Histochemical demonstration of monoamine oxidase in the hypothalamo-hypophysial system of the tree sparrow and the rat

Lack of effect of bromocriptine and domperidone on arginine-vasopressin response to insulin-induced hypoglycemia in normal men

In order to establish whether the arginine-vasopressin (AVP) increase evoked by insulin-induced hypoglycemia is mediated by a dopaminergic pathway, 16 normal men were given an insulin tolerance test (ITT) under basal conditions and after treatment with the dopamine agonist bromocriptine (2.5 mg by mouth) (eight subjects) or the dopamine antagonist domperidone (10 mg i.v.) (eight subjects). In addition, five out of eight subjects in each group were treated with higher doses of bromocriptine (5 mg by mouth) or domperidone (20 mg i.v.). Basal and ITT-stimulated AVP secretion was not modified by the drug treatments, suggesting that dopamine is not involved in the regulation of AVP secretion in response to insulin-induced hypoglycemia.

A functional analysis of dopaminergic innervation of the neurohypophysis

Dopaminergic neurons arising from cell bodies in the rostral arcuate nucleus of the hypothalamus have been shown to make axoaxonic contact with neurohypophyseal neurosecretory axons. In this study, electrical stimulation of the rostral arcuate nucleus depresses multiunit electrical activity recorded from neurosecretory axons within the neurohypophysis. After a single 5-s stimulus train, neurohypophyseal electrical activity is reduced to 6% of control. The superfusion of dopamine (5 micrograms/microliters) onto the neurohypophysis also has an inhibitory effect. Superfusion directly onto the neurohypophysis of the dopamine-receptor antagonist, pimozide (1 micrograms/microliters), abolishes the inhibitory effect of arcuate nucleus stimulation. These findings suggest that the dopaminergic innervation of the neurohypophysis may have an inhibitory influence on the release of neurohypophyseal hormones.

Histochemical characterization of monoamine oxidase in ependyma of rat hypothalamus

Monoamine oxidase (MAO) activity has been demonstrated histochemically in rat hypothalamic ependyma using the sulphate-tetrazolium and coupled peroxidatic techniques with tryptamine, tyramine, 5-hydroxytryptamine and benzylamine as substrates. Both methods were applied to cryostat sections with and without exposure to selective amine oxidase inhibitors, including the selective A-MAO inhibitor clorgyline, and the B-MAO inhibitor deprenyl. Our results show that both cuboidal-columnar and tanycyte ependyma contain one or more forms of MAO not generally present in the hypothalamus. It is suggested that ependymal MAO may form an amine-barrier system modulating the movement and effect within the hypothalamus of specific cerebrospinal fluid or blood monoamines.

Distribution of monoamine oxidase (MAO) in relation to the hypothalamo-hypophyseal system of the teleost Clarias batrachus (L.)

Varying degrees of monoamine oxidase (MAO) activity were noticed all along the hypothalamo-hypophyseal neurosecretory tract in C. batrachus. Intense activity was evident on the wall of the secretory cells of the adenohypophysis, neurons of the nucleus preopticus (NPO) and nucleus lateralis tuberis (NLT). However, MAO activity was also observed in the cytoplasm of some neurons in the ventromedian region of the NLT and groups of cells present in the neurohypophyseal ramifications of the pars intermedia. A strong accumulation of formazan granules in the median eminence (ME) and the neuroadeno interface lends further support to the view that C. batrachus has a ME of both the tetrapodan and the teleostean type. The distributional pattern of MAO in the hypothalamus and hypophysis suggests that monoaminergic fibres are probably involved in the secretory mechanism of the releasing factors and pituitary hormones.