The hypothalamo-neurohypophysial system of native Australian desert rodents. The vasopressin and oxytocin contents of hypothalamus and posterior pituitary of Notomys alexis and Pseudomys australis compared with those of the laboratory rat and mouse in different states of water balance.

The neurohypophysial hormones vasopressin (the antidiuretic hormone) and oxytocin have been quantified in xerophilic and mesophilic rodents under normal and dehydrated conditions in order to study the relative contribution of these hormones to survival under drought conditions. The ability to cope adequately with water deprivation or saline ingestion was found to be closely related to the magnitude (relative to body size) of pre-existing hormone stores in the neural lobe and to the subsequent ability to enhance hypothalamic biosynthesis of vasopressin and oxytocin sufficiently to maintain those stores at near-normal levels despite a continuing high level of demand for vasopressin and, apparently, also oxytocin. The possible role(s) of the release of oxytocin in these conditions is discussed. Attempts have been made to explain differences observed between laboratory-bred Notomys and those taken from the wild in their relative abilities to withstand water deprivation, the former seeming to be more dependent upon acutely accelerated hormone biosynthesis than on previous adaptation via renal medullary 'work-load' hypertrophy.

The hypothalamo-neurohypophysial system of native Australian desert rodents. The catecholamine contents of the hypothalamus and posterior pituitary of Notomys alexis, Pseudomys australis and the laboratory rat.

The putative central catecholamine neurotransmitters, adrenaline, noradrenaline and dopamine, have been quantified by enzyme radiochemical methods in the hypothalamo-neurohypophysial system (HNS) of the xeric rodents, Notomys alexis and Pseudomys australis, using the laboratory rat for comparison. All three catecholamines were found (noradrenaline greater than dopamine greater than adrenaline) in the hypothalami of the three species. The hypothalami of the two xeric rodents contained some 5-10 times as much of each of the catecholamines (when expressed as ng per body weight) as the rat. The ratio of noradrenaline to dopamine was constant (3 X 8-4 X 2) between species. Posterior pituitary contents of the catecholamines were generally inconsistent within and between species, with only dopamine being invariably and consistently present. It was also found that, like the antidiuretic hormone content, the dopamine content of the neural lobe was significantly correlated with the body surface area of the animal. Withdrawal of drinking water or its replacement with 2 X 5% saline increased hypothalamic noradrenaline synthesis and dopamine utilisation. In the neural lobe, saline ingestion produced a significant increase in all three catecholamines, but with water deprivation only dopamine was increased (relative to body weight). The results of these studies are interpreted as indicating a possible role for catecholamine neurotransmitters, particularly dopamine, in the control of neurohypophysial hormone release in the HNS.

The effect of dopamine on neurohypophysial hormone release in vivo and from the rat neural lobe and hypothalamus in vitro.

1. The rat hypothalamus (containing the supra-optic nuclei, paraventricular nuclei, median eminence and proximal pituitary stalk) has been incubated in vitro and shown to be capable of releasing the neurohypophysial hormones, oxytocin and arginine vasopressin, at a steady basal rate about one twentieth that of the rat neural lobe superfused in vitro. 2. The hypothalamus and neural lobe in vitro released both hormones in a similar arginine vasopressin/oxytocin ratio of about 1-2:1. However, when release was expressed relative to tissue hormone content, the hypothalamus was shown to release about three times as much arginine vasopressin and six times as much oxytocin as the neural lobe. 3. Dopamine in a concentration range of 10(-3)-10(-9)M caused graded increases in hormone release from the hypothalamus in vitro to a maximum fivefold increase over preceding basal levels. The demonstration that apomorphine also stimulated hormone release whereas noradrenaline was relatively ineffective suggested that a specific dopamine receptor was involved. A separate cholinergic component in the release process was indicated by the finding that acetylcholine stimulated release to a maximum fivefold increase in concentrations of 10(-3)-10(-9)M. 4. The fact that the isolated hypothalamus can be stimulated by dopamine and acetylcholine to release increased amount of oxytocin and arginine vasopressin raises the question of the origin and fate of the hormones released in this way. The possibility that they could be released into the hypophysial portal circulation from median eminence to affect the anterior lobe of the pituitary is discussed. 5. In similar doses, both dopamine and noradrenaline injected into the lateral cerebral ventricles of the brain of the anaesthetized, hydrated, lactating rat caused the release of arginine vasopressin and oxytocin. Apomorphine release both hormones but at a higher dose level and to less effect than the catecholamines. 6. The hormone release induced in vivo by dopamine could be prevented by the prior administration of haloperidol or phentolamine and these antagonists were equally effective in blocking the hormone release due to noradrenaline. The involvement of a specific dopamine receptor was more clearly implicated by the use of pimozide which completely inhibited the hormone release due to dopamine and apomorphine but not that due to noradrenaline. 7. It is suggested that the release of neurohypophysial hormones can be stimulated via a dopaminergic nervous pathway in addition to a cholinergic one. The possibility that the osmoreceptor mechanism for the release of antidiuretic hormone from the neural lobe of the pituitary may involve such a dopaminergic pathway is discussed.