1997]. A putative mechanism and argument The exact mechanisms
by which SSRIs render hyperprolactinemia and cause several clinical consequences such as amenorrhea and galactorrhea (neuroendocrine effects) remain elusive. There is evidence that serotonin might stimulate prolactin release directly via postsynaptic 5-HT receptors in the hypothalamus [Nicholas et al. 1998], or indirectly via 5-HT-mediated inhibition of tuberoinfundibular Inhibitors,research,lifescience,medical dopaminergic neurons [Arya, 1994]. The serotoninergic neurons project from the dorsal raphe nucleus to the medial basal hypothalamus and exert their action via 5HT1A and 5HT2 receptors and paraventricular Inhibitors,research,lifescience,medical nucleus containing different populations of neurosecretory cells producing oxytocin, vasopressin, vasoactive intestinal peptide (VIP), thyrotropin-releasing hormone (TRH) and other neuropeptides [Emiliano and Fudge, 2004; Aizawa and Hinkle, 1985; Benker et al. 1990; Bjoro et al. 1990]. It is known that serotonin affects the prolactin level through the action of one or more of these prolactin-releasing factors (PRFs), among which the VIP pathway is the best studied. VIP acts both via hypothalamic afferents and direct paracrine and autocrine PD0332991 mouse mechanisms, through lactotroph cell receptors binding, enhancing adenylate Inhibitors,research,lifescience,medical cyclase activity and increasing prolactin gene transcription.
Oxytocin seems to participate in VIP-induced Inhibitors,research,lifescience,medical prolactin release and could act through the inhibition of the tuberoinfundibular dopamine pathway (TIDA) [Emiliano and Fudge, 2004; Ben-Jonathan, 1994; Wanke and Rorstad, 1990; McCann et al. 1984; Lightman and Young, 1987; Mogg and Samson, 1990; Samson et al. 1986, Inhibitors,research,lifescience,medical 1989]. However, there is little synaptic contact between serotonin fibers and dopaminergic cells. Hence, if direct inhibition of dopaminergic cells occurs, it is rather through this serotonin that volume transmission occurs in this region [Kiss and Halasz, 1986]. A wealth of scientific and clinical evidence also supports the concept that direct
stimulation of GABAergic neurons in the vicinity of dopamine cells, the tuberoinfundibular–GABA (TI-GABA) system, is involved in modulating prolactin secretion in humans, possibly through serotoninergic stimulation of GABA interneurons via the 5HT1A membrane receptor, resulting in inhibition of only TIDA cells and causing the tonic inhibition of prolactin release [Ondo and Dom, 1986; Wagner et al. 1994; Fuchs et al. 1984; Mirkes and Bethea, 2001] (Figure 1). Figure 1. Putative pathways through which fluoxetine and norfluoxetine may stimulate prolactin releasing factors (PRF) such as oxytocin (OT) and vasoactive intestinal peptide (VIP) via 5HT2A receptor predominantly on PVN (neurosecretory magnocellular cell) via …