Effect of the 5-HT receptor agonist DOI on female rat sexual behavior

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.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Female Reproductive Behavior

Reproductive behavior is the behavior related to the production of offspring and includes all aspects from the establishment of mating systems, courtship, sexual behavior, and parturition to the care of young. In this chapter, I outline the hormonal regulation of the estrous cycle, followed by a description of the neural regulation of female sexual behavior. Ovarian hormones play an important role in the induction of ovulation and behavioral estrus, in which they interact closely with several neurotransmitters and neuropeptides to induce sexual behavior. This chapter discusses the latest research on the role of estrogen, progesterone, serotonin, dopamine, noradrenaline, oxytocin, and GABA in female mating behavior. In addition, the most relevant brain areas, such as the preoptic area and the ventromedial nucleus of the hypothalamus, in which these regulations take place, are discussed.

Neuroanatomical dichotomy of sexual behaviors in rodents: a special emphasis on brain serotonin

Much of the social behavior in which rodents engage is related to reproduction, such as maintaining a breeding territory, seeking mates, mating, and caring for their young. Rodents belong to the internally fertilizing species that require sexual behavior for reproduction. The dyadic, heterosexual patterns of most mammalian species are sexually dimorphic, but they also share mutual components in both sexes: sexual attraction is reciprocal, sexual initiative is assumed, appetitive behavior is engaged in, and mating involves consummatory and postconsummatory phases in females as well as in males. Serotonin, a phylogenetically ancient molecule, is the most widely distributed neurotransmitter in the brain and its signaling pathways are essential for numerous functions including sexual behavior. Since the late 1960s, brain serotonergic neurotransmission has been considered to exert an inhibitory influence on the neural mechanisms mediating sexual behavior. This contention was based mainly on the observations that a decrease in central serotonergic activity facilitated the elicitation of sexual behavior, whereas an increase in central serotonergic activity attenuated it. However, the discovery of over 14 types of serotonin receptors has added numerous layers of complexity to the study of serotonin and sexual behavior. Evidence shows that, upon activation, certain receptor subtypes facilitate, whereas some others suppress, sexual behavior, as well as sexual arousal and motivation. Furthermore, the role of these receptors has been shown to be different in the male and female sexes. The use of serotonergic pharmacological interventions, mouse strains with genetic polymorphisms causing alterations in the levels of brain serotonin, and animal models with genetic manipulations of various serotonin effectors has helped delineate the fundamental role of this neurotransmitter in the regulation of sexual behavior. This review aims to examine the basics of the components of female and male sexual behavior and the participation of the serotonin system in the modulation of these behaviors, with emphasis on rodents.

Pharmacology of serotonin and female sexual behavior

In this review, first a historical perspective of serotonin's (5-HT) involvement in female sexual behavior is presented. Then an overview of studies implicating 5-HT is presented. The effect of drugs that increase or decrease CNS levels of 5-HT is reviewed. Evidence is presented that drugs which increase 5-HT have negative effects on female sexual behavior while a decrease in 5-HT is associated with facilitation of sexual behavior. Studies with compounds that act on 5-HT₁, 5-HT₂ or 5-HT₃ receptors are discussed. Most evidence indicates that 5-HT₁A receptor agonists inhibit sexual behavior while 5-HT₂ or 5-HT₃ receptors may exert a positive influence. There is substantial evidence to support a role for 5-HT in the modulation of female consummatory sexual behavior, but studies on the role of 5-HT in other elements of female sexual behavior (e.g. desire, motivation, sexual appetite) are few. Future studies should be directed at determining if these additional components of female sexual behavior are also modulated by 5-HT.

Flibanserin treatment increases appetitive sexual motivation in the female rat

INTRODUCTION

Flibanserin is a mixed 5-HT1A agonist/5-HT2A antagonist that has been developed for the treatment of hypoactive sexual desire disorder in women.

AIM

To assess the acute and chronic dose-response effects of flibanserin on measures of sexual desire and copulation in ovariectomized rats primed with estradiol benzoate (EB) alone or in combination with progesterone (P).

METHODS

In Experiment 1, sexually experienced ovariectomized (OVX) rats at one testing site were rendered fully sexually receptive with EB + P priming and tested weekly with a sexually active male in bi-level pacing chambers following daily flibanserin treatment for 28 days. In Experiment 2, sexually experienced OVX rats at a different testing site received EB alone and were tested weekly with sexually active males following daily flibanserin treatment.

MAIN OUTCOME MEASURES

Female appetitive behaviors (solicitations, hops and darts, anogenital investigations), defensive behaviors, pacing, lordosis, and male copulatory responses (intromissions and ejaculations) were measured during each 30-minute copulation test.

RESULTS

Acute flibanserin or 1 week of chronic flibanserin treatment did not modify sexual responses in fully (EB + P) or partially (EB-alone) primed females. After 2 weeks of chronic treatment, fully primed females displayed significantly more solicitations than the three other groups. After 3 weeks of chronic treatment, a significant increase in female solicitations was observed in both hormone-treatment groups.

CONCLUSION

This study shows the first evidence that chronic, but not acute, flibanserin treatment augments appetitive sexual behaviors in OVX female rats primed with EB + P or EB alone. Given the positive effect of flibanserin in clinical trials, these results confirm previous reports that solicitations in the female rat are a predictive animal model of human female sexual desire.

Paroxetine-induced reduction of sexual incentive motivation in female rats is not modified by 5-HT1B or 5-HT2C antagonists

RATIONALE

Clinical data show that paroxetine causes sexual dysfunction in a substantial proportion of women taking this compound.

OBJECTIVES

This work was conducted to determine whether chronic paroxetine reduces sexual incentive motivation in female rats and whether this compound can modify any aspect of paced mating. The role of the 5-HT(1B) and 5-HT(2C) receptors in any potential effects was also evaluated.

METHODS

Ovariectomized female rats were implanted with osmotic minipumps releasing 10 mg/kg per day of paroxetine or vehicle for 28 days. Tests for sexual incentive motivation and paced mating were performed just before implantation and at regular intervals thereafter. The females were primed with estradiol benzoate (25 μg/rat) and progesterone (1 mg/rat) before each of these tests. On days 25-27 of treatment, the females were injected with the 5-HT(1B) antagonist GR125,743 (5 mg/kg), the 5-HT(2C) antagonist SB206,553 (5 mg/kg) and vehicle in counterbalanced order. Preinjection time was 30 min.

RESULTS

Paroxetine reduced sexual incentive motivation on day 20 of treatment without affecting any aspect of paced mating. None of the antagonists modified the inhibitory effect of paroxetine on sexual incentive motivation. In the group chronically treated with vehicle, SB206,553 reduced proceptive behaviors in the paced mating test. No other effect was obtained.

CONCLUSION

The effects of paroxetine seen in female rats are similar to those observed in women, suggesting that disturbances of sexual incentive motivation in rats are predictive of sexual dysfunction in women. The 5-HT(1B) and 5-HT(2C) receptors do not seem to be of any importance for paroxetine's inhibitory effect.

Serotonin 5-HT2A and 5-HT5A receptors are expressed by different motoneuron populations in rat Onuf's nucleus

Motoneurons of Onuf's nucleus innervate the pelvic striated muscles, which play a crucial role in erection, ejaculation, and urinary continence. Serotonergic descending projections from the brain are involved in the modulation of Onuf's motoneuron activity. However, conflicting results regarding the effects of spinal serotonin (5-HT) on pelvi-perineal functions have been reported. They may be partly accounted for by the multiplicity of neuronal targets and receptor subtypes on which 5-HT is acting. In order to provide comparative data regarding 5-HT receptor expression in various groups of Onuf's motoneurons, we used retrograde tracing techniques from different pelvic muscles combined with immunocytochemistry of 5-HT2A and 5-HT5A receptors in male and female rats. In males, 5-HT2A receptor immunolabeling was very dense in motoneurons innervating the ischiocavernosus muscle. By contrast, in female rats, 5-HT2A receptor expression in Onuf's nucleus was very weak. In both genders, 5-HT5A receptor immunoreactivity was found in motoneurons innervating the external urethral sphincter. In males, a moderate or low 5-HT5A immunolabeling was observed in motoneurons innervating the bulbospongiosus and ischiocavernosus muscles, respectively. These data show a preferential localization of 5-HT2A and 5-HT5A receptors to motoneurons controlling the striated muscles located at the penile crus and sphincter muscles, respectively, suggesting a specific serotoninergic control on different pelvic functions. In addition, the subcellular distribution of receptors suggests a different mode of action of 5-HT, paracrine at 5-HT2A receptors and synaptic at 5-HT5A receptors. This might have implications for pharmacological research targeting different pelvic functions e.g., micturition and ejaculation.