A stereotaxic atlas of the medulla oblongata of the goat's brain.

A stereotaxic atlas has been prepared for the medulla oblongata of the adult goat's brain using the technique described previously (Tindal et al. 1968). The atlas consists of transverse stereotaxic planes passing caudally at 1 mm intervals from posterior 10 mm (P10) at the level of the junction between brainstem and cerebellum to posterior 20 mm (P20) at the level of the obex.

Central inhibition of oxytocin release in the rabbit: role of the midbrain.

Transection of the ventral central grey and surrounding midbrain tegmentum in anaesthetized lactating rabbits caused repeated milk ejections which, on comparison with the effects of i.v. infusions of synthetic oxytocin, synthetic arginine-vasopressin or a mixture of the two, were attributed to continuous release of 1.25-2.5 mu. oxytocin/min, although it is not known whether lesser amounts of vasopressin might also have been released. It is suggested that the ascending midbrain reticular formation, which is known to project rostrally through this region, controls the central inhibition of oxytocin release via the previously described septo-hippocampo-subicular route to the hypothalamus.

Inhibition of growth hormone release by rumen distension in female goats.

Six rumen-fistulated, overnight-fasted, female British Saanen goats were used. In preliminary experiments on two of the goats, evidence was obtained that inflation of a balloon with water in the cranial sac of the rumen was accompanied by inhibition of GH release. In a definitive series of experiments on the other four goats (one goat on 1 day = one experiment), a total of 96 experiments were carried out, 48 of which were balloon inflations with 2.5 litres water at 37 degrees C in the cranial sac of the rumen for 40 min and 48 were control experiments on alternate days. It was found that the mean plasma GH concentration of the samples taken at the end of the 40-min inflation period was significantly (P less than 0.05) lower than the mean value of the samples taken at the start of inflation. Mean control values at the start and finish of inflation were not significantly different. There were no significant differences between mean plasma insulin concentrations at the start and finish of inflation in experimental and control groups. It is concluded that distension of the cranial rumen in the goat is a signal which can inhibit the release of GH.

Central inhibition of milk ejection in the rabbit: involvement of hippocampus and subiculum.

Removal of discrete parts of the forebrain of the anaesthetized lactating rabbit by suction caused repeated milk-ejection responses. These were associated with transection or removal of the subiculum, which represents the major efferent pathway from the hippocampal formation to the diencephalon. It is suggested that the hippocampus may exert a tonic, inhibitory influence over oxytocin release, that this inhibition may represent a cascade of events which originates in the brainstem and culminates in the subiculum and that removal of this inhibition may lead to release of oxytocin.

Control of growth hormone release in goats: effect of vagal cooling, feeding and artificial distension of the rumen.

It was confirmed that, as in overnight-fasted castrate male sheep, plasma growth hormone (GH) levels also fell in overnight-fasted castrate male goats shortly after they were given hay the following morning and that GH levels could also fall in anticipation of being fed. A further observation was that plasma GH levels usually remained low for 2--3 hours if the goats were allowed to continue eating but that if the hay was removed after only one hour of eating the plasma GH level rose 20--50 min later. Although initial attempts to determine the role of visceral stimuli in the regulation of GH release in castrate male goats by bilateral cooling of exteriorized vagi were inconclusive because of side-effects, they drew attention to the role of rumen distension and activation of stretch receptors in the modulation of GH release. This was investigated by artificial distension of the cranial sac of the rumen with a balloon in a castrate male, overnight-fasted goat prepared with a rumen fistula. Inflation of the balloon with warm water was followed by a fall, and deflation of the balloon by a rise, in plasma GH levels. It is concluded that rumen distension and hence activation of ruminal stretch receptors is a sufficient stimulus to explain the immediate inhibition of GH release by feeding in the goat. As a result of these, and other, observations during this work, it is also suggested that unrestrained release of GH, and hence large oscillations in plasma GH level, may occur in the goat unless it is suppressed by any of several factors, which include activation of rumen stretch receptors by feeding or inflation, anticipation of being fed, stress and metabolic satiety.

A neural basis for central inhibition of milk ejection in the rabbit.

Discrete, transverse stereotaxic cuts in th forebrain of the anesthetized lactating rabbit caused spontaneous milk-ejection responses repeated at intervals of 1-5 min. The region of the brain in which cuts were effective included the diagonal band of Broca, the entire rostrocaudal extent of the dorsal medial septum, part of the cingulum and part of the hippocampal fimbria. It is proposed that severance of a septo-hippocampal patway frees the hippocampus from inhibition and allows oxytocin release to occur. It remains to be determined whether the hippocampus itself is directly responsible for periodic activation of neurosecretory cells.

Release of growth hormone in lactating and non-lactating goats in relation to behaviour, stages of sleep, electroencephalograms, environmental stimuli and levels of prolactin, insulin, glucose and free fatty acids in the circulation.

Recording electrodes were implanted in contact with the dura mater overlying the parietal cortex of six female goats, four of which were lactating. After recovery from surgery and complete familiarization with the housing conditions, the personnel and the recording technique, each goat was observed continuously for 24 h with simultaneous recording of the cortical electroencephalogram (EEG). Remote blood sampling was carried out every 30 min without disturbing the animal. Apart from the release of growth hormone (GH) associated with morning milking in two of the goats, there was no consistent relationship between the apparently spontaneous, episodic release of GH and behvaiour, stages of sleep, cortical EEG, air temperature, time of day or night, obvious environmental stimuli which arose from the normal husbandry routine, or the levels of porlactin, insulin, glucose or free fatty acids in the blood. There was also no relationship between the release of prolactin and the stages of sleep.

Pathways in the forebrain of the rat concerned with the release of prolactin.

Circulating prolactin levels were measured by radioimmunoassay after electrical stimulation of discrete sites in the forebrain of 420 lactating rats. The animals were prepared under initial anaesthesia, were decorticated except for frontal cortex and their brain stems were transected at the mid-collicular level.

Further studies on the afferent path of the milk-ejection reflex in the brain stem of the rabbit.

When the afferent pathway of the milk-ejection reflex, which we had previously reported, was surgically severed bilaterally in the mid-brain of the lactating rabbit, the reflex release of oxytocin in response to suckling was blocked for up to 11 days; unilateral severance did not block the reflex. The position and discrete nature of the pathway were also further substained by electrical stimulation experiments in acute studies in the anaesthetized rabbit. Some animals, however, did not release oxytocin in response to stimulation of the pathway. Furthermore, whereas stimulation of this reflex pathway in the guinea-pig brain at intervals of a few minutes evokes release of oxytocin after each stimulation, in the present study the release of oxytocin in the rabbit in response to repeated electrical stimulation was either progressively attenuated or did not occur at all after the initial release. There appears, therefore, to be a powerful overriding central inhibitory mechanism in the rabbit which can prevent release of oxytocin, even when the appropriate stimulus for release is applied.

Effects of lesions of the pituitary stalk on lactation in the rabbit.

The daily milk yields and the yields of fat, lactose and protein in milk were studied in New Zealand White rabbits in which the pituitary stalk and its portal vessels had been interrupted by a radiofrequency lesion placed during the second or third weeks of lactation. In comparison with yields from control rabbits i.e. animals in which the lesion did not completely interrupt the stalk, the yields of milk anf of milk constituents were depressed after interruption of the stalk on day 11 or day 15 of lactation. When the lesion was placed on day 19 of lactation the molk yield was depressed whether or not the stalk was completely disrupted; the decline in yield exaggerated the normal decline of yield that occurs at this stage of lactation. After interruption of the pituitary stalk changes in milk composition were associated with changes in milk production, not evidence being obtained to support anearlier suggestion that section of the pituitary stalk in the rabbit alters the milk compostion whilst leaving milk output unaffected.