Neural substrates of opiate withdrawal.

Drug withdrawal is an integral part of most types of dependence and, to a large extent, opiate withdrawal has been considered the prototypic, classic measure of opiate dependence. The opiate withdrawal syndrome is characterized by multiple behavioral and physiological signs such as behavioral activation, ptosis, diarrhea, 'wet dog' shakes and motivational dysfunction, which may be represented in the CNS at multiple sites. It seems that the activating effects associated with the opiate withdrawal syndrome may be mediated by the nucleus locus coeruleus. Other signs such as wet dog shakes may involve sites in the hypothalamus important for temperature regulation. Certain other signs such as diarrhea and lacrimation may be dependent on peripheral opiate receptors. The motivational aspects of opiate withdrawal as demonstrated by the aversive stimulus effects or negative reinforcing effects (e.g. disrupted lever-pressing for food and place aversions) may involve those elements of the nucleus accumbens that are known to be important for the acute reinforcing effects of opiates in nondependent rats. Evidence exists at the cellular and molecular level for both 'within-system' and 'between-system' adaptations to dependence. Elucidation of the neural networks, cellular mechanisms and molecular elements involved in opiate withdrawal may provide not only a model for our understanding of the adaptive processes associated with drug dependence but also of those associated with other chronic insults to CNS function.