The effect of chronic ritanserin and clorgyline administration on 5-HT2 receptor linked inositol phospholipid hydrolysis

The separate and combined effects of monoamine oxidase inhibition and nicotine on P50 sensory gating

INTRODUCTION

The cognitive effects of nicotine in humans remain a topic of great interest, due to the continued prevalence of cigarette smoking in society as well as the hypothesis that cognitively impaired populations such as schizophrenia patients use nicotine as a means of self-medicating against deficits of sensory gating. However, chronic smoking can predispose individuals to robust monoamine oxidase (MAO) inhibition, and thus far, the effect of MAO inhibition on human sensory gating is unknown.

METHODS

In this study, we investigated the effects of both nicotine (6-mg gum) and pharmacologically induced MAO-A inhibition via moclobemide (75 mg) on P50 event-related potential-indexed sensory gating in a sample of 24 healthy non-smoking males.

RESULTS

Ratio score (rP50) measured gating revealed significant improvement in auditory stimulus suppression after combined nicotine and MAO-A inhibition compared to placebo and to the nicotine-alone condition. This nicotine + MAO-A inhibition-induced efficient gating was consistent regardless of participants' baseline (placebo) gating efficiency, despite the observation that nicotine in the absence of MAO-A inhibition exhibited a detrimental effect on gating in participants with high baseline suppression ratios.

CONCLUSION

Nicotine and monoamine oxidase-inhibiting agents in tobacco smoke appear to exert a synergistic effect on sensory gating, which may contribute to the elevated dependence rates seen in populations with cognitive deficits such as schizophrenia.

Monoamine oxidase: from genes to behavior

Cloning of MAO (monoamine oxidase) A and B has demonstrated unequivocally that these enzymes are made up of different polypeptides, and our understanding of MAO structure, regulation, and function has been significantly advanced by studies using their cDNA. MAO A and B genes are located on the X-chromosome (Xp11.23) and comprise 15 exons with identical intron-exon organization, which suggests that they are derived from the same ancestral gene. MAO A and B knock-out mice exhibit distinct differences in neurotransmitter metabolism and behavior. MAO A knock-out mice have elevated brain levels of serotonin, norephinephrine, and dopamine and manifest aggressive behavior similar to human males with a deletion of MAO A. In contrast, MAO B knock-out mice do not exhibit aggression and only levels of phenylethylamine are increased. Mice lacking MAO B are resistant to the Parkinsongenic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine. Both MAO A and B knock-out mice show increased reactivity to stress. These knock-out mice are valuable models for investigating the role of monoamines in psychoses and neurodegenerative and stress-related disorders.

Sensitive measurement of agonist-stimulated [3H]inositol monophosphate accumulation in rat cortical miniprisms

Phosphoinositide (PI) breakdown is an important transmembrane signaling mechanism in rat brain and numerous transmitter receptors are linked to this mechanism. Since agonist-stimulated PI breakdown is often changed after drug pretreatment, assessment of changes in PI breakdown represents an important tool in drug development. PI breakdown is commonly monitored by assaying [3H]inositol monophosphate ([3H]IP1) accumulation in the presence of lithium as an inhibitor of inositol monophosphatase. The present protocol presents a lithium-inhibited [3H]IP1 accumulation assay that enhances relatively weak agonist-stimulated [3H]IP1 accumulation signals by thorough oxygenation during agonist stimulation. The protocol is particularly useful in the measurement of [3H]IP1 accumulation after in vitro exposure to relatively weak stimulants, such as serotonin (5-HT), and/or after animal pretreatments that decrease the response to the agonist. In the case of 5-HT stimulation the monoamine oxidase (MAO) inhibitor, tranylcypromine, was added to the incubation medium to inhibit breakdown of exogenous serotonin. We used this protocol to measure 5-HT- and carbachol-stimulated [3H]IP1 accumulation in cortical miniprisms obtained from rats pretreated with D-fenfluramine. D-Fenfluramine is a drug that acutely releases 5-HT into the synaptic cleft and blocks its reuptake.

Effects of subchronic pretreatment with D-fenfluramine or p-chloroamphetamine on [3H]inositolmonophosphate accumulation in rat cortical miniprisms

Phosphatidylinositol (PI) breakdown in rat cerebral cortex is stimulated by serotonin (5-HT), acting via 5-HT2 and possibly 5-HT3 receptors and by acetylcholine or carbachol, acting via muscarinic M1 and M3 receptors. Serotoninergic neurons have been described as tonically inhibiting cortical acetylcholine release. We studied the effects of subchronic pretreatment with high doses of D-fenfluramine (10 mg/kg, i.p., daily for 4 days), which releases 5-HT and blocks its reuptake, on 5-HT-and carbachol-stimulated PI breakdown, as measured by [3H]inositolmonophosphate ([3H]IP1) accumulation in cortical miniprisms. This pretreatment decreased 5-HT-stimulated [3H]IP1 accumulation, suggesting that a prolonged increase of 5-HT in the synaptic cleft reduces the activity of the transducing system used by postsynaptic 5-HT receptors. Carbachol-stimulated PI breakdown was unaltered by pretreatment with D-fenfluramine. Pretreatment with a single dose of p-chloroamphetamine (5 mg/kg), a serotoninergic neurotoxin, which depleted cortical 5-HT by 85%, did not change [3H]IP1 accumulation after stimulation by 5-HT or by the muscarinic agonist carbachol. Subchronic pretreatment, which depleted cortical 5-HT by 90%, decreased both 5-HT- and carbachol-stimulated [3H]IP1 accumulation. The mechanism by which p-chloroamphetamine, but not D-fenfluramine, diminishes the PI response to carbachol might involve impairment of the tonic serotoninergic inhibition of acetylcholine release.

Further evidence for enigmas in adaptation mechanisms for the DOI-induced behaviors

The acute and chronic effects of the 5-hydroxytryptamine2/1C (5-HT2/1C) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the antagonist ketanserin were evaluated on the DOI-induced 5-HT2 receptor-mediated ear-scratch response (ESR) in mice. A challenge dose of DOI (2.5 mg/kg) administered 24 h following its first injection reduced the ESR frequency by 80-97%. The ESR score attained first injection value when the time lag between the first and the second injection was greater than 72 h. On the other hand, a single administration of ketanserin (1.0 mg/kg) caused no significant effect at 24 or 48 h but significantly reduced (51%, p < 0.05) the DOI-induced ESR 120 h following its injection. Chronic once-daily DOI injections reduced the ESR score by 80-97% throughout the treatment regimen. Following cessation from chronic treatment, the DOI-induced ESR frequency returned to control levels in a time-dependent manner. Repeated ketanserin administration significantly reduced the DOI-induced ESR score by 46% when tested 24 or 48 h following cessation of antagonist administration but had no effect at 78 h. Recently, we reported that 48 h following either a single DOI injection or termination from repeated DOI or ketanserin administration the DOI-induced head-twitch response (HTR) in mice exhibited supersensitivity. Thus, it appears that the DOI-induced behaviors exhibit differential adaptation mechanisms following either agonist or antagonist exposure. These studies further support our hypothesis that serotonergic drugs may have the ability to change independently the 5-HT-receptor sensitivity (signal transduction) and receptor density in the same or opposite directions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic ritanserin or clorgyline on amine and metabolite levels in rat frontal cortex

As part of an investigation of ritanserin-induced receptor down-regulation, monoamine and metabolite levels in rat frontal cortex were measured following chronic ritanserin (2 mg/kg per day) or clorgyline (10 mg/kg per day) administration. Clorgyline increased 5-hydroxytryptamine (5-HT) by 83%, noradrenaline (NA) by 54%, and dopamine (DA) by 16% and decreased 5-hydroxyindoleacetic acid (5-HIAA) by 28%, homovanillic acid (HVA) by 57% and 3,4-dihydroxyphenylacetic acid (DOPAC) by 67%. All these changes were statistically significant (P less than 0.001) except for the increase in DA. Ritanserin increased 5-HT by 30%, NA by 33% and DA by 26% and decreased 5-HIAA by 22%, HVA by 23% and DOPAC by 40%; however, only the increases in 5-HT and NA reached statistical significance (P less than 0.05). Monoamine oxidase (MAO) activity in cortical homogenates was also measured following the chronic ritanserin and clorgyline regimens and also following ritanserin administration in vitro. Chronic clorgyline and ritanserin inhibited MAO activity by 60 and 39%, respectively. In vitro, ritanserin administration at concentrations of less than 10(-6) M had no effect on MAO activity but at doses higher than 10(-6) M, MAO activity was inhibited in a dose-dependent manner from 18 +/- 0.5% at 3 x 10(-6) M to 63 +/- 9% at 10(-4) M. Thus, ritanserin appears to act as an MAO inhibitor in addition to being a 5-HT2 antagonist and this may be related to its ability to induce 5-HT2 receptor down-regulation.