The effects of cholinergic drugs support an avoidance learning hypothesis of brief footshock-induced analgesia.

Rats were tested for tail-flick responses and then immediately subjected to footshock for 30 sec. This procedure induced analgesia, i.e. prolonged the latency of the tail-flick response, which was maximal immediately after the shock and decayed to normal levels within 2 hr. No analgesia occurred if either the analgesia test before the shock or the shock itself was omitted. The dependence of the analgesia on the association between the test before the shock plus the shock, suggests that this was a form of avoidance learning. The effects of drugs injected immediately after the shock were determined on latency of the tail-flick response, measured 2 hr later. Drugs known to improve memory, including physostigmine, pramiracetam and the muscarinic agonists, oxotremorine and RS 86, selectively induced analgesia in rats subjected to test before the shock plus the shock, thereby supporting a hypothesis of avoidance learning. Neostigmine and atropine methyl nitrate had no effect, indicating that the effects were mediated centrally. The learning effects were distinguishable from analgesia induced by drugs, since morphine increased analgesia regardless of the presence or absence of the test before shock or the shock. Also, naloxone, which had no effect per se, blocked analgesia induced by morphine but enhanced physostigmine-induced analgesia. Neither chlordiazepoxide nor D-amphetamine produced any changes in the latency of the tail-flick responses indicating that neither anxiolytic/muscle relaxant nor stimulant actions were involved.