Prolactin Secretion Preceding Delayed Pseudopregnancy in Rats after Electrical Stimulation of the Hypothalamus*

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Abstract
Delayed pseudopregancy (PSP) was induced by electrical stimulation of the dorsomedial-ventromedial hypothalamus under 2 different experimental conditions. To induce long delay PSP, 4 day cycling rats were stimulated on the 3rd day of pentobarbital blockade of ovulation. A similar stimulus on the afternoon of diestrus day 2 under pentobarbital resulted in short delay PSP. The parameters of electrical stimulation were as follows: biphasic 1 m pulse pairs, 100 Hz, 200 .mu.A peak to peak, 30/60 s for 30 min. In animals stimulated on the 3rd day of ovulation blockade, blood samples (.ltoreq. 0.4 ml each; .ltoreq. 3 samples/24 h. rat) were taken from a chronic intracardiac cannula to 0300, 0600, 0900, 1200, 1500, 1800, or 2400 h for 3 days beginning on the morning after the stimulus. By contrast with sham-stimulated rats and stimulated rats continuing to cycle, those rats later manifesting PSP presented twice daily PRL [prolactin] surges similar to those of PSP at 0300-0600 and 1500-1800 h through the 3rd day of the long delay interval. After similar stimulation on diestrus day 2 to induce the short delay PSP, trunk blood was collected after rapid decapitation at 2400 or at 0300, 0600, or 0900 h during the next morning. A modest but significant (P < 0.05) increase in PRL above control levels occurred at 0300 h. These findings are in contrast to earlier study of short delay PSP induced by cervical stimulation in which there was no immediate rise in PRL during the delay interval. This difference may be related either to the difference in locale of the stimulus (hypothalamus vs. cervix) or, quantitatively, to the relatively prolonged stimulus used in the present experiments. Surging of PRL may or may not occur during the delay interval and is not essential for initiating the subsequent PSP.