The physiological role of neuropeptides in the brain has received much attention, but we still have a rudimentary understanding of their synaptic roles. Recently we have found that the opioid peptide dynorphin is released during high-frequency stimulation from mossy fibers (mfs), and causes a presynaptic inhibition of neighboring mf synapses by acting on kappa 1 opioid receptors. It also raises the threshold for the induction of mf long-term potentiation (LTP). This grant proposes to examine 7 questions: (1) how does dynorphin inhibit transmitter release from mfs? Experiments will examine the effects of selective Ca channel blockers on the action of dynorphin as well as the effect of dynorphin on the frequency of miniature excitatory postsynaptic currents; (2) what are the properties controlling the release of dynorphin from mf synapses? We will apply AM esters of slow Ca buffers, such as EGTA, to selectively block peptide release; (3) what factors are responsible for the remarkably slow time course of the synaptically-released dynorphin? We will apply a brief puff of dynorphin and terminate the action of dynorphin by rapidly applying opioid antagonists. Application of a cocktail of peptidase inhibitors will determine if the time course is governed by enzymatic degradation; (4) are kappa 1 receptors actually present in the CA3 region? We will design experiments to localize the action of dynorphin, (5) what is the potency of the various opioid peptides that are contained in mfs? This information will help identify the most likely candidate for the synaptic effects; (6) does activation of kappa 2 receptors inhibit mf fiber evoked N-methyl-D-aspartate receptor mediated responses, and if so, what is the mechanism? (7) are there similarities between our results with dynorphin at mf synapses and the possible role of enkephalins at the lateral perforant path in the dentate gyrus? We will test the hypothesis that enkephalin controls the induction of LTP at this synapse by causing disinhibition during the tetanus. These studies will involve characterizing opioid mediated synaptic effects on identified interneurons. While neuropeptides have received much attention, particularly opioid peptides, the role of this class of signaling molecule remains very unclear. The present study will help define the physiological role of neuropeptides in the CNS at the cellular level which is absolutely essential for understanding the role of these peptidergic synapses in affect, cognition, substance abuse, and the management of pain.
