Sodium Pentobarbitone and the Suppression of Luteinizing Hormone Pulses in the Female Rat: the Role of Hypothermia

Abstract

The role of hypothermia in the suppression of pulsatile luteinizing hormone (LH) release by the barbiturate sodium pentobarbitone has been investigated in ovariectomized rats. Each animal was fitted with an intraperitoneal miniature radio transmitter to monitor core temperature and with an indwelling intravenous and intraperitoneal catheter. During the 6-h sampling period the animal's core temperature was recorded automatically every 5 min and a 25 microliters blood sample was obtained concurrently using an automated system. After the initial 3 h of sampling either the drug or the vehicle was administered via the intraperitoneal cannula from outside the cage, thus ensuring minimal disturbance to the animal. Administration of sodium pentobarbitone (40 mg/kg) at an ambient temperature of 21 degrees C resulted in a significant hypothermia throughout the 3-h post-injection period. During this period there was a significant reduction in mean LH concentration, and in the frequency and amplitude of the LH pulses. When the drug was administered at an ambient temperature of 35 degrees C there was no reduction in core temperature and no significant change in the LH pulse parameters. Vehicle treatment was without significant effect on core temperature or on the LH pulse parameters when administered at an ambient temperature of either 21 degrees C or 35 degrees C. These results indicate that the effects of this barbiturate on the pulsatile release of LH are secondary to the induced hypothermia and suggest that hypothermia per se may be able to disrupt LH pulses. It is therefore imperative to reassess the significance of previous studies that have implicated particular neurotransmitter systems in the control of LH pulses; unrecognized hypothermic effects of the treatments may have been the primary cause of the pulse suppression, rather than a direct involvement of the neurotransmitter in question in the regulation of LH releasing hormone. The neurophysiological process by which the hypothermic state may inhibit LH pulses remains to be determined.

Effects of Anesthetics and Terminal Procedures on Biochemical and Hormonal Measurements in Polychlorinated Biphenyl Treated Rats

“…These may include (1) different animal handling procedures eliciting activation of the hypothalamo-pituitary-adrenal axis (HPA), (2) delay between exposure to specific anesthetics and loss of responsive reflex (eg, toe pinch), (3) hypoxic changes due to lengthy exsanguination procedures or CO 2 exposure, and (4) hypothermia induced by the anesthetic and effects of the anesthetic itself. 47…”

Endocrine Responses to Heat and Cold Stress

“…Chronic heat exposure (33°C) for 30 days elevated the Tre by 1.3°C and induced no changes in plasma LH and FSH levels in pigs, although ovary size was decreased and the LH and FSH responses after ovariectomy were suppressed [104]. At 21°C, pentobarbital anesthesia decreased the T eore by 6°C and reduced the mean plasma LH level and the frequency and amplitude of the LH pulse in female rats [105]. The PO/AH as the thermoregulatory center is also the principal site of the LH-releasing hormone (LHRH) cell bodies.…”

Estrogen Receptor Immunoreactivity Is Present in the Majority of Central Histaminergic Neurons: Evidence for a New Neuroendocrine Pathway Associated with Luteinizing Hormone-Releasing Hormone-Synthesizing Neurons in Rats and Humans1

“…The whole blood LH concentrations were measured in a single RIA as described previously (18). Within group comparisons were made using one-way repeated measures ANOVA followed by the Tukey multiple comparison test; between group comparisons were made using the unpaired Student's t test.…”