SUMMARY1. The expression of c-fos mRNA in the rat hypothalamus was examined by in situ hybridization following systemic administration of cholecystokinin (CCK), a procedure known to activate magnocellular oxytocin neurones but not magnocellular vasopressin neurones.2. Conscious male rats were given a single i.P. injection of 50 jug/kg CCK. c-fos mRNA signal was apparent in the supraoptic and paraventricular nuclei in rats killed 10 min after injection but not in uninjected or saline-(vehicle) injected rats. The density of c-fos mRNA at both sites was further elevated in rats killed 30 min or 60 min following injection, and was absent in rats killed 4 h after injection.3. In the paraventricular nucleus the most dense expression of c-fos mRNA following CCK administration was in the medial, mainly parvocellular portion of the nucleus, in an area corresponding to the distribution ofcorticotrophin-releasing factor mRNA determined by in situ hybridization in adjacent sections.4. The i.P. injection of CCK increased plasma oxytocin concentrations, measured by specific radiommunoassay from 13 ± 5 pg/ml in control rats to 107 + 9 pg/ml in the rats killed 10 min after injection, a similar response to that observed previously in urethane-anaesthetized rats.5. In each of six urethane-anaesthetized rats, recordings were made from single neurones in the supraoptic nucleus, identified antidronomically as projecting to the posterior pituitary and identified electrophysiologically as putative oxytocin neurones. Following i.P. injection of 50 jug/kg CCK, the neurones increased their firing rate by a mean of 1-3 +0-2 spikes/s averaged over the O min following injection.6. From the appearance of c-fos mRNA in supraoptic neurones following CCK administration we conclude that this message is expressed in magnocellular oxytocin neurones, since vasopressin neuronal activity and vasopressin release is known to be t Permanent address:

1 Intravenous administration of cholecystokinin (CCK) results in a transient activation of oxytocin neurones in the rat, and hence to oxytocin secretion: this activation is followed by expression of c-fos mRNA and of Fos-like immunoreactivity (Fos-LI) in magnocellular oxytocin neurones. Fos-like immunoreactivity is also induced in the regions of the brainstem that are thought to relay information from the periphery to the hypothalamus. 2 Administration of the selective CCKA receptor antagonist MK-329, but not the CCKB receptor antagonist L-365,260, prior to CCK injection, prevented oxytocin release as measured by radioimmunoassay and oxytocin neuronal activation as measured by electrophysiology and by the lack of induction of c-fos mRNA.3 MK-329 abolished the release of adrenocorticotrophic hormone (ACTH) following injection of CCK. 4 MK-329 prevented the expression of Fos-LI in the hypothalamic magnocellular nuclei and in the area postrema and dorsal vagal complex of the brainstem.5 L-365,260 had no effect on the expression of Fos-LI in the brainstem, but attenuated that seen in the hypothalamic magnocellular nuclei. 6 We conclude that CCK acts on CCKA receptors, either in the area postrema or on peripheral endings of the vagus nerve, to cause the release of hypothalamic oxytocin and ACTH. Information may be carried to the hypothalamus in part by CCK acting at CCKB receptors.

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Effects of footshocks (FS) on antidiuretic hormone (vaso-pressin, VP) in the plasma were studied in rats. Continuously applied FS of 60 s period with 5 ms pulses at 50 Hz frequency significantly increased VP as well as adrenocorticotrophic hormone (ACTH) in the plasma in a time-and shock intensity-dependent manner. Contrarily, the 50 Hz FS of 2 s period as repeated intermittently at every 15 s for over the period of 2, 10, and 30 min were much less effective for increasing plasma VP, whereas these intermittent FS increased plasma ACTH to an extremely high level. During the inter-shock intervals of 13s between successive two shock periods rats exhibited a "freezing" behavior. Hypertonic saline or ure-thane injected I.P, immediately after termination of the intermittent FS significantly increased VP as well as ACTH in the plasma. These data clearly indicate that FS potentiate VP secretion and suggest the possibility that emotional stress may suppress the noxious stimuli-induced VP secretion. In an earlier bioassay study (MIRSKY et al., 1954), footshocks (FS) were shown to increase antidiuretic activity in the rat plasma. Recently, however, it has been demonstrated with radioimmunoassay (RIA) technique that FS of 0.5 Hz do not significantly elevate plasma level of antidiuretic hormone (vasopressin, VP) in rats (KNEPEL et al., 1982). On the other hand, our previous electrophysiological data have shown that VP-secreting neurosecretory cells in the rat supraoptic nucleus are excited by noxious stimuli (HAMAMURA et al., 1984; SHIBUKI and MAGI, 1986) and suggest the possibility that FS potentiate VP secretion by the pituitary. In addition ineffectiveness of FS with relatively low frequency for potentiation of VP secretion (KNEPEL et al., 1982) also suggest the possibility that the temporal pattern of footshock pulses may influence the effectiveness of FS. In an attempt to test these possibilities we have conducted a series of experiments with continuously or