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Do you know the sex of your cells? Not a question that is frequently heard around the lab bench, yet thanks to recent research is probably one that should be asked. It is self-evident that cervical epithelial cells would be derived from female tissue and prostate cells from a male subject (exemplified by HeLa and LnCaP, respectively), yet beyond these obvious examples, it would be true to say that the sex of cell lines derived from non-reproductive tissue, such as lung, intestine, kidney, for example, is given minimal if any thought. After all, what possible impact could the presence of a Y chromosome have on the biochemistry and cell biology of tissues such as the exocrine pancreatic acini? Intriguingly, recent evidence has suggested that far from being irrelevant, genes expressed on the sex chromosomes can have a marked impact on the biology of such diverse tissues as neurons and renal cells. It is also policy of AJP-Cell Physiology that the source of all cells utilized (species, sex, etc.) should be clearly indicated when submitting an article for publication, an instruction that is rarely followed (http://www.the-aps.org/mm/Publications/Info-For-Authors/Composition). In this review we discuss recent data arguing that the sex of cells being used in experiments can impact the cell's biology, and we provide a table outlining the sex of cell lines that have appeared in AJP-Cell Physiology over the past decade.

The illuminance threshold for maintenance of testicular function was found to be considerably higher in Syrian hamsters kept in continuous light (LL) than in hamsters on long-day (14-hr) photoperiods (LD 14:10), or in a similar-length skeleton photoperiod (LDSK); the threshold lay between 3 and 30 lux in LL and at approximately 0.3 lux in LD 14:10 or LDSK. The threshold for testicular maintenance in LL was related to the capacity of LL to suppress nocturnal melatonin secretion: 400 lux totally suppressed, 30 or 3 lux partially suppressed, and 0.3 lux failed to suppress melatonin secretion. Hamsters in the LD and LDSK groups, whose locomotion was entrained into a pattern characteristic of long-day exposure, maintained full testicular function; those whose locomotion free-ran or assumed a pattern of entrainment characteristic of short-day exposure underwent testicular regression. These results suggest that light signals entrain the circadian rhythms of locomotion and melatonin secretion in a similar manner, and that LL is less effective than LD or LDSK in shortening the duration of melatonin secretion. For hamsters in LL, a direct relationship was seen between the free-running period (tau) of locomotion and log10 illuminance at 0.3, 3.0, and 30 lux, but tau at 400 lux was no longer than tau at 30 lux. Splitting of locomotion did not occur at 0.3 or 3.0 lux, and occurred in 43% and 62% of hamsters in 30 and 400 lux, respectively.

Feedback lighting (LDFB), which illuminates an animal cage in response to active wheel running, exposes only the photosensitive portion of the phase-response curve to light. In the hamster, the photoinducible zone of the circadian rhythm of photoperiodic photosensitivity occurs during the interval of active wheel running. Since LDFB exposes the photoinducible zone almost as much as constant light (LL), we predicted that LDFB would maintain gonadal function just as LL does. Surprisingly, 10 male hamsters exposed to 1-sec pulses of LDFB for 8 wk had regressed testes similar to those of hamsters in continuous darkness (DD) and significantly smaller than hamsters exposed to LL (P less than 0.01). Two of 5 male hamsters exposed to 2-min pulses of LDFB underwent complete testicular regression and two had partially regressed testes. All females exposed to LDFB or to DD ceased showing cyclic signs of ovulation within 20 days, whereas most hamsters exposed to LL continued to show signs of cyclic ovulation. Six of the 8 hamsters exposed to LL had ova in their oviducts at autopsy, and also had significantly larger uteri (P less than 0.01) than hamsters exposed to DD or LDFB. None of the latter two groups (n = 6 and 9, respectively) had oviductal ova at autopsy. These results demonstrate that considerable exposure of the photoinducible zone to light does not necessarily maintain gonadal function. Light delivered to the photoinducible zone by LDFB may disrupt the normal alignment (internal coincidence) of circadian rhythms, thereby causing gonadal regression. Gonadal induction can occur when the photoinducible zone is exposed to light; however, it may not be the light itself, but rather the action of the light to alter the phase relationships of several oscillators, that causes induction and maintenance of the gonads.

Using feedback circuits, light exposure was linked to wheel-running activity in female albino rats. Because the photosensitive portions of the circadian cycle are known to coincide with wheel-running activity, the feedback circuits concentrated light on the photosensitive portions of the cycle. In this type of lighting, the free-running period of locomotor activity was directly proportional to the light intensity (i.e., the Aschoff effect), and at an intensity of 100 1x, cyclic ovulation caused. Both these effects, which were previously thought to result only from exposure to continuous light (LL), occurred even though these rats were exposed to only 4 h of light per circadian cycle. These results indicate that the consequences of LL are not due to the continuity of the light per se but represent the effects of light falling on discrete photosensitive portions of the circadian cycle.

Immature female rats exposed daily to a-14 h photoperiod were induced to ovulate precociously by administering pregnant mares' serum gonadotrophin (PMS) at 30 days of age (day 30). In these rats ovulation occurs on day 33 between 0130 and 0330 (midpoint of photoperiod equals 1200). The acute effect on the timing of ovulation of abruptly lengthening the photoperiod by 6 h was investigated in this preparation. When compared with controls kept on a 14-h photoperiod, adding 6 h of light to the beginning of the daily photoperiod (i.e., AM light) advanced ovulation by 2 h; adding 6 h of light to the end of the photoperiod (i.e., PM light) delayed ovulation by 5.5 h; adding 3 h of light to the beginning and to the end of the photoperiod delayed ovulation by 2 h. These results suggest that the time of release of an ovulatory quota of pituitary gonadotrophin may be advanced by exposure to AM light and delayed by exposure to PM light, but the PM light appears to have a stronger effect on the time of gonadotrophin secretion.

Controversy exists regarding whether or not the circadian rhythms of nocturnal rodents can be entrained by red light. Female albino rats that were free-running in continuous darkness (DD) were exposed to 2 h of red light (> 650 nm) near the beginning (dusk signals) or the end (dawn signals) of their active 12-h period of running. Red dawn signals advanced and red dusk signals delayed the onset of running on subsequent days. Because the altered onsets of running persisted in DD, the red light had produced a true entrainment of the circadian rhythm of running activity. The fact that the time of ovulation was similarly shifted by red light suggests that the circadian rhythm of luteinizing hormone secretion was also entrained by red light.

To further explore the validity of the non-parametric model of entrainment for predicting the phase-shifting effects of light pulses, we exposed rats to several intensities of continuous light (LL) and feedback lighting (LDFB). LDFB is a lighting condition that exposes the animal to light only during the interval of active locomotion; this interval is coincident with the photosensitive portion of the circadian cycle as defined by the phase-response-curve. This is achieved by linking lights on with locomotor activity. In addition to the comparison of LL with LDFB, the duration of the LDFB pulse was also varied in four rats. Whether rats were exposed to LL or LDFB, as light intensity increased, the free-running period (tau) of the locomotor activity rhythm also increased. This intensity-related increase in tau, which is known as the Aschoff effect, was similar for LL and LDFB 2 min at each light intensity (0.1, 1, and 100 lux). However, when the LDFB pulses were shortened from a duration of 2 min to a duration of approximately 1 sec, tau shortened significantly. These results demonstrate that the non-parametric model of entrainment adequately explains the major period-lengthening effects of LL. However, there are temporal limits to the light pulses that can be used to simulate the effects of LL (i.e., one second light pulses fail to produce the effects brought about by longer pulses).

Female rats exposed to low intensities (0.1-1.5 lx) of continuous light (LL), displayed regular estrous cycles and free-running circadian rhythms of locomotor activity. In most rats, as the intensity of LL was increased to greater than 2.0 lx, components within the active portion (alpha) of the locomotor rhythm remained synchronized as the periodicity of the rhythm lengthened. However, in a few rats alpha split into two components; one of which free-ran with a period shorter than 24 h, while the other free-ran with a period longer than 24 h. As soon as the two components became maximally separated they spontaneously rejoined. In most rats, estrous cycles ceased shortly after the intensity of LL was increased to greater than 2.0 lx even though the locomotor activity rhythm retained its unsplit free-running nature. These observations suggest that the multiple oscillators that control the rhythms of locomotor activity and the estrous cycle are normally coupled to one another. In certain intensities of LL, these oscillators uncouple and free-run with different periodicities, a condition which causes estrous cycles to cease and sometimes produces a split locomotor activity rhythm.

Continuous monitoring of wheel-running activity and determination of the time of ovulation in rats by serial laparotomies revealed that ovulation followed the onset of running at prooestrus by approximately 9 h (range 7--1 h). This temporal relationship held in rats in which the period of the circadian rhythm had been modified (entrained) by daily exposure to 14 h photoperiods, and in rats in dim continuous light whose rhythms were non-entrained (free-running). Knowledge of this temporal relationship between the two rhythms made it possible to give bright light signals at known points in the circadian cycle of the rat and to observe the effects on the timing of running and ovulation in subsequent cycles. Giving daily light signals near the onset of running (i.e. at subjective dusk) delayed, whereas giving signals near the end of running (i.e. at subjective dawn) advanced, the time of running and ovulation in subsequent cycles. These results indicate that in rats exposed to the usual laboratory photoperiod the delaying effect of dusk light and the advancing effect of dawn light balance one another; thus the preovulatory surge of LH occurs at a relatively consistent time at prooestrus.

In immature female rats injected with PMSG at 30 days of age (day 30), ovulation occurs between the hours of 02.00 and 03.00 on day 33. If progesterone is injected at 10.00 h on day 32, the onset of ovulation is advanced by 1-2 h. In rats that were not given progesterone, ovulation was blocked by phenobarbitone sodium administered on day 32 before 13.50 h. However, pretreatment with progesterone at 10.00 h caused ovulation to occur in spite of phenobarbitone treatment at 13.50 h. An early release of ovulatory gonadotrophin from the anterior pituitary gland cannot completely account for progesterone's capacity to reverse the blockade of ovulation by phenobarbitone, because when phenobarbitone treatment was advanced by 2-4 h, ovulation still occurred in most progesterone-treated rats.

A phase-response curve (PRC) for the circadian rhythm of locomotor activity was constructed for female Sprague-Dawley-derived rats kept in continuous darkness (DD) except when given a 1-h light pulse (150 lx) once each 2 wk. By use of the circadian onset of wheel running as the phase-reference point, the free-running period (tau) in DD was 24.09 h. Maximum phase delays and phase advances occurred in response to light pulses given during the first 5 and last 6 h of activity, respectively. The delay-to-advance ratio (D/A) of the PRC was 1.5. In a separate group of rats exposed to continuous light, tau increased by 1.45 h as illuminance was increased in log steps from 0.1 to 10 lx, thus demonstrating the Aschoff effect in rats. This increase in tau was large, considering the relatively low D/A of the PRC, suggesting that factors in addition to the D/A contribute to the Aschoff effect.

This paper describes an electronic device through which environmental lighting conditions are linked to locomotor activity thus allowing only the photosensitive portions of a nocturnal rodents phase-response-curve to be exposed to light. In the past, this type of lighting schedule has been difficult, if not impossible, to present with an exogenously controlled lighting system due to the phase shifting ability of the rodent's circadian system. The feedback lighting system is made from components which can be purchased at most electronics outlets for less than $100.

These experiments were undertaken to determine if the pineal gland is involved in the physiological mechanism by which the rat alters its free-running period (tau) in response to changes in illuminance. Spontaneous wheel-running activity was recorded from pinealectomized or sham-operated female Charles River rats. The tau of running activity was determined in continuous darkness (DD) or in continuous dim light (LL). Pinealectomized rats and sham-operated rats lengthened their tau's to approximately the same extent when shifted from DD to LL and shortened their tau's when shifted back to DD. Continuous melatonin administration via Silastic capsules failed to alter tau of rats kept in dim LL. These results indicate that the pineal is not primarily involved in the mechanism by which the rat alters tau in response to changes in illuminance.