Role of 5-hydroxytryptamine in amphetamine effects on punished and unpunished behaviour

Serotonin-Related Gene Polymorphisms and Asymptomatic Neurocognitive Impairment in HIV-Infected Alcohol Abusers

HIV-infected individuals continue to experience neurocognitive deterioration despite virologically successful treatments. While the cause remains unclear, evidence suggests that HIV-associated neurocognitive disorders (HAND) may be associated with neurobehavioral dysfunction. Genetic variants have been explored to identify risk markers to determine neuropathogenesis of neurocognitive deterioration. Memory deficits and executive dysfunction are highly prevalent among HIV-infected adults. These conditions can affect their quality of life and HIV risk-taking behaviors. Single nucleotide polymorphisms in the SLC6A4, TPH2, and GALM genes may affect the activity of serotonin and increase the risk of HAND. The present study explored the relationship between SLC6A4, TPH2, and GALM genes and neurocognitive impairment in HIV-infected alcohol abusers. A total of 267 individuals were genotyped for polymorphisms in SLC6A4 5-HTTLPR, TPH2 rs4570625, and GALM rs6741892. To assess neurocognitive functions, the Short Category and the Auditory Verbal Learning Tests were used. TPH2 SNP rs4570625 showed a significant association with executive function in African American males (odds ratio 4.8, 95% CI, 1.5–14.8; P = 0.005). Similarly, GALM SNP rs6741892 showed an increased risk with African American males (odds ratio 2.4, 95% CI, 1.2–4.9; P = 0.02). This study suggests that TPH2 rs4570625 and GALM rs6741892 polymorphisms may be risk factors for HAND.

Treatment with the atypical antipsychotic, olanzapine, prevents the expression of amphetamine-induced place conditioning in the rat

Place conditioning (PC) experiments were conducted as a means to further elaborate the treatment potential of the atypical antipsychotic, olanzapine (OLZ), for stimulant abuse. The resulting preference/aversion provides an indirect measure of the incentive salience (i.e., euphoria/dysphoria) produced by a drug. Male Sprague-Dawley rats (n=48) were conditioned in two unique environments (i.e., vertical vs. horizontal stripped walls, large vs. small grid flooring) using injections (1.0 mg/kg ip) of either amphetamine (AMPH) or saline (SAL). On average, animals displayed a significant preference for the AMPH-paired location after 2.5 weeks of conditioning (five pairings each of AMPH and SAL). Once the preference was established, animals were pretreated (60 min) with a single dose of OLZ (0.0, 0.56, 1.0 or 1.5 mg/kg sc) given on the test (AMPH-free) day. For the following week's test, animals were injected with SAL (1.0 mg/kg ip) in an attempt to recapture the side preference exhibited before OLZ treatment. OLZ treatment prevented the expression of the AMPH-conditioned preference and reduced locomotor activity. Inhibition of preference resulted from the highest dose of OLZ (1.5 mg/kg), while the inhibition of locomotor activity occurred across all three doses. Additionally, while the effects on preference were no longer apparent by the SAL test the following week (reversible), the activity was still depressed during the SAL tests in animals that had experienced the highest dose of OLZ (1.5 mg/kg). Control experiments, in which OLZ was used as the conditioning drug, suggest that OLZ itself possesses no aversive effects in the PC paradigm, and may even produce a preference for the drug-paired chamber. Because the AMPH preference is dependent on dopamine (DA) release in the nucleus accumbens (NAcc), these experiments suggest that OLZ pretreatment interferes with the rewarding, as well as the subjective effects of AMPH.

Effects of tryptophan loading on verbal, spatial and affective working memory functions in healthy adults

Serotonin (5-HT) appears to modulate affective behaviours by providing a homeostatic threshold around which other transmitters respond. This general principle of activity should hold for other types of behaviour, including cognition, but has not been extensively examined. We hypothesized, based on past findings, that increased 5-HT would constrain prefrontally guided working memory functions that are mediated by catecholamine neurotransmitters. Healthy adults ingested amino acid compounds designed to deplete and load systemic tryptophan levels in a repeated-measures crossover design. Outcome variables included total plasma tryptophan, serum prolactin levels and self-report measures of mood, as well as measures of motor skill, attention, memory span and working memory for verbal, spatial and affective stimuli. Our findings indicate decrements in working memory for verbal and affective stimuli following tryptophan loading versus depletion, as well as subtle changes in vigilant attention and motor coordination. Implications for the aetiology and treatment of affective disorders and psychosis are discussed.

5-Hydroxytryptamine-interacting drugs in animal models of anxiety disorders: more than 30 years of research

An overview of the behavioral data arising from the vast literature concerning the involvement of 5-hydroxytryptamine (5-HT) neurotransmission in the regulation of anxiety is presented. More than 1300 experiments were carried out in this area and they provide evidence that: (1) results obtained in ethologically based animal models of anxiety with drugs stimulating 5-HT transmission are most consistent with the classic 5-HT hypothesis of anxiety in that they show an increase in animals' emotional reactivity; (2) no category of anti-anxiety models are selectively sensitive to the anxiolytic-like effects of drugs targetting 5-HT1A, 5-HT2A or 5-HT2C receptor subtypes; (3) anxiolytic-like effects of 5-HT3 receptor antagonists, in the great part, are revealed by models based on spontaneous behaviors. Taken together, these observations lead to the conclusion that different 5-HT mechanisms, mediated by different receptor subtypes, are involved in the genesis of anxiety.

Actions of 5-hydroxytryptophan to inhibit and disinhibit mouse behaviour in the light/dark test

The involvement of 5-HT receptors in behavioural responding to an aversive situation was investigated in the mouse light/dark test. The administration of 5-hydroxytryptophan (5-HTP) (12.5-50 mg/kg i.p.) increased brain 5-HT turnover and inhibited mouse behaviour in the light/dark test box. The 5-HT2C/5-HT2A receptor antagonists methysergide (1.0 and 5.0 mg/kg i.p.) and ritanserin (0.1-1.0 mg/kg i.p.) antagonised (methysergide) or reversed (ritanserin) the effects of 5-HTP to an increased exploration of the light compartment; a low dose of the 5-HT3 receptor antagonist ondansetron (0.01 mg/kg i.p.) had a similar effect. The disinhibitory effect of the 5-HTP/ritanserin interaction was antagonised by the 5-HT3/5-HT4 receptor antagonists SDZ205-557 (0.001-0.1 mg/kg) and a high dose of tropisetron (1.0 mg/kg i.p.) but not by ondansetron (1.0 mg/kg i.p.). At these doses tropisetron and ondansetron had no effect in their own right. Thus the dominant effect of 5-HTP in the mouse is to inhibit behaviour, a response mediated via 5-HT2C/5-HT2A and 5-HT3 receptors. A 5-HT4 receptor may effect an opposing disinhibitory potential as revealed by ritanserin.

Evidence that 5-HT2c receptor antagonists are anxiolytic in the rat Geller-Seifter model of anxiety

Four non-selective 5-HT2C/5-HT2A receptor antagonists, mianserin (2-8 mg/kg), 1-naphthyl piperazine (1-NP) (0.5-1 mg/kg), ICI 169,369 (20 mg/kg) and LY 53857 (5 mg/kg), increased punished responding for a food reward in the rat Geller-Seifter test 30 min after subcutaneous (SC) administration. This property was shared by the benzodiazepine anxiolytic chlordiazepoxide (5 mg/kg SC). However, the selective 5-HT2A receptor antagonists ketanserin (0.2-1 mg/kg SC) and altanserin (0.5, 1 mg/kg SC) had little effect. The 5-HT1A, 5-HT1B and beta-adrenergic receptor antagonists pindolol and cyanopindolol (6 mg/kg SC) did not affect punished responding either, nor did the 5-HT1D receptor partial agonist and alpha 2 adrenergic receptor antagonist yohimbine (2.5 mg/kg SC) or the histamine H1 receptor antagonist mepyramine (1 mg/kg SC). Unpunished responding was also modestly increased after some doses of the 5-HT2C/5-HT2A receptor antagonists. However, this effect was inconsistent and was also seen after chlordiazepoxide. Furthermore, it was not associated with the increase in punished responding observed in rats orally treated with mianserin (10, 20 mg/kg), 1-NP (10, 20 mg/kg) or ICI 169,369 (50 mg/kg). The action of the 5-HT2C/5-HT2A receptor antagonists tested is therefore consistent with anxiolysis. The results also strongly suggest that this effect is mediated by blockade of the 5-HT2C receptor, although the possibility of 5-HT2B receptor mediation is discussed.

5-HT1C receptor antagonists have anxiolytic-like actions in the rat social interaction model

The effects of a range of 5-HT receptor antagonists were examined in an animal model of anxiety--the social interaction test. Six antagonists with high affinity for 5-HT1C receptors; mianserin, (+) mianserin, 1-naphthyl piperazine, ICI 169 369, pizotifen and LY 53857 all increased the time spent in active social interaction by pairs of weight-matched rats under high light unfamiliar conditions. As locomotion was only increased by 1-NP and then only at high doses, the effect of the drugs is consistent with anxiolysis. These properties were shared by the benzodiazepine anxiolytic chlordiazepoxide but not by the specific 5-HT2 antagonists ketanserin and altanserin, nor by the 5-HT1A and 5-HT1B antagonists cyanopindolol and pindolol. Similarly, neither the adrenergic alpha 2 antagonist idazoxan, the alpha 2 antagonist and putative 5-HT1D partial agonist yohimbine nor the H1 antagonist mepyramine had any significant effect. Since (+)mianserin, LY 53857 and ICI 169 369 at least have low affinity for 5-HT3 receptors these receptors are also unlikely to be involved. The results therefore imply that the observed anxiolytic effects of the drugs are likely to be mediated by 5-HT1C receptor blockade.

Selective agonists and antagonists for 5-hydroxytryptamine receptor subtypes, and interactions with yohimbine and FG 7142 using the elevated plus-maze test in the rat

The effects of some 5-HT receptor ligands were investigated on measures of anxiety in an elevated plus-maze test in the rat. Quipazine (2 and 4 mg kg-1), a non-specific 5-HT agonist and ritanserin (0.25-10 mg kg-1), a 5-HT2 receptor antagonist displayed anxiogenic profiles by reducing both of the measures of anxiety used in this test. Two 5-HT1A receptor ligands, buspirone (4 and 8 mg kg-1) and ipsapirone (2.5-10 mg kg-1) and the 5-HT1 agonist, RU 24969 (0.1875-1.5 mg kg-1) significantly reduced only the percentage of time spent on the open arms. (-)-Propranolol (5 and 10 mg kg-1), a 5-HT1 receptor antagonist significantly reduced only the percentage of entries made onto the open arms. Metergoline (4 mg kg-1), a non-specific 5-HT antagonist displayed anxiolytic effects in this test by increasing both measures of anxiety. The 5-HT1A receptor agonist, 8-OH-DPAT (0.0625-0.25 mg kg-1) had no effect on either of the measures of anxiety. The results from the non-specific ligands (quipazine and metergoline) are consistent with the theory that a reduction in 5-HT function reduces anxiety. However, in spite of their more selective effects on 5-HT receptors the results in this test from the more specific ligands are not consistent with a strong involvement of any single receptor subtype. The interaction studies with yohimbine and FG 7142 (beta-carboline-3-carboxylate methylamide) provided no clear evidence for a major role of 5-HT pathways in the mediation of their anxiogenic effects.

Anti-aversive role of serotonin in the dorsal periaqueductal grey matter

Microinjection of 5, 10, and 20 nmol serotonin (5-HT) and of 0.5, 1, and 2 nmol 5-methoxy-N, N-dimethyltryptamine (5-MeODMT) into the dorsal midbrain of rats bearing chronically implanted chemitrodes raised the electrical threshold for inducing escape behaviour following stimulation of the dorsal periaqueductal grey matter (DPAG). Linear regressions of log dose against drug-induced increase in aversive threshold were obtained for 5-HT and 5-MeODMT. The 5-MeODMT dose-effect curve was steeper and lay to the left of the 5-HT dose-effect curve. Local pre-treatment with 10 nmol metergoline or ketanserin blocked the anti-aversive effect of 10 nmol 5-HT, whereas pre-treatment with 100 nmol zimelidine potentiated this effect of 5-HT. The same dose of zimelidine raised the aversive threshold when given alone. These results suggest that 5-HT plays an inhibitory role in the DPAG controlling aversion, probably mediated by 5-HT2 receptors.

Effect of metergoline on human anxiety

In order to assess the role played by serotonin (5-HT) in subjective anxiety, three groups of 12 healthy volunteers were given 12 mg metergoline (MET), 10 mg diazepam (DZ) or placebo (PB), under double-blind conditions, and submitted to a simulated public speaking (SPS) test. MET increased state-anxiety scores, measured by Spielberg's State-Trait Anxiety Inventory. The effect of MET was significantly different from both the PB and DZ groups immediately before the SPS test (prestress) as well as 24 h after medication, and from the DZ group only, 2.5 h after the test (poststress). In contrast, DZ did not significantly affect subjective anxiety. The SPS test significantly increased anxiety in DZ- or PB-treated subjects as compared to prestress scores, whereas the increases in the MET group were not significant, probably because pretest levels were already high. No drug effect on heart rate, skin electrical conductance and quality of sleep during the night following medication was found. In addition, the drugs did not cause bodily symptoms that could secondarily affect mood. Since MET is a 5-HT receptor antagonist, active on the central nervous system, an inhibitory role of 5-HT on subjective anxiety might be suggested.