Testicular hormones modulate circadian rhythms of the diurnal rodent, Octodon degus

Sex differences have been identified in a variety of circadian rhythms, including free-running rhythms, light-induced phase shifts, sleep patterns, hormonal fluctuations, and rates of reentrainment. In the precocial, diurnal rodent Octodon degus, sex differences have been found in length of free-running rhythm (tau), phase response curves, rates of reentrainment, and in the use of social cues to facilitate reentrainment. Although gonadal hormones primarily organize circadian rhythms during early development, adult gonadal hormones have activational properties on various aspects of circadian rhythms in a number of species examined. Gonadectomy of adult female O. degus did not influence tau, phase angle of entrainment, or activity patterns in previous experiments. The present experiment examined the role of gonadal hormones in adult male degus' circadian wheel-running rhythms. We predicted that male gonadal hormones would have an activational effect on some aspects of circadian rhythms, particularly those in which we see sex differences. Phase angles of entrainment, tau, length of the active period (alpha), maximum and mean activity levels, and activity amplitude were examined for intact and castrated males housed in LD 12:12. Responses to light pulses while housed in constant darkness (DD) were also compared. Castration had no significant effect on tau or light-induced phase shifts. However, castration significantly increased phase angle of entrainment and decreased activity levels. The data indicate that adult gonadal steroids are not responsible for the sex differences in endogenous circadian mechanisms of O. degus (tau, PRC), although they influence activity level and phase angle of entrainment. This is most likely due to masking properties of testosterone, similar to the activity-increasing effects of estrogen during estrus in O. degus females.     

Scheduled voluntary wheel running activity modulates free-running circadian body temperature rhythms in Octodon degus

Entrainment of the circadian pacemaker to nonphotic stimuli, such as scheduled wheel-running activity, is well characterized in nocturnal rodents, but little is known about activity-dependent entrainment in diurnal or crepuscular species. In the present study, effects of scheduled voluntary wheel-running activity on circadian timekeeping were investigated in Octodon degus, a hystricomorph rodent that exhibits robust crepuscular patterns of wakefulness. When housed in constant darkness, O. degus exhibited circadian rhythms in wheel-running activity and body temperature (Tb) with an average period length (tau) of 23.39 +/- 0.11 h. When wheel running was restricted to a fixed 2-h schedule every 24 h, tau increased on average 0.39 +/- 0.09 h but did not result in steady-state entrainment. Instead, relative coordination between the fixed running schedule and circadian timing was observed. Tau was greatest when scheduled wheel running occurred at CT 20.5 (0.4 h greater than DD baseline tau). Scheduled running activity also influenced Tb waveform symmetry, reflecting concomitant changes in the circadian activity-rest ratio (alpha:rho). Aftereffects of the scheduled wheel-running paradigm were also observed. In 2 animals, tau lengthened from 23.20 and 23.80 h to 24.14 and 24.15 h, respectively, and remained relatively stable for approximately 1 month during the wheel schedule. Although behavioral activity appears to be a weak zeitgeber in this species, these data suggest that nonphotic stimuli can phase delay the circadian pacemaker in O. degus at similar times of the day as in nocturnal hamsters and mice, and in humans.     

Testosterone suppresses circadian responsiveness to social cues in the diurnal rodent Octodon degus

The diurnal, social rodent Octodon degus displays a robust sex difference in the ability to use social cues to facilitate reentrainment following a phase advance of the light cycle. Adult females housed with a female social cue donor reentrained 25% to 40% faster than did females reentraining alone. However, reentrainment rates of males were unaffected by exposure to female social cues during reentrainment. The authors hypothesized that males were less sensitive to the reentrainment-enhancing effects of social cues and that their higher threshold to the stimuli could be overcome if the social cues were either increased in strength or salience. Housing a male with two females significantly shortened the time to reentrain following a 9-h phase advance (p = 0.002). Housing with a sister had no effect on reentrainment. Therefore, male degus are able to respond to social cues but require the stimulus to be stronger than that for females. The effect of testosterone was tested by comparing reentrainment rates of castrated males before and after testosterone replacement both with and without a female social cue donor. Castrated males responded to a single female social cue donor, reentraining 35% faster than when housed alone (p = 0.006), whereas the time to reentrainment of intact males and males with testosterone capsule implants did not differ. Intact females were also implanted with testosterone and phase shifted with and without donors. Testosterone treatment eliminated the increase in reentrainment rates in the presence of social cues. The authors conclude that the rate of recovery from odor-enhanced phase shifts is modulated by activational effects of testosterone in male degus. Testosterone is also effective in suppressing social cue responsiveness in females, suggesting that testosterone's effects on responsiveness are not sexually dimorphic. This hormonal effect likely occurs by altering sensory system functions or CNS response to sensory information.     

Odor-specific effects on reentrainment following phase advances in the diurnal rodent, Octodon degus

Reentrainment following phase shifts of the light-dark (LD) cycle is accelerated in Octodon degus in the presence of olfactory social cues (i.e., odors) produced by conspecifics. However, not all odors from conspecifics were effective in facilitating reentrainment after a phase advance. In the current experiments, we examined whether nonanimal odors, odors from another species, or conspecific odors, including those manipulated by steroid hormones, can cause the same increased reentrainment of wheel-running activity as odors from an intact, adult female degu. A variety of odors, each selected to probe a particular aspect of the reentrainment acceleration phenomenon, were presented to a group of phase-shifting female degus. The shifting females (test animals) responded to odors of intact, female degu donors with decreased reentrainment time, but odors of ovariectomized (OVX), OVX with a single hormone replacement capsule (estradiol or progesterone) or phase-shifting females had no effect. Multiple males were effective odor donors, whereas a single male was ineffective in earlier studies. Rats and cloves were not effective in accelerating reentrainment. Furthermore, odors from rats delayed reentrainment. We conclude that the odors that effectively accelerate degu reentrainment after a phase advance of the LD cycle are species specific. We also report that repeated phase shifts, followed by complete recovery of phase relationships, do not alter the rate of recovery from a phase shift over time. These data suggest that in O. degus, a social species, odors may reinforce and strengthen the salience of the photic zeitgeber and/or facilitate synchronization of rhythms between animals.     

Circadian organization of the diurnal Caviomorph rodent, Octodon degus

The Octodon degus, or degu, is an excellent animal model for studying the theoretical and neural underpinnings of diurnality. The power of this model comes from their unique evolutionary lineage, long lives, and relative ease of care in the laboratory for a non-domesticated species. We have summarized the field and laboratory data indicating the critical variables that influence the degus' phase preference and the possible mechanisms for the phase flexibility observed in the field and laboratory. We also review studies examining the physiology and anatomy of light and non-photic inputs to the degu circadian system and studies of the circadian pacemaker itself, with particular emphasis placed on characteristics that appear to be convergent adaptations to a diurnal niche. Finally, we begin to seek the origin for the diurnally-phased activity output of the degu, although we conclude that significant work remains to be done.     

Ovarian hormones influence olfactory cue effects on reentrainment in the diurnal rodent, Octodon degus

Octodon degus, a social hystricomorph rodent, responds to olfactory cues from a gonadally intact female entrained to a light-dark cycle (LD) by accelerating reentrainment of running wheel activity following a 6-h phase advance of the LD cycle. In this study, we examined the role of ovarian hormones in the production of and responsiveness to olfactory social cues in females. Experiment 1: intact females were sequentially phase-advanced 6 h with photic cues alone, or in the presence of an intact female donor, ovariectomized (OVX) donor, a castrated male, or a castrated male with testosterone replacement. Acceleration of reentrainment occurred only in the presence of the intact female donor while reentrainment was delayed by OVX donors. Experiment 2: OVX females undergoing a 6-h phase advance did not accelerate reentrainment in the presence of an intact female donor compared to reentrainment with photic cues alone. Thus, ovarian hormones are necessary for both the production of and responsiveness to olfactory cues. Experiment 3: OVX females implanted with estrogen-filled Silastic capsules did not accelerate reentrainment following the 6-h phase advance in the presence of an intact donor, whereas animals implanted with a combination of estrogen- and progesterone-filled capsules (Experiment 4) reduced the length of time needed to reentrain in the presence of an intact donor. Therefore, combined progesterone and estrogen are sufficient for responsiveness to the effective olfactory cue in intact donor females. These data clarify that the sex difference in sensitivity to non-photic odor effects on circadian reentrainment is caused by both the testosterone's inhibitory effects (Octodon degus. J. Biol. Rhythms 18 (2003) 43-50) and the enhancing effects of progesterone and estrogen.     

Seasonal variation in the range areas of the diurnal rodent Octodon degus

Both breeding activity and abundance and quality of available food are expected to influence daily movements of animals. Animals are predicted to range over large areas to meet high energy demands associated with reproduction (females) or to increase mating success (males). However, animals should expand their range areas whenever food conditions deteriorate. To examine the extent to which breeding activity versus food availability influence space use, we compared the size and location of range areas (home ranges) of the degu (Octodon degus), a diurnal rodent from semiarid environments of north-central Chile, during the austral winter and summer seasons. Degus produce young during the austral spring (September-October) when high-quality food is readily available. In contrast, degus do not breed during the austral summer (January-March) when food is scarce and of low quality. We predicted that degus would range over smaller areas in winter if the availability of food has a greater influence on space than breeding activity. Individuals were radiotracked in winter and the following summer over a 3-year period. Surveys of herbaceous cover were conducted during winter and summer to determine seasonal changes in the abundance and quality of primary food. In summer degus expanded and moved the location of their range areas to locations with available food. Given that preferred food was less abundant in summer than winter, we suggest that degu range areas are strongly influenced by food conditions.     

Octodon degus: a diurnal, social, and long-lived rodent

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.     

Phase response curve to melatonin in a putatively diurnal rodent, Octodon degus

The degu (Octodon degus) is a diurnal rodent, although phase inversions to nocturnal behavior have been reported under specific laboratory conditions. The reliability of this animal as a diurnal model of sleep therefore requires further characterization of intrinsic circadian pacemaker properties. A phase response curve to light has been reported in the degu, and is consistent with other diurnal animals. This study reports a phase response curve to melatonin in the degu, which is distinct in orientation from the light curve.     

Epigenetic modulation of the developing serotonergic neurotransmission in the semi-precocial rodent Octodon degus

Environmental influences during early life periods, particularly those provided by the mother or parents, are generally considered to have a strong impact on the development of brain and behaviour of the offspring. In the semi-precocial South American species Octodon degus, a rodent becoming increasingly popular in different laboratory research fields, the present study aimed to examine the consequences of the disturbance of the parent-offspring interaction induced by parental separation on the serotonergic neurotransmission. Based on a quantitative neurochemical approach using brain homogenates obtained from cortical regions and the hippocampus our results revealed that (i) the tissue levels of serotonin and 5-hydroxyindoleacetic acid showed in both sexes a moderate, around two-fold increase until adulthood, indicating relatively matured cortical and hippocampal serotonergic systems at birth. In addition, we found an age-, region- and sex-specific pattern of changes in the serotonergic system induced by (ii) an acute stress challenge early in life (1-h parental separation at the postnatal day 3, 8, 14 or 21) with the most pronounced effects at earlier ages (between postnatal days 3 and 14) in the female cortex and (iii) repeated stress exposure (1h daily) during the first 3 weeks of life affecting cortical regions of both sexes. Taken together, these data indicate that early life stress (i.e. parental separation) influences the developing serotonergic system in the semi-precocial O. degus, even if the brain is relatively well matured at the early stages of postnatal development.     

Olfactory cues accelerate reentrainment following phase shifts and entrain free-running rhythms in female Octodon degus (Rodentia).

Social interactions between conspecifics is a type of nonphotic zeitgeber common to several species. In the diurnal rodent Octodon degus, social interactions enhance reentrainment after phase shifts and can act as a weak zeitgeber. Olfactory stimuli appear necessary for these effects since bulbectomy eliminates socially enhanced reentrainment. In Experiment 1, the authors examined whether stimulation of the main olfactory system was sufficient to enhance reentrainment after 6-h phase advances and delays in the adult female O. degus. When test animals received conspecific odor cues during reentrainment, they entrained 39% faster after phase advances (p < 0.05) and 33% faster after phase delays (p < 0.001) than when they did not receive odor cues. Thus, olfactory cues from distant female donors were sufficient to enhance rates of entrainment in female O. degus and provided results equivalent to earlier studies with donors and shifters housed in the cages together. In Experiment 2, the authors examined whether discrete 3-h and 1-h daily pulses of airborne odors from a group of 5 entrained female degus would be sufficient to produce entrainment of wheel-running activity in adult female conspecifics. During the period of exposure to 3-h pulses, 50% (4/8) of the subjects temporarily entrained to a 24-h cycle, while 12.5% (1/8) of the subjects fully entrained. Exposure to 1-h pulses allowed 37.5% (3/8) of the subjects to temporarily entrain and 12.5% (1/8) of the subjects to fully entrain. Duration of entrained episodes was positively correlated with psi, daily onset of activity with respect to the timing of odor exposure (Pearson r = 0.731; p < 0.05), such that animals with the entraining odor pulse beginning during subjective day (psi = 7.8 h, CT 7.8 +/- 1.4) had longer periods of entrainment (22.2 +/- 5.6 days) than animals with the entraining pulse occurring during subjective night (psi = -4.6 h; CT 19.4 +/- 0.9; 5.6 +/- 0.9 days; p < 0.001). In addition, for each animal, the combined duration of all episodes of 24-h entrainment correlated with increased period length (tau) of free-running rhythms (Pearson r = 0.733; p < 0.05). Thus, daily discrete pulses of odors with durations of either 1 or 3 h from female conspecifics were sufficient to produce both temporary and full entrainment to a 24-h cycle in the majority of female O. degus, and the likelihood of long periods of entrainment correlated with long taus and coordination of the odor pulse with mid subjective day.     

Photoperiodic control of circadian activity rhythms in diurnal rodents

The responses of red squirrels(Tamiasciurus hudsonicus) and eastern chipmunks(Tamias striatus) to complete and skeleton light-dark (LD) cycles were compared. The skeletons, comprised of two 1-h pulses of light per day, effectively simulated the complete photoperiods in the squirrels, but not the chipmunks. Skeleton photoperiods greater than 12-h caused the chipmunks to shift activity from the longer to the shorter of the two intervals between the pulses. To interpret the mechanism of phase control, squirrels and chipmunks were kept in continuous darkness and exposed to 1-h light pulses every 10 days. The time-course of entrainment was also quantified. Both techniques produced light-response curves. The data suggest that the parametric and non-parametric contributions to entrainment are different in these two rodent species.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Burrow limitations and group living in the communally rearing rodent, Octodon degus

Group living is thought to evolve whenever individuals attain a net fitness advantage due to reduced predation risk or enhanced foraging efficiency, but also when individuals are forced to remain in groups, which often occurs during high-density conditions due to limitations of critical resources for independent breeding. The influence of ecological limitations on sociality has been studied little in species in which reproduction is more evenly shared among group members. Previous studies in the caviomorph rodent Octodon degus (a New World hystricognath) revealed no evidence that group living confers an advantage and suggest that burrow limitations influence formation of social groups. Our objective was to examine the relevance of ecological limitations on sociality in these rodents. Our 4-year study revealed no association between degu density and use of burrow systems. The frequency with which burrow systems were used by degus was not related to the quality of these structures; only in 1 of the 4 years did the frequency of burrow use decrease with decreasing abundance of food. Neither the number of females per group nor total group size (related measures of degu sociality) changed with yearly density of degus. Although the number of males within social groups was lower in 2008, this variation was not related clearly to varying density. The percentage of females in social groups that bred was close to 99% and did not change across years of varying density. Our results suggest that sociality in degus is not the consequence of burrow limitations during breeding. Whether habitat limitations contribute to variation in vertebrate social systems is discussed.     

Freerunning and entrained circadian rhythms in activity,eating and drinking in the cat

Abstract

Because cats with pontile lesions exhibit an abnormal behavior that is under photoperiodic control, and because circadian rhythms are implicated in photoperiodic control mechanisms, an effort was made to detect circadian rhythms in the cat. Cats were isolated from all extraneous stimuli in soundproof chambers for extended periods of time. Photocells were used to monitor activity, eating and drinking in different LD cycles, in constant light at two intensities, and in constant dark. Freerunning circadian rhythms were found in the constant conditions, and entrained nocturnal patterns occurred in most of the LD cycles. The higher intensities of ambient illumination disrupted the freerunning rhythms. The freerunning rhythms were always greater than 24 h, ranging from 24.2 to 25 h. Measurements of food intake of cats living in a large colony room, obtained by weighing the food, revealed that a nocturnal pattern of entrainment was not present in the majority of the cats. Instead, most cats in the colony exhibited a random pattern of eating throughout the light and dark period of the LD cycle. However, the variation among the cats in the colony was considerable, extending from nocturnal to diurnal patterns of eating. A diurnal pattern of human activity was present in the colony and may account for the disruption of a basic nocturnal pattern. The presence of circadian rhythms in the cat leads us to consider the coincidence models for photoperiodic induction as possible explanations of the photoperiodic control of the lesion‐induced abnormal behavior.     

Ontogenic development of intestinal disaccharidases in the precocial rodent Octodon degus (Octodontidae)

We studied the ontogeny of the intestinal brush border disaccharidases sucrase and lactase in the precocial rodent Octodon degus. Sucrase hydrolyze sugars from plants while lactase hydrolyzes sugars from milk. Enzyme expression varied inversely with dietary changes according to the developmental pattern. All new-born pups had high lactase and low sucrase activities. Also, a negative correlation between sucrase and lactase activity was found, supporting the economic design hypothesis for the intestinal tract. Profiles for development of sucrase expression exhibit some differences among precocial species, and in O. degus is correlated with the slower transition from milk to solid food consumption at weaning.     

Sociality, glucocorticoids and direct fitness in the communally rearing rodent, Octodon degus

While ecological causes of sociality (or group living) have been identified, proximate mechanisms remain less clear. Recently, close connections between sociality, glucocorticoid hormones (cort) and fitness have been hypothesized. In particular, cort levels would reflect a balance between fitness benefits and costs of group living, and therefore baseline cort levels would vary with sociality in a way opposite to the covariation between sociality and fitness. However, since reproductive effort may become a major determinant of stress responses (i.e., the cort–adaptation hypothesis), cort levels might also be expected to vary with sociality in a way similar to the covariation between sociality and fitness. We tested these expectations during three years in a natural population of the communally rearing degu, Octodon degus. During each year we quantified group membership, measured fecal cortisol metabolites (a proxy of baseline cort levels under natural conditions), and estimated direct fitness. We recorded that direct fitness decreases with group size in these animals. Secondly, neither group size nor the number of females (two proxies of sociality) influenced mean (or coefficient of variation, CV) baseline cortisol levels of adult females. In contrast, cortisol increased with per capita number of offspring produced and offspring surviving to breeding age during two out of three years examined. Together, our results imply that variation in glucocorticoid hormones is more linked to reproductive challenge than to the costs of group living. Most generally, our study provided independent support to the cort–adaptation hypothesis, according to which reproductive effort is a major determinant, yet temporally variable, influence on cort–fitness covariation.     

Chronotype changes during puberty depend on gonadal hormones in the slow-developing rodent, Octodon degus

During puberty, human adolescents develop a later chronotype, exhibiting a delay in the timing of rest and activity as well as other daily physiological rhythms. The purpose of this study was to determine whether similar changes in chronotype occur during puberty in a laboratory rodent species, and, if so, to determine whether they are due to pubertal hormones acting on the circadian timekeeping system. To test this hypothesis, we carefully tracked daily activity rhythms across puberty in the slow-developing rodent Octodon degus. We confirmed that male degus showed a large reorganization of activity rhythms that correlated with secondary sex development during puberty, including a loss of bimodality and a 3-5 h phase-advance. Similar to humans, this circadian reorganization showed distinct sex differences, with females showing little change during puberty in two separate experiments. Prepubertal gonadectomy (GDX) eliminated the changes, whereas SHAM gonadectomy had little impact. Therefore, gonadal hormones are likely to play a role in pubertal changes in chronotype in this rodent species. Using evidence from a variety of species, including our recent studies in the rat, we conclude that chronotype changes during puberty are a well-demonstrated phenomenon in mammals.     

The effects of low-dose 0.5-mg melatonin on the free-running circadian rhythms of blind subjects

Exogenous melatonin (0.5-10 mg) has been shown to entrain the free-running circadian rhythms of some blind subjects. The aim of this study was to assess further the entraining effects of a daily dose of 0.5 mg melatonin on the cortisol rhythm and its acute effects on subjective sleep in blind subjects with free-running 6-sulphatoxymelatonin (aMT6s) rhythms (circadian period [tau] 24.23-24.95 h). Ten subjects (9 males) were studied, aged 32 to 65 years, with no conscious light perception (NPL). In a placebo-controlled, single-blind design, subjects received 0.5 mg melatonin or placebo p.o. daily at 2100 h (treatment duration 26-81 days depending on individuals' circadian period). Subjective sleep was assessed from daily sleep and nap diaries. Urinary cortisol and aMT6s were assessed for 24 to 48 h weekly and measured by radioimmunoassay. Seven subjects exhibited an entrained or shortened cortisol period during melatonin treatment. Of these, 4 subjects entrained with a period indistinguishable from 24 h, 2 subjects continued to free run for up to 25 days during melatonin treatment before their cortisol rhythm became entrained, and 1 subject appeared to exhibit a shortened cortisol period throughout melatonin treatment. The subjects who entrained within 7 days did so when melatonin treatment commenced in the phase advance portion of the melatonin PRC (CT6-18). When melatonin treatment ceased, cortisol and aMT6s rhythms free ran at a similar period to before treatment. Three subjects failed to entrain with initial melatonin treatment commencing in the phase delay portion of the PRC. During melatonin treatment, there was a significant increase in nighttime sleep duration and a reduction in the number and duration of daytime naps. The positive effect of melatonin on sleep may be partly due to its acute soporific properties. The findings demonstrate that a daily dose of 0.5 mg melatonin is effective at entraining the free-running circadian systems in most of the blind subjects studied, and that circadian time (CT) of administration of melatonin may be important in determining whether a subject entrains to melatonin treatment. Optimal treatment with melatonin for this non-24-h sleep disorder should correct the underlying circadian disorder (to entrain the sleep-wake cycle) in addition to improving sleep acutely.     

Relatedness does not predict vigilance in a population of the social rodent Octodon degus

The possibility that social foragers adjust and coordinate their scanning activity when in the presence of close relatives to attain inclusive fitness benefits remains controversial and scarcely examined. To this aim, we first tested the null hypothesis of no association between foraging individuals of the diurnal rodent, Octodon degus and their pairwise relatedness (six microsatellite loci), under natural conditions. Secondly, we examined the influence of relatedness on scan effort (percent overlapping) and temporal distribution of scanning using linear regression. Finally, we evaluated whether temporal distributions of scanning were significantly lower (coordination) or higher (synchrony) than random expectations using bootstrapping. We found that pairwise relatedness between focal degus and their foraging partner did not influence the scan effort or the temporal distribution of scanning. These original, field-based findings imply that vigilance behavior in socially foraging degus is unlikely to be kin-selected and adds to results from previous lab studies in that kinship remains a poor predictor of social behavior in these animals. Overall, our study adds to others revealing that kin selection may not have had an impact on aspects of social behavior such as vigilance during social foraging.     

Distribution of aquaporins in the colon of Octodon degus,a South American desert rodent

Octodon degus is a desert rodent of northern Chile, adapted to survive with a limited supply of water. This rodent has a high degree of fecal dehydration, related to colon water absorption. With the hypothesis that aquaporins (AQPs) might be present in the colon epithelium of O. degus and involved in fluid absorption, we studied colon water absorption in vivo and the distribution of AQPs and Na(+) transporters by immunocytochemistry. AQP-1 was found in apical and basolateral membranes of surface-absorptive and crypt epithelial cells. AQP-8 was found in the cytoplasm of enterocytes of surface colon. AQP-3 immunolabeling, on the other hand, was absent from the epithelium but present in a subepithelial fibroblast layer, pericryptal cells, and muscularis mucosae. The hydration state did not modify the amount of immunostaining for any of the AQPs. Colon water absorption was markedly decreased by the mercurial agent p-chloromercuribenzenesulfonic acid and was not affected by water deprivation. The NHE3 isoform of Na(+)/H(+) exchanger and alpha-1 subunit of the Na(+)-K(+)-ATPase were found in apical and basolateral membranes of surface-absorptive cells, respectively. These results suggest that colon water absorption is mostly transcellular and mediated by water channels like AQP-1. Apical Na(+)/H(+) exchanger and basolateral Na(+)-K(+)-ATPase in surface cells could be part of the Na(+) absorption pathway. It is hypothesized that this transport is necessary to provide an osmotic gradient for water absorption. The roles of AQP-8 and AQP-3 in water absorption remain to be established.