Two groups of rats have a small pellet implanted under the skin. In Group A rats the pellet contains morphine, which is slowly absorbed over a 72 hour period

Question

In Group B rats the pellet contains an inert substance. The rats are kept in their home cages for the three days after the pellet implantation. Both groups are then tested for analgesia by being injected intraperitoneally with a small dose of morphine. This test consists of the rats being placed on a hot surface and they are timed to see how long it takes them to lick their paws. (It is assumed that the longer it takes the rats to lick their paws, the more the analgesia.) Group A rats display much less of an analgesic response than Group B rats. Is this tolerance phenomenon more compatible with a pharmacodynamic or a context specific mechanism for tolerance development? Explain your reasoning.

What will be an ideal response?

Answer 1

The fact that the Group A rats, which had been exposed to morphine prior to their exposure in the analgesia test, exhibited less analgesia than Group B rats indicates the development of tolerance to morphine's analgesic properties. Because the Group A rats given the morphine pellet (1) were tested for morphine analgesia in a context totally different from that in which they experienced the effects of morphine and (2) never practiced the paw-licking behavior in response to heat while under the influence of morphine, it is unlikely that this is a context-specific form of tolerance, e.g., the development of a compensatory CR to the contextual/conditioned stimuli (CS) or an instrumental compensatory behavior. On the other hand, three days of chronic exposure to the morphine should be sufficient to induce some pharmacodynamic neuroadaptations in the Group A rats.