The role of the habenular complex in the elevation of dorsal raphe nucleus serotonin and the changes in the behavioral responses produced by uncontrollable stress
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Abstract
Previous research indicates that the serotonergic neurons of the caudal dorsal raphe
nucleus (DRN) are activated to a greater degree by inescapable shock (IS) as compared
to escapable shock (ES), causing a greater release of serotonin (5-HT) in the DRN
and in target regions. This differential activation is necessary for the behavioral
changes that occur after exposure to IS, but not to ES (i.e. 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
unknown. One structure that may be involved in the activation of the DRN and the 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 severely attenuating the rise in 5-HT for both
groups. In experiment 2, sham operated and habenula lesioned rats were exposed to
either ES, IS or no stress (home cage control; HCC). Twenty-four hours later, sham
rats previously exposed to IS exhibited longer escape latencies as compared to both
ES and HCC rats (i.e. learned helplessness). The habenular lesion eliminated the differences
in escape latency between groups, thus eliminating the induction of learned helplessness/behavioral
depression. These results suggest that the habenula is necessary for the differential
activation of the DRN and the escape deficits produced by IS.