Dihydrexidine produces hypothermia in rats via activation of dopamine D1 receptors

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.

A 68930 and dihydrexidine inhibit locomotor activity and d-amphetamine-induced hyperactivity in rats: a role of inhibitory dopamine d1/5 receptors in the prefrontal cortex?

The behavioral and biochemical effects of the full dopamine D(1/5) receptor agonists, dihydrexidine and (1R,3S)-1-aminomethyl-5,6-dihydroxy-3-phenylisochroman HCl (A 68930), were examined in rats. Both A 68930 (0-4.6 mg kg(-1), s.c.) and dihydrexidine (0-8.0 mg kg(-1), s.c.) caused a dose-dependent suppression of locomotor activity, as assessed in an open-field. This locomotor suppression was dose-dependently antagonized by the selective dopamine D(1/5) receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCl (SCH 23390; 0-5.0 microg kg(-1), s.c.), but not by the selective dopamine D(2/3) receptor antagonist raclopride (0-25.0 microg kg(-1), s.c.). Furthermore, A 68930 and dihydrexidine did not cause any locomotor activity in habituated rats that displayed a very low base-line activity. Neither did A 68930 nor dihydrexidine produce any excessive stereotypies that could possibly interfere with and mask ambulatory activity. In fact, both A 68930 and dihydrexidine potently blocked hyperactivity produced by d-amphetamine (0-4.0 mg kg(-1), s.c.). Such findings traditionally would be interpreted as a sign of potential antipsychotic properties of A 68930 and dihydrexidine. Examination of neuronal activation, as indexed by the immediate early gene c-fos, showed that A 68930 and dihydrexidine caused a highly significant expression of c-fos in the medial prefrontal cortex. This c-fos expression was sensitive to treatment with SCH 23390, but not with raclopride. The effects of A 68930 and dihydrexidine on c-fos expression in caudate putamen or nucleus accumbens were less marked, or undetectable. The results indicate that stimulation of dopamine D(1/5) receptors, possibly in the medial prefrontal cortex, is associated with inhibitory actions on locomotor activity and d-amphetamine-induced hyperactivity. Assuming an important role of prefrontal dopamine D(1/5) receptors in schizophrenia, such inhibitory actions of dopamine D(1/5) receptor stimulation on psychomotor activation may have interesting clinical implications in the treatment of schizophrenia.

Temperature set-point changes induced by DA D2/3 and 5-HT1A receptor agonists in the rat

The DA D2/3 receptor agonist 7-OH-DPAT (2 micromol kg(-1)) and the 5HT1A receptor agonist 8-OH-DPAT (0.6 micromol kg(-1)) both produced a marked and similar decrease in core temperature of 3-4 degrees C at 10 and 20 degrees C ambient temperature. At 30 degrees C there were no, or weak, effects. The decrease in core temperature was accompanied by a sudden increase in tail temperature, followed by a decrease as core temperature returned to basal values. The results suggest that the hypothermia produced by the respective DA D2/3 and the 5-HT1A receptor agonists 7-OH-DPAT and 8-OH-DPAT is an active process, in all probability due to changes in a hypothalamic set-point for temperature regulation.