One for all or one for one: does co-transmission unify the concept of a brain galanin "system" or clarify any consistent role in anxiety?

Abstract

Galanin (GAL) is a potential target for novel antidepressant or anti-anxiety drug development. However, no integrated role for a "brain galanin system" in anxiety has yet emerged. It is possible that such a function may be revealed by examining the interaction of GAL with norepinephrine (NE), with which it is prominently co-localized. We showed previously that enhancing stress-activation of the NE system by yohimbine (YOH) pretreatment induced the release of GAL in central amygdala (CeA) to exert an anxiolytic effect on the elevated plus-maze. However, it remained to be demonstrated conclusively that GAL was co-released from NE terminals in CeA in this context, or if a multi-synaptic circuit activated GAL release from another afferent to CeA, or from local GAL neurons in the vicinity of CeA. In studies presented at the Third International Symposium on Galanin and Its Receptors, we utilized a combination of behavioral pharmacological approaches, testing the effects of YOH on the behavioral response to stress on the plus-maze after lesioning NE afferents to CeA with 6-OHDA, and anatomical approaches to identify GAL afferents to CeA that are activated in the context of stress with yohimbine pretreatment, to address these alternatives. Our results suggest that GAL was not co-released from noradrenergic terminals innervating CeA to exert an anxiolytic influence when noradrenergic activation was amplified by yohimbine pretreatment. Rather, it most likely originated from GAL neurons immediately adjacent to CeA that were activated by a non-noradrenergic afferent arising from elsewhere in the brain, itself activated by increasing NE activity. Thus, any role for brain GAL in anxiety remains region-specific, pathway specific, response specific and context-specific, which is likely to continue to present challenges to the development of novel agents targeting brain GAL for treatment of depression or anxiety.