Cocaine cues retain silent traces of an excitatory history after conversion into conditioned inhibitors

Abstract

The present experiment investigated the extent to which the A+/AB− conditioned inhibition procedure could counteract an excitatory drug-related conditioning history. In two groups of rats, a light stimulus was established as a signal for the absence of cocaine. For the History Group, the light had previously been a discriminative stimulus (SD) that occasioned cocaine self-administration and could thus be classified as a cocaine excitor. In comparison, the No-History Group first encountered the light during conditioned inhibition training. During conditioned inhibition training, both groups self-administered cocaine during tone as well as in click SDs, while drug seeking was eliminated in click-plus-light wherein cocaine was not available (A+/AB−). Drug seeking was essentially eliminated in both groups. Nevertheless, on a summation test the light reduced cocaine seeking occasioned by the tone SD by 95% in the No-History Group, but by less than 50% in the History Group. This summation test result showed that the effects of a drug-related history persisted even after the light was converted into an effective conditioned inhibitor on the training baseline through the powerful A+/AB− procedure. Future research should seek procedures that produce even stronger conditioned inhibition that eliminates such residual “silent” drug excitation, the “ghost in the addict” (Siegel, 2002, 2005).