[In-vitro evaluation of morphine dependence. I. Response of the rat "fundus" to ACh]

The kinetic of cholinergic receptor systems was studied in an in vitro model for opiate tolerance and dependence. The fundus strip of rats chronically treated with morphine and sacrificed at different time intervals and in different conditions (with and without abstinence signs) was used. In evaluating the responses of fundus to ACh, the cholinergic receptor system showed the same response both in a condition of abstinence and in a condition of non-abstinence in vivo and in vitro. The response does vary however, according to the duration of the treatment: after an initial increase it comes back to its normal values, thus reproducing the tolerance pattern.