Circannual rhythms of ground squirrels: a test of the frequency demultiplication hypothesis.

The frequency demultiplication hypothesis (FDH) posits that circannual rhythms are generated from circadian cycles by frequency transformation to the lower-frequency rhythm. To test the FDH, we determined the periods of the circannual body mass and estrous cycles of golden-mantled ground squirrels with circadian locomotor activity rhythms entrained to 23-, 24-, or 25-hr days (T-cycles). Circannual period length did not differ among squirrels entrained to the different T-cycles; intergroup ranges were 298-314 days and 303-312 days, respectively, for body mass and estrus. These results are not consistent with the FDH and suggest instead that separate mechanisms generate circadian and circannual rhythms. In ground squirrels the circannual system influences circadian organization, but a reciprocal influence of circadian on circannual rhythms has yet to be demonstrated.

Plasma oxytocin increases in the human sexual response.

The purpose of this study was to determine whether plasma oxytocin (OT) levels change during human sexual responses and, if so, to demonstrate the temporal pattern of change. Plasma OT levels were measured by RIA before, during, and after private self-stimulation to orgasm in normal men (n = 9) and women (n = 13). Blood samples were collected continuously through indwelling venous catheters. The subjects pressed a signal to indicate the start and finish of orgasm/ejaculation. Objective assessment of sexual arousal and orgasm was obtained by measuring blood-pulse amplitude and electromyographic activity, recorded continuously throughout testing from an anal device containing a photoplethysmograph and electromyograph electrodes connected to a polygraph located in an adjacent room. These measures allowed collection of data from men and women of changes in blood flow and muscle activity in the lower pelvic/pubic area. Plasma OT levels increased during sexual arousal in both women and men and were significantly higher during orgasm/ejaculation than during prior baseline testing. We suggest that the temporal pattern of secretion could be related to smooth muscle contractions of the reproductive system during orgasm.

Hamster activity and estrous cycles: control by a single versus multiple circadian oscillator(s).

Running activity onset and estrous onset were recorded for hamsters exposed to progressively shorter daily light/dark (T) cycles. The period of the estrous cycle was a quadruple multiple of the period of the activity rhythm during entrainment to T cycles of 23.5-21.5 hr. There was no evidence of desynchronization of the activity and estrus rhythms. The very short estrous periods shown during exposure to short T cycles indicate that an intrinsic 96-hr interval for ovarian follicular maturation does not determine the period of the estrous cycle. Dissociation of estrous and running activity onsets occurred for all hamsters: estrous onset generally preceded running activity onset for T greater than or equal to 23.0 hr; for shorter T cycles, estrous onset generally lagged behind running activity onset. Wheel-running activity was intermittently split into entrained and free-running components for one female: at T = 22.0 hr, estrous onset was coupled first with one and then with the other of the split activity components. These findings suggest that two or more separate circadian oscillators may control timing of the activity and estrous rhythms.

Circannual variations in circadian rhythms of ground squirrels.

There was a circannual rhythm in the times of activity onset (psi o) and activity termination (psi T) and in the duration of the active phase (alpha) in female ground squirrels (Spermophilus lateralis) maintained in a light-dark (LD) 14:10 photoperiod for 22 mo. Activity onset occurred 2.3 h after light onset in the 4 mo before estrus, 0.5 h before light onset during the month encompassing estrus, and progressively later during each of the ensuing 4 mo. Termination of activity occurred later during the month of estrus than in the 4 mo preceding estrus; alpha nearly doubled during the month of estrus and decreased gradually over the next 3 mo. The psi o and psi T remained entrained to the daily illumination cycle, but the phase angles of activity onset and offset and the duration of the active phase varied seasonally and recurred with a circannual period of 9-11 mo. The LD 14:10 photoperiod entrained the circadian activity rhythm throughout the year; however, circannual variations in activity as well as in estrus and body mass were not entrained by this photoperiod but free-ran with periods of less than 12 mo.

Relationships among cardiovascular, muscular, and oxytocin responses during human sexual activity.

To determine the psychophysiological correlates of hormonal response during sexual activity, systolic blood pressure (SBP), anal electromyography (EMG), and anal photoplethysmography (APG) were monitored continuously throughout testing in 13 women and 10 men. Each subject completed two or more tests of self-stimulation to 5 min beyond orgasm. Blood samples were obtained continuously for measurement of oxytocin (OT) levels. In both men and women, very high positive correlations were observed between the percentage change in levels from baseline through orgasm of: OT and SBP; OT and EMG intensity prior to and during orgasm; APG and EMG. The number of anal contractions and duration of orgasm were also highly correlated. Two patterns of orgasm were defined by the presence or absence of a quiescent period between orgasmic contractions. EMG and APG amplitudes correlated with the pattern of orgasm. Subjective orgasm intensity correlated significantly with increased levels of OT in multiorgasmic women only. The positive correlations between measures are consistent with a possible functional role for OT in human sexual response.