Desensitization and down-regulation of brain alpha 2-adrenoceptors by centrally acting antihypertensive drugs

Consequences of maternal cocaine on cerebral microvascular functions in piglets

Maternal cocaine abuse is associated with fetal and neonatal neurological abnormalities. Prolonged exposure to cocaine can induce blood flow disorders, growth restriction, and hypoxia in the newborn. We investigated the impact of chronic fetal cocaine exposure on cerebral microvascular reactivity and autonomic function in the piglets. Pregnant pigs received cocaine (1 mg/kg i.v.; twice weekly) or saline throughout the last trimester. Prenatal exposure to cocaine did not have any significant effect on the birth weight of the piglets as compared to the control. Following delivery, effects of recurrent prenatal cocaine exposure on cerebral microvascular functions were examined in piglets (3-6 days old). Pial arteriolar responses to applications of 5-hydroxytryptamine (5-HT), endothelin-1 (ET-1), and clonidine were examined using closed cranial windows. Functional effects of prenatal cocaine exposure on changes in mean arterial pressure (MAP) and pial arteriolar diameter induced by intracisternal injection (i.c.) of clonidine (1 microg/kg) were also determined. Topical applications of 5-HT, ET-1, and clonidine dose-dependently decreased pial arteriolar diameter in the control and these constrictions were significantly enhanced in the in utero cocaine-exposed piglets. Prenatal cocaine exposure did not have any significant effects on the resting MAP and heart rate as there were no differences between the groups. IC clonidine caused sustained decrease in MAP in both groups but the decrease was more pronounced in the cocaine than the control group. IC clonidine causes cerebral microvascular dilation coincident with the development of hypotension. Such dilation was severely attenuated in the cocaine group, even though the hypotension was much more pronounced than in the control. In conclusion, prenatal cocaine exposure resulted in attenuated autoregulatory vasodilation and potentiated responses to vasoconstrictor agents. The mechanisms behind the effects of in utero cocaine exposure on alteration of newborn cerebral functions need further investigation. Such actions may be important in development of cerebral pathologies associated with recurrent prenatal cocaine exposure.

Guanabenz, an antihypertensive centrally acting alpha2-agonist, suppresses morning elevations in aggregation of human platelets

To determine whether the antihypertensive agent guanabenz affects the circadian rhythm in the hemorheologic properties of the platelet, we evaluated the aggregability of platelets collected from 11 healthy subjects in the morning and the evening after the oral administration of this agent, daily for 2 weeks. We analyzed platelet aggregation by the turbidimetric method. In an in vitro study, guanabenz, 10 nM-100 microM, did not affect platelet aggregation, whereas epinephrine induced platelet aggregation at an EC50 of 1.5 microM. The healthy volunteers demonstrated a diurnal variation in platelet aggregability that was high in the morning and low in the evening (66 +/- 10% and 56 +/- 11% respectively, of the percent platelet aggregation induced by epinephrine). The same variation was seen with the platelet aggregation induced by adenosine diphosphate (ADP) (62 +/- 8% [morning] vs. 51 +/- 7% [evening]). After the administration of guanabenz, platelet aggregability was significantly reduced in the morning compared with that before drug administration, when platelet aggregation was induced by epinephrine (49 +/- 9%, p < 0.05) or ADP (48 +/- 7%, p < 0.05), although the plasma levels of catecholamine were unchanged. A suppressive effect of guanabenz on platelet aggregability was observed in the evening, as the platelets were stimulated by epinephrine (38 +/- 9%, p < 0.05), but not by ADP (49 +/- 5%). Findings suggest that guanabenz mainly suppressed the morning enhancement in platelet aggregability, which contributes to the formation of intravascular thrombi. Thus, in addition to its antihypertensive actions, guanabenz may help to reduce the risk of vascular accidents, which frequently occur in the morning.

Four consecutive serines in the third intracellular loop are the sites for beta-adrenergic receptor kinase-mediated phosphorylation and desensitization of the alpha 2A-adrenergic receptor

During short term agonist exposure, the alpha 2A-adrenergic receptor (alpha 2AAR) undergoes rapid functional desensitization caused by phosphorylation of the receptor by the beta-adrenergic receptor kinase (beta ARK). This signal quenching is similar in nature to that found with a number of G-protein coupled receptors in which agonist-promoted desensitization is due to beta ARK phosphorylation; like these other receptors, the precise molecular determinants of the receptor required for beta ARK phosphorylation are not known. To delineate such a motif in the human alpha 2AAR (alpha 2C10), we constructed six mutated receptors consisting of deletions or substitutions of Ser-296-299 in the EESSSS sequence of the third intracellular loop of the receptor. These were expressed in Chinese hamster ovary and COS-7 cells, and agonist-promoted desensitization and receptor phosphorylation were assessed. Deletion of the EESSSS sequence and substitution of alanine for all four serines resulted in a total loss of phosphorylation and desensitization. Mutant receptors that retained two of the original serines (AASS and SSAA) underwent agonist-promoted phosphorylation of 55 +/- 7% and 57 +/- 8% of the phosphorylation found for wild type alpha 2C10. Additional substitution mutants (SSSA and SAAA) underwent 77 +/- 1% and 27 +/- 4% of wild type phosphorylation, respectively. Thus, substitution of alanine for each additional serine decreased overall phosphorylation as compared with wild type alpha 2C10 by approximately 25%, which is consistent with all 4 serines being phosphorylated. Mutated receptors that only partially phosphorylated (as compared with wild type) failed to undergo agonist-promoted desensitization. Thus, beta ARK-mediated phosphorylation of alpha 2C10 occurs at Ser-296-299 in the third intracellular loop, and this represents the critical step in rapid agonist-promoted desensitization. A number of other G-protein coupled receptors that undergo desensitization have a sequence motif similar to that which we have found for beta ARK-mediated phosphorylation of alpha 2C10, suggesting that these receptors may also be substrates for beta ARK.

Analgesic effects of detomidine in thoroughbred horses with chronic tendon injury

This study was undertaken to assess the analgesia provided by detomidine (20 micrograms kg-1 intravenously) in thoroughbred horses. Pain thresholds to a mechanical noxious stimulus were measured before and after a period of mild chronic pain in one foreleg. Detomidine was a good analgesic in control animals; their pain thresholds were significantly elevated for about 60 minutes. After injury, the injured leg had a significantly lower pain threshold and the intensity and duration of analgesia provided by detomidine were significantly reduced. The analgesia in the opposite (sound) leg was also reduced, indicating that there were both central and peripheral aspects to this increased sensitivity to painful stimuli. Detomidine deserves to be considered as a potent analgesic in the horse rather than a sedative with analgesic side effects.

The role of imidazoline receptors in blood pressure regulation

Using the ligands [3H] clonidine and [3H] idazoxan, nonadrenergic imidazoline preferring binding sites have been identified in a range of tissues from several species including man. These sites may represent a new family of receptors. An endogenous ligand and potential clonidine displacing substance has been identified. There is strong evidence for an involvement of the nonadrenergic imidazoline [3H] clonidine labelled sites in the nucleus reticularis lateralis in blood pressure regulation, and some evidence for a role in sodium regulation in the kidney for the [3H] idazoxan labelled sites. Some drugs which were previously thought to act via alpha 2-adrenoceptors, may mediate their effects in part via these imidazoline sites.