Action potential broadening and frequency-dependent facilitation of calcium signals in pituitary nerve terminals.

Abstract

Hormone release from nerve terminals in the neurohypophysis Is a sensitive function of action potential frequency. We have investigated the cellular mechanisms responsible for this frequency-dependent facilitation by combining patch clamp and fluorimetric Ca2+ measurements in single neurosecretory terminals in thin slices of the rat posterior pituitary. In these terminals both action potential-induced changes in the intracellular Ca2+ concentration ([Ca2+] Little is known about the mechanisms that regulate secretion from nerve terminals, despite the importance of these mechanisms to many physiological processes. For example, a presynaptic site has been proposed as an important locus for change in several forms of use-dependent synaptic plasticity, including facilitation and posttetanic potentiation at neuromuscular synapses (1) and long-term potentiation in the mammalian hippocampus (2, 3). However, in these and most other systems one can do little more than infer that some presynaptic change occurs, because technical difficulties preclude a direct physiological characterization of the nerve terminals. Although a direct physiological characterization is possible in some invertebrate systems (4, 5), until recently there was no comparable vertebrate system for the study of presynaptic physiology.In an effort to bridge this technical gap, some investigators have made electrical and optical recordings from the nerve terminals of the vertebrate neurohypophysis (6-12). These terminals release the neuropeptides vasopressin and oxytocin and have received much attention as a model system for the study of secretion (6-12). Peptide secretion from these terminals also exhibits a striking form of use-dependent plasticity: secretion is a sensitive function of action potential frequency (11,(13)(14)(15)(16)(17). This frequency dependence is essential to the input-output properties of the hypothalamichypophyseal axis (18) and has been proposed to be due to frequency-dependent modulation of action potential duration (10,11,13,14,(19)(20)(21)(22)(23). In the present study, we use patch clamp techniques and fluorimetric calcium indicators to test this hypothesis in individual nerve terminals in pituitary slices. A preliminary account of this work has appeared (24). METHODSNeurohypophysial Slices. The neurointermediate lobe ofthe pituitary was removed from male rats with ages ranging from 2 to 4 months and placed in carbogen (95% 02/5% C02)-saturated rat Ringer's solution (125 mM NaCl, 2.5 mM KCl, 1.25 mM NaH2PO4, 26 mM NaHCO3, 2 mM CaCl2, 1 mM MgCl2, and 20 mM glucose) at 0WC for at least 1 min. Slices 70-80 j&m thick were then cut with an FTB vibratome, with continued bathing in 0WC rat Ringer's solution. Slices were either kept in a 34WC bath or transferred to a recording chamber at room temperature (20-220C) tTo whom reprint requests should be addressed. 380The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with...

Presynaptic plasticity: The regulation of Ca2+-dependent transmitter release

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Omega‐conotoxin‐sensitive and ‐resistant transmitter release from the chick ciliary presynaptic terminal.

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Cell Surface Targeting and Clustering Interactions between Heterologously Expressed PSD-95 and the Shal Voltage-gated Potassium Channel, Kv4.2

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