The effect of morphine on monoamine release and content in guinea-pig brain slices

Effects of naloxone on amphetamine induced striatal dopamine release in vivo: a microdialysis study

The opiate antagonist naloxone (NX) alters amphetamine (AMPH) induced behaviors including locomotor activity, rearing and stereotypy. However, the exact nature of the NX induced alteration of AMPH induced behaviors is controversial, with some studies using high (5-40 mg/kg) doses of NX reporting an inhibition, and others using low (< or = 1-2 mg/kg) doses observing a potentiation. As these behaviors are mediated by AMPH induced dopamine (DA) release, the effect of NX on the latter was examined by microdialysis in an effort to resolve the controversy. Saline and NX pretreated groups subsequently administered AMPH were compared in vivo across nine separate 10 min intervals as well as by grouped intervals. NX alone (0.8 mg/kg) and saline exerted statistically equivalent effects on striatal DA release with the exception of the fifth interval, where a small but significant increase was seen after NX. On the other hand, the same dose of NX significantly enhanced AMPH induced striatal DA release relative to saline pretreated animals during each of four separate intervals, from 30 to 70 minutes following AMPH (1.5 mg/kg), and across all nine intervals combined. NX pretreatment (0.8 mg/kg) followed by a higher dose of AMPH (3.0 mg/kg) produced a significantly greater cumulative effect on DA release than saline pretreatment over the last six combined intervals (30-90 min) and over two grouped intervals (30-50 min and 40-60 min inclusive). However, a comparison of single rather than paired or grouped intervals revealed no significant differences. Previous studies have also examined the effect of NX on AMPH induced striatal DA release using in vivo microdialysis. However, the doses used were invariably high (5 mg/kg) and the results on striatal DA release always inhibitory. The present results suggest that NX potentiates AMPH induced striatal DA release when lower doses of NX are used. These results combined with those of previous studies also suggest that NX exerts a biphasic effect on AMPH induced DA release, with lower doses potentiating release and higher doses inhibiting release. This is close agreement with behavioral observations and may be due to the effect of low versus high doses of NX on intraterminal calcium influx.

Clonidine inhibition of norepinephrine release from normal and morphine-tolerant guinea pig cortical slices

Endogenous norepinephrine (NE) release in cerebral cortex slices taken from normal and morphine-tolerant guinea pigs was measured by HPLC. In normal slices, a linear relationship was found between electrically evoked NE release and the log of the frequency of stimulation in the range of 1-20 Hz. The efficiency of the alpha 2-mediated autofeedback was tested by adding the alpha 2-agonist clonidine and the alpha 2 agonist idazoxan. NE release was dose-dependently reduced by clonidine (1 nmol/L-1 mumol/L) and increased by idazoxan (10-100 nmol/L). The inhibition by clonidine was significantly greater at 1 Hz than at 3 Hz, whereas the absolute increase in NE release induced by idazoxan was greater at 3 Hz than at 1 Hz. Morphine at 1 mumol/L (a concentration per se ineffective) shifted to the left the clonidine concentrations able to inhibit NE release at 3 and 1 Hz (1-10 nmol/L), but at both frequencies, the opiate reduced the maximal inhibition induced by clonidine at 1 mumol/L. In slices taken from morphine-tolerant guinea pigs (in the presence of morphine at 1 mumol/L), clonidine (1 nmol/L-1 mumol/L) was ineffective at the stimulation rate of 3 Hz, but it was more active than in normal slices at 1 Hz. Such a response pattern suggests a reduced availability of alpha 2 receptors and an increase in their sensitivity to clonidine. However, chronic morphine treatment did not influence the physiological autoinhibition because the increase in NE release elicited by idazoxan (10-100 nmol/L) at 1 and 3 Hz was the same in normal and in "morphine-tolerant" slices.(ABSTRACT TRUNCATED AT 250 WORDS)