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Thesis (Ph. D.)--University of Adelaide, Dept. of Genetics, 1962. / Typewritten. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1961. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 24-27).

1. There is a population of alpha-bungarotoxin-resistant cholinergic receptors on the soma of the first basalar motoneurone of the locust, Schistocerca gregaria. 2. The receptors have a broadly muscarinic pharmacology: the response to pressure-applied ACh can be reversibly reduced by the global muscarinic antagonists atropine and dexetimide, and the vertebrate muscarinic subtype-selective antagonists pirenzepine (M1 subtype-selective), 4-DAMP (M3), p-F-HHSiD (M3 > M1 > M2) and methoctramine (M2). 3. In the presence of alpha-bungarotoxin, however, nicotine evokes a current similar to those induced by the muscarinic agonists muscarine, McN-A-343 and carbachol under voltage clamp; the nicotine-induced current can be blocked by the muscarinic antagonist scopolamine, suggesting that these 'muscarinic' receptors have a 'mixed' cholinergic pharmacology. 4. Under voltage clamp, activation of the receptors by the muscarinic agonists muscarine and McN-A-343, and the cholinergic agonist carbachol (in the presence of alpha-bungarotoxin) evokes an inward current at command potentials between -100 and 0 mV. This current is time- and voltage-dependent: the current component evoked at potentials more negative than -40 mV is relatively small, and either increases continuously during 15 min exposure to agonist, or increases to a maximum after 5 or 10 min before decreasing. At potentials more positive than -40 mV, the current component is larger, and increases continuously in the presence of drug. In approximately 50% of neurones voltage clamped at a holding potential of -50 mV, the inward current is preceded by an initial outward current at potentials positive to -40 mV. After 1 min exposure to agonist, the outward current reverses and becomes inwardly-directed, increasing in amplitude in the presence of agonist. 5. The apparent inward current evoked by McN-A-343 at potentials positive to -40 mV is due to the inhibition of a voltage-dependent K+ current, and may involve the production of cAMP. Activation of the 'mixed' receptors also stimulates the release of Ca2+ from intracellular stores. 6. The input resistance of the neurone, measured under current clamp, is increased by McN-A-343 in some preparations and decreased in others. The agonist decreases the spike threshold of the neurone, and therefore the physiological role of these 'mixed' receptors may be to modulate the excitability of the neurone.

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Acidification of rectal fluid in Schistocerca gregaria is due probably to active secretion of hydrogen ions, although bicarbonate absorption from H₂CO₃ leaving behind hydrogen ions cannot be excluded. This was concluded after the following mechanisms were eliminated: introduction of acid from some anterior region of the gut; bacterial fermentation; slow release of hydrogen ions from fecal material; preferential absorption of the basic form of the phosphate buffer pair; secretion of acid phosphate; release of hydrogen ions from the intima. Any absorption of phosphate in the rectum must be as the monovalent ion since the cuticular intima was found to limit severely passage of the divalent form, as shown by studies of intimal permeability at various pH values. The relationship of acidification of rectal contents to total acid-base regulation of the locust is not clear, although there is some evidence that excess acidity is lost via the excreta. Regulation of the hemolymph pH is rather slow, with a halfway return to normal in about one day when the tolerance level has not been exceeded. / Science, Faculty of / Zoology, Department of / Graduate