The serotonergic dorsal raphe nucleus in opiate dependence and stress-induced relapse

Abstract

Opioids are used for the clinical treatment of pain, but can lead to tolerance and addiction. In this project we examined the role of the serotonin (5-HT) system originating from the dorsal raphe nucleus (DRN) during morphine exposure, withdrawal, abstinence and following an acute stressor capable of initiating behavioral relapse. Following four days of morphine exposure rats showed a preference for the morphine paired side of the conditioned place preference (CPP) chamber. After four days of morphine abstinence, rats showed no net preference for the morphine paired side. The next day rats were exposed to forced swim stress and returned to the CPP chamber where they demonstrated stress-induced reinstatement. Utilizing whole-cell patch-clamp we demonstrated an increase in the amplitude of inhibitory post-synaptic currents (IPSCs) in 5-HT DRN neurons, but not non 5-HT DRN neurons of morphine-conditioned subjects. Next the stress neurohormone corticotrophin releasing factor (CRF) was administered in vitro instead of forced swim. We found an increase in CRF-R2-mediated inward current of 5-HT DRN neurons in animals with a morphine history. From this experiment we concluded that morphine history sensitizes 5-HT DRN neurons to the GABAergic inhibitory effects of stress and to some of the effects of CRF. In the next series of experiments we surgically implanted either morphine or placebo pellets in rats for 72 hours to create physical dependence. The pellets were subsequently removed, and animals experienced up to seven days of abstinence with and without forced swim stress exposure. Real time quantitative PCR was used to measure the mRNA levels of genes at multiple points across this timeline. We examined genes involved in trophic support, stress responses and 5-HT regulation. We determined that mRNA levels for brain-derived neurotrophic factor (BDNF) and the BDNF receptor TrkB were downregulated after opiate exposure, and again following seven days of abstinence. Following seven days of abstinence there was a decrease in mRNA levels of the CRF-R1 receptor and an increase in mRNA levels of the CRF-R2 receptor. During acute opiate exposure there was a decrease in mRNA levels for the autoregulatory 5-HT1A receptor. Finally following forced swim, there was an increase in mRNA levels of the 5-HT synthesis enzyme TPH2. Collectively these results indicate that a morphine history in abstinent subjects may produce hypofunctioning of the 5-HT DRN system induced by multiple neurochemical mechanisms and this dysregulation may enhance vulnerability to stress-induced relapse.