Hyperthermia induced by the dopamine D1 receptor agonist SK&F38393 in combination with the dopamine D2 receptor agonist talipexole in the rat

Analysis of the Involvement of NMDA Receptors in Analgesia and Hypothermia Induced by the Activation of TRPV1 Ion Channels

NMDA glutamate receptors play an important role in normal and pathophysiological nociception. At the periphery, they can interact with TRPV1 ion channels. The blockade of TRPV1 ion channels decreases NMDA-induced hyperalgesia, and NMDA receptor antagonists suppress the pain response to the TRPV1 agonist capsaicin. Since TRPV1 ion channels and NMDA receptors can functionally interact at the periphery, it would be interesting to investigate the possibility that they interact in the CNS. A single subcutaneous injection of 1 mg/kg of capsaicin was found to raise the thermal pain threshold in the tail flick test in mice, which reproduces the spinal flexion reflex, owing to the ability of capsaicin to cause long-term desensitization of nociceptors. Preventive administration of either noncompetitive NMDA receptor antagonists (high-affinity MK-801 20 μg/kg and 0.5 mg/kg subcutaneously; low-affinity hemantane 40 mg/kg intraperitoneally) or the selective TRPV1 antagonist BCTC (20 mg/kg intraperitoneally) inhibit the capsaicin-induced increase in the pain threshold. Capsaicin (1 mg/kg, subcutaneous injection) induces transient hypothermia in mice, which is brought about by hypothalamus-triggered vegetative reactions. This effect is prevented by BCTC but not by the noncompetitive NMDA receptor antagonists.

Methanandamide attenuates cocaine-induced hyperthermia in rats by a cannabinoid CB1-dopamine D2 receptor mechanism

Evidence implicates anandamide in dopamine-related cocaine function. In the present study, we investigated the effect of methanandamide (5 mg/kg, i.p.), a stable anandamide analog, on the hyperthermia and hyperactivity induced by a fixed dose of cocaine (15 mg/kg,i.p.). Cocaine administered to rats produced hyperthermia and hyperactivity whereas methanandamide was ineffective. For combined administration, methanandamide attenuated the hyperthermia, but not hyperactivity, induced by cocaine. The effect of methanandamide was abolished by pretreatment with a cannabinoid CB1 receptor antagonist, SR141716A (5 mg/kg, i.p.), or dopamine D2 receptor antagonist, S(−)-raclopride(5 mg/kg, i.p.) but not by capsazepine (40 mg/kg, i.p.), a transient receptor potential vanilloid 1 cation channel antagonist. Methanandamide also attenuated the hyperthermia caused by a dopamine D1 receptor agonist, SKF 38393 (10 mg/kg, s.c.), indicating that it reduces hyperthermia produced by dopamine D1 receptor activation. URB597 (0.25 mg/kg, i.p.), an inhibitor of anandamide metabolism, did not alter cocaine-induced hyperthermia. Our results demonstrate that methanandamide activates cannabinoid CB1 receptors to attenuate cocaine-induced hyperthermia, and that dopamine D2 receptor activation plays a permissive role in the thermoregulatory effects of methanandamide.

Dihydrexidine--the first full dopamine D1 receptor agonist

The functional role of dopamine D(1) receptors is still controversial. One reason for this controversy is that for a long time the only available agonists for in vivo characterization of dopamine D(1) receptors were benzazepines. Among them was the prototype dopamine D(1) receptor partial agonist, SKF 38393. The lack of a selective and fully efficacious dopamine D(1) receptor agonist hampered basic research on dopamine D(1) receptors and left the potential clinical utility of dopamine D(1) receptor agonists elusive. The research situation improved when the first potent full dopamine D(1) receptor agonist dihydrexidine, a phenanthridine, was introduced in the late 1980s. In contrast to SKF 38393, dihydrexidine was shown to stimulate cyclic AMP synthesis just as well or better than dopamine, and potently displaced [(3)H]SCH 23390 from rat and monkey striatal membranes. Also, dihydrexidine was the first dopamine D(1) receptor agonist that had potent antiparkinsonian activity in a primate model of Parkinson's disease. This finding suggested clinical utility for dopamine D(1) receptor agonists in Parkinson's disease and that this utility might be critically dependent on the intrinsic efficacy of the drug. Clinical utility for dopamine D(1) receptor agonists in other central nervous disorders might also be dependent on the intrinsic efficacy of the drug. However, even though studies with dihydrexidine as a pharmacological tool have pointed to the clinical use for dopamine D(1) receptor agonists, dihydrexidine's unfavorable pharmacokinetic profile and various adverse effects are likely to restrict or even preclude its use in humans. This review article provides an updated overview of the pharmacology of dihydrexidine and discusses possible clinical utility of dopamine D(1) receptor agonists in various central nervous system disorders.

Dopaminergic modulation of rat pup ultrasonic vocalizations

The dopamine D(1) receptor agonist, R(+)-6-chloro-7, 8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 81297), the dopamine D(2)/D(3) receptor agonist, trans-(-)-4aR-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo[3, 4-g]quinoline hydrochloride (quinpirole), and the dopamine D(3) receptor agonist, (+/-)-7-hydroxy-dipropylaminotetralin hydrobromide (7-OH-DPAT) all reduced the frequency of isolation-induced infant rat ultrasonic vocalizations and lowered body temperature when compared to saline-injected controls. Ultrasonic vocalization rate was not affected by either the dopamine D(1) receptor antagonist, R(+)-2,3,4, 5-tetrahydro-8-iodo-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrochloride (SCH 23390) or the dopamine D(2)/D(3) receptor antagonist, S(-)-raclopride-L-tartrate (raclopride) when given alone, nor did these antagonists block the ultrasonic vocalization reductions caused by the dopamine D(1) receptor agonist or the dopamine D(2)/D(3) receptor agonist. The dopamine D(2)/D(3) receptor antagonist but not the dopamine D(1) receptor antagonist blocked the dopamine D(3) receptor agonist's ultrasonic vocalization reduction. SKF 81297 reduced general activity while quinpirole and 7-OH-DPAT increased activity. Raclopride reversed quinpirole's reduction in body temperature, as well as 7-OH-DPAT's effects on body temperature, ultrasonic vocalizations, and activity. These results indicate that dopamine D(1), D(2)/D(3), and D(3) receptor agonists all reduce ultrasonic vocalizations by as yet undetermined mechanisms.

Independent roles of dopamine D1 and D2/3 receptors in rat thermoregulation

The present study was designed to examine a possible interaction between dopamine D1 and D2/3 receptors involved in thermoregulation in rats. The dose-dependent hypothermia produced by the dopamine D1 receptor agonist A 68930 (0.9-15.0 micromol kg-1, s.c.), was augmented in an additive manner by pretreatment with the dopamine D2/3 receptor agonist 7-OH-DPAT (0.06 micromol kg-1, s.c.). The dose-dependent hypothermia produced by 7-OH-DPAT (0.06-1.00 micromol kg-1 s.c.) was also augmented in an additive manner by pretreatment with A 68930 (0.9 micromol kg-1 s.c.). In contrast to these observations, locomotor activity measurements disclosed a marked interaction between the dopamine D1 and D2/3 receptor agonists. Thus, A 68930 (0.9-15.0 micromol kg-1, s.c.) produced a dose-dependent suppression of open-field locomotor activity. The addition of 7-OH-DPAT (1.00 micromol kg-1, s.c.), which by itself produced a weak suppression of locomotor activity, resulted in a gradual reversal of the A 68930-induced suppression of the locomotor activity. Thus, the present results provides strong support for an independent role of dopamine D1 receptors in rat thermoregulatory mechanisms, distinct from effects mediated via the dopamine D2 receptor family.

Dihydrexidine produces hypothermia in rats via activation of dopamine D1 receptors

The selective dopamine D1 receptor agonist dihydrexidine (2.0-8.0 mg/kg, s.c.) caused a dose-dependent decrease in core temperature in rats. The hypothermia produced by dihydrexidine (4.0 mg/kg), was completely blocked by the dopamine D1 receptor antagonists SCH 23390 (0.1 mg/kg, s.c.) or NNC 687 (4.0 mg/kg, s.c.), but not by the dopamine D2/3 receptor antagonist raclopride (0.2 mg/kg, s.c.). Neither of the dopamine antagonists by themselves produced any effects on core temperature. The present results provide important evidence for the notion that activation of dopamine D1 receptors induces hypothermia in rats.

Procaine microinjection into the lower midbrain increases brown fat and body temperatures in anesthetized rats

A tonic inhibitory mechanism on heat production was studied by microinjecting procaine into various regions of the brain while recording temperature changes of the interscapular brown adipose tissue (IBAT) and rectum in urethane-anesthetized rats at room temperature of 23-25 degrees C. Procaine microinjected bilaterally (10%, 1.0 mu l/site, 1.5 mm to midline) into the midbrain and the upper- to mid-pontine area of the reticular formation increased temperatures of the IBAT and rectum. The highest temperature rise (1.02 +/- 0.11 degrees C for IBAT, 0.64 +/- 0.06 degrees C for rectum) with the shortest onset latency (1.5 +/- 0.3 min for IBAT, 4.6 +/- 1.1 min for rectum) was observed when procaine was injected into the lower midbrain (the area between 6 and 7 mm posterior to the bregma, and 6.5 to 8.5 mm deep from the cortical surface). These regions include the retrorubral field, pedunculopontine tegmental nucleus, and rubrospinal tract. Procaine-induced IBAT and rectal temperature increases were dose-dependent, and reproduced reliably from the same injection site of the same animal. Intravenous indomethacin, a prostaglandin H synthase inhibitor, did not affect procaine-induced temperature rise, and propranolol, a beta-blocker, completely blocked it. These results suggest that microinjected procaine exerts its local anesthetic effect and release a tonic inhibition resulting in a disinhibition-induced temperature increase through the enhanced central sympathetic outflow. They support the hypothesis that a bilateral tonic inhibitory mechanism on heat production exists in the lower midbrain.

Effects of putative dopamine D3 receptor agonists, 7-OH-DPAT, and quinpirole, on yawning, stereotypy, and body temperature in rats

7-OH-DPAT ((+/-)-2-(dipropylamino)-7-hydroxy-1,2,3,4-tetrahydronaphthalene) was recently identified as a dopamine receptor agonist having a > 100-, 1,000- and > 10,000-fold higher affinity for dopamine D3 than for D2, D4 and D1 receptors, respectively. Quinpirole (LY 171555) has also been reported to have a 113-fold greater affinity for dopamine D3 receptors than for D2 receptors. Therefore, we investigated the effects of these putative dopamine D3 receptor agonists on yawning, stereotypy and rectal temperature in rats (N = 424). 7-OH-DPAT and quinpirole administered subcutaneously (SC) at respective low doses of 10-250 micrograms/kg and 25-500 micrograms/kg elicited yawning behavior. The yawning induced by these agents was blocked by spiperone (0.5 mg/kg, SC) and scopolamine (0.5 mg/kg, SC) but was increased by intraperitoneal (IP) administration of pindolol (20 mg/kg). The yawning was also potentiated after treatment with reserpine. 7-OH-DPAT and quinpirole at respective high doses of 0.25 mg/kg (SC) and 0.5 mg/kg (SC) evoked slight stereotypy such as sniffing and licking, and this effect was enhanced by a selective dopamine D1 receptor agonist, SK&F 38393 (1-phenyl-2,3,4,5,-tetrahydro-(1H)-3-benzazepine-7,8-diol). 7-OH-DPAT (0.5 mg/kg, SC) and quinpirole (0.5 mg/kg, SC) decreased, but SK&F 38393 (10 mg/kg, SC) increased body temperature. However, the hyperthermia induced by SK&F 38393 was interestingly enhanced by 7-OH-DPAT and quinpirole.(ABSTRACT TRUNCATED AT 250 WORDS)