Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents: II

Role of synthetical amynoquinone ethyl 2-(1,4-dioxo-1,4-dihydronaphthalen-2-ylamino) acetate in inhibition of Ehrlich's tumor

Quinones are naturally or synthetically occurring secondary metabolites that have various bio-dynamics, highlighting their antitumor potential. This has been explored through their selective cytotoxicity, and studies in medicinal chemistry about the relation between biological activity versus chemical structure may lead to the solution of the toxicity problems associated with quinones. In this context, the antitumor effect of a synthetic naphthoquinone, named Ethyl 2-(1,4-Dioxo-1,4-Dihydronaphthalen-2-Ylamino) Acetate, was tested using mice transplanted with Ehrlich ascitic tumor as an experimental model. The acute toxicity test was performed using 30 mice that received the aminoquinone at doses of 100, 200, 300, and 600 mg/kg. After evaluation of the clinical findings in the spontaneous activity tests, the LD50 calculation for the test substance showed low levels of toxicity at doses lower than 244.11 ± 23.29 mg/kg. Thus, three experimental groups were established, where animals transplanted with tumor cells received NaCl vehicle solution (control, n = 6), and the others were treated with 71.7 mg/kg of Methotrexate (n = 6) or 20 mg/kg of Aminoquinone (n = 6). All administrations were intraperitoneal, in a single dose. Three days after the implantation of the tumor cells the animals were weighed daily and evaluated for tumor biometry and development. The treatments occurred five days after the implantation of the tumor cells and were extended for 7 more days. At the end of the 12-day experimental period, all animals were euthanized for biochemical and histopathological analyses of the tumors and vital organs. The spontaneous activity test showed that the amount of responses associated with the nervous system tends to increase with the increase in dosage, highlighting the excitatory effect on the central nervous system in almost all dosages employed, followed by depressant activities on this system. There was a significant tumor reduction, both in animals treated with methotrexate (71.7 %) and in those treated with aminoquinone (91.6 %) in the control group. There was no significant difference in tumor volume between the animals treated with aminoquinone or methotrexate. The histopathological analysis revealed that in both treatments there were fewer mitoses in the tumor mass compared to the control group. However, there was apparent toxicity to the liver, heart, and left kidney in the treatment with methotrexate compared to aminoquinone. The significant capacity for tumor reduction presented by aminoquinone allows pointing it as a promising alternative for the development of a more efficient drug to control tumor development, being necessary for the development of new studies to deepen the knowledge about its mechanisms of action.

Comparison of the effects of morphine-lidocaine-ketamine and fentanyl-lidocaine-ketamine combinations administered as constant rate infusions on postprocedure rectal temperature in dogs

OBJECTIVE

To compare the effects of morphine-lidocaine-ketamine (MLK) and fentanyl-lidocaine-ketamine (FLK) combinations administered as constant rate infusions (CRIs) during and after veterinary procedures on postprocedure rectal temperature in dogs.

ANIMALS

32 clinically normal client-owned dogs undergoing nonemergent procedures.

PROCEDURES

Dogs were randomly assigned to receive an MLK or FLK combination (16 dogs/group). During the procedure, each dog received 2% lidocaine hydrochloride (1 mg/kg/h; both groups), ketamine hydrochloride (0.6 mg/kg/h; both groups), and morphine (0.36 mg/kg/h; MLK group) or fentanyl (4 μg/kg/h; FLK group) via CRI for analgesia; esophageal temperature was maintained at 37° to 39°C. At extubation, each drug dose in each assigned combination was halved and administered (via CRI) for 12 additional hours for postprocedure analgesia. Rectal temperature and other data were recorded at baseline (prior to administration of premedicants), extubation (0 hours), and 0.5, 1.5, 3, 6, and 12 hours thereafter.

RESULTS

Mean postprocedure rectal temperature was significantly lower at each postextubation time point for the MLK group, compared with corresponding values for the FLK group. Compared with the baseline value, mean postprocedure rectal temperature was significantly lower at 0, 0.5, 1.5, and 3 hours for the FLK group and at all postprocedure time points for the MLK group. Hypothermia (rectal temperature < 37°C) was detected at ≥ 1 postprocedure time point more often in dogs in the MLK group (9/16) than in the FLK group (1/16).

CONCLUSIONS AND CLINICAL RELEVANCE

Dogs that received an MLK combination for analgesia during and after a veterinary procedure developed hypothermia more commonly than did dogs that received an FLK combination under similar conditions.

The histaminergic system is involved in psychological stress-induced hyperthermia in rats

The histaminergic system modulates numerous physiological functions such as wakefulness, circadian rhythm, feeding, and thermoregulation. However, it is not yet known if this system is also involved in psychological stress-induced hyperthermia (PSH) and, if so, which histamine (H) receptor subtype mediates the effect. Therefore, we investigated the effects of pretreatments with intraperitoneal injections of mepyramine (an H1 receptor inverse agonist), cimetidine (an H2 receptor antagonist), and ciproxifan (an H3 receptor inverse agonist) on cage-exchange stress-induced hyperthermia (a model of PSH) by monitoring core body temperature (T_c) during both light (10:00 am-12:00 pm) and dark (10:00 pm-12:00 am) phases in conscious, freely moving rats. We also investigated the effects of these drugs on stress-induced changes in locomotor activity (L_a) to rule out the possibility that effects on T_c are achieved secondary to altered L_a Cage-exchange stress increased T_c within 20 min followed by a gradual decrease back to baseline T_c during both phases. In the light phase, mepyramine and cimetidine markedly attenuated PSH, whereas ciproxifan did not affect it. In contrast, in the dark phase, mepyramine dropped T_c by 1°C without affecting cage-exchange stress-induced hyperthermia, whereas cimetidine and ciproxifan did not affect both postinjection T_c and PSH Cage-exchange stress induced an increase in L_a, especially in the light phase, but none of these drugs altered cage-exchange stress-induced L_a in either circadian rhythm phase. These results suggest that the histaminergic system is involved in the physiological mechanisms underlying PSH, particularly through H1 and H2 receptors, without influencing locomotor activity.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Hypothermia in mouse is caused by adenosine A1 and A3 receptor agonists and AMP via three distinct mechanisms

Small mammals have the ability to enter torpor, a hypothermic, hypometabolic state, allowing impressive energy conservation. Administration of adenosine or adenosine 5'-monophosphate (AMP) can trigger a hypothermic, torpor-like state. We investigated the mechanisms for hypothermia using telemetric monitoring of body temperature in wild type and receptor knock out (Adora1^-/-, Adora3^-/-) mice. Confirming prior data, stimulation of the A₃ adenosine receptor (AR) induced hypothermia via peripheral mast cell degranulation, histamine release, and activation of central histamine H₁ receptors. In contrast, A₁AR agonists and AMP both acted centrally to cause hypothermia. Commonly used, selective A₁AR agonists, including N⁶-cyclopentyladenosine (CPA), N⁶-cyclohexyladenosine (CHA), and MRS5474, caused hypothermia via both A₁AR and A₃AR when given intraperitoneally. Intracerebroventricular dosing, low peripheral doses of Cl-ENBA [(±)-5'-chloro-5'-deoxy-N⁶-endo-norbornyladenosine], or using Adora3^-/- mice allowed selective stimulation of A₁AR. AMP-stimulated hypothermia can occur independently of A₁AR, A₃AR, and mast cells. A₁AR and A₃AR agonists and AMP cause regulated hypothermia that was characterized by a drop in total energy expenditure, physical inactivity, and preference for cooler environmental temperatures, indicating a reduced body temperature set point. Neither A₁AR nor A₃AR was required for fasting-induced torpor. A₁AR and A₃AR agonists and AMP trigger regulated hypothermia via three distinct mechanisms.

Peripheral Adenosine A3 Receptor Activation Causes Regulated Hypothermia in Mice That Is Dependent on Central Histamine H1 Receptors

Adenosine can induce hypothermia, as previously demonstrated for adenosine A1 receptor (A1AR) agonists. Here we use the potent, specific A3AR agonists MRS5698, MRS5841, and MRS5980 to show that adenosine also induces hypothermia via the A3AR. The hypothermic effect of A3AR agonists is independent of A1AR activation, as the effect was fully intact in mice lacking A1AR but abolished in mice lacking A3AR. A3AR agonist-induced hypothermia was attenuated by mast cell granule depletion, demonstrating that the A3AR hypothermia is mediated, at least in part, via mast cells. Central agonist dosing had no clear hypothermic effect, whereas peripheral dosing of a non-brain-penetrant agonist caused hypothermia, suggesting that peripheral A3AR-expressing cells drive the hypothermia. Mast cells release histamine, and blocking central histamine H1 (but not H2 or H4) receptors prevented the hypothermia. The hypothermia was preceded by hypometabolism and mice with hypothermia preferred a cooler environmental temperature, demonstrating that the hypothermic state is a coordinated physiologic response with a reduced body temperature set point. Importantly, hypothermia is not required for the analgesic effects of A3AR agonists, which occur with lower agonist doses. These results support a mechanistic model for hypothermia in which A3AR agonists act on peripheral mast cells, causing histamine release, which stimulates central histamine H1 receptors to induce hypothermia. This mechanism suggests that A3AR agonists will probably not be useful for clinical induction of hypothermia.

Effects of anti-histaminic and anti-cholinergic substances on human thermoregulation during cold provocation

The roles of histaminergic and cholinergic neuron systems in the regulation of body temperature have been studied almost exclusively in animals. Recently, we have found that motion sickness, i.e. a condition where hippocampal cholinergic mismatch signals induce a release of histamine in the vomiting centre, accelerates the decline in body temperature in men during exposure to cold. In the present study we measured the thermoregulatory effects of two substances commonly used against motion sickness, i.e. the histamine (H1) receptor blocker dimenhydrinate (DMH) and the muscarine receptor blocker scopolamine (SCOP). In three trials, control (CN), DMH and SCOP, 10 male subjects were immersed in 15 degrees C water for a maximum of 90 min. The trials were separated by a minimum of three days and their order was alternated between subjects. In all trials the subject received, in a double blind fashion, a transdermal patch (SCOP or placebo) 12-14 h before immersion and a tablet (DMH or placebo) 1h before immersion. Mean skin temperature, rectal temperature (T(rec)), the difference in temperature between the non-immersed right forearm and 3rd finger of the right hand (T(ff)), and oxygen uptake (VO(2)) were recorded. The fall in T(rec) was smaller in the DMH than in the CN and SCOP conditions. The recordings of T(ff) and VO(2) suggest that SCOP attenuates peripheral vasoconstriction while DMH increases shivering thermogenesis. Notably, thermal discomfort was reduced in the SCOP condition. Findings are thoroughly discussed in the context of animal studies on the neuropharmacology and neurophysiology of thermoregulation and motion sickness.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Pharmaceutical metabolic down-regulation by protein synthesis inhibition in a conscious rat model

Pharmaceutically induced metabolic down-regulation may be a useful therapeutic adjunct when tissue oxygen supply is restricted. We hypothesized that protein synthesis inhibition in a non-hibernating species should lower oxygen demand, resulting in aerobic metabolic rate depression at the whole animal level. We compared metabolic responses and measures of systemic oxygenation of conscious catheterized rats given either protein synthesis inhibition (PSI) agents or carrier controls (normal saline and DMSO). Core temperature was measured by implanted transmitters, and VO2 was determined in an open flow-through metabolic chamber at 25 degrees C. Mean arterial pressure MAP and heart rate HR were determined from arterial pressure transducer tracings; arterial blood gases and lactate were sampled every 15 min. PSI rats exhibited an immediate transient decline in VO2, followed by a secondary decline to new resting levels; VO2 for the first hour was significantly lower than that for rats receiving DMSO vehicle. Unlike controls, PSI rats showed an overall 3.5 degrees C decline in core temperature, coupled with increased arterial lactate. There were no differences in MAP and HR of PSI rats compared to controls. Although hypothermic response to toxic agents typical of rodents cannot be ruled out completely, the mild hypothermia and reduced VO2 exhibited by PSI rats may be partially attributed to the action of protein synthesis agents.

Evidence for tolerance following repeated dosing in rats with ciproxifan, but not with A-304121

Blockade of presynaptic histamine H(3) receptors with potent and selective ligands improves cognitive function in rodents and there is significant interest in developing such drugs for long-term symptomatic treatment of CNS disorders such as attention deficit hyperactivity disorder (ADHD). Unfortunately, little is known about the effects of repeated exposure to H(3) receptor antagonists/inverse agonists. We therefore investigated the effects of acute and repeated daily administration of two potent, brain penetrating H(3) receptor antagonists/inverse agonists, ciproxifan and A-304121, on rat body weight, food and water intake, core temperature and locomotor activity, as well as H(3) receptor density and gene expression levels. Methylphenidate, used clinically for the treatment of ADHD, was included as an additional comparator. Ciproxifan, an imidazole-based compound, decreased food intake over the first 10 days and locomotor activity acutely, but these effects were lost after further repeated administration. The ex vivo binding studies revealed increased H(3) receptor density in rats following repeated administration of ciproxifan for 10 or 15 days; however, H(3) receptor gene expression was not changed. In contrast, rats treated with the non-imidazole, A-304121, did not differ from controls on any measure during the observation period, while rats treated with methylphenidate exhibited hyperthermia and hyperactivity. The implications for potential long-term treatment with H(3) receptor antagonists in CNS disorders such as ADHD are discussed.

Effects of morphine on circadian rhythms of motor activity and body temperature in pig-tailed macaques

Previous studies of the effects of opiates on motor activity and body temperature in nonhuman primates have been limited in scope and typically only conducted with restrained animals. The present study used radio-telemetry devices to continuously measure activity and temperature in unrestrained pig-tailed macaques for 24 h following morphine administration. Two dose-response functions (0.56 to 5.6 mg/kg, i.m.) were determined, one with morphine administered at 9 a.m. and one with morphine administrated at 3 p.m. Under both the 9 a.m. or 3 p.m. administration schedules, body temperature and activity were increased acutely. Activity was also reduced the following morning after morphine administered at either time. In other regards, morphine's effects on both temperature and activity differed between 9 a.m. and 3 p.m. injection, including periods of decreased activity immediately after the acute increases after 9 a.m. but not 3 p.m. administration. Surprisingly, motor activity also increased 9-12 h post-injection following morphine administered at 9 a.m., but not at 3 p.m. These results clearly show an interaction between timing of morphine administration and effects on temperature and activity. These results also underscore the fact that single injections of drugs may have multiple and delayed effects on circadian rhythms in macaques.

NMDA receptors modulate morphine-induced hyperthermia

An accumulating body of evidence indicates that activation of NMDA receptor complexes modulates a number of morphine-induced responses. Because a single injection of morphine increases extracellular glutamate levels and downregulates NMDA receptors, acute morphine appears to increase glutamatergic transmission. On the basis of those data and the fact that morphine and glutamate induce hyperthermia, we investigated whether NMDA receptors modulate the hyperthermic effects of acute morphine in male Sprague-Dawley rats. Subcutaneous injection of morphine (0.1-15 mg/kg) evoked dose-dependent hyperthermia, which was rapid in onset and peaked 45-60 min post-injection. Pretreatment with LY 235959 (0.1-1 mg/kg, s.c.), a highly selective and competitive NMDA antagonist, or dextromethorphan (5-15 mg/kg, s.c.), a noncompetitive NMDA antagonist, attenuated the hyperthermic effect of morphine (4 mg/kg). In contrast, administration of LY 235959 (1 mg/kg) 15 min after morphine (4 mg/kg) did not reverse the hyperthermia. LY 235959 (1 mg/kg) depressed the hyperthermia caused by DAMGO (1 micro g/rat, i.c.v.), a selective mu agonist, confirming that NMDA receptor activation maximizes mu receptor-induced hyperthermia. Neither LY 2359595 nor dextromethorphan by itself significantly altered body temperature. These data indicate that NMDA receptors modulate morphine-induced hyperthermia and suggest that increases in glutamatergic transmission maximize the hyperthermia evoked by morphine.

Enprostil, a prostaglandin-E(2) analogue, inhibits interleukin-8 production of human colonic epithelial cell lines

We previously reported that intracolonic administration of enprostil, a prostaglandin-E(2) (PGE(2)) analogue, had therapeutic effects on acute colitis induced in rodents by dextran sulfate sodium (DSS). In addition, production of growth-regulated gene product/cytokine-induced neutrophil chemoattractant-1 [GRO/CINC-1; an interleukin(IL)-8 like cytokine] was suppressed in the inflamed tissues. In the present study we used a human colon cancer cell line (HT-29) to investigate enprostil effects on the IL-8 production of intestinal epithelial cells stimulated by various stimulants. In a MTT assay, concentrations of enprostil >10(-5)M had cytotoxitic effects on HT-29 cells. Furthermore, 10(-6) M enprostil suppressed IL-8 production in HT-29 cells, SW620 and CaCo2 stimulated with interleukin-1 beta (IL-1 beta) or lipopolysaccharide (LPS), but did not suppress this response when cells were stimulated with tumour necrosis factor (TNF)-alpha. These results suggest that enprostil affects a point in the pathway between the IL-1 receptor or LPS receptor and nuclear factor-kappa B(NF-kappa B), without affecting the pathway between the TNF receptor and NF-kappa B, with the latter factor being required for the IL-8 gene transcription. The therapeutic effect of exogenous enprostil on DSS colitis may involve the inhibition of IL-8 production in colonic epithelial cells stimulated by IL-1 beta or LPS.

Brain eicosanoids and LPS fever: species and age differences

The results of the present study, summarized in Table 2, demonstrate that different species and strains of rodents (rats and mice) and birds (chickens) exhibit rather specific fever response. Systemic administration of LPS caused monophasic elevation in Tb of chickens, biphasic changes in Tb of rats (initial drop followed by an increase in Tb), whereas mice failed to develop hyperthermia and responded by a decreased Tb. The LPS-induced alterations in hypothalamic prostanoid synthesis were also rather species-specific and differ markedly even between the two strains of mice. We failed to find a common direct correlation between LPS-induced changes in Tb and hypothalamic prostanoid production in rodents (rats and mice). This observation is supported by our recent study on age-related changes in fever response in rats, where we found that hypothalami of LPS-treated old and young adult rats produced similar amounts of PGE2 and PGI2, in spite of more pronounced and prolonged hypothermia, and a delayed elevation in Tb of old rats, as compared with young (Fraifeld et al., 1995b). Moreover, the hypothalamus of febrile chickens did not display any detectable activation of PGE2 production, suggesting that PGE2 is not a common central mediator of fever in homeotherms (Fraifeld et al., 1995a). Apparently, the actual body temperature not always reflects the functional state of central thermostat, and increased PGE2 production in hypothalamus would not directly, at least in rodents, lead to body temperature elevation. Furthermore, peripheral effects, including PG-mediated ones, of pyrogens can interfere and even overcome their centrally-mediated effects (Morimoto et al., 1991; Burysek et al., 1993). Previously, we have shown that no additional elevation in hypothalamic PGE2 production occurs in response to doses of LPS over 10 micrograms in rats and 25 micrograms in mice, while the increased doses led to further changes in Tb response (Kaplanski et al., 1993). Morimoto et al. (1991) have considered that PGE2 acts centrally to cause fever and peripherally to cause hypothermia, and, hence, these opposing actions, both being induced by LPS, may act together to determine the final thermoregulatory response. Other possibilities could be related to counterbalance of endogenous antipyretics (Kluger, 1991; Kozak et al., 1995), that may occur not only at the level of thermoregulatory center but also outside the CNS (Klir et al., 1995), and to the existence of PG-independent mechanisms of LPS fever. The latter have been shown for IL-8 (Rothwell et al., 1990; Zampronio et al., 1994) and MIP-1 (Davatelis et al., 1989; Minano et al., 1990; Hayashi et al., 1995; Lopez-Valpuesta and Myers, 1995), which are, apparently, mediated via CRF (Strijbos et al., 1992; Zampronio et al., 1994), and INF-alpha, mediated via the opioid receptor mechanisms (Hori et al., 1991, 1992). However, it has been shown recently that in different species the same pyrogenic cytokines (IL-8) may induced fever via different, PG-independent (in rats; Zampronio et al., 1994) or PG-dependent (in rabbits; Zampronio et al., 1995) mechanisms. It should be noted that fever response is not always accompanied by an elevation in Tb. The final effect of pyrogens on body temperature depends upon the balance between heat production and heat loss, which in turn is highly dependent upon body size and ambient temperature, especially in small animals. Perhaps, the hypothermic response observed in our mice and rats at 22 degrees C may be in part attributed to ambient temperature, which was below a thermoneutral zone. The reduced febrile response is considered, at least in part, to contribute to an increased mortality and prolonged recovery from infections (Kluger, 1986). From this point, it is difficult to suggest whether the hypothermia observed in our mice and rats could be of somewhat adaptive significance. It has been shown that at the ambient temperature of 30 degrees C, Swiss Webster mice can re

The role of histamine H1 receptors in the thermoregulatory effect of morphine in mice

Morphine is known to release histamine from mast cells and increase the turnover of neuronal histamine. It is also known that histamine receptors mediate some of the morphine effects. The contribution of histamine H1 and H2 receptors to the thermoregulatory effect of morphine in mice was investigated in the present experiments. Morphine produced a hypothermic effect, especially at the dose of 10 mg/kg. Although the histamine H1 receptor antagonist, dimethindene (0.1 mg/kg, i.p.), attenuated the hypothermic effect of morphine (10 mg/kg), a histamine H2 receptor antagonist, ranitidine (100 mg/kg, i.p.), had no effect. These results suggest that the hypothermic effect of morphine in mice is mediated, at least partly, through histamine H1 receptors.

Thermoregulatory aspects of environmental exposure to anticholinesterase agents

Anticholinesterase (antiChE) agents can be highly toxic to birds and mammals and constitute a major proportion of the pesticides used throughout the world. AntiChEs consist of the organophosphates (OP), which irreversibly inhibit the enzyme acetylcholinesterase (AChE), and the carbamates (CB), which reversibly inhibit AChE. AChE inhibition elicits cholinergic stimulation in the central nervous system and in peripheral tissues and organs, which can lead to marked dysfunction of homeostatic systems, including temperature regulation. The control of body temperature uses cholinergic pathways in the integration and central processing of thermal information, as well as in the control of thermoeffector responses. Hence, the cholinergic stimulation elicited from exposure to antiChEs has profound effects on body temperature at rest as well as during exercise. Ambient heat and cold stress can also modulate the animal's sensitivity to antiChE exposure. After exposure to most OPs, rodents and other small species undergo a marked hypothermic response lasting up to 24 hours. On the other hand, humans exposed to OP pesticides rarely become hypothermic but rather experience a fever that may last many days. Recent studies monitoring body temperature in OP-exposed, telemetered rats demonstrated that the initial hypothermic response is followed by a period of hyperthermia lasting several days. That the hyperthermia can be blocked with administration of sodium salicylate suggests that the hyperthermia is a fever. Thus, the antiChE-induced effects on body temperature and other physiological systems cannot be explained solely by the immediate consequences of AChE inhibition and stimulation of cholinergic systems. Research into the mechanisms of action of antiChE toxicity will be improved with a better understanding of their effects on temperature regulation.

Effect of dermorphin on thermoregulation in rats at selected ambient temperatures

Intraperitoneal administration of dermorphin caused dose-dependent changes in rats core temperature and tail skin temperature (indicative of compensatory thermoregulatory vasoreactions in rats). The character of these changes depended strongly on the environmental temperature at which the inversion of the dermorphin-induced thermoregulatory effect was observed. In the cold environment (4-7 degrees C) dermorphin caused a significant, stable, dose-dependent hypothermia. In the thermoneutral environment (27-28 degrees C) dermorphin also caused hypothermia, but this effect was less pronounced. In the hot environment (31-32 degrees C) dermorphin caused hyperthermia. Dermorphin-induced changes in tail skin temperature indicate that dermorphin suppresses the thermoregulatory peripheral compensatory vasomotor reactions. Pretreatment with naloxone attenuated dermorphin-induced effects on core temperature and partially enhanced vasomotor effects of dermorphin. The data obtained indicate that dermorphin affects the core temperature regulation via mu-opiate receptors, whereas vasomotor effects of the peptide are probably mediated via naloxone-insensitive receptors.

Nicotine-induced hypothermia through an indirect dopaminergic mechanism

The effect of nicotine on core body temperature was studied in mice. Intraperitoneal (i.p.) injection of nicotine (0.5, 1 and 2 mg/kg) induced a dose-dependent hypothermia. The response was inhibited by reserpine (5 mg/kg), the centrally active nicotinic receptor antagonist mecamylamine (0.1-1 mg/kg) and the D-2 dopamine receptor antagonist sulpiride (25-100 mg/kg). The β-adrenoceptor antagonist propranolol (5 and 10 mg/kg) and the serotonergic blocker methysergide (5 and 10 mg/kg) did not inhibit but increased the nicotine response. The α-adrenoceptor antagonist phenoxybenzamine, the antimuscarinic agent atropine, the D-1 dopamine receptor antagonist SCH 23390, the peripheral dopamine antagonist domperidone and the peripheral nicotinic antagonist hexamethonium did not alter the nicotine-induced hypothermia. It is concluded that nicotine may cause a fall in core body temperature through a central dopaminergic mechanism.

On the mechanism(s) of morphine-induced hypothermia

The effects of morphine on core body temperature of mice in the presence or absence of catecholamine receptor antagonists were examined. Administration of different doses of morphine (20, 30 and 40 mg/kg) to mice caused a hypothermic effect. Pre-treatment of animals with the opioid receptor antagonist naloxone (1, 1.5 and 3 mg/kg), the D-2 receptor antagonists sulpiride (25 and 50 mg/kg), pimozide (0.0625, 0.125 and 0.25 mg/kg) and the adenosine receptor antagonist theophylline (25 and 50 mg/kg) decreased the morphine-induced hypothermia. The D-1 receptor antagonist SCH 23390 (0.1 and 0.2 mg/kg), the peripheral D-2 antagonist domperidone (10 and 30 mg/kg), the serotonin (5-HT) antagonist methysergide (5 and 10 mg/kg), the adrenoceptor antagonist phenoxybenzamine (2.5 and 5 mg/kg) and the β-adrenoceptor antagonist propranolol (5 and 10 mg/kg) did not inhibit the morphine response. The antimuscarinic drug atropine (5 and 10 mg/kg) caused a slight decrease in the morphine response. In animals pre-treated with reserpine (5 mg/kg), a hyperthermic response was observed after morphine injection. It is concluded that indirect dopaminergic or adenosine receptor mechanism(s) may be involved in the morphine-induced hypothermia in mice.

Effects of different factors in lead- and cadmium-induced hypothermia in mice

The aim of this study was to investigate the relationship between the hypothermic effect induced by lead or cadmium chloride treatments and the cerebral metal levels attained. Mice were injected intraperitoneally with different doses of lead acetate or cadmium chloride at ambient temperatures of 22 degrees C and 35 degrees C, and rectal temperatures and brain metal levels were determined. At 22 degrees C, doses of lead acetate exceeding 25 mg/kg caused significant hypothermia and a rise of lead levels in the brain; this hypothermic effect was significantly inhibited at 35 degrees C. In mice repeatedly treated with 2 or 5 mg/kg lead acetate and exposed to 22 degrees C or 35 degrees C, no significant changes were observed in body temperature after the repeated exposure, although brain lead levels increased significantly. Cadmium chloride at 22 degrees C produced a significant fall in body temperature, and a rise in brain cadmium levels at doses of 2-4 mg/kg; the decrease in body temperature was partially inhibited at 35 degrees C. The results of the present study suggest that the hypothermic effect elicited by these heavy metals is related to both the level of metal absorbed by the brain and its rate of uptake.

Effect of mu-, kappa-, and delta-selective opioid agonists on thermoregulation in the rat

The effect of selective mu-, kappa-, and delta-agonists on brain surface temperature (Tb), oxygen consumption (Vo2), and heat exchange (Q) was studied in unrestrained, male Sprague-Dawley rats using whole-body calorimetry. Hyperthermia, produced by PL-017 (1.86 nM) given ICV, resulted from increased Vo2 and reduced Q during the first 15-45 min postinjection. Tb returned to control levels due to a combination of increased Q and reduced Vo2. PL-017-induced hyperthermia was abolished by the mu-selective antagonist CTAP (0.75 nM). Dynorphin A1-17 (4.65 nM), a kappa-selective agonist, reduced both Vo2 and Q, resulting in hypothermia that was blocked by the kappa-selective antagonist nor-binaltorphimine (25 nM). The delta-selective agonist DPDPE (4.64 nM) caused no significant changes in Tb, Vo2, or Q. The data indicate that central stimulation of the mu- and kappa-opioid receptors affects both oxidative metabolism and heat exchange, which result in a change in Tb. These alterations can be prevented with selective opioid antagonist pretreatment.

Influence of pertussis toxin on thermic responses to morphine and neurotensin in rats

The influence of pertussis toxin (PTX) on thermic responses elicited by morphine and neurotensin was evaluated in unrestrained rats kept at 22 degrees C. High doses of morphine (9-36 micrograms/rat i.c.v.) lowered body temperature and low doses (1.25, 2.5 micrograms/rat i.c.v.) produced hyperthermia. The hyperthermic effect was more resistant than the hypothermic effect to naloxone antagonism. Neurotensin (50, 100 micrograms/rat i.c.v.) induced marked hypothermia followed by hyperthermia. I.c.v. injection of PTX (1 microgram), six days before morphine (18 micrograms/rat i.c.v.), replaced the opiate hypothermia by consistent hyperthermia and reduced by 60% the hyperthermia elicited by morphine (2.5 micrograms/rat i.c.v.). The toxin also affected the thermic responses induced by neurotensin (50 micrograms/rat i.c.v.) administered six days after PTX (1 microgram/rat i.c.v.). The initial hypothermia was enhanced by 173% and the late hyperthermia was fully antagonized. It thus appears that PTX-sensitive G-proteins play different roles in the molecular events underlying the thermoregulatory responses to morphine and neurotensin.

Dimaprit--induced neurotoxicity

The neurotoxicity of the histamine H2 agonist dimaprit was characterized. Dimaprit (100 micrograms administered into the lateral cerebral ventricle) induced a large area of brain necrosis 1-3 days later which was uniformly lethal. Lower doses caused dose - related effects on survival, gross brain pathology and body weight. Experiments with other H2 agonists and H2 antagonists, together with studies by others demonstrating a similar toxicity of the congener homodimaprit suggest that the neurotoxicity of dimaprit is independent of brain H2 receptors. Although dimaprit is a useful tool for the characterization of H2 receptor responses, the present results show that this agent must be used with caution, if at all, in classifying brain H2-receptor mediated events.

Hypothalamic-midbrain dysregulation syndrome: hypertension, hyperthermia, hyperventilation, and decerebration

Certain decerebrate lesions of brain stem or hypothalamus induce pharmacologically reversible hypertension and hyperthermia in animals. We observed three young patients with episodic decerebration, hyperthermia, hypertension, and hyperventilation during recovery from comas of different etiologies. The shared pathology on neurologic examinations and computed tomographic scans was hypothalamic-mesencephalic dysfunction, suggesting a diencephalic-brain-stem disconnection syndrome or brain-stem release mechanism. Propranolol was the most effective drug tested, but only two patients responded, one dramatically. This novel clinical syndrome may have localizing and therapeutic significance in pediatric coma that needs to be further defined in future studies.

Antagonism of physostigmine induced hypothermia and neuroendocrine changes following exposure to different environmental temperatures

1. The magnitude of physostigmine-induced hypothermia increased with decreasing environmental temperature. 2. The hypothermic response was accompanied by significant changes in plasma levels of corticosterone, glucose and fatty acids. 3. Central cholinergic mediation appears to be a significant component of physostigmine-induced hypothermia and neuroendocrine changes at moderate temperature. 4. At lower ambient temperatures cholinergic blockers produced less antagonism of physostigmine-induced effects. 5. The decreased effectiveness of cholinergic blockers at low environmental temperatures and the increased plasma fatty acid levels under almost all conditions studied may be of importance in considering long term therapy with cholinergic agonists.

GABAergic mechanisms in morphine-induced hypothermia

Morphine produced a hypothermic effect in restrained rats which was antagonized by naloxone. This effect was completely inhibited by gamma-acetylenic-GABA, an inhibitor of GABA transaminase and by baclofen, a specific GABAB agonist. Pretreatment with diazepam, an agonist of the benzodiazepine receptor, partially inhibited morphine-induced hypothermia. Flumazenil, a benzodiazepine receptor blocker, potentiated the action of morphine on body temperature. A role of brain GABA in the thermoregulatory effects of morphine is proposed on the basis of these results.

Potentiating effect of morphine upon d-methamphetamine-induced hyperthermia in mice. Effects of naloxone and haloperidol

We have examined changes in rectal temperature of mice after subcutaneous administrations of d-methamphetamine alone or methamphetamine plus morphine. Methamphetamine 5 mg/kg produced slight hyperthermia, while simultaneous administration of morphine (25-100 mg/kg), which alone produces hypothermia, potentiated markedly the increase in body temperature by methamphetamine. Methamphetamine showed a hyperthermic effect in a dose-dependent manner in the presence of morphine. The hyperthermia due to methamphetamine plus morphine was avoided by pretreatment with 10 mg/kg naloxone. When animals were pretreated with 2.5 mg/kg haloperidol, hyperthermia due to methamphetamine alone was completely abolished, while that due to methamphetamine plus morphine was still observed. These results showed that dopamine may be implicated in methamphetamine hyperthermia and a haloperidol-nonsensitive mechanism may be involved in the methamphetamine-morphine hyperthermia.

Intraocular hypothalamic transplants containing histaminergic neurons: innervation of host iris and hippocampal cografts

Hypothalamic tissue containing the tuberomammillary nucleus was dissected from fetuses of Embryonic Day 17 and inserted into the anterior chamber of the eye of young adult recipient rats. The growth of hypothalamic grafts was monitored through the translucent cornea and transplants were found to double in size over the 8 first weeks in oculo. After 4 weeks fetal hippocampal formation (Embryonic Day 18) was inserted into the eye chamber in half of the previously grafted animals and placed in contact with the first grafts. Double grafts were allowed to mature for up to 18 weeks before sacrifice. Recipient rats were anesthetized and superfused with carbodiimide and paraformaldehyde, after which transplants were removed, frozen, sectioned on a cryostat, and incubated with histamine antibodies. Immunohistochemical evaluations revealed a large number of histamine-positive nerve cell bodies with processes innervating the entire hypothalamic graft with a dense plexus of varicose fibers. Such histamine-positive fibers were also seen to invade the surrounding host iris in some cases with thick axon bundles as well as with single fibers. When hypothalamic transplants were combined with hippocampal grafts numerous histamine-immunoreactive fibers invaded the hippocampal tissue to form a plexus of varicose terminals throughout the cografts. After 4 weeks in oculo only a sparse histamine-positive innervation of hippocampal grafts was found, while 18-week-old double grafts contained a considerably larger amount of immunoreactive neurites.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermal biology of the laboratory rat

The purpose of this paper is to thoroughly review the literature and present a data base of the basic thermoregulatory parameters of the laboratory rat. This review surveys the pertinent papers dealing with various aspects of the thermal biology of the laboratory rat, including: metabolism, thermoneutrality, core and brain temperature, thermal tolerance, thermal conductance and insulation, thermoregulatory effectors (i.e., thermogenesis, peripheral vasomotor tone, evaporation, and behavior), thermal acclimation, growth and reproduction, ontogeny, aging, motor activity and exercise, circadian rhythm and sleep, gender differences, and other parameters. It is shown that many facets of the thermoregulatory system of the laboratory rat are typical to that of most homeothermic species. However, is several instances the rat exhibits unique thermoregulatory responses which are not comparable to other species.

Components of the opioid withdrawal syndrome in mice are thermoregulatory responses

C57BL/6J mice were rendered physically dependent on morphine by giving them ad lib access to a drinking fluid containing 0.2% saccharin and morphine for 14 days at 20-22 degrees C. Core body temperatures were monitored by radio telemetry, which obviated the need for restraint, handling, or otherwise disturbing the animals. Consistent hyperthermia was present throughout the morphine intoxication phase, followed by hypothermia after the withdrawal syndrome had been precipitated by naloxone challenge (2.0 mg/kg, IP) at 22.5 degrees C. The hypothermia could be blocked by exposing the animals to a 34.5 degrees C ambient temperature, which also prevented the occurrence of tremor and "wet dog shakes." In contrast, the other withdrawal signs monitored were not significantly affected. In a second experiment, mice were given the same morphine-saccharin drinking fluid as before, except that a choice was provided between two interconnected home cages (23 degrees C vs. 35 degrees C) throughout the experiment. A marked preference for the 35 degrees C cage was seen during intoxication, which served to enhance the hyperthermia due to morphine. Following withdrawal, when hypothermia is evident, the preference for the 35 degrees C cage declined to control levels. These results suggest that hypothermia is both a consequence and a contributor to the opioid withdrawal syndrome.

Neurobiologic effects of bright artificial light

Bright light is an effective treatment of winter depression. Study of the effects of this treatment on mechanisms thought to be involved in the pathophysiology of depressive disorders is reviewed. Measurement of a physiological parameter, namely the change in core temperature using an intraperitoneally implanted radio transmitter sensitive to temperature in freely moving rats, indicates that treatment with bright light under various experimental conditions tends to powerfully subsensitize muscarinic and nicotinic mechanisms. Pulses of bright light during the phase delay portions of the PRC blunt sensitivity to clonidine. Our studies with bright light are consistent with those indicating that heterocyclic antidepressants and a monoamine oxidase inhibitor produce subsensitivity to the thermic effects of nicotine. Reports of the influences of full-spectrum bright light and its impact on targeted neurotransmitter mechanisms call attention to the anatomical substratum mediating its effects. Possible receptor changes are measurable using receptor binding techniques and quantitative autoradiography. The physiological effects of this interesting treatment raises questions of its impact on coupling mechanisms and second messengers.

Inhibition of histamine release from rat hypothalamic slices by omega-conotoxin GVIA, but not by nilvadipine, a dihydropyridine derivative

Histamine release in response to 40 mM high K+-stimulation from the rat hypothalamic slice preparations perifused in vitro was significantly inhibited by 1.0 nM-1.0 microM omega-conotoxin GVIA, a peptide modulator of N- and L-type voltage-sensitive calcium channels, but not by similar concentrations of nilvadipine, a dihydropyridine derivative of L-type calcium channel antagonist. These results indicate that the voltage-sensitive calcium channel controlling histamine release from hypothalamic slices is omega-conotoxin-sensitive but dihydropyridine-insensitive.

Comparative pyrogenic potency of endogenous prostanoids and of prostanoid-mimetics injected into the anterior hypothalamic/preoptic region of the cat

In both pyrogen-induced fever and fever subsequent to acute hypothalamic trauma, pyrexia is believed to be mediated by cyclooxygenase products acting within the anterior hypothalamic/preoptic (AH/PO) region of the brain. The goal of the present study was to assess, through a potency analysis, the likely contributions of various prostanoids to pyrexia production. Prostanoids and prostanoid-mimetics were injected bilaterally into the AH/PO region of conscious, indomethacin pretreated cats, and partial dose-response curves for pyrexic activity were obtained. ED1 degrees doses (doses producing a 1 degree C fever) for PGE2, PGE1 and 6-keto-PGE1 (a metabolite of PGI2 and/or of the PGI2 hydrolysis product, 6-keto-PGF1 alpha) ranged between 2 and 15 pmol. PGF2 alpha and the stable PGI2-mimetics, iloprost and 6-beta-PGI1, required doses of 900-1100 pmol. PGD2 and 6-keto-PGF1 alpha had ED1 degrees doses of 2200-2400 pmol. PGI2, thromboxane (TX) B2 and the TXA2/PGH2-mimetics, SQ26655, 9,11-azo-PGH2 and U46619, were incapable of producing a 1 degrees C rise at the maximum dose of 30,000 pmol. The results offer no support for an involvement in fever of PGF2 alpha, PGD2, TXA2, TXB2, PGH2, PGI2 or 6-keto-PGF1 alpha. Only the 3 E-series prostaglandins were sufficiently potent to merit serious consideration as mediators of pyrexia. Of these, only PGE2 is known to be produced in abundance by cat brain; no information is available regarding PGE1 production, and our results with PGI2 and 6-keto-PGF1 alpha indicate that cat brain may not synthesize 6-keto-PGE1. The results thus suggest an important role for PGE2 in fever production in the cat and are compatible with an involvement of PGE1.

Involvement of histamine H1 and H2 receptors in hypothermia induced by ionizing radiation in guinea pigs

Radiation-induced hypothermia was examined in guinea pigs. Exposure to the head alone or whole-body irradiation induced hypothermia, whereas exposure of the body alone produced a small insignificant response. Systemic injection of disodium cromoglycate (a mast cell stabilizer) and cimetidine (H2-receptor antagonist) had no effect on radiation-induced hypothermia, whereas systemic and central administration of mepyramine (H1-receptor antagonist) or central administration of disodium cromoglycate or cimetidine attenuated it, indicating the involvement of central histamine through both H1 and H2 receptors in this response. Serotonin is not involved, since the serotonin antagonist methysergide had no effect on radiation-induced hypothermia. These results indicate that central histaminergic systems may be involved in radiation-induced hypothermia.

Time course of physostigmine effects on neuroendocrine responding at varying environmental temperatures

1. Hypothermia was found to be related to both the dose of physostigmine and the environmental temperature. 2. Plasma corticosterone levels were elevated above controls regardless of dose of physostigmine or environmental temperature. 3. Plasma free fatty acid levels appeared to be inversely related to physostigmine-induced hypothermia. 4. A hyperglycemic response was observed under all experimental conditions at 0.5 hours and 1.0 hour post injection. 5. Significant inhibition of brain acetylcholinesterase was observed, whereas, plasma and erythrocyte acetylcholinesterase activity was inconsistent.

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

The neuromechanism mediating the hyperthermia induced by injection of PGE2 into the preoptic/anterior hypothalamus (PO/AH) was investigated in the rat. Pretreatment of rats with intraperitoneal injection of atropine sulfate blocked, whereas pretreatment with atropine methyl bromide had no significant effect on the hyperthermia. In a second series of experiments, atropine sulfate was microinjected into different regions of the hypothalamus and the thalamus in an attempt to locate the central cholinergic synapses involved in the PGE2-induced hyperthermia. The hyperthermia was blocked by atropine injection into the dorsal/dorsomedial hypothalamic area (DH), but was not significantly affected by injection into the PO/AH, ventromedial hypothalamus, or the thalamic area above the DH. Moreover, microinjection of the cholinergic agonist carbachol (0.5 microgram) into the DH could also elicit hyperthermia. Thus, our data suggest that the hyperthermia induced by PGE2 administration into the PO/AH is mediated by a cholinergic mechanism in the DH.

Changes in body temperature after administration of antipyretics, LSD, delta 9-THC and related agents: II

Antipyretics, in particular acetaminophen, aspirin and ibuprofen, constitute the single most important class of drugs used therapeutically for an effect on body temperature. Hallucinogens exert prominent actions on the central nervous system, and it is not surprising that, like so many other centrally-acting agents, they too often affect temperature. This compilation primarily covers the considerable amount of data published from 1981 through 1985 on the interactions of these drugs and thermoregulation, but data from many earlier papers not included in a previous compilation are also tabulated. The effects of agents not classically considered as antipyretics on temperatures of febrile subjects are also covered. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent.

Effects of scopolamine on locomotor activity and metabolic rate in mice

Reduction of metabolic rate occurs in rodents in response to intoxication with several chemicals, including amphetamine. In the present study, cholinergic mediation of locomotor activity and metabolic rate was investigated by measuring the effects of scopolamine on the frequency of photobeam breaks, the rate of CO2 production, and rectal temperature in unrestrained mice. Increasing doses of scopolamine (0, 0.3, 1.0, 3.0, and 10.0 mg/kg IP) increased locomotor activity over a 72-min observation period. CO2 production (as minute volume exhaled CO2, VECO2), measured simultaneously with locomotor activity, was suppressed equally at all doses of the drug. Rectal temperatures taken 72 min after scopolamine declined slightly in a dose-related manner. These results parallel earlier findings with d-amphetamine and suggest that divergent effects on metabolic rate and locomotor activity may be induced by centrally-acting compounds acting on more than one neurochemical system.

Cyclosporine and alpha-interferon do not attenuate morphine withdrawal in rats but do impair thermoregulation

Immunomodulating drugs as diverse as alpha-interferon and cyclosporine have been reported to attenuate physical signs of morphine withdrawal in rats. On the basis of these results, the immune system has been claimed to be involved in opiate addiction. To assess whether this is the case, the effects of alpha-interferon and cyclosporine were studied on objective signs of morphine withdrawal in morphine-dependent rats. Rats made dependent upon morphine by implantation of a 75-mg morphine pellet were challenged three days later by naloxone (1 mg/kg). Pretreatment with alpha-interferon (150 U/g) or cyclosporine (15 mg/kg) did not attenuate the reduction in body weight or the behavioral suppression induced by naloxone in morphine-dependent rats trained to press a lever for food reinforcement on a fixed-ratio 10 schedule. Alpha-interferon pretreatment blocked the capacity of naloxone to decrease body temperature in these rats and actually induced an hyperthermic response. In contrast, cyclosporine tended to enhance the drop in body temperature induced by naloxone. This last effect was more striking when the rats were placed in a cold room at 3.5 degrees C. Cyclosporine by itself induced a drop in body temperature in normal rats exposed to 3.5 degrees C. These results indicate that alpha-interferon and cyclosporine impair thermoregulation but do not directly interfere with morphine withdrawal signs.

Changes in body temperature after administration of adrenergic and serotonergic agents and related drugs including antidepressants: II

This survey continues a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published from 1980 to 1984 but data from many earlier papers are also tabulated.

Histamine and histamine receptors: behavioral thermoregulation in the salamander Necturus maculosus

Low doses (0.01, 0.1 mg/kg, i.p.) of histamine (HA) caused selection of significantly lower temperatures, and higher doses (0.5, 1.0 mg/kg) increased temperatures by mudpuppies in linear thermal gradients. Injection of the HA precursor, L-histidine (500 mg/kg) produced an increase in the temperatures selected. Results from injections of HA H1-receptor agonist (2-pyridylethylamine) and antagonist (pyrilamine), and H2-receptor agonist (dimaprit) and antagonist (cimetidine) had significant effects on thermoregulation; H1-receptors may mediate behavioral hyperthermia and H2-receptors behavioral hypothermia. Responses to these histaminic compounds are significantly influenced by the time of day at which the responses are measured and by season and acclimation temperature. The equivalent behavioral responses in both endotherms and ectotherms to agents which produce physiological hyperthermia and hypothermia are probably behavioral hypothermia ("cold seeking") and behavioral hyperthermia ("heat seeking"), respectively.