Jorge A. Colombo

Effects of Electrical Stimulation of the Amygdala on Gonadotropin Release and Ovulation in the Rat

Summary Electrical stimulation of the amygdala in acute experiments under ether on the afternoon of proestrus in cycling rats blocked ovulation whereas similar stimulation on the morning of proestrous did not alter the afternoon surges of the gonadotropic hormones. “Acute” stimulation on the afternoon of proestrous also failed to overcome the ovulatory blockade exerted by pentobarbital or reserpine. Furthermore, serum LH levels were temporarily reduced by stimulation of chronic electrodes in the amygdala of ovariectomized rats, but such “chronic” stimulation did not inhibit the proestrous rat from releasing an ovulatory surge of LH. The results indicate that the amygdala can exert an inhibitory influence on LH release in acute experiments in cycling rats and under chronic conditions in ovariectomized rats. Our appreciation is extended to Mrs. Virginia Hoover for expert technical assistance and Mrs. Franoes Smith for secretarial assistance.

Local changes in multiple unit activity induced by electrochemical means in preoptic and hypothalamic areas in the female rat

Abstract Local effects of applying anodic direct current (ADC) thorugh stainless steel electrodes were studied by recording multiple unit activity with various electrode configurations in female rats under urethane anesthesia. The medial preoptic area (MPO) and anterior hypothalamic area (AHA) usually showed an increase in activity attaining its maximum by 10–25 min after ADC and lasting for 30–60 min. Some cases showed increases lasting for 2–4 h, without any consistent relationship to the current applied, brain location, or stage of estrous cycle. In the lateral preoptic area-medial forebrain bundle region only a consistent drop in activity was observed. Different current strengths induced changes in amplitude but not in duration of the increase in the MPO. No differences in responsiveness could be observed with respect to the estrous cycle. Similar increases in the MPO were obtained under pentobarbital anesthesia. These results are discussed in relation to neuroendocrine mechanisms. Caution is recommended in assuming that ADC applied through stainless steel electrodes necessarily induces a large increase in activity irrespective of the particular brain location to which it is applied. Studies regarding extradiencephalic structures are currently in progress.

Timing of the Estrogen-Induced Release of LH in Ovariectomized Rats Under an Altered Lighting Schedule

Summary Previous results from this laboratory (4) have shown that rats submitted to a 0500-1900 hr “lights-on” schedule responded to a single estradiol administration 48 hr after ovariectomy with plasma LH rises only during the afternoon (1630-1700 hr) of the following days. Under otherwise similar experimental conditions, it is now reported that if the “lights-on” period is shifted to 2100-1100 hr, the estrogen-induced LH surges occur at 1100-1230 hr. Cycling rats submitted to the altered illumination schedule show similarly altered times of the proestrous surge. The results reveal that the estrogen-induced LH rise in ovarieetomized animals is synchronized by light, and suggest that similar mechanisms control this estrogen-induced LH surge in ovariectomized rats and the spontaneous proestrous LH surge in cycling rats. We thank Ms. Francis Smith and Kathy Bangs and Mr. David Whitmoyer for valuable technical assistance and Mr. Bob McAlister for drawing the figures.

Effects of Cortical Spreading Depression on Multiunit Activity in the Preoptic Area and Hypothalamus of the Female Rat

A study was made of the effects of cortical spreading depression (SD) on the multiunit activity (MUA) of the medial preoptic area and ventromedial hypothalamus in female rats under acute (cycling) and

Effects of Limiting Water Intake on the Rat Estrous Cycle with Observations on LH, Prolactin, and Corticosterone

Summary By the fifth week of restricting water intake at 1100 to a 15-min period per day, estrous cycle lengths had shifted in most animals (31/40) from 4 to 5 days with the 5-day cycles usually consisting of 3 days of diestrus. In the 5-day cycling rats, plasma corticosterone peaked at 1100 and also at 1900 hr, the proestrous surge in plasma LH was not significantly changed, although the variation in individual animals was greater and the prolactin surge was prolonged. Similar prolonged cycles followed water restriction at 1800 hr with animals returning to regular 4-day cycles when given water ad libitum. These results suggest that the alteration of the normal corticosteroid rhythm and/or changes in prolactin secretion may account, in part at least, for the changes noted in cycle length after restricting the water intake. The authors thank Miss Katherine Bangs, Mr. Mike Profit, and Mrs. Frances Smith for valuable technical and secretarial assistance, and Mr. Bob McAlister for drawing the figures. We would also like to express our thanks to Dr. Anna Taylor and Berilyn Branch for their assistance in the Fluoro-metric determinations.

Effects of spreading depression on stress-induced changes in plasma prolactin and LH.

Summary Female rats rendered “pseudo-pregnant” by treatment with PMS and hCG and ovariectomized rats injected with estradiol and progesterone (OVX-E2-P) were subjected to cortical spreading depression (SD). Within 7-10 min under ether anesthesia in a stereotaxic instrument a frontal craniotomy was performed and a cotton ball saturated with physiological saline (control) or 25% KC1 was applied to the exposed dura, covered with dental cement and skin sutured. The animals were then placed in separate containers in an isolated room and decapitated for collection of trunk blood at 0, 15, 30, or 60 min after surgery. In PMS-hCG saline-treated control animals, prolactin levels had dropped by 15 and 30 min when compared with the zero-time values but by 60 min had increased significantly above the 30-min level. At that time (60 min), prolactin values in the KC1 group were significantly lower than in the controls. Corticosterone levels were high at both 15 and 60 min in control and KC1 groups. In OVX-E2-P control animals, plasma prolactin levels also rose at 60 min compared with 15- and 30-min samples and at 60 min were significantly higher than in the KC1 group. In control animals, LH levels were lower at 15 and 60 min than at zero time, but they remained unchanged in the KC1 group. The data are interpreted as indicating that cortical SD suppresses the stress responses observed in saline-treated control animals. The authors thank Ms. Katherine Bangs, Ms. Frances Smith and Ms. Lois Fels for valuable technical and secretarial assistance and Mr. Bob McAllister for drawing the figures.

Subcortical Multiple Unit Activity and Plasma Levels of Prolactin and LH During Strychninization of the Cerebral Cortex in the Female Rat

Summary Under urethane anesthesia the application of 2% strychnine sulfate to the cerebral cortex of female rats induced seizure activity. Multiple unit electrical activity recorded simultaneously from the medial preoptic area and basal hypothalamus showed significant changes only when the amplitude of the tonic EEG activity rose or fell. This suggests that tonic cortical activity is the crucial factor relating EEG events with preoptic or hypothalamic MUA during cortical strychninization. Plasma prolactin and LH levels in PMS-hCG and OVX-E2-P treated animals were not modified by the application of strychnine to the cerebral cortex. The results suggest that spreading depression is a more potent influence on pituitary function by virtue of its demonstrated effects on subcortical activity in preoptic and hypothalamic areas.

Pituitary LH Response to LHRH in Rats After Treatment with PMS-hCG or Ovariectomy Followed with Estrogen and Progesterone

Summary Long–term ovariectomized rats primed with estrogen and progesterone responded to exogenous LHRH at dosages of 40 and 100 ng with a sharp rise in plasma LH, peaking at 10–20 min after injection and gradually returning to control levels. In contrast, intact rats treated a week earlier with PMS followed by hCG completely failed to respond to even the higher dosage of LHRH: plasma LH levels remained almost undetecable in these animals. Assays of pituitary extracts revealed a similar concentration and content of prolactin but very depressed values for pituitary LH in these PMS–hCG–treated rats compared with earlier studies in OVX–E2–P animals. We thank Dr. A. V. Schally for supplying the synthetic LHRH and Dr. A. F. Parlow and the NIAMDD Hormone Distribution Program for making the RIA kits available. Ms. Frances Smith, Virginia Hoover, and Katherine Bangs and Mr. David Whitmoyer supplied valuable technical assistance.

Serum estradiol and progesterone concentrations in ovariectomized estrogen- and progesterone-primed and PMS-hCG-treated rats.

Summary Estradiol (E2) and progesterone (P) were measured by radioimmunoassay in peripheral blood serum on the second day of diestrus (D–2) and estrus (E) in normal cycling rats, in ovariectomized (OVX) animals, and in PMS–hCG– and OVX–E2–P–treated rats. PMS–hCG and OVX–E2–P animals had significantly higher estradiol and progesterone levels than cycling rats on D–2 or E. Progesterone was significantly higher in the PMS–hCG group while estradiol was higher in OVX–E2–P–treated rats. These results may provide an explanation of previous findings relative to LH dynamics in OVX–E2–P– and PMS–hCG–treated animals. We thank Mrs. Ruth Penardi, Miss Katherine Bangs, and Mrs. Frances Smith for technical and secretarial help and Mr. Bobby McAlister for drawing the figure.