Robert Newcomb

Role of Q-type Ca2+ Channels in Vasopressin Secretion From Neurohypophysial Terminals of the Rat

1 The nerve endings of rat neurohypophyses were acutely dissociated and a combination of pharmacological, biophysical and biochemical techniques was used to determine which classes of Ca2+ channels on these central nervous system (CNS) terminals contribute functionally to arginine vasopressin (AVP) and oxytocin (OT) secretion. 2 Purified neurohypophysial plasma membranes not only had a single high‐affinity binding site for the N‐channel‐specific ω‐conopeptide MVIIA, but also a distinct high‐affinity site for another ω‐conopeptide (MVIIC), which affects both N‐ and P/Q‐channels. 3 Neurohypophysial terminals exhibited, besides L‐ and N‐type currents, another component of the Ca2+ current that was only blocked by low concentrations of MVIIC or by high concentrations of ω‐AgaIVA, a P/Q‐channel‐selective spider toxin. 4 This Ca2+ current component had pharmacological and biophysical properties similar to those described for the fast‐inactivating form of the P/Q‐channel class, suggesting that in the neurohypophysial terminals this current is mediated by a ‘Q’‐type channel. 5 Pharmacological additivity studies showed that this Q‐component contributed to rises in intraterminal Ca2+ concentration ([Ca2+]i) in only half of the terminals tested. 6 Furthermore, the non‐L‐ and non‐N‐component of Ca2+‐dependent AVP release, but not OT release, was effectively abolished by the same blockers of Q‐type current. 7 Thus Q‐channels are present on a subset of the neurohypophysial terminals where, in combination with N‐ and L‐channels, they control AVP but not OT peptide neurosecretion.

Neurotoxins of Cone Snail Venoms

Cone snails are predatory marine mollusks that rely on their venom components to immobilize and capture fish, worms, or other mollusks. Cones employ a number of prey-hunting strategies, ultimately involving the injection of venom through a hollow, harpoon-like, modified-tooth structure (1). Like most animal venoms, those of the cones can contain proteins and small molecules, as well as a variety of peptides that are most often conformationally constrained by internal disulfide bridges. When envenomated, a cone’s piscine, molluscan, or vermicular prey is rapidly subdued by the concerted, high-affinity binding of the venom’s protein and peptide toxins to voltage- and ligand-gated ion channels essential for the proper function of the prey’s nervous and muscular systems.

Simulation of peptide processing, compartmentation and release in neurosecretory cells

Abstract A theoretical framework is presented which mathematically describes the transport of secretory granules in neurons and the enzymatic processing of a prohormone and subsequent intermediates within such granules. The transport system represents the synthesis of individual granules, the migration of the granules among various “diffusionally”-connected compartments, and the release of granules from designated compartmental release sites. The processing of prohormone is represented by a multiple-site cleavage reaction with separate rate constants for each cleavage site on each peptide fragment. The effects of neuron growth, processing enzyme activity decay, and changes in release probability with granule age are taken into account. Computer models were constructed based on these underlying assumptions. Simulations are used to illustrate how the distribution of peptide fragments contained in a neuron will depend on interactions of peptide processing kinetics and cellular transport and storage processes. The models provide a tool for testing how multiple cell-biological variables will interact to control the chemical mixture secreted from a peptidergic neuron.

Quantitative analysis and computer simulation of oxytocin-neurophysin processing in the rat neurohypophysis

Abstract A chromatographic procedure is used to obtain data on amounts of the oxytocin-neurophysin (OT-NP, or “A”) and its more completely processed form (OT-NP′, or “B”), which lacks the carboxyl-terminal glutamic acid residue of the A form. Measurements of B + A and B/(B + A) are made for single neurohypophyses under varying physiological conditions, in release obtained by perfusion of isolated neurohypophysis with saline containing increased external potassium concentrations, and in isolated neural lobe nerve endings and nerve swellings. The chromatographic procedure is also combined with injection of [35S]cysteine over the supraoptic nucleus. This approach is used to obtain the in vivo A to B conversion rate, and to investigate the trafficking of newly synthesized secretory granules in different axonal compartments. The manner with which the conversion rate changes with secretory activity is investigated by comparing the conversion rate in control animals to that in animals subjected to osmotic stress. Generalized computer models of peptide processing, storage, and release in neurons are used to test various hypotheses of the control of secretory granule movements in the neurohypophysis for sufficiency in explaining the changes in B + A and B/(B + A) with changing physiological state.