Some studies on the mechanism by which d-amphetamine antagonizes guanethidine induced adrenergic neurone blockade

Alterations in the noradrenergic projection to the cerebellum of the dystonic (dt) rat

The genetically dystonic rat (dt) has elevated resting levels of cerebellar norepinephrine (NE) in comparison with phenotypically normal littermates. This difference is not secondary to cerebellar hypoplasia. Increased NE is observed as early as postnatal day 12, when clinical symptoms have become evident. The elevation in cerebellar NE levels in the dt rat involves all cerebellar areas, but is not generalized to all terminal fields of the locus coeruleus. Elevations in cerebellar NE are followed developmentally by a reduction in sensitivity to the NE-depleting effects of reserpine, a change which is also confined to the cerebellum. The effects of amphetamine and the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine were similar in normal and dt rats. Levels of the major cerebellar metabolite of NE, 3-methoxy-4-hydroxyphenylglycol, did not differ between mutant and normal animals. Nor were any changes noted in the number or affinity of beta-adrenergic receptors. These data indicate that there is a regional alteration in NE storage. Cerebellar morphology appears normal in the dt rat, except for a decrease in Purkinje cell size. This change and other evidence of biochemical abnormalities in the Purkinje cells suggest that the alterations in cerebellar NE in the dt mutant may be a secondary response to a functional change in the target neuron for this system, the Purkinje cell.

Effects of fenfluramine on the adrenergic system

The influence of fenfluramine on sympathetic transmission has been examined in various intact and isolated preparations. Fenfluramine both facilitated and inhibited the responses elicited by the sympathetic stimulation according to the dose administered and the preparation used. Fenfluramine, to a lesser extent than amphetamine, antagonized, in some preparations, the neuron blocking action of guanethidine. Fenfluramine did not antagonize the sympathomimetic effects elicited by amphetamine in the isolated atria and the isolated vas deferens unless very high concentrations of both drugs were used.

Catecholamine-containing neurones and electrical self-stimulation. 1. A review of some data

SYNOPSIS Experimental data on the location in the brain of electrode sites supporting electrical self-stimulation are reviewed. All previous studies are in agreement that the behaviour is most easily elicited from lateral hypothalamic sites corresponding approximately to the fibre pathways known as the ‘medial forebrain bundle’. Some brainstem electrode sites will support self-stimulation. Experiments are summarized which indicate that there is a correlation between the sites of electrode tips positive for self-stimulation and the location of catecholamine-containing neurones in two regions: (1) in the region of the dopamine-containing cells in the ventral mesencephalon (cell-body groups A9 and AIO of Dahlstr¨m and Fuxe, 1964), and (2) in the region of the noradrenaline-containing cells of the locus coeruleus (A6 group). These two cell-body groups give rise to axons which pass through the medial forebrain bundle to form terminal systems with a widespread distribution in the prosencephalon. The hypothesis is advanced that electrical self-stimulation results in the majority of cases from activation of one or both of these catecholamine-containing systems.

Prejunctional actions of some beta-adrenoreceptor antagonists in the vas deferens preparation of the guinea-pig

1. The beta-adrenoceptor antagonists propranolol, pronethalol, MJ 1999 and Ciba 39089-Ba reduced responses to field stimulation of the guinea-pig isolated vas deferens preparation without significantly affecting responses to exogenously added noradrenaline.2. This prejunctional blocking action of the drugs cannot be correlated with their action as beta-adrenoceptor antagonists or non-specific depressants.3. The blockade produced was more pronounced at low (5-20 Hz) than at high (50 Hz) frequencies of stimulation.4. The blockade was slow in onset, and once established was poorly reversed by washing the preparation over a period of 1 to 2 h.5. The blockade produced could be reversed by dexamphetamine and cocaine.6. These experiments suggest that the beta-adrenoceptor antagonists may have some actions which closely resemble those of the adrenergic neurone blocking agent guanethidine.