Morphine history sensitizes postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats.

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Previous work has shown that the dorsal raphe nucleus (DR)-5-HT system is inhibited by swim stress via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor (CRF). Additionally, the DR 5-HT system is regulated by opioids. The present study tests the hypothesis that the DR 5-HT system regulates stress-induced opioid relapse. In the first experiment, electrophysiological recordings of GABA synaptic activity in 5-HT DR neurons were conducted in brain slices from Sprague-Dawley rats that were exposed to swim stress-induced reinstatement of previously extinguished morphine conditioned place preference (CPP). Behavioral data indicate that swim stress triggers reinstatement of morphine CPP. Electrophysiology data indicate that 5-HT neurons in the morphine-conditioned group exposed to stress had increased amplitude of inhibitory postsynaptic currents (IPSCs), which would indicate greater postsynaptic GABA receptor density and/or sensitivity, compared to saline controls exposed to stress. In the second experiment, rats were exposed to either morphine or saline CPP and extinction, and then 5-HT DR neurons from both groups were examined for sensitivity to CRF in vitro. CRF induced a greater inward current in 5-HT neurons from morphine-conditioned subjects compared to saline-conditioned subjects. These data indicate that morphine history sensitizes 5-HT DR neurons to the GABAergic inhibitory effects of stress as well as to some of the effects of CRF. These mechanisms may sensitize subjects with a morphine history to the dysphoric effects of stressors and ultimately confer an enhanced vulnerability to stress-induced opioid relapse.

Topographical distribution of corticotropin-releasing factor type 2 receptor-like immunoreactivity in the rat dorsal raphe nucleus: co-localization with tryptophan hydroxylase.

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides are involved in the regulation of stress-related physiology and behavior. Members of the CRF family of neuropeptides bind to two known receptors, the CRF type 1 (CRF1) receptor, and the CRF type 2 (CRF2) receptor. Although the distribution of CRF2 receptor mRNA expression has been extensively studied, the distribution of CRF2 receptor protein has not been characterized. An area of the brain known to contain high levels of CRF2 receptor mRNA expression and CRF2 receptor binding is the dorsal raphe nucleus (DR). In the present study we investigated in detail the distribution of CRF2 receptor immunoreactivity throughout the rostrocaudal extent of the DR. CRF2 receptor-immunoreactive perikarya were observed throughout the DR, with the highest number and density in the mid-rostrocaudal DR. Dual immunofluorescence revealed that CRF2 receptor immunoreactivity was frequently co-localized with tryptophan hydroxylase, a marker of serotonergic neurons. This study provides evidence that CRF2 receptor protein is expressed in the DR, and that CRF2 receptors are expressed in topographically organized subpopulations of cells in the DR, including serotonergic neurons. Furthermore, these data are consistent with the hypothesis that CRF2 receptors play an important role in the regulation of stress-related physiology and behavior through actions on serotonergic and non-serotonergic neurons within the DR.