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.
A stereotaxic atlas h a s been prcpared of the forebrain of the guinea pig (body weight 700-830 9 ) using a standard cat-monkey stereotaxic instrument.The coordinates used were t h e vertical interaural planc (APO) and the horizontal orbital-interaural planc ( H O ) . The atlas consists of transverse sections of the brain taken a t 0.5 m m intervals from A5 through A15.Although stereotaxic atlases have been published for many species of laboratory animals, a search of the literature failed to reveal a n atlas of the guinea pig brain. In this department it has recently become desirable to use the guinea pig for studies on the neuroendocrine control of lactation. This species is well suited to the purpose, since unlike the rabbit or rat, it has only two mammary glands, each with a discrete vascular and nervous supply. To facilitate our studics a stereotaxic atlas has been prepared of the forebrain of the guinea pig. MATERIALS AND METHODSFemale guinea pigs weighing 700-830 g were used. Each animal was anesthetized with sodium pentobarbital and the brain was perfused with 0.9% saline followed by neutral 10% formalin. The head was then severed from the body and was fixed in a cat-monkey stereotaxic instrument with ear bars in the external auditory rneati, eye bars in the medial canthi of the eyes. J. COMP. N E U R , 124: 259-266resting on the lower borders of the orbits, and "palate clamp" under the mandibles. The scalp was reflected, holes were drilled in the skull, size 00 stainless steel insect pins were inserted vertically 5; 10 and 15 mm anterior to the vertical interaural plane 2.5 mm lateral to the midline, and one horizontal pin was inserted 2 mm above the orbital-interaural horizontal plane 1.5 mm lateral to the midline. The head was removed from the instrument, immersed in neutral 10% formalin for ten days, after which the brain was removed from the skull leaving the implanted pins in situ. After immersion in 20% ethanol for 24 hours the brain was sectioned histologically. Transverse serial frozen sections, 80 11 thick, were cut by the dry ice freezing method of Marshall ('40) and alternate sections were stained with the thionine and Weil techniques for nuclear and fiber structures respectively. Pairs of sections taken at 0.5 mm intervals from A 5 through A15 were projected X 10 and the outlines of principal structures were traced on graph paper. Finer detail was added later by microscopic examination of sections. The brains of six animals were used in the initial tentative determination of coordinates and a further five were used to confirm, and where necessary to correct, the final coordinates. When the initial spacing of the vertical wires was compared with the spacing of tracks in the histological sections, it was found that only trivial shrinkage of brain tissue had occurred.
SUMMARY The afferent path of the milk-ejection reflex has been studied in the brain of the anaesthetized lactating rabbit. Electrical stimulation was applied between a monopolar electrode in the brain and an indifferent electrode in the scalp. The brain was transected at the mid-cerebellar level to eliminate sympathetico-adrenal activation, and intramammary pressure and arterial blood pressure were monitored to detect release of neurohypophysial hormones. In the mid-brain, the afferent path of the reflex is compact, lying in the lateral tegmentum of each side and passing forwards to lie medio-ventral to the medial geniculate body. On entering the diencephalon, the pathway on each side bifurcates: a dorsal path passing forwards in association with the extreme rostral central grey and periventricular region, and a ventral path ascending through the subthalamus. The dorsal and ventral paths reunite in the posterior hypothalamus. Delineating the pathway further forward in the hypothalamus, using a simple stimulation technique, was not possible because at this level it intermingles with efferent fibres descending from the paraventricular nucleus to the pituitary stalk. The afferent path of the reflex is concerned with the preferential release of oxytocin from the neurohypophysis, is not a major pathway for the release of vasopressin and its neural substrate in the mid-brain is believed to be the spinothalamic system of fibres.
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.
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