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) conce ntrations in microdissected regions of the guinea-pig brain. Guinea-pi gs, which were treated with a single dose of morphine sulphate (15 mg/ kg s.c.), received naloxone hydrochloride (15 mg/kg s.c.) after 2 h. C ontrol animals received injections of saline, saline and naloxone, or morphine and saline. Locomotor and behavioural activities were measure d throughout the experiments. Animals were killed 0.5 h after naloxone administration, brains were removed and SP-like immunoreactivity (SP- LI) was measured in microdissected regions using radioimmunoassay. Mor phine significantly increased the concentration of SP-LI in the centra l nucleus of the amygdala, but reduced SP-LI overall in the mesencepha lon. Guinea-pigs pretreated with morphine and then given naloxone to p recipitate withdrawal showed no change in SP-LI concentrations in any brain region, compared with control animals, despite an increase in lo comotor activity and other behaviours characteristic of opioid withdra wal in guinea-pigs.