Effect of thiazol-4-ylmethoxyamine, a new inhibition of histamine biosynthesis on brain histamine, monoamine levels and behavior

H1-histamine receptor affinity predicts short-term weight gain for typical and atypical antipsychotic drugs

As a result of superior efficacy and overall tolerability, atypical antipsychotic drugs have become the treatment of choice for schizophrenia and related disorders, despite their side effects. Weight gain is a common and potentially serious complication of some antipsychotic drug therapy, and may be accompanied by hyperlipidemia, hypertension and hyperglycemia and, in some extreme cases, diabetic ketoacidosis. The molecular mechanism(s) responsible for antipsychotic drug-induced weight gain are unknown, but have been hypothesized to be because of interactions of antipsychotic drugs with several neurotransmitter receptors, including 5-HT(2A) and 5-HT(2C) serotonin receptors, H(1)-histamine receptors, alpha(1)- and alpha(2)-adrenergic receptors, and m3-muscarinic receptors. To determine the receptor(s) likely to be responsible for antipsychotic-drug-induced weight gain, we screened 17 typical and atypical antipsychotic drugs for binding to 12 neurotransmitter receptors. H(1)-histamine receptor affinities for this group of typical and atypical antipsychotic drugs were significantly correlated with weight gain (Spearman rho=-0.72; p<0.01), as were affinities for alpha(1A) adrenergic (rho=-0.54; p<0.05), 5-HT(2C) (rho=-0.49; p<0.05) and 5-HT(6) receptors (rho=-0.54; p<0.05), whereas eight other receptors' affinities were not. A principal components analysis showed that affinities at the H(1), alpha(2A), alpha(2B), 5-HT(2A), 5-HT(2C), and 5-HT(6) receptors were most highly correlated with the first principal component, and affinities for the D(2), 5-HT(1A), and 5-HT(7) receptors were most highly correlated with the second principal component. A discriminant functions analysis showed that affinities for the H(1) and alpha(1A) receptors were most highly correlated with the discriminant function axis. The discriminant function analysis, as well as the affinity for the H(1)-histamine receptor alone, correctly classified 15 of the 17 drugs into two groups; those that induce weight gain and those that do not. Because centrally acting H(1)-histamine receptor antagonists are known to induce weight gain with chronic use, and because H(1)-histamine receptor affinities are positively correlated with weight gain among typical and atypical antipsychotic drugs, it is recommended that the next generation of atypical antipsychotic drugs be screened to avoid H(1)-histamine receptors.

Effects of tryptophan and of 5-hydroxytryptamine receptor subtype agonists on feeding

Feeding or food withdrawal can affect the supply of tryptophan to the brain and hence (in some circumstances) 5-HT synthesis therein. Also fenfluramine which releases 5-HT to postsynaptic receptors suppresses appetite and there are reports that tryptophan can have a similar effect. Furthermore, feeding is reported to release hypothalamic 5-HT. Therefore 5-HT could have a role in the normal termination of feeding and perhaps also in disorders of appetite. The recognition of various 5-HT receptor subtypes has stimulated research in this area. We have now investigated the involvement of the subtypes in the pharmacological control of feeding. Thus, 5-HT1A agonists (8-OH-DPAT, buspirone, gepirone etc.) stimulate intake in freely feeding rats, probably by activating autoreceptors on the cell bodies of 5-HT neurons so that 5-HT release at terminals is decreased. The hyperphagia is not explicable by increased activity or gnawing and is strikingly manifest against carbohydrate in carbohydrate vs. protein choice experiments. Feeding in previously food deprived rats is decreased by the 5-HT agonists RU 24969, 1-(3-chlorophenyl)piperazine (mCPP) and 1-[3-(trifluoromethyl) phenyl]piperazine (TFMPP). Effects of antagonists suggest that RU 24969-induced hypophagia depends on 5-HT1B receptors only while mCPP and TFMPP induce hypophagia at 5-HT1C sites, though this effect also requires 5-HT1B receptors for its expression. Responsible sites occur in the paraventricular nucleus of the hypothalamus as infusing either RU 24969 or TFMPP therein causes hypophagia. On systemic injection, the hypophagic drugs are particularly active in female rats, an effect of conceivable relevance to human anorexic illness.

Serotonin and appetite

Feeding or food withdrawal can affect the supply of tryptophan to the brain and hence (in some circumstances) 5-HT synthesis therein. Also fenfluramine which releases 5-HT to postsynaptic receptors suppresses appetite, and there are reports that tryptophan can have a similar effect. Furthermore, feeding is reported to release hypothalamic 5-HT. Therefore 5-HT could have a role in the normal termination of feeding and perhaps also in disorders of appetite. The recognition of various 5-HT receptor subtypes has stimulated research in this area. We have now investigated the involvement of the subtypes in the pharmacological control of feeding. Thus, 5-HT1A agonists (8-OHDPAT, buspirone, gepirone, etc.) stimulate intake in freely feeding rats, probably by activating autoreceptors on the cell bodies of 5-HT neurons so that 5-HT release at terminals is decreased. The hyperphagia is not explicable by increased activity or gnawing and is strikingly manifest against carbohydrate in carbohydrate vs. protein choice experiments. Feeding in previously food-deprived rats is decreased by the 5-HT agonists RU 24969, 1-(3-chlorophenyl) piperazine (mCPP) and 1-(3-trifluoromethyl) phenyl) piperazine (TFMPP). Effects of antagonists on these properties suggest that RU 24969-induced hypophagia depends on 5-HT1B receptors only, while mCPP and TFMPP induce hypophagia at 5-HT1C sites, though this effect also requires 5-HT1B receptors for its expression. Responsible sites occur in the paraventricular nucleus of the hypothalamus as infusing either RU 24969 or TFMPP therein causes hypophagia. On systemic injection, the hypophagic drugs are particularly active in female rats, an effect of conceivable relevance to human anorexic illness.

Excitatory effect of histamine on the arousal system and its inhibition by H1 blockers

To clarify whether the sedative effect of H1 blockers is exerted in relation to H1 receptors in the brain, EEG activity recorded from the cortex and thalamus of rats was studied by power spectral analysis. EEG processing was performed by the FFT method and displayed as compressed spectral arrays. When a train of low frequency electrical stimulation was applied to the midbrain reticular formation of conscious rats, there was an increase in spectral power recorded at the cortex and thalamus, especially in the low frequency bands (0-6 Hz). The intraventricular administration of histamine suppressed the increase in power; this inhibition was antagonized by simultaneous administration of pyrilamine or diphenhydramine, though not in in combination with cimetidine or ranitidine. As in the case of histamine, the administration of 2-methylhistamine decreased power in the slow wave region, while administration of 4-methylhistamine did not. It was assumed that the arousal effect of histamine is exerted via H1 and not related to H2 receptors. Adverse effects of H1 blockers, such as drowsiness, may be caused by their inhibition of histamine's arousal effect.

Evidence that hypophagia induced by mCPP and TFMPP requires 5-HT1C and 5-HT1B receptors; hypophagia induced by RU 24969 only requires 5-HT1B receptors

Male Sprague-Dawley rats deprived of food for 18 h were injected with the 5-HT agonists RU 24969, 1-(3-chlorophenyl)piperazine (mCPP) or 1-[3-(trifluoromethyl)phenyl)]piperazine (TFMPP) and 20 min later presented with their normal diet. Food intake was determined 1, 2 and 4 h later. All three drugs reduced intake over 1 and 2 h. Three out of four drugs with high affinity for 5-HT1C receptors (metergoline, mianserin, and mesulergine but not cyproheptadine) opposed hypophagia caused by mCPP. Another drug reported to have high affinity for the 5-HT1C site, 1-naphthyl-piperazine (1-NP), also blocked the hypophagic response to mCPP at doses which attenuated mCPP-induced hypolocomotion. Only one of the above drugs (metergoline) which also has high affinity for other 5-HT sites opposed hypophagia caused by RU 24969. Two out of three 5-HT1B receptor antagonists [(+/-) cyanopindolol, (-) propranolol, but not (-) pindolol)] which oppose hypophagia caused by RU 24969 (Kennett et al. 1987) also opposed hypophagia caused by mCPP. The 5-HT2 antagonists ketanserin and ritanserin, the 5-HT3 antagonist ICS 205-930 and the alpha 2 adrenoceptor antagonist idazoxan did not oppose the hypophagic effect of mCPP. In agreement with results for mCPP, hypophagia caused by TFMPP was opposed by both, mianserin and (+/-) cyanopindolol. Given alone, mianserin 1-NP and cyproheptadine but not ICS 205-930 increased food consumption of normally fed rats. The results suggest that RU 24969-induced hypophagia depends on 5-HT1B receptors but not on 5-HT1C receptors, while mCPP (and TFMPP)-induced hypophagia may depend on both receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Food intake and selection after peripheral tryptophan

Two studies investigated the effects of peripheral (IP) administration of the dietary indispensible amino acid tryptophan, on food intake and macronutrient selection in rats adapted to a 12 hr nocturnal feeding period and a choice of 10% and 60% casein diets. In a dose-response study (35, 55, 75, 95, 115 mg/kg), the threshold dose of 75 mg/kg produced a significant reduction in total food intake (3.6 to 2.3 g, p less than 0.05) during the first hour of feeding. The reduction in carbohydrate intake (2.1 vs. 1.2 g, p less than 0.05) was greater than that for protein intake (1.6 vs. 1.1 g, p less than 0.05). Twelve hr total food intake was also decreased (20.9 to 19.5 g, p less than 0.05) and this was attributable to decreased carbohydrate intake (13.2 to 11.8 g, p less than 0.05). In a second study designed to determine if tryptophan's effects were mediated by the central nervous system, brain tryptophan uptake was blocked by co-injecting valine with tryptophan. The significant reduction in first hour total food intake by tryptophan was not prevented by co-injection of an equal quantity of valine (3.5 to 1.8 g, p less than 0.05). Again the suppression of carbohydrate intake (2.0 to 0.9 g p less than 0.05) was greater than that for protein intake (1.5 to 0.9 g, p less than 0.05). This dose of valine significantly reduced brain tryptophan uptake by 16% (21.3 to 17.8 micrograms/g, p less than 0.05) and when administered alone did not affect first hour total food intake (3.1 vs 3.2 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Food intake suppression by histidine

Following injection of histidine (as 1-histidine monohydrochloride, 500 mg/kg, IP) rats showed a suppression of total food intake within the first 2 hours of a 12 hour daily feeding period but not if the rats were adapted to a 4 hour daily feeding period. Furthermore, rats adapted to a nocturnal as compared to a diurnal 12 hour feeding period showed a greater response (50% vs. 20% suppression of feeding) to histidine. Overall, within an experiment, food intake suppression correlated with the histidine dose (0, 125, 250, 375 and 500 mg/kg; for mean response r(3) = 0.90, p less than 0.05) although the lowest dose measured to be effective in a cross-over design experiment was 375 mg/kg. No differential effect upon protein or carbohydrate intake was observed in any of the studies. The effects of injections of 250 and 500 mg/kg histidine on food intake were associated with significant elevations of brain histidine and histamine. We conclude that histidine, possibly by changes in brain histidine, influences total food intake but not macronutrient selection.

The inhibition of histamine formation in vivo

Histamine formation was measured by the [14C]histamine content of the stomach following injection of [14C]histidine into rats. Carbonyl reagents (e.g. oxyamines) which inactivate pyridoxal phosphate, the co-enzyme of histidine decarboxylase, were the most effective inhibitors of histamine formation in vivo. Although the oximes prepared from imidazol-4(5)-ylmethoxyamine and pyridoxal or pyridoxal phosphate cannot inactivate the co-enzyme, they proved sufficiently potent to merit further study as inhibitors of histamine formation in vivo.