Role of L-type calcium channels on yohimbine-precipitated clonidine withdrawal in vivo and in vitro

Diazepam augments gender differences in cutaneous LD flux response to local cooling

Cutaneous vasoconstriction in response to local cooling is normally greater in females than in males. Cold induces amplification of alpha2-adrenoceptor affinity for norepinephrine and increases reflex sympathetic thermoregulatory output. Benzodiazepines are drugs with very well-known binding to the central and peripheral benzodiazepine receptors. Besides these effects they decrease sympathetic output and as it was shown in the last decade they act synergistically with alpha-adrenoceptors. In the present study we tested the hypothesis that the benzodiazepine diazepam interacts with an alpha-adrenoceptor mechanism at the level of microcirculation. We measured laser-Doppler blood flux changes provoked by local cooling before and after oral application of a low dose of diazepam (5 mg) in 9 healthy males and in 11 healthy females with regular menstrual cycles. The results of our experiments show that in females there is a significant reduction (ANOVA, p < 0.05) in laser-Doppler flux during the first four minutes of cooling after taking of diazepam. In males, there is no significant difference in the responses to cold before and after an application of diazepam. Our results suggest that diazepam, in addition to its well-known effect on BZ receptors may also interact with alpha2C-adrenoceptors in the vessel wall during local cooling.

Interaction between brain L-type calcium channels and alpha2-adrenoceptors in the inhibition of sodium appetite

Calcium channels mediate the actions of many drugs. The present work investigated whether diltiazem, an L-type calcium channel blocker, alters the inhibition of sodium appetite induced by noradrenaline and the alpha2-adrenoceptor agonist clonidine. Adult male Holtzman rats (N=4-8) with cannula implanted into the third cerebral ventricle were submitted to sodium depletion (furosemide sc+24-h removal of ambiente sodium). Sodium depleted control animals that received 0.9% NaCl as vehicle injected intracerebroventricularly (i.c.v.) ingested 13.0+/-1.5 ml/120 min of 1.8% NaCl. Intracerebroventricular injection of either noradrenaline (80 nmol) or clonidine (20 nmol) inhibited 1.8% NaCl intake from 70 to 90%. Prior i.c.v. injection of diltiazem (6-48 nmol) inhibited from 50 to 100% the effect of noradrenaline and clonidine in a dose-response manner. Diltiazem alone at 100 nmol inhibited, but at 50 nmol had no effect on, sodium appetite. The results suggest: (1) common ionic mechanisms involving calcium channels for the inhibition that noradrenaline and clonidine exert on sodium appetite and (2) a dual role for the benzothiazepine site of L-type calcium channels in the control of sodium appetite.

Lithium inhibits the development of physical dependence to clonidine in mice

Based on our previous finding that chronic lithium treatment reduced naloxone-precipitated withdrawal syndrome in morphine-treated mice, the effect of chronic lithium treatment was evaluated on the development of dependence to clonidine. Dependence was induced by injection of either morphine (50, 50 and 75 mg/kg, intraperitoneally with 3 hr interval for 3 consecutive days), or clonidine (2 mg/kg/day, intraperitoneally for 10 days). Naloxone (4 mg/kg, intraperitoneally) precipitated withdrawal signs in both morphine- and clonidine-treated mice. Yohimbine (5 mg/kg, intraperitoneally) precipitated withdrawal signs in the clonidine-treated mice, similar to morphine withdrawal signs; but failed to precipitate any significant sign in the morphine-treated mice. Coadministration of lithium was carried out by adding lithium chloride to drinking water (600 mg/l for 20 days; 10 days before the beginning of clonidine administration and 17 days before the administration of morphine to allow the lithium concentration to reach steady-state). The results indicated that chronic lithium administration significantly attenuated the withdrawal signs, precipitated either by yohimbine or naloxone, in clonidine-treated mice. As a conclusion, clonidine withdrawal signs are very similar to opioid withdrawal signs, and lithium is able to prevent the development of physical dependence to clonidine.

Omega-agatoxin IVA blocks spinal morphine/clonidine antinociceptive synergism

Involvement of P-type voltage-dependent Ca2+ channels in spinal morphine- or clonidine-induced antinociception and in the synergistic interaction between morphine and clonidine was examined in the present studies. Coadministration of the selective P-type antagonist, omega-agatoxin IVA (25 ng) intrathecally (i.t.) to mice along with morphine or clonidine enhanced the tail flick antinociception of each agonist 5-6-fold. The greater-than-additive (synergistic) interaction that occurred when morphine and clonidine were coadministered i.t. decreased to an additive interaction in the presence of omega-agatoxin IVA. In mice pretreated with pertussis toxin (10 ng) to inactivate G proteins, omega-agatoxin IVA did not alter the morphine/clonidine synergism. Surprisingly, omega-agatoxin IVA reversed the additive morphine/clonidine interaction that occurs in morphine-tolerant mice back to synergism. These results suggest that functional P-type Ca2+ channels play an essential role in the antinociceptive synergism between spinal morphine and clonidine.

Effects of diltiazem, a Ca2+ channel blocker, on naloxone-precipitated changes in dopamine and its metabolites in the brains of opioid-dependent rats

The effects of diltiazem, an L-type Ca2+ channel blocker, on naloxone (an opioid receptor antagonist)-precipitated withdrawal signs and changes in extracellular levels of dopamine (DA) and its metabolites in various brain regions of morphine (a mu-opioid receptor agonist) or butorphanol (a mu/delta/kappa mixed opioid receptor agonist) dependent rats were investigated using high performance liquid chromatography fitted with an electrochemical detector (HPLC-ED). Rats were rendered opioid-dependent by continuous intracerebroventricular (i.c.v.) infusion with morphine (26 nmol/microliters per h) or butorphanol (26 nmol/microliters per h) for 3 days. The expression of physical dependence produced by these opioids, as evaluated by naloxone (5 mg/kg. i.p.)-precipitated withdrawal signs, was reduced by concomitant infusion of diltiazem (10 and 100 nmol/microliters per h). Under the same condition, naloxone decreased the levels of: DA in the cortex, striatum, and midbrain; 3,4-dihydroxyphenylacetic acid (DOPAC) in the cortex, striatum, limbic areas, and midbrain: and homovanilic acid (HVA) in the striatum, limbic areas, and midbrain regions. In animals rendered dependent on butorphanol, the results obtained were similar to those of morphine-dependent rats except for the changes in DOPAC levels. Furthermore, concomitant infusion of diltiazem and opioids blocked the decreases in levels of DA, DOPAC, and HVA in a dose-dependent manner. These results suggest that the augmentation of intracellular Ca2+ mediated through L-type Ca2+ channels during continuous opioid infusion results in a decrease in extracellular levels of DA and its metabolites in some specific regions, which are intimately involved in the expression of withdrawal syndrome precipitated by naloxone.

Ca2+ channel blocker, diltiazem, prevents physical dependence and the enhancement of protein kinase C activity by opioid infusion in rats

The influence of an L-type Ca2+ channel blocker, diltiazem [(2S-cis)-3-(acetyloxy)-5-[2-(dimethylamino)-ethyl]-2,3-dihydro-2- (4- methoxyphenyl)-1,5-benzothiazepin-4(5H)-one], on the behavioral signs of naloxone (opioid receptor antagonist)-precipitated withdrawal syndrome and the enhancement of protein kinase C activity in the pons/medulla regions of rats rendered dependent on morphine (mu-opioid receptor agonist) or butorphanol (mu/delta/kappa mixed opioid receptor agonist) was investigated. The expression of physical dependence produced by continuous intracerebroventricular (i.c.v.) infusion of morphine (26 nmol/microliters per h) or butorphanol (26 nmol/microliters per h) for 3 days, as evaluated by naloxone (5 mg/kg, i.p.)-precipitated withdrawal signs, was dose dependently attenuated by concomitant infusion of diltiazem (10 and 100 nmol/microliters per h). Furthermore, diltiazem (100 nmol/microliters per h) completely inhibited the enhancement of cytosolic protein kinase C activity in the pons/medulla regions in rats rendered dependent by continuous infusion with morphine or butorphanol. These results suggest that the augmentation of intracellular Ca2+ concentration mediated through L-type Ca2+ channels during continuous opioid infusion leads to the enhancement of cytosolic protein kinase C activity in the pons/medulla region which is intimately involved in the development and/or expression of physical dependence on opioids.