Learned helplessness increases 5-hydroxytryptamine1B receptor mRNA levels in the rat dorsal raphe nucleus

Learned helplessness is a behavioral condition induced by exposure to inescapable stress that models aspects of stress-related disorders including depression and posttraumatic stress disorder, and has been associated with diminished serotonin release in the rat frontal cortex. Our hypothesis was that presynaptic 5-hydroxytryptamine1B (5-HT1B) receptors, which inhibit the synthesis and release of serotonin in nerve terminals, may be increased in learned helplessness. Postsynaptic 5-HT1B mRNA hybridization levels in the hippocampus or frontal cortex were unchanged following induction of learned helplessness; however, presynaptic 5-HT1B mRNA hybridization signal in the dorsal raphe nucleus of helpless rats was 25% higher than control values. There was no change in dorsal raphe serotonin transporter mRNA level. The detection of increased 5-HT1B mRNA levels in the dorsal raphe nucleus suggests an increased capacity to synthesize presynaptic 5-HT1B receptors and could account for diminished serotonin neurotransmission in learned helplessness.