Fenfluramine-induced hypothermia is associated with cutaneous dilation in conscious rats

Select panicogenic drugs and stimuli induce consistent increases in tail skin flushes and decreases in core body temperature

Panic attacks (PAs) are episodes of intense fear or discomfort that are accompanied by a variety of both psychological and somatic symptoms. Panic induction in preclinical models (e.g. rats) has largely been assayed through flight and avoidance behavioral tests and cardiorespiratory activity. Yet, the literature pertaining to PAs shows that thermal sensations (hot flushes/heat sensations and chills) are also a common symptom during PAs in humans. Considering that temperature alterations are objectively measurable in rodents, we hypothesized that select panicogenic drugs and stimuli induce consistent changes in thermoregulation related to hot flushes and chills. Specifically, we challenged male rats with intraperitoneal injections of the GABAergic inverse agonist FG-7142; the α2 adrenoceptor antagonist yohimbine; the serotonin agonist D-fenfluramine, and 20% CO2 (an interoceptive homeostatic challenge). We assayed core body temperature and tail skin temperature using implanted radiotelemetry probes and tail thermistors/thermal imaging camera, respectively, and found that all challenges elicited rapid, high-amplitude (~7-9°C) increase in tail skin temperature and delayed decreases (~1-3°C) in core body temperature. We propose that thermal sensations such as these may be an additional indicator of a panic response in rodents and humans, as these panicogenic compounds or stimuli are known to precipitate PAs in persons with panic disorder.

5-hydroxytryptamine (5-HT) reduces total peripheral resistance during chronic infusion: direct arterial mesenteric relaxation is not involved

Serotonin (5-hydroxytryptamine; 5-HT) delivered over 1 week results in a sustained fall in blood pressure in the sham and deoxycorticosterone acetate (DOCA)-salt rat. We hypothesized 5-HT lowers blood pressure through direct receptor-mediated vascular relaxation. In vivo, 5-HT reduced mean arterial pressure (MAP), increased heart rate, stroke volume, cardiac index, and reduced total peripheral resistance during a 1 week infusion of 5-HT (25 µg/kg/min) in the normotensive Sprague Dawley rat. The mesenteric vasculature was chosen as an ideal candidate for the site of 5-HT receptor mediated vascular relaxation given the high percentage of cardiac output the site receives. Real-time RT-PCR demonstrated that mRNA transcripts for the 5-HT_2B, 5-HT_1B, and 5-HT₇ receptors are present in sham and DOCA-salt superior mesenteric arteries. Immunohistochemistry and Western blot validated the presence of the 5-HT_2B, 5- HT_1B and 5-HT₇ receptor protein in sham and DOCA-salt superior mesenteric artery. Isometric contractile force was measured in endothelium-intact superior mesenteric artery and mesenteric resistance arteries in which the contractile 5- HT_2A receptor was antagonized. Maximum concentrations of BW-723C86 (5- HT_2B agonist), CP 93129 (5-HT_1B agonist) or LP-44 (5-HT₇ agonist) did not relax the superior mesenteric artery from DOCA-salt rats vs. vehicle. Additionally, 5-HT (10^–9 M to 10^–5 M) did not cause relaxation in either contracted mesenteric resistance arteries or superior mesenteric arteries from normotensive Sprague- Dawley rats. Thus, although 5-HT receptors known to mediate vascular relaxation are present in the superior mesenteric artery, they are not functional, and are therefore not likely involved in a 5-HT-induced fall in total peripheral resistance and MAP.

Serotonin and blood pressure regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.