Modulation of the amount of neuropeptide released from a neurosecretory tissue may be achieved by different means. These include alterations in the quantity secreted from each active nerve terminal or in the actual number of terminals activated. From the vertebrate hypothalamus, magnocellular neurons project their axons as bundles of fibers through the median eminence and infundibular stalk to arborize extensively and terminate in the neurohypophysis, where the neurohypophysial peptides and proteins are released into the circulation by a Ca-dependent mechanism. Elevating [Ca2+]o increases the magnitude of an intrinsic optical change in the neurohypophysial terminals that is intimately related to the quantity of neuropeptide released. Similarly, the addition of micromolar concentrations of 4-aminopyridine to the bathing solution enhances this change in large angle light scattering. However, we show here that, while these effects are superficially similar, they reflect different mechanisms of action. Evidence from intrinsic optical signals (light scattering) and extrinsic (potentiometric dye) absorption changes suggests that calcium increases the amount of neuropeptide released from each active terminal in the classical manner, while 4-aminopyridine exerts its secretagogue action by enhancing the invasion of action potentials into the magno-cellular neuron's terminal arborization, increasing the actual number of terminals activated. Physiologically, electrical invasion of the complex terminal arborization in the neurohypophysis may represent an extremely sensitive control point for modulation of peptide secretion. This would be especially effective in a neurohaemal organ like the posterior pituitary, where, in contrast with a collection of presynaptic terminals, the precise location of release is less important than the quantity released.

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