Acute effects of morphine on Substance P concentrations in microdissected regions of guinea-pig brain.

The present study investigated the effects of acute morphine treatment and of naloxone-induced morphine withdrawal on Substance P (SP) concentrations in microdissected regions of the guinea-pig brain. Guinea-pigs, which were treated with a single dose of morphine sulphate (15mg/kg s.c.), received naloxone hydrochloride (15mg/kg s.c.) after 2h. Control animals received injections of saline, saline and naloxone, or morphine and saline. Locomotor and behavioural activities were measured throughout the experiments. Animals were killed 0.5h after naloxone administration, brains were removed and SP-like immunoreactivity (SP-LI) was measured in microdissected regions using radioimmunoassay. Morphine significantly increased the concentration of SP-LI in the central nucleus of the amygdala, but reduced SP-LI overall in the mesencephalon. Guinea-pigs pretreated with morphine and then given naloxone to precipitate withdrawal showed no change in SP-LI concentrations in any brain region, compared with control animals, despite an increase in locomotor activity and other behaviours characteristic of opioid withdrawal in guinea-pigs.