A fundamental problem in neurobiology is the excitability of presynaptic nerve terminals, which is critical for synaptic release of neurotransmitter. The factors regulating excitability of terminals are poorly understood. This is due to the extremely small size of vertebrate nerve terminals, which make intracellular recordings impossible to obtain. It appears that many classical and peptide neurotransmitter, as well as nitric oxide and carbon monoxide, can influence the electrical properties of presynaptic terminals. However, the existing evidence for such influence is largely indirect, relying on changes in postsynaptic responses to released transmitter. In this proposal, a unique model system will be used to study the electrical properties of vertebrate nerve terminals. The parasympathetic chick ciliary contains "giant" presynaptic nerve terminal that form cup- like or calyciform endings on ciliary neurons. We have made intracellular recordings from these nerve terminals, and find evidence for a number of ion channels and neurotransmitter receptors on the terminals. The following fundamental questions will be addressed, Which ion channels are present on vertebrate nerve terminals, and what role do they play in regulating the membrane potential of the terminal region? How do presynaptic neurotransmitter receptors alter the excitability of nerve terminal? Can neurotransmitter receptors located on presynaptic nerve terminals be functionally coupled to different ion channels than are identical receptors situated on the same neuron's soma and dendrites? Intracellular (sharp) and whole-cell patch clamp recording will be made from calyciform terminals in intact ciliary ganglia in vitro. Using a brain slice preparation, similar recordings will be made from neurons in the chick lateral Edinger-Westphal nucleus, since the large terminal in the ciliary ganglion originate from cell bodies in this terminal receptors and similar somatic/dendritic receptors on the same class of neurons. Both opioids and tachykinins will be used for these studies, as receptors for both classes of neuropeptides are present on Edinger- Westphal somas and on calyciform terminals. OF particular interest ar mu and delta opioid receptors, since our initial results indicate that these receptors are coupled to different ion channels in the terminal region than in the somatic region. The mechanisms producing these disparate responses will be examined using a variety of agents that have inhibitory or stimulatory effects on G-protein-mediated intracellular pathways. This proposal seeks to obtain new information regarding factors that can alter the excitability of presynaptic nerve terminal. The model system to be used provides a unique opportunity to address several fundamental questions in neurobiology that are directly relevant to the function of presynaptic nerve terminals located throughout the nervous system. Alterations in normal presynaptic terminal excitability may be present in a number of neurological disease, including epilepsy and Alzheimer's disease.