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Severe hemorrhage lowers arterial pressure by suppressing sympathetic activity. The central mechanism that initially triggers the fall in arterial pressure evoked by hemorrhage is not well understood, although opioid neurons are thought to play a role. This study tested the hypothesis that hemorrhagic hypotension is mediated by delta opioid receptors in the ventrolateral periaqueductal gray (vlPAG), a region importantly involved in opioid analgesia. Depressor sites were first identified by microinjecting DL-homocysteic acid (20 nmol/0.1 microl) or beta-endorphin (0.5 nmol/0.1 microl) into the vlPAG of halothane-anesthetized rats. Consistent with earlier reports, DL-homocysteic acid injection into the caudal vlPAG lowered arterial pressure and heart rate; beta-endorphin evoked a comparable depressor response, but did not affect heart rate. Naloxone or selective opioid receptor antagonists were subsequently injected into the vlPAG 5 min before hemorrhage (1.9 or 2.5 ml/100 g of body weight over 20 min) was initiated using the same stereotaxic coordinates. Naloxone injection into the caudal vlPAG completely prevented the fall in arterial pressure evoked by hemorrhage. The response was dose-dependent and evident with both fixed volume and fixed pressure hemorrhage. The delta opioid receptor antagonist naltrindole inhibited hemorrhagic hypotension significantly in both conscious and anesthetized rats but mu and kappa receptor antagonists were ineffective. beta-Endorphin(1--27), an endogenous opioid receptor antagonist, was also significantly inhibitory. Naltrindole was ineffective when injected into the dorsolateral periaqueductal gray and did not influence cardiovascular function in nonhemorrhaged animals. These data support the hypothesis that hemorrhagic hypotension is mediated by delta opioid receptors in the vlPAG.

Alpha-MSH has been implicated in changing attention behavior following peripheral injections, but no brain sites were studied. In the present report, alpha-MSH was injected directly into specific sites in the medial anterior hypothalamic/preoptic area (MAHPOA) while measuring performance in a visually cued discrimination task. Alpha-MSH injections resulted in reduced errors, indicated by decreased responding during noncued intervals, but no change in responding to correct cues. The improved error rate was consistent with attentional changes in a variety of paradigms. Attentional and motivational parameters were differentiated. The injected alpha-MSH appears to act on an inhibitory component of an attentional mechanism.

Chronic administration of aminotriazole (0.5 g/kg food) to rats was accompanied by a reduced responsiveness to acute administration of the beta-adrenergic agonist, l-isoproterenol (50-100 mug/kg, sc). The responses tested included water intake, change in heart rate in the anesthetized and unanesthetized rat, change in mean blood pressure, and change in blood glucose concentration. In addition, the increase in tail skin temperature accompanying administration of epinephrine (1 mg/kg, sc) was significantly reduced in the hypothyroid group. Administration of l-thyroxine (25 mug/kg per day, ip) to aminotriazole-treated rats prevented the reduction in responsiveness to beta-adrenergic stimulation. Thus, an interaction appears to exist between the level of thyroid activity and responsiveness to beta-adrenergic agonists in rats.

We have investigated the ability of three hyperthermic stimuli (PGE2, 5-HT and ACh) to elicit hyperthermia in the Helium-Cold (He-Cold) hypothermic hamster. Hamsters in these conditions are poikilothermic and will passively follow room temperature in a regulated cold room. Animals were injected centrally at AH/POA sites via an indwelling guide tube at body temperatures maintained between 9-12 degrees C. Active sites in the AH/POA were determined prior to the experiment by PGE2 injection. PGE2 injection at an effective AH/POA site immediately reversed the anesthetic induced hypothermia in warm air. Hamsters were induced into hypothermia by the He-Cold induction method and body temperatures were maintained in a 9 degrees C cold room. Colonic temperatures were monitored at 5 minute intervals by a YSI thermistor probe and telethermometer. Central injections of 5-HT (2 micrograms/microliter) and ACh (50 micrograms/microliter) at effective AH/POA sites evoked significant increases in colonic temperature in He-Cold hamsters. PGE2 injections were not different from saline control injections and did not elicit pronounced temperature changes in these animals. Specific blockade of the 5-HT and ACh temperature increases was demonstrated with specific antagonist injections. The results suggest that the central organization of heat-gain mechanisms in the AH/POA is the same as normothermic animals even at temperatures well below those previously investigated.

Hamsters in deep experimentally induced hypothermia, at body temperatures between 7 degrees C and 11.5 degrees C, were microinjected with 5-HT and ACh at brain sites in the anterior-preoptic area of the hypothalamus (AH/POA). ACh or 5-HT was injected into an AH/POA site at different starting core temperatures in different groups of hypothermic hamsters. Colonic temperatures (Tc) were maintained, following He-Cold induction, in a temperature controlled environmental chamber and measured with a YSI thermister probe and YSI telethermometer. Injections of either 5-HT or ACh at Tc's between 7.0 degrees C and 9.0 degrees C elicited only modest increases in Tc i.e., 0.3 degrees C--0.6 degrees C, respectively. As Tc increased, however, to ranges between 9.1 degrees C--10.0 degrees C and in different animals to greater than 10 degrees C both ACh and 5-HT at the same sites elicited significant increases in Tc, 1.5 degrees C for 5-HT and 2.2 degrees C for ACh compared to saline injections. These data suggest that at the lowest Tc's we are observing a "cold block" of temperature sensitive sites in the AH/POA. Increasing the starting Tc beyond 9.0 degrees C however, evokes significant increases in heat-gain following AH/POA injection of either ACh or 5-HT. These data are consistent with Myers' observations concerning the organization of heat-gain mechanisms at AH/POA sites. In addition, they suggest that both the afferent limb of the heat-gain circuit (5-HT) and the efferent limb of the circuit (ACh) are functionally impaired when Tc is close to the physiological limit in the He-Cold hypothermic hamster.

The present work describes a combination of techniques for the identification of neurochemicals released within the cuneate nucleus. During electrical stimulation of the superficial radial nerve, the extracellular fluid of the nucleus is continuously sampled by push-pull perfusion. In addition, the population electrical activity of peripheral nerve as well as the activity of cuneate neurons are recorded. Subsequently, the neurochemical content of the sampled fluid is assessed by HPLC analysis. The comparison of sampled fluid content during control (no stimulation) versus stimulation runs indicates that somatosensory stimulation elicits the release of specific neurochemicals within the cuneate nucleus. The possible sources of released neurochemicals are discussed.

The beta-adrenergic agonist isoproterenol (100-200 mug/kg body wt sc) induces vasodilation and an increase in skin temperature of the tail of the euthyroid but not the hypothyroid rat. Administration of thyroxine (25 mug/kg body wt per day) to rats made hypothyroid by means of the antithyroid drug aminotriazole (0.5 g/kg of foof) returned responsiveness to control level. Reduced responsiveness to isoproterenol occurred between 1 and 5 wk of treatment with aminotriazole. Increase in tail skin temperature induced in euthyroid rats by isoproterenol was blocked by administration of propranolol (7.5 mg/kg ip). Other beta-adrenergic-induced responses, including increased water intake and increased plasma glucose concentration, also were reduced in hypothyroid rats and returned to control level by administration of thyroxine. Thus, hypothyroidism appears to be accompanied by a reduced beta-adrenergic responsiveness as assessed by changes in tail skin temperature, water intake, and plasma glucose concentration after injection of isoproterenol. Since administration of thyroxine returned the responses of hypothyroid rats to control levels, it appears that thyroxine is important in maintaining beta-adrenergic responsiveness under these conditions.

Helium-cold hypothermic hamsters, colonic temperature (Tc) 7 to 11 degrees C, injected with acetylcholine (ACH) at a preoptic-anterior hypothalamic (AHPOA) site responded with a rise in colonic temperature while remaining in a cold environmental chamber. The He-Cold hamster does not thermoregulate at these body temperatures. In contrast to central ACH-elicited responses, the injection of alpha and beta adrenergic drugs into the systemic circulation of the He-Cold hamster did not elicit a rise in colonic temperature. The data describe a different animal model of rewarming than has previously been described that is under pharmacologic control by the experimenter. The use of exogenous neurotransmitter provides the potential to understand the mechanisms of thermoregulation in deep experimental hypothermia.