Possible DA agonist properties of naloxone

Dopaminergic supersensitivity factors in Tourette's syndrome: a hypothesis

Alterations in striatal dopamine receptor functions are thought to underlie the major symptoms of Tourette's syndrome (TS). In this communication, evidence is presented to suggest that the striatal dopamine receptor supersensitivity in TS may be mediated by alterations in the release and function of dopamine supersensitive factors including prolactin, estrogens, beta-endorphin, melanocyte stimulating hormone (MSH), and adrenal corticotrophic hormone (ACTH). It is suggested that treatment of TS should be directed towards rectifying these endocrine and neuropeptide imbalances.

Naloxone requires circulating catecholamines to attenuate the cardiovascular suppression of endotoxic shock

The opiate antagonist naloxone (NAL) improves cardiovascular performance in canine hemorrhagic and endotoxic shock. If the release of neural and adrenal catecholamines is attenuated, NAL does not produce the expected improvement in cardiovascular function in canine hemorrhagic shock. This study tests the hypothesis that an endorphin-catecholamine interaction at the heart is responsible for a part of the cardiovascular depression of endotoxic shock. Two groups of five dogs were instrumented to measure mean arterial pressure (MAP), the first derivative of left ventricular pressure over time (LV dP/dt max), cardiac output, and heart rate (HR); they were then subjected to bilateral adrenalectomy and given chlorisondamine to produce ganglionic blockade. At t = 0 min the dogs were given Escherichia coli endotoxin at 1 mg/kg (LD80). Group I animals received NAL at 2 mg/kg + 2 mg/kg.hr iv from t = 30 to t = 60. At t = 45 these animals were treated with epinephrine (EPI) at 20 micrograms/kg.hr iv until t = 60. Group II animals got EPI from t = 30 to t = 60 and NAL from t = 45 to t = 60 at the same doses as Group I. In Group I, NAL alone had no effect on MAP, LV dP/dt max, or HR. EPI significantly increased (P less than 0.002) cardiovascular parameters with MAP increasing from 52 +/- 7 to 159 +/- 14 mm Hg. In Group II, EPI produced a significant increase in all parameters, and the addition of NAL produced a further significant increase; MAP increased from 37 +/- 3 to 126 +/- 16 mm Hg with EPI and then to 175 +/- 11 mm Hg with NAL. These data support the above hypothesis and indicate that circulating catecholamines need to be present to allow naloxone to reverse the cardiovascular depression in endotoxic shock.

Effects of nimodipine on the discriminative stimulus properties of d-amphetamine in rats

The discriminative stimulus (DS) properties of d-amphetamine (AMP) are thought to be mediated by enhanced release of catecholamines, which may involve neuronal calcium influx through voltage sensitive channels. The present study examined the influence of nimodipine, a calcium channel blocker, on the DS properties of AMP. Rats (N = 8) were trained to discriminate AMP (0.5 mg/kg, IP) from saline in a two-lever, food-reinforced, drug discrimination paradigm. Nimodipine alone (2.0-5.6 mg/kg, IP) did not substitute for AMP. When given in combination with AMP, 2.0 mg/kg nimodipine increased by less than 2-fold the AMP dose necessary to induce AMP-appropriate responses. Higher doses of nimodipine combined with AMP did not increase the magnitude of this effect. Nimodipine enhanced the effects of AMP on response rate. Haloperidol (0.125 mg/kg) increased by approximately 4-fold, whereas diazepam (0.5 or 1.0 mg/kg) and morphine (5.0 mg/kg) increased by approximately 2-fold the AMP dose necessary to induce AMP-appropriate responses. The interaction with AMP was associated with enhanced reduction of response rate in the tests with diazepam and morphine but not haloperidol. These results suggest that nimodipine attenuates the DS properties of AMP, probably in a non-specific way, due to the ability of nimodipine itself to induce a discriminable internal state.

Sleep pattern alterations by naloxone. Partial prevention by haloperidol

A continuous intravenous infusion of the opiate receptor antagonist naloxone, at doses ranging from 0.7 to 1.0 micrograms/kg/min (total dose 8.8-13.2 mg), was performed in eight normal subjects during the first 240 min of night sleep. A significant reduction in stage REM and in stage 4 percent duration was observed. Pretreatment with the dopamine receptor blocking agent haloperidol (2 mg intramuscularly before sleep onset) partially prevented the naloxone effect, suggesting that it is due, at least in part, to the stimulation of dopamine receptors.