Neural mechanisms mediating the hyperthermia elicited by prostaglandin E2 injected into the preoptic-anterior hypothalamus

The dorsomedial hypothalamus: a new player in thermoregulation

Neurons in the dorsomedial hypothalamus (DMH) play key roles in physiological responses to exteroceptive (“emotional”) stress in rats, including tachycardia. Tachycardia evoked from the DMH or seen in experimental stress in rats is blocked by microinjection of the GABAA receptor agonist muscimol into the rostral raphe pallidus (rRP), an important thermoregulatory site in the brain stem, where disinhibition elicits sympathetically mediated activation of brown adipose tissue (BAT) and cutaneous vasoconstriction in the tail. Disinhibition of neurons in the DMH also elevates core temperature in conscious rats and sympathetic activity to least significant difference interscapular BAT (IBAT) and IBAT temperature in anesthetized preparations. The latter effects are blocked by microinjection of muscimol into the rRP, while microinjection of muscimol into either the rRP or DMH suppresses increases in sympathetic nerve activity to IBAT, IBAT temperature, and core body temperature elicited either by microinjection of PGE2 into the preoptic area (an experimental model for fever), or central administration of fentanyl. Neurons concentrated in the dorsal region of the DMH project directly to the rRP, a location corresponding to that of neurons transsynaptically labeled from IBAT. Thus these neurons control nonshivering thermogenesis in rats, and their activation signals its recruitment in diverse experimental paradigms. Evidence also points to a role for neurons in the DMH in thermoregulatory cutaneous vasoconstriction, shivering, and endocrine adjustments. These directions provide intriguing avenues for future exploration that may expand our understanding of the DMH as an important hypothalamic site for the integration of autonomic, endocrine, and behavioral responses to diverse challenges.

Membrane depolarization by activation of prostaglandin E receptor EP3 subtype of putative serotonergic neurons in the dorsal raphe nucleus of the rat

A whole-cell current-clamp study using a thin slice preparation of the rat brain was carried out to elucidate the function of prostaglandin E (PGE) receptor EP3 subtype in the dorsal raphe nucleus (DR), where mRNA of this subtype is highly expressed. Bath application of PGE2 or M&B 28767, an EP3 agonist, depolarized the membrane of the large DR neurons in a concentration-dependent manner between 10(-9) and 10(-6) M. These neurons showed hyperpolarization of membrane potential to 10 or 50 microM serotonin. Neither an EP2 receptor agonist, butaprost, an EP2/EP4 receptor agonist, 11-deoxy-PGE1, nor an EP1 receptor agonist, 17-phenyl-PGE2, had any effect on large DR neurons between 10(-9) and 10(-6) M. The M&B 28767-induced depolarization was observed in a Ca(2+)-free, high Mg2+ (5 mM) solution containing 0.3 microM tetrodotoxin, and occurred equally well when intracellular Cl- was replaced by gluconate. These results suggest that direct agonist-activation of EP3 receptor depolarizes the membrane by a cationic conductance, leading to excitation of DR neurons, and indicate a physiological implication that EP3 receptor may modulate the serotonergic inhibition of neuronal activities.

Prostaglandins and hypothalamic neurotransmitter receptors involved in hyperthermia: a critical evaluation

The role of a prostaglandin of the E series (PGE) in the hypothalamic mechanisms underlying a fever continues to be controversial. This paper reviews the historical literature and current findings on the central action of the PGEs on body temperature (Tb). New experiments were undertaken to examine the local effect of muscarinic, nicotinic, serotonergic, alpha-adrenergic, or beta-adrenergic receptor antagonists at hypothalamic sites where PGE1 caused a rise in Tb of the primate. Guide tubes for microinjection were implanted stereotaxically above sites in and around the anterior hypothalamic, preoptic area (AH/POA) of male Macaque monkeys. Following postoperative recovery, 30-100 ng of PGE1 was micro-injected unilaterally in a volume of 1.0-1.5 microliter at sites in the AH/POA to evoke a rise in Tb, and once identified, pretreated with a receptor antagonist. PGE1 hyperthermia was significantly reduced by microinjections of the muscarinic and nicotinic antagonists, atropine, or mecamylamine, at PGE1 reactive sites in the AH/POA. The serotonergic antagonist, methysergide, injected at PGE1 sensitive sites in the ventromedial hypothalamus also attenuated the rise in Tb. However, the 5-HT reuptake blocker, fluoxetine, and the beta-adrenergic receptor antagonist, propranolol, injected in the AH/POA failed to alter the PGE1 hyperthermia. In contrast, the alpha-adrenergic antagonist, phentolamine, potentiated the increase in Tb at all PGE1 reactive sites in the hypothalamus. An updated model is presented to explain how the concurrent actions of aminergic neurotransmitters acting on their respective receptors in the hypothalamus can interact with a PGE to elicit hyperthermia. Finally, an evaluation of the current literature including recent findings on macrophage inflammatory protein (MIP-1) supports the conclusion that a PGE in the brain is neither an obligatory nor essential factor for the expression of a pyrogen fever.

Localization of central prostaglandin E2 antisecretory effects

Intracerebroventricular prostaglandin E2 (PGE2) inhibits stimulated gastric acid secretion; however, the central site of action is unknown. Specific PGE2 binding sites have been localized to the ventromedial hypothalamic nucleus and central amygdala (A). The nuclear accumbens has been shown to play a role in central neurotensin-induced antisecretory effects. These studies tested the hypothesis that microinjections of PGE2 into the ventromedial hypothalamic nucleus, central amygdala, and nuclear accumbens inhibit stimulated gastric acid secretion. The hippocampus served as a cerebral control region. Two days before the experiments, metal cannulas were stereotaxically positioned bilaterally into specific areas of the brain, and metal gastric cannulas were operatively implanted, under nembutal anesthesia, in male 250-g Sprague-Dawley rats. On the experimental day, the rats, fasted for 14 hours, were given saline or PGE2 (0.1-1.0 micrograms in 0.2 microL/side) through the central cannulas 10 minutes before administering pentagastrin (40 micrograms/kg SC). Gastric secretion was measured at 30-minute intervals and expressed as acid output, micromoles per hour. Acid output (mean +/- SE) in control animals was 161 +/- 14 mumol/h. Prostaglandin E2 administration at doses of 0.10, 0.50, and 1.0 micrograms/side (a) into ventromedial hypothalamic nucleus reduced acid output to 53 +/- 11,* 36 +/- 10,* and 27 +/- 11* mumol/h regularly; (b) into NACB reduced acid output to 157 +/- 36, 60 +/- 12,* and 38 +/- 12* mumol/h; and (c) into A reduced acid output to 144 +/- 31, 141 +/- 26, and 90 +/- 19* mumol/h, respectively (*P less than 0.05 by Neuman-Keuls test). Prostaglandin E2 (0.50 micrograms/side) administration into hippocampus had no significant effect on acid output (134 +/- 28 mumol/h). Although central PGE2 administration was associated with hyperthermia, this occurred at lower doses than those required to inhibit acid secretion. Prostaglandin E2 administration into specific brain areas known to have PGE2 receptors, the central amygdala and ventromedial hypothalamic nucleus, and into nuclear accumbens inhibits stimulated gastric acid secretion. These observations suggest that PGE2 may have a physiological role in the central control of gastric acid secretion.

E-series prostaglandins and arginine vasopressin in the modulation of male sexual behavior

Studies carried out recently in the author's laboratory have suggested that fever accompanies copulation in the male rat. Given the action of prostaglandin-E (PGE) in the genesis of fever and given the integrative role of the medial preoptic area (MPOA) in the expression of both fever and male sexual behavior, two hypotheses were advanced concerning male copulation. The first concerns PGE in facilitating transmission in MPOA pathways mediating mounting, intromission and ejaculation. The second concerns arginine vasopressin, a presumed "natural antagonist" of PGE, in inhibiting such transmission and eventually making the male refractory to the receptive female. Several experiments were suggested for testing each hypothesis.