Stimulation electrically and by acetylcholine of the rat hypothalamus in vitro.

1. The hypophysiotrophic area of the rat hypothalamus was studied in vitro. The preparation remained viable for at least 3 hr and showed oxygen consumption varying between 68.9-120 mumole/g.hr. The tissue potassium ion content (per unit wet weight) fell to about 50% of the in vivo concentration during this time compared with 15% in the presence of ouabain (10(-4) M). Histological examination of tissue incubated for 3 hr showed variable perineuronal oedema but the nuclei were of normal appearance and none showed the pyknotic changes that would be associated with cell degeneration.2. Corticotrophin releasing hormone (CRH) in the medium pooled from five to twenty hypothalami was assayed in five to twelve rats which were median-eminence lesioned 48 hr earlier. In vitro corticosterone production of quartered adrenals was used as the end point of the assay. Regression lines of the dose-response curves for ACTH, crude CRH and different volumes of medium from electrically stimulated hypothalami were parallel. CRH output was maximal at 75 Hz and 100 muA when the square-wave pulses lasted for 1 msec. No CRH activity was found on stimulation of cerebral cortex or thalamic tissue pieces of equivalent size.3. Hypothalami taken from rats, adrenalectomized 7-14 days previously, released several-fold more CRH into the medium during electrical stimulation than the initial content of the tissue, showing that the tissue was capable of synthesizing CRH in vitro. The hypothalami taken from intact rats released considerably less CRH into the medium than tissue taken from 12 to 14 day adrenalectomized rats. The hyper-secretion of CRH observed in hypothalami taken from adrenalectomized rats was abolished by pre-treatment with 5 mg/100 g s.c. of corticosterone 24 hr before removal of the tissue. It is therefore proposed that the delayed negative feed-back action of corticosterone at the hypothalamic level is by the suppression of CRH synthesis and that the effect of secretion is secondary to the effect on synthesis.4. The presence of Ca(2+) in the medium was essential for the release of CRH.5. CRH secretion increases linearly with doses of acetylcholine from 5.5 x 10(-15)-5.5 x 10(-14) M. Cerebral cortex incubated with acetylcholine showed no CRH activity. The effect of acetylcholine was reduced by atropine (3.5 x 10(-13) M). Median eminence-pituitary stalk fragments (which contain mainly terminal axons of neurones) incubated with acetylcholine showed no CRH stimulation in the doses that activate the release of CRH using the hypophysiotrophic hypothalamus. Acetylcholine may act as a neurotransmitter at the dendritic level in the CRH neurone.