Serotonin (5-hydroxytryptamine, 5-HT) is an abundant neurotransmitter found predominantly in enteric gastrointestinal enterochromaffin cells and platelets but also in the central and peripheral nervous system neurons [1]. Serotonin (5-HT) receptors are classified into a complex of four families based upon molecular characteristics. Each family is further subdivided into fourteen subtypes. Of these subtypes, 5-HT₃ is exclusively a ligand gated ion channel [2] and therefore distinct from the predominance of 5-HT subtypes which couple to various GTP-binding proteins [3]. The serotonergic system’s role in the processing of pain has been extensively delineated. Bulbospinal and supratentorial serotonergic pathways suppress spinal pain afferents and nociception while peripheral serotonergic effects are strongly pro-algesic.

Table 1: 5HT Family of receptors

Location and action of 5-HT₃ receptors

The 5-HT₃ receptor family consists of a ligand-gated ion channel seritonergic receptor existing in two subtypes, A and B (table 1). The subtype 5-HT₃a in distributed both centrally and peripherally while the subtype 5-HT₃b is exclusively peripherally distributed [4]. 5-HT₃b however, functions only in conjunction with the 5-HT₃a subtype and therefore is not a homomeric receptor [5]. Serotonin 5-HT₃ receptors have been localized peripherally, in vagus nerve afferents, in the inferior ganglia of the vagus nerve (nodose), in pre- and postganglionic autonomic fibers, and in dorsal root ganglia nociceptive afferents [6,7]. Centrally they are found in Rexed’s lamina II (nucleus Substantia Gelatinosa) of the spinal gray dorsal horn and in several supraspinal loci including the area postrema, nuclei of the solitary tract, the nuclei ambiguous, the nuclei accumbens, amygdala, and habenula, the hippocampus and diffusely throughout the cortex [8,9].

Table 1: 5-HT Receptors.

Action of peripheral 5-HT₃ receptors

Injection of 5-HT into peripheral tissue initiates tissue trauma and the release of arachidonic acid, prostaglandins, bradykinin and the interleukins (IL-1,2& 6) in a dose-dependent response of inflammation and pain [10]. The release of kinins precipitates vasodilation and the release extravascular circulatory components such as platelets and free blood-borne serotonin [11]. Platelets release further serotonin into the surrounding tissues activating 5-HT₃ receptors on free nerve endings which are apparently responsible for nociceptive effects and probably also the perpetuation of a delayed secondary inflammation [7,10,12]. Delineation of the role of peripheral 5-HT₃ receptors was provided by peripheral intraplantar injections of 5-HT₃ antagonists ICS 205-930 and MDL 72222 which produced dose-dependent anti-nociceptive effects against inflammatory pain [12].

Action of central 5-HT₃ receptors

The central 5-HT system is a major participant in analgesia. Seritonergic fibers from the raphe nuclei contribute to the dorsolateral funiculi which innervate the superficial dorsal horn of the spinal cord [13]. 5-HT₃ receptors have been identified in the superficial laminae of the spinal dorsal horn [9,4]. Activation of these descending 5-HT pathways causes the release of serotonin at synaptic connections with both nociceptive afferents and interneurons within the cord to produce analgesia [12,14]. Administration of 5-HT₃ receptor antagonists blocked 5-HT-induced analgesia and produced a moderate hyperalgesic response [15]. The reduction of dorsolateral funiculus modulated anti-nociception by administration of 5-HT₃ receptor antagonists suggested that 5-HT₃ receptors are involved with the expression of feed back, serotonergic pain modulation at the spinal level [16]. Further, Intraspinal administration of the 5-HT₃ receptor agonist, 2-methylserotonin produced significant dose-dependent analgesia against inflammatory and thermal pain, but not mechanical pain [17].

Published data supports that 5-HT, released from descending dorsolateral funicular pathways binds to spinal gray dorsal horn interneuronal 5-HT₃ receptors, depolarizing them [18]. Interneuron depolarization releases GABA and opioids which inhibit primary and/ or second-order nocisponsive neurons [17].

Table 2: Location of 5-HT₃ receptors

Antagonism of 5-HT₃ receptors by gonadal steroids

Gonadal steroids probably combine allosterically with 5-HT₃ receptors at the receptor-membrane surface [19] (table 2). The competition between serotonin and gonadal steroids to react with 5-HT₃ receptor sites has been repeatedly demonstrated therefore establishing gonadal steroids as serotonin antagonists and potentiating analgesia [19]. Specifically, 17-beta-estradiol, testosterone and progesterone have been shown to act on the surface of the cell membrane as noncompetitive antagonists for 5-HT₃ receptors [20-22]. Further, ovarian hormones play a role in regulating 5-HT₃ receptor expression in stressinduced bowel disfunction [23]. Both the 5-HT₃ receptor channel and the voltage-gated sodium channel are steroid targets. This is compatible with a common mechanistic principle in steroid-induced inhibition of the two channels [19].

Table 2: 5-HT₃ receptor locations.

A search of the current and historical literature was conducted.

Early estrus influence on inflammatory pain

Many studies have suggested that a gender difference exists in the processing of painful stimuli in both humans and rats [24-27]. Further, several studies conclude that pain processing is estrus cycle stage dependant in rats [23,28]. 5-HT₃ receptors have been shown to mediate inflammatory pain both [1] peripherally, by depolarizing C-fibers which terminate in the dorsal horn and [2] in the spinal cord dorsal horn hyperpolarizing GABA and opoidergic interneurons which terminate on ascending secondary pain afferent (spinothalamic) neurons [21]. As previously stated, 5-HT₃ receptors are non-competitively antagonized by gonadal steroids [27,29].

During the initial fourteen days of the female menstral cycle estrogen levels rise in response to follicle stimulating hormone (FSH) effects on ovarian follicular cells. Though 70% of circulating estrogens are bound to sex steroid-binding globulin and 25% to plasma albumin, free estrogens are highest on the thirteenth day of mensus [30]. With circulating estrogen levels high, making it more available for binding with 5-HT₃ receptors, it seems logical that females would be less responsive to inflammatory pain during that period and particularly at ovulation as a result of 5-HT₃ receptor competition as it has been shown in mice [31]. Circulating progesterone levels, in response to leutenizing hormone (LH) increase from day fourteen until approximately day twenty one, maintaining the potential for some level of continued inflammatory pain analgesia. However, there exists an obvious need for further investigation into the effects of gonadal steroids on pain resulting from tissue inflammation.

Competition between serotonin and gonadal steroids for the 5-HT₃ receptor site reduces the propagation of peripheral inflammatory nociception during periods of increased circulating gonadal steroids. This conclusion may be a consideration when pain treatment regimens include analgesia.