We studied the temporal aspects of endocrine signaling between the pituitary gland and testes by measuring moment to moment changes in blood LH and testosterone levels in individual male rats. Each rat was fitted with an indwelling vascular cannula, and blood was withdrawn every 5 min for 8-12 h. Rats were maintained throughout the intensive blood-sampling period with an isotonic blood replacement mixture containing rat red blood cells and a human plasma protein preparation. LH and testosterone measurements were made in plasma volumes of 50 and 60 microliters. Most rats released LH in well defined pulses, characterized by a rapid increase in plasma LH within 5-10 min and a gradual decline lasting for the next 50-70 min. LH pulses occurred singly or in trains of two to four. Episodes of testosterone secretion spanned 3-6 h and were marked by a slowly graded rise and fall of plasma testosterone. In several instances, testosterone episodes were preceded (1-2 h) by a train of closely coupled LH pulses. Within a particular animal on different days, hormone episodes varied in number, amplitude, and timing. A particular hormone profile did not serve as a reliable hormone signature for an individual rat. Many rats displayed a characteristic sequence of 1) multiple LH pulses, 2) a sustained testosterone episode, and 3) a period of no LH pulses. This tripartite sequence of events is viewed as the essence of pituitary-testicular stimulation, and testicular negative feedback. Intermittent, short term fluctuations in peripheral levels of LH and testosterone represent the blood-borne, gland to gland signals controlling hypothalamic-pituitary-testicular function in the normal rat.

We compared the effects of light pulses in constant darkness (DD) and dark pulses in constant light (LL) on the free-running rhythm of locomotor activity in male golden hamsters. Light pulses yielded advances, delays, or no change in the rhythm of activity. These data conform to a typical phase-response curve; this curve was unaffected by pinealectomy. Dark pulses occurring either late in the subjective night or early in the subjective day had little effect. In contrast, dark pulses occurring either late in the subjective day or early in the subjective night altered the rhythm in one of three ways: advance of the rhythm; splitting into two components; or induction of a new component, in phase with the pulse. Because dark pulses in LL perturb the circadian system in a different manner than do light pulses in DD, they may have value in identifying heretofore unknown aspects of circadian systems. As such, the use of dark pulses to perturb circadian rhythmicity will be a useful tool in examining the formal properties of circadian systems.

Distinct, short-term pulses of pituitary luteinizing hormone (LH) release are a characteristic feature of tonic LH secretion in normal rats. LH pulses are more pronounced, more frequent, and more regular in castrated rats. Using castrated rats, we sought to identify the basis of the pulsatile discharge of LH by the pituitary gland. Indwelling atrial cannulae were used to obtain frequent blood samples through 3–4 h from three cohorts of conscious, freely moving, orchidectomized male rats. Rats were (1) untreated castrates, (2) infused with ovine antiserum to LHRH or control serum, or (3) infused with a luteinizing hormone releasing hormone (LH-RH) analog, [D-pGlu¹, D-Phe², D-Trp³⁶]-LH-RH. Castrates exhibited a pulsatile pattern of circulating LH levels; the mean (±SE) peak level of LH pulses was 640 ± 15 ng/ml, with a mean (±SE) pulse period of 18.5 ± 0.8 min. LH-RH antiserum arrested pulsatile LH secretion immediately, leaving plasma LH levels at 80–120 ng/ml. The LH-RH analog caused a similar suppression of LH release, although this effect was of a shorter duration than the suppression of LH pulses induced by LH-RH antiserum. The obliteration of LH pulses by anti-LH-RH and suppression of LH release by an LH-RH antagonist indicate that the pulsatile secretion of LH is due to corresponding stimulation of the pituitary gland by hypothalamic LH-RH. Anti-LH-RH and an LH-RH antagonist are identified as valuable probes for the experimental dissection of blood-borne signals within the rat hypothalamic-pituitary-testicular axis.

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We charted the development of pulsatile luteinizing hormone (LH) secretion as a function of the time elapsed after removal of the testes. On seven occasions between the moment of castration and 80 days afterwards, we obtained consecutive blood samples at frequent (2.5- to 5-min) intervals from cannulated male rats. Orchidectomy increased both the amplitude and frequency of LH release within 1 day after surgery. Amplitude: From 19 h through 80 days postcastration, peak LH levels rose steadily, and LH pulses grew progressively more pronounced in nadir-to-peak amplitude. Frequency: Our findings offer new evidence establishing an increase in LH pulse frequency from less than 1 per h to 2-3 per h within 1 day after orchidectomy. Once deprived of testicular influences, the frequency of pulsatile LH discharges remained static through 80 days. The sudden onset (less than 1 day after castration) and temporal uniformity of high-frequency LH pulses demonstrate that LH release is governed by an intrinsic, 20- to 30-min neural periodicity in castrate rats. Most important, these findings imply that the testes mask or modulate the expression of an intrinsic, 20- to 30-min neural generator directing the periodic discharge of LH in the intact male rat.