Serotonin1A (5-HT1A) receptors mediate diverse signaling cascades via different G-proteins that are involved in various physiological functions and play a role in the etiology and/or treatment of mood disorders. Mood disorders are associated with increased activation of the HPA axis that is normalized by antidepressants. In neuroendocrine neurons in the adult hypothalamus, fluoxetine (Prozac(R)) and other serotonin-selective reuptake inhibitors (SSRIs) desensitize 5-HT1A activation of plasma hormones such as ACTH and oxytocin. The long-term objective of this proposal is to develop better treatment strategies by understanding the mechanisms invoked by various classes of agonists in mediating 5-HT receptor signaling of multiple intracellular pathways upon acute versus long-term repetitive drug administration. Because the clinical effectiveness of various drugs including the SSRIs is associated with adaptive changes in 5-HT1A receptor signaling, it is critical to understand the mechanisms by which 5-HT1A receptors mediate signaling in neuroendocrine neurons in response to the acute and chronic exposure to different classes of 5-HT agonists. Based on our initial findings that 5-HT1A receptors can couple to different populations of G proteins to activate both MAP kinase and hormone responses via pathways that act independently upon acute receptor activation but exhibit """"""""cross-talk"""""""" resulting in desensitization of 5-HT1A mediated neuroendocrine response upon repetitive drug administration, we HYPOTHESIZE: MAP kinase activation plays an integral role in the neuroadaptive changes in 5-HT1A receptor-mediated hormone signaling in oxytocin and CRF containing neuroendocrine neurons in response to SSRIs and SNRI drugs. This will be tested by the four aims of this project;
Aim 1 will determine effectiveness of different classes of agonists to traffick 5-HT1A receptors to Gai/o proteins and activate hypothalamic MAP kinase and/or Gaz-proteins to stimulate hormone responses;
Aim 2; will determine the specific Ga-protein subtypes that mediate 5-HT1A receptor agonist activation of MAP kinase in the hypothalamic PVN and the localization of 5-HT1A activated MAP kinase (pERK) to oxytocin- and CRF-positive neurons;
Aim 3 will determine the mechanisms by which serotonin/norepinephrine reuptake inhibitors (SNRIs) desensitize 5-HT1A receptor signaling of hormone responses in oxytocin and CRF-containing neuroendocrine neurons and the requirement of MAP kinase in mediating SNRI-induced desensitization, and Aim 4 will determine the efficacy of repetitive administration of different classes of serotonergic agonists to desensitize hypothalamic 5-HT1A receptor mediated MAP kinase signaling. These studies will provide important new information regarding the novel mechanisms of 5-HT1A receptor signaling pathways in hypothalamic neurons and the mechanisms by which clinically used antidepressant drugs may produce adaptive changes in signaling pathways mediated by different G proteins. These studies will elucidate role of MAP kinase in regulating the responsiveness of neuroendocrine neurons to various classes of drugs used clinically. This information is critical to identifying the mechanisms that may contribute to delays in the onset of clinical efficacy or the """"""""side effects"""""""" of SSRIs, SNRIs or other drugs developed to treat various psychopathologies and mood disorders involving 5-HT1A receptors.
Alterations in serotonergic and hypothalamic function are associated with mood disorders and can be effectively treated by drugs that target 5-HT function. In the previous project, we demonstrated that acute activation of 5-HT1A receptors in the paraventricular nucleus of the hypothalamus can concomitantly stimulate plasma hormone responses and activate extracellular signal regulated kinase (i.e. ERK or MAP kinase) via distinct G protein subtypes in mutually independent pathways and that each exhibit differential responsiveness to drug-induced desensitization. Despite the independence of these pathways to acute activation, activation of MAP kinase is required for chronic treatment with serotonin selective reuptake inhibitors(SSRIs) to desensitize 5-HT1A signaling of hormones from oxytocin and ACTH containing neurons. This represents the first evidence in neuroendocrine neurons in vivo of multiple 5-HT1A signal tranduction pathways that are independent with respect to acute activation but exhibit crosstalk between pathways in a heterologous manner upon chronic drug exposure. The studies proposed will fill a number of gaps in our understanding 5-HT1A signaling and regulation in neuroendocrine neurons and the critical role of MAP kinase in drug-induced desensitization. We propose to investigate: the dose-dependence of different classes of agonists to traffick via one or multiple 5-HT1A receptor signaling pathways, the subtypes of G proteins required for MAP kinase activation in PVN and at the cellular level (on oxytocin and CRF positive cells);the mechanisms of desensitization mediated by serotonin/norepinephrine reuptake inhibitors (SNRIs) and the requirement of MAP kinase for SNRI desensitization of the 5-HT1A signaling of hormones;and the sensitivity or resilience of MAP kinase signaling to desensitization by repetitive activation of 5-HT1A receptors by different classes of serotonergic agonists. .
