The role of the habenular complex in the elevation of dorsal raphe nucleusserotonin and the changes in the behavioral responses produced by uncontrollable stress

Previous research indicates that the serotonergic neurons of the caudal dor sal raphe nucleus (DRN) are activated to a greater degree by inescapable sh ock (IS) as compared to escapable shock (ES), causing a greater release of serotonin (5-HT) in the DRN and in target regions. This differential activa tion is necessary for the behavioral changes that occur after exposure to I S, but not to ES (Le. learned helplessness/behavioral depression). Although the critical role of the DRN in learned helplessness is clear, the neural inputs to the caudal DRN which result in this selective activation are unkn own. One structure that may be involved in the activation of the DRN and th e induction of learned helplessness/behavioral depression is the habenular complex. In experiment 1, habenula lesions eliminated the differential rise in DRN extracellular 5-HT levels in response to IS and ES exposure by seve rely attenuating the rise in 5-HT for both groups. In experiment 2, sham op erated and habenula lesioned rats were exposed to either ES, IS or no stres s (home cage control; HCC). Twenty-four hours later, sham rats previously e xposed to IS exhibited longer escape latencies as compared to both ES and H CC rats (i.e. learned helplessness). The habenular lesion eliminated the di fferences in escape latency between groups, thus eliminating the induction of learned helplessness/behavioral depression. These results suggest that t he habenula is necessary for the differential activation of the DRN and the escape deficits produced by IS. (C) 2001 Elsevier Science BY All rights re served.