Chemical Coding Of Neurotransmission

National Institute Of Mental Health

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Abstract

The Section on Molecular Neuroscience studies slow (metabotropic) transmission in the nervous system, to identify novel molecular components underlying cellular plasticity following synaptic activity and slow transmitter release and post-synaptic action. The neuropeptide PACAP, acting through its G-protein coupled receptor PAC1 acts at the adrenomedullary synapse, as well as in the brain, to simultaneously elevate calcium and cyclic AMP signaling, and thus uniquely stimulate simultaneous signaling for secretion, and signaling for transcription, in target cells. The subtype of the PAC1 receptor responsible for this unique mode of combinatorial signaling has been shown by T. Mustafa to be the 'hop' variant of the PAC1 receptor. Features of the third intracellular loop of the receptor impart signaling for calcium mobilization, while independent determinants impart signaling through cAMP. Simultaneous activation of calcium and cAMP elevation by PACAP signaling directs activation of ERK within target cells via a non-canonical (PKA-independent) pathway that M. Gerdin has demonstrated can be mimicked by simultaneous treatment of neuroendocrine cells with agents that elevate cAMP and increase intracellular calcium, without participation of protein kinase A. This type of combinatorial signaling in neuroendocrine cells activates target genes that are not stimulated by either calcium or cyclic AMP alone, and include the apoptosis regulatory factor ier-3, and members of the secretogranin family of neurosecretory proteins. PACAP knock-out mice, previously used to demonstrate a requirement for PACAP signaling at the adrenomedullary synapse in the periperal nervous system, were employed to demonstrate a requirement for PACAP in neuroprotective responses to ischemic damage in stroke. Several target genes activated in cerebral cortex in stroke are PACAP-dependent (regulated in wild-type but not PACAP knock-out mice) including ier-3, and the neuropeptides enkephalin, neurotensin and substance P, and thus are potentially actors in neuroprotective effects of PACAP in ischemia. Up-regulation of several neuropeptides in the adrenal medulla during stress is likewise abrogated in PACAP-deficient mice, suggesting that PACAP's role as a regulator of neuropeptide expression is widespread throughout the nervous system during synaptic activation and/or stress. [unreadable] Our further work will be directed towards demonstrating the molecular components of the pathway linking cAMP signaling to ERK and downstream target genes activated by PACAP in neuroendocrine cells, extending this analysis to central nervous system target genes, and determining the physiological relevance of this novel signaling system to synaptic transmission and cellular plasticity in the brain as well as at the adrenomedullary synapse.