, both competitive (AP-5 and CPP) and noncompetitive (MK-801, ketamine, dextrorphan and dextromethorphan) NMDA receptor antagonists markedly enhanced 5-HT-induced selective serotonergic behavior, HTR, in p-chlorophenylalanine (PCPA)-treated mice which were devoid of any involvement of indirect serotonergic function, to establish the involvement of the NMDA receptor in 5-HT-induced HTR at the postsynaptic 5-HT2 receptors. In addition, the enhancement of 5-HT-induced HTR was inhibited by a dopamine agonist, apomorphine, NMDA receptor antagonist, NMDA and a serotonin 5-HT2 receptor antagonist, cyproheptadine, in PCPA-treated mice. Therefore, the present results support our previous conclusion that the NMDA receptors play an important role in the glutamatergic modulation of serotonergic function at the postsynaptic 5-HT2 receptors.