Evidence of place conditioning after chronic intrathecal morphine in rats

The acute administration of either systemic or intrathecal morphine produced an antinociceptive reaction on the tail flick test; only systemic morphine produced a sedative effect on motor activity. Rats receiving chronic intrathecal injections were hyperactive relative to saline injected control subjects. After a series of four injections, a place preference was induced by both routes of administration: rats who had previously received morphine either by subcutaneous or intrathecal injection spent significantly more time in the context in which the drug had been given, relative to rats who were comparably injected with saline. These data suggest that chronic intrathecal morphine in rats may elicit a discriminable cue, either through direct pharmacological stimulation of supraspinal sites, or, indirectly, as a consequence of spinal opiate action.