Effects of ketanserin on DOI-, MCPP- and TRH-induced prolactin secretion in estrogen-treated rats

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI): From an Obscure to Pivotal Member of the DOX Family of Serotonergic Psychedelic Agents - A Review

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI, or DOX where X = -I) was first synthesized in 1973 in a structure-activity study to explore the effect of various aryl substituents on the then newly identified, and subsequently controlled, hallucinogenic agent 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM, or DOX where X = -CH3). Over time, DOI was found to be a serotonin (5-HT) receptor agonist using various peripheral 5-HT receptor tissue assays and later, following the identification of multiple families of central 5-HT receptors, an agonist at 5-HT2 serotonin receptors in rat and, then, human brain. Today, classical hallucinogens, currently referred to as serotonergic psychedelic agents, are receiving considerable attention for their potential therapeutic application in various neuropsychiatric disorders including treatment-resistant depression. Here, we review, for the first time, the historical and current developments that led to DOI becoming a unique, perhaps a landmark, agent in 5-HT2 receptor research.

An endogenous serotonergic rhythm acting on 5-HT(2A) receptors may be involved in the diurnal changes in tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in female rats

The central serotonergic system has long been known to have a stimulatory role on the secretion of prolactin (PRL). The integrity of serotonergic neurotransmission is essential for the expression of the estrogen-induced afternoon PRL surge. Whether its effect on PRL involves change in the activity of tuberoinfundibular dopaminergic (TIDA) neurons has not been ascertained. In adult ovariectomized rats treated with estrogen, depletion of central serotonin (5-HT) by 5,7-dihydroxytryptamine (5,7-DHT, 200 microg/rat, i.c.v.) effectively prevented the afternoon fall in TIDA neuronal activity (using the levels of 3,4-dihydroxyphenylalanine and 3, 4-dihydroxyphenylacetic acid (DOPAC), and the ratio of DOPAC/dopamine in the median eminence as indices), and blunted the afternoon PRL surge. A single injection of a 5-HT(2A) receptor antagonist, ketanserin (5 mg/kg, i.p. at 12.00 h), also had the same effects on the diurnal changes in TIDA neuronal activity and PRL secretion as the treatment with 5,7-DHT did. Intracerebroventricular injection of a 5-HT(2) receptor agonist 2, 5-dimethoxy-4-iodoamphetamine (DOI) in the morning inhibited the TIDA neuronal activity and stimulated PRL secretion in a dose-dependent manner; while injection of a 5-HT(1) agonist, 8-hydroxy-dipropylaminotetralin, was without effect. Injection of DOI in 5,7-DHT-pretreated rats at 14.30 h also lowered the TIDA neuronal activity and reinstated the PRL surge. In all, endogenous 5-HT, acting through the 5-HT(2A) receptor, appears to exhibit an inhibitory effect on TIDA neuronal activity during the afternoon, which is essential for the PRL surge.

Clomipramine challenge responses covary with Tridimensional Personality Questionnaire scores in healthy subjects

Cloninger's Unified Biosocial Theory of Personality postulates a relationship between the relative functional activity of central serotonergic, dopaminergic, and noradrenergic neurotransmitter systems, and the strength of three elemental dimensions of personality. These dimensions are Harm Avoidance, Novelty Seeking, and Reward Dependence, respectively. Accordingly, we predicted that neuroendocrine responses to serotonergic challenge would correlate with Harm Avoidance scores, but not with Novelty Seeking or Reward Dependence scores. We examined the relationship between the prolactin and cortisol responses to a 12.5-mg intravenous clomipramine challenge and these personality dimensions as measured by Cloninger's Tridimensional Personality Questionnaire in 32 healthy subjects. The cortisol response correlated only with Harm Avoidance scores, as predicted; however, prolactin response did not correlate with Harm Avoidance scores. Instead, it demonstrated an inverse relationship with Novelty Seeking scores. There was a positive relationship of baseline prolactin with Harm Avoidance in a post hoc analysis. Cortisol response to serotonergic challenge may be a better indicator for responsivity of serotonergic systems as they relate to the personality dimension of Harm Avoidance than is prolactin. Prolactin responses may be overly affected by dopaminergic influences; however, baseline prolactin may still be a valid indicator of serotonergic tone.

Central administration of 8-OH-DPAT and mCPP stimulates prolactin secretion in ovariectomized, estrogen-treated rats: lack of an effect on tuberoinfundibular dopaminergic neuron activity

The effects of central administration of two serotonin receptor agonists, 8-OH-DPAT and mCPP, on tuberoinfundibular dopaminergic (TIDA) neuron activity and serum prolactin (PRL) levels in ovariectomized, estrogen-treated rats were determined. 8-OH-DPAT dose-dependently (0.1-10 microgram/rat, icv) stimulated serum PRL levels, and depressed serotonergic neuron activity in 30 min. However, the TIDA neuron activity was not affected at all. Similar treatment of mCPP was less effective than 8-OH-DPAT: only the highest dose of mCPP (10 microgram) stimulated PRL secretion and inhibited serotonergic neuron activity. No change in TIDA neuron activity was observed either. We conclude that central serotonin acts on 5-HT1 receptors to stimulate the PRL secretion, which may not involve the TIDA neurons.

Involvement of the 5-HT2 receptor in the 5-HT receptor-mediated stimulation of prolactin release

The present study examined the involvement of 5-HT1C and 5-HT2 receptors in the regulation of prolactin release in the rat. Both the mixed 5-HT2/5-HT1C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and the preferential 5-HT1C agonist, 2-chloro-6-(1-piperazinyl)pyrazine monohydrochloride (MK-212), elicited a significant increase in plasma prolactin concentration with DOI about 20 times more potent than MK-212. Treatment with DOI, but not with MK-212, induced head twitching in the rat, and this behavior was inhibited by both the mixed 5-HT2/5-HT1C receptor antagonist, ritanserin, and the selective 5-HT2 receptor antagonist, amperozide. DOI-induced prolactin release was also antagonized by ritanserin and amperozide, whereas only ritanserin affected MK-212-induced prolactin release. Furthermore, amperozide did not attenuate d-fenfluramine-elicited prolactin release, which is known to be antagonized by ritanserin. These data suggest that the pharmacological stimulation of prolactin release by DOI is mediated via the 5-HT2 receptor and that the 5-HT1C receptor may be of importance for the physiological regulation of prolactin release.