Transcriptional effects of melatonin on the gut commensal bacterium Klebsiella aerogenes.

Klebsiella (nee Enterobacter) aerogenes is the first human gut commensal bacterium with a documented sensitivity to the pineal/gastrointestinal hormone melatonin. Exogenous melatonin specifically increases the size of macrocolonies on semisolid agar and synchronizes the circadian clock of K. aerogenes in a concentration dependent manner. However, the mechanisms driving these phenomena are unknown. In this study, we applied RNA sequencing to identify melatonin sensitive transcripts during culture maturation. This work demonstrates that the majority of melatonin sensitive genes are growth stage specific. Melatonin exposure induced differential gene expression of 81 transcripts during exponential growth and 30 during early stationary phase. This indole molecule affects genes related to biofilm formation, fimbria biogenesis, transcriptional regulators, carbohydrate transport and metabolism, phosphotransferase systems (PTS), stress response, metal ion binding and transport. Differential expression of biofilm and fimbria-related genes may be responsible for the observed differences in macrocolony area. These data suggest that melatonin enhances Klebsiella aerogenes host colonization.

Melatonin duration gates photoperiodic vocal state change in a songbird.

Seasonally breeding animals concentrate courtship to a particular time of year such that their offspring will be reared in a favorable environment. In house sparrows, Passer domesticus, primary (gonads) and secondary (song, plumage, beak color, etc) sexual characteristics are expressed differentially depending on the photoperiod. Removal of the pineal gland (PINX) has no effect on seasonal rhythms in gonad size but alters the photostimulated increase in vocal rate and complexity. Administration of long durations of melatonin, indicative of short days of winter, prevents seasonal recrudescence of song control nuclei in photostimulated house sparrows. In this study, male PINX house sparrows were exposed to three durations of melatonin, while vocalization and locomotor behavior were recorded as they were transitioned from short photoperiod to equinoctial photoperiods of spring. Birds receiving short duration melatonin or vehicle control increased dawn and dusk choruses as well as call complexity. Long durations of melatonin prevented this expansion to a spring-like vocal state observed in birds receiving the short duration of melatonin or vehicle control. The daily distribution of locomotor activity, beak color, and testis size was unaffected by treatment. Vocal state change was defined by our measures in two capacities: (i) increased dawn and dusk choruses, and (ii) an increase in calls associated with territory and mate attraction compared to the winter-like "social song." We conclude that house sparrows use the calendar information provided by melatonin duration to control seasonal vocalization behavior, independent of effects on and of the gonads.

Aging, melatonin biosynthesis, and circadian clockworks in the gastrointestinal system of the laboratory mouse.

The gastrointestinal (GI) system is vital in its capacities for nutrient and water uptake, immune function, metabolism and detoxification, and stem-cell derived regeneration. Of significance to human health are a myriad of GI disorders associated with aging that integrate with the circadian clock. Here we present data from three groups of mice: young (3 mo old), middle aged (12 mo old), and old aged (24 mo old). Small intestine and colon samples taken every 4 h under light-dark (LD) conditions were assayed for gene expression related to molecular circadian rhythmicity, transcription, cell signaling, and immune function. Transcripts related to melatonin biosynthesis and signaling, as well as melatonin content from stool, were also included, as GI melatonin and aging have been associated in contexts outside of the circadian clock. With respect to circadian genes, the data here are congruent with data from other peripheral tissues: age does not affect the rhythmic expression of core clock genes in the gut. The same can be said for several clock-controlled transcripts. In contrast, diurnal patterns in the expression of nitric oxide synthase 1 and of immune factors irak4 and interleukin-8 were observed in the colon of young mice that were lost in middle-aged and aged animals. Furthermore, the diurnal pattern of melatonin synthesis genes was altered by age, and stool melatonin levels showed significant decline between young mice and aged cohorts. These data expand the evidence for the persistence of the circadian clock throughout the aging process and highlight its importance to health.

Avian circadian organization: a chorus of clocks.

In birds, biological clock function pervades all aspects of biology, controlling daily changes in sleep: wake, visual function, song, migratory patterns and orientation, as well as seasonal patterns of reproduction, song and migration. The molecular bases for circadian clocks are highly conserved, and it is likely the avian molecular mechanisms are similar to those expressed in mammals, including humans. The central pacemakers in the avian pineal gland, retinae and SCN dynamically interact to maintain stable phase relationships and then influence downstream rhythms through entrainment of peripheral oscillators in the brain controlling behavior and peripheral tissues. Birds represent an excellent model for the role played by biological clocks in human neurobiology; unlike most rodent models, they are diurnal, they exhibit cognitively complex social interactions, and their circadian clocks are more sensitive to the hormone melatonin than are those of nocturnal rodents.

The role of the pineal gland in the photoperiodic control of bird song frequency and repertoire in the house sparrow, Passer domesticus.

Temperate zone birds are highly seasonal in many aspects of their physiology. In mammals, but not in birds, the pineal gland is an important component regulating seasonal patterns of primary gonadal functions. Pineal melatonin in birds instead affects seasonal changes in brain song control structures, suggesting the pineal gland regulates seasonal song behavior. The present study tests the hypothesis that the pineal gland transduces photoperiodic information to the control of seasonal song behavior to synchronize this important behavior to the appropriate phenology. House sparrows, Passer domesticus, expressed a rich array of vocalizations ranging from calls to multisyllabic songs and motifs of songs that varied under a regimen of different photoperiodic conditions that were simulated at different times of year. Control (SHAM) birds exhibited increases in song behavior when they were experimentally transferred from short days, simulating winter, to equinoctial and long days, simulating summer, and decreased vocalization when they were transferred back to short days. When maintained in long days for longer periods, the birds became reproductively photorefractory as measured by the yellowing of the birds' bills; however, song behavior persisted in the SHAM birds, suggesting a dissociation of reproduction from the song functions. Pinealectomized (PINX) birds expressed larger, more rapid increases in daily vocal rate and song repertoire size than did the SHAM birds during the long summer days. These increases gradually declined upon the extension of the long days and did not respond to the transfer to short days as was observed in the SHAM birds, suggesting that the pineal gland conveys photoperiodic information to the vocal control system, which in turn regulates song behavior.

Circadian regulation of metabolic, cell division, and cation transport promoters in the gastrointestinal bacterium Klebsiella aerogenes.

Introduction: All eukaryotes and at least some prokaryotes express the capacity to anticipate and adapt to daily changes of light and temperature in their environments. These circadian programs are fundamental features of many forms of life. Cyanobacteria were the first prokaryotes to have demonstrated circadian gene expression. Recently, a circadian rhythm was also discovered in an unrelated bacterium, Klebsiella aerogenes, a human gut commensal and nosocomial pathogen. Methods: Here we characterize new clock-controlled genes with spatial differences in expression using a bacterial luciferase reporter. These include dephospho-coenzyme A kinase (coaE), manganese transporter, H-dependent (mntH) and a gene identified as filamenting temperature-sensitive mutant Z (ftsZ). Results and Discussion: The data show that all three reporter constructs exhibited circadian variation, although only PmntH::luxCDABE reporter strains were synchronized by melatonin. Additionally, we show that K. aerogenes divides rhythmically in vitro and that these bacteria may alternate between exponential and stationary cells. Together, these findings provide a deeper understanding of K. aerogenes.

Circadian rhythms of gastrointestinal function are regulated by both central and peripheral oscillators.

Circadian clocks are responsible for daily rhythms in a wide array of processes, including gastrointestinal (GI) function. These are vital for normal digestive rhythms and overall health. Previous studies demonstrated circadian clocks within the cells of GI tissue. The present study examines the roles played by the suprachiasmatic nuclei (SCN), master circadian pacemaker for overt circadian rhythms, and the sympathetic nervous system in regulation of circadian GI rhythms in the mouse Mus musculus. Surgical ablation of the SCN abolishes circadian locomotor, feeding, and stool output rhythms when animals are presented with food ad libitum, while restricted feeding reestablishes these rhythms temporarily. In intact mice, chemical sympathectomy with 6-hydroxydopamine has no effect on feeding and locomotor rhythmicity in light-dark cycles or constant darkness but attenuates stool weight and stool number rhythms. Again, however, restricted feeding reestablishes rhythms in locomotor activity, feeding, and stool output rhythms. Ex vivo, intestinal tissue from PER2::LUC transgenic mice expresses circadian rhythms of luciferase bioluminescence. Chemical sympathectomy has little effect on these rhythms, but timed administration of the ß-adrenergic agonist isoproterenol causes a phase-dependent shift in PERIOD2 expression rhythms. Collectively, the data suggest that the SCN are required to maintain feeding, locomotor, and stool output rhythms during ad libitum conditions, acting at least in part through daily activation of sympathetic activity. Even so, this input is not necessary for entrainment to timed feeding, which may be the province of oscillators within the intestines themselves or other components of the GI system.

Context and novelty increase strength of auditory cues as a weak circadian zeitgeber in songbirds.

Light is the best-studied external cue (zeitgeber) for the entrainment of circadian rhythms. Non-photic entrainment is also possible; some organisms can entrain to rhythmic temperatures, drug administration, feeding, water turbulence, exercise and social cues. One such social cue that has the capacity to act as a weak zeitgeber to songbirds is the rhythmic presentation of conspecific vocalization. To better characterize this phenomenon, we performed several trials in which male and female zebra finches were maintained in constant dim light and allowed to free-run for 1 week before being presented with different audio cues of various lengths of playback and audio design every day at the same time of day for 15-31 days. Live audio monitoring from a nearby colony housed in light: dark (LD) conditions proved the strongest zeitgeber we tested, suggesting the phenomenon is enhanced with dynamic, context-appropriate vocalizations. Live colony playback was more efficacious than was a 2 h or 4 h presentation of the same, single zebra finch song but not a 1 h presentation, suggesting that habituation may have occurred in some of these experiments. The monitoring of the colony was also not statistically different from a 4 h playback of that same song, reversed, suggesting that social context is not required. It was, however, more effective than a 4 h presentation of synthesized, pseudorandom tones. When birds entrained to the period of the zeitgebers, their expressed period closely matched 24 h with phases closely matched to the onset of the zeitgeber. Masking was not evident in contrast to masking observed following transfer from constant dim light to LD and vice versa.This series of experiments could prove a means of quantifying the capacity for reciprocal social interaction, a state which can be dynamic in songbirds, as well as the integration between sociality and the circadian clock.

Rhythmic changes in colonic motility are regulated by period genes.

Human bowel movements usually occur during the day and seldom during the night, suggesting a role for a biological clock in the regulation of colonic motility. Research has unveiled molecular and physiological mechanisms for biological clock function in the brain; less is known about peripheral rhythmicity. This study aimed to determine whether clock genes such as period 1 (per1) and period2 (per2) modulate rhythmic changes in colonic motility. Organ bath studies, intracolonic pressure measurements, and stool studies were used to examine measures of colonic motility in wild-type and per1per2 double-knockout mice. To further examine the mechanism underlying rhythmic changes in circular muscle contractility, additional studies were completed in neuronal nitric oxide synthase (nNOS) knockout mice. Intracolonic pressure changes and stool output in vivo, and colonic circular muscle contractility ex vivo, are rhythmic with greatest activity at the start of night in nocturnal wild-type mice. In contrast, rhythmicity in these measures was absent in per1per2 double-knockout mice. Rhythmicity was also abolished in colonic circular muscle contractility of wild-type mice in the presence of N(omega)-nitro-L-arginine methyl ester and in nNOS knockout mice. These findings suggest that rhythms in colonic motility are regulated by both clock genes and a nNOS-mediated inhibitory process and suggest a connection between these two mechanisms.

Transcriptional profiling of mRNA expression in the mouse distal colon.

BACKGROUND & AIMS: Intestinal epithelial cells and the myenteric plexus of the mouse gastrointestinal tract contain a circadian clock-based intrinsic time-keeping system. Because disruption of the biological clock has been associated with increased susceptibility to colon cancer and gastrointestinal symptoms, we aimed to identify rhythmically expressed genes in the mouse distal colon. METHODS: Microarray analysis was used to identify genes that were rhythmically expressed over a 24-hour light/dark cycle. The transcripts were then classified according to expression pattern, function, and association with physiologic and pathophysiologic processes of the colon. RESULTS: A circadian gene expression pattern was detected in approximately 3.7% of distal colonic genes. A large percentage of these genes were involved in cell signaling, differentiation, and proliferation and cell death. Of all the rhythmically expressed genes in the mouse colon, approximately 7% (64/906) have been associated with colorectal cancer formation (eg, B-cell leukemia/lymphoma-2 [Bcl2]) and 1.8% (18/906) with various colonic functions such as motility and secretion (eg, vasoactive intestinal polypeptide, cystic fibrosis transmembrane conductance regulator). CONCLUSIONS: A subset of genes in the murine colon follows a rhythmic expression pattern. These findings may have significant implications for colonic physiology and pathophysiology.

Disruption of period gene expression alters the inductive effects of dioxin on the AhR signaling pathway in the mouse liver.

The aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) are transcription factors that express Per-Arnt-Sim (PAS) DNA-binding motifs and mediate the metabolism of drugs and environmental toxins in the liver. Because these transcription factors interact with other PAS genes in molecular feedback loops forming the mammalian circadian clockworks, we determined whether targeted disruption or siRNA inhibition of Per1 and Per2 expression alters toxin-mediated regulation of the AhR signaling pathway in the mouse liver and Hepa1c1c7 hepatoma cells in vitro. Treatment with the prototypical Ahr ligand, 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), had inductive effects on the primary targets of AhR signaling, Cyp1A1 and Cyp1B1, in the liver of all animals, but genotype-based differences were evident such that the toxin-mediated induction of Cyp1A1 expression was significantly greater (2-fold) in mice with targeted disruption of Per1 (Per1(ldc) and Per1(ldc)/Per2(ldc)). In vitro experiments yielded similar results demonstrating that siRNA inhibition of Per1 significantly increases the TCDD-induced expression of Cyp1A1 and Cyp1B1 in Hepa1c1c7 cells. Per2 inhibition in siRNA-infected Hepa1c1c7 cells had the opposite effect and significantly decreased both the induction of these p450 genes as well as AhR and Arnt expression in response to TCDD treatment. These findings suggest that Per1 may play a distinctive role in modulating AhR-regulated responses to TCDD in the liver.

Pineal melatonin acts as a circadian zeitgeber and growth factor in chick astrocytes.

Melatonin is rhythmically synthesized and released by the avian pineal gland and retina during the night, targeting an array of tissues and affecting a variety of physiological and behavioral processes. Among these targets, astrocytes express two melatonin receptor subtypes in vitro, the Mel(1A) and Mel(1C) receptors, which play a role in regulating metabolic activity and calcium homeostasis in these cells. Molecular characterization of chick astrocytes has revealed the expression of orthologs of the mammalian clock genes including clock, cry1, cry2, per2, and per3. To test the hypothesis that pineal melatonin entrains molecular clockworks in downstream cells, we asked whether coculturing astrocytes with pinealocytes or administration of exogenous melatonin cycles would entrain metabolic rhythms of 2-deoxy [14C]-glucose (2DG] uptake and/or clock gene expression in cultured astrocytes. Rhythmic secretion of melatonin from light-entrained pinealocytes in coculture as well as cyclic administration of exogenous melatonin entrained rhythms of 2DG uptake and expression of Gallus per2 (gper2) and/or gper3, but not of gcry1 mRNA. Surprisingly, melatonin also caused a dose-dependent increase in mitotic activity of astrocytes, both in coculture and when administered exogenously. The observation that melatonin stimulates mitotic activity in diencephalic astrocytes suggests a trophic role of the hormone in brain development. The data suggest a dual role for melatonin in avian astrocytes: synchronization of rhythmic processes in these cells and regulation of growth and differentiation. These two processes may or may not be mutually exclusive.

Time's arrow flies like a bird: two paradoxes for avian circadian biology.

Biological timekeeping in birds is a fundamental feature of avian physiology, behavior and ecology. The physiological basis for avian circadian rhythmicity has pointed to a multi-oscillator system of mutually coupled pacemakers in the pineal gland, eyes and hypothalamic suprachiasmatic nuclei (SCN). In passerines, the role of the pineal gland and its hormone melatonin is particularly important. More recent molecular biological studies have pointed to a highly conserved mechanism involving rhythmic transcription and translation of "clock genes". However, studies attempting to reconcile the physiological role of pineal melatonin with molecular studies have largely failed. Recent work in our laboratory has suggested that melatonin-sensitive physiological processes are only loosely coupled to transcriptional oscillations. Similarly, although the pineal gland has been shown to be critical for overt circadian behaviors, its role in annual cycles of reproductive function appears to be minimal. Recent work on the seasonal control of birdsong, however, suggests that, although the pineal gland does not directly affect gonadal cycles, it is important for seasonal changes in song. Experimental analyses that address these paradoxes will shed light on the roles the biological clock play in birds and in vertebrates in general.

Modulation of metabolic and clock gene mRNA rhythms by pineal and retinal circadian oscillators.

Avian circadian organization involves interactions between three neural pacemakers: the suprachiasmatic nuclei (SCN), pineal, and retina. Each of these structures is linked within a neuroendocrine loop to influence downstream processes and peripheral oscillations. However, the contribution of each structure to drive or synchronize peripheral oscillators or circadian outputs in avian species is largely unknown. To explore these interactions in the chick, we measured 2-deoxy[(14)C]-glucose (2DG) uptake and mRNA expression of the chick clock genes bmal1, cry1, and per3 in three brain areas and in two peripheral organs in chicks that underwent pinealectomy, enucleation, or sham surgery. We found that 2DG uptake rhythms damp under constant darkness in intact animals, while clock gene mRNA levels continue to cycle, demonstrating that metabolic rhythms are not directly driven by clock gene transcription. Moreover, 2DG rhythms are not phase-locked to rhythms of clock gene mRNA. However, pinealectomy and enucleation had similar disruptive effects on both metabolic and clock gene rhythms, suggesting that both of these oscillators act similarly to reinforce molecular and physiological rhythms in the chicken. Finally, we show that the relative phasing of at least one clock gene, cry1, varies between central and peripheral oscillators in a tissue specific manner. These data point to a complex, differential orchestration of central and peripheral oscillators in the chick, and, importantly, indicate a disconnect between canonical clock gene regulation and circadian control of metabolism.

Duration of melatonin regulates seasonal changes in song control nuclei of the house sparrow, Passer domesticus: independence from gonads and circadian entrainment.

Avian behavior and physiology are temporally regulated by a complex circadian clock on both a daily and an annual basis. The circadian secretion of the hormone melatonin is a critical component of the regulation of circadian/daily processes in passerine birds, but there is little evidence that the gland regulates annual changes in primary reproductive function. Here it is shown that locomotor rhythms of house sparrows, Passer domesticus, which are made arrhythmic by either pinealectomy or maintenance in constant light, can be synchronized by daily administration of melatonin of different durations to simulate the melatonin profiles indicative of long and short photoperiods. Pinealectomized male sparrows maintained in constant darkness were entrained by both melatonin regimens. In both cases, testes were regressed and the song control nuclei were small. Intact male house sparrows maintained in constant light were also entrained to both melatonin regimens. However, sparrows that received a long duration melatonin cycle exhibited small song control nuclei, while sparrows that received short duration melatonin or no melatonin at all exhibited large song control nuclei. The data indicate that seasonal changes in melatonin duration contribute to the regulation of song control nuclei.

The effects of aging on sleep parameters in a healthy, melatonin-competent mouse model.

BACKGROUND: Sleep disturbances are common maladies associated with human age. Sleep duration is decreased, sleep fragmentation is increased, and the timing of sleep onset and sleep offset is earlier. These disturbances have been associated with several neurodegenerative diseases. Mouse models for human sleep disturbances can be powerful due to the accessibility to neuroscientific and genetic approaches, but these are hampered by the fact that most mouse models employed in sleep research have spontaneous mutations in the biosynthetic pathway(s) regulating the rhythmic production of the pineal hormone melatonin, which has been implicated in human sleep. PURPOSE AND METHOD: The present study employed a non-invasive piezoelectric measure of sleep wake cycles in young, middle-aged and old CBA mice, a strain capable of melatonin biosynthesis, to investigate naturally-occurring changes in sleep and circadian parameters as the result of aging. RESULTS: The results indicate that young mice sleep less than do middle-aged or aged mice, especially during the night, while the timing of activity onset and acrophase is delayed in aged mice compared to younger mice. CONCLUSION: These data point to an effect of aging on the quality and timing of sleep in these mice but also that there are fundamental differences between control of sleep in humans and in laboratory mice.

Entrainment of the Circadian Clock of the Enteric Bacterium Klebsiella aerogenes by Temperature Cycles.

The gastrointestinal bacterium Klebsiella (née Enterobacter) aerogenes expresses an endogenously generated, temperature-compensated circadian rhythm in swarming motility. We hypothesized that this rhythm may be synchronized/entrained in vivo by body temperature (TB). To determine entrainment, cultures expressing bioluminescence were exposed to temperature cycles of 1°C (35°C-36°C) or 3°C (34°C-37°C) in amplitude at periods (T-cycles) of T = 22, T = 24, or T = 28 h. Bacteria entrained to all T-cycles at both amplitudes and with stable phase relationships. A high-amplitude phase response curve (PRC) in response to 1-h pulses of 3°C temperature spike (34°C-37°C) at different circadian phases was constructed, revealing a Type-0 phase resetting paradigm. Furthermore, real-time bioluminescence imaging revealed a spatiotemporal pattern to the circadian rhythm. These data are consistent with the hypothesis that the K. aerogenes circadian clock entrains to its host via detection of and phase shifting to the daily pattern of TB.

Circadian genomics of the chick pineal gland in vitro.

BACKGROUND: Chick pinealocytes exhibit all the characteristics of a complete circadian system, comprising photoreceptive inputs, molecular clockworks and an easily measured rhythmic output, melatonin biosynthesis. These properties make the in vitro pineal a particularly useful model for exploring circadian control of gene transcription in a pacemaker tissue, as well as regulation of the transcriptome by primary inputs to the clock (both photic and noradrenergic). RESULTS: We used microarray analysis to investigate the expression of approximately 8000 genes within cultured pinealocytes subjected to both LD and DD. We report that a reduced subset of genes was rhythmically expressed in vitro compared to those previously published in vivo, and that gene expression rhythms were lower in amplitude, although the functional distribution of the rhythmic transcriptome was largely similar. We also investigated the effects of 6-hour pulses of light or of norepinephrine on gene expression in free-running cultures during both subjective day and night. As expected, both light and norepinephrine inhibited melatonin production; however, the two treatments differentially enhanced or suppressed specific sets of genes in a fashion that was dependent upon time of day. CONCLUSION: Our combined approach of utilizing a temporal, photic and pharmacological microarray experiment allowed us to identify novel genes linking clock input to clock function within the pineal. We identified approximately 30 rhythmic, light-responsive, NE-insensitive genes with no previously known clock function, which may play a role in circadian regulation of the pineal. These are candidates for future functional genomics experiments to elucidate their potential role in circadian physiology. Further, we hypothesize that the pineal circadian transcriptome is reduced but functionally conserved in vitro, and supports an endogenous role for the pineal in regulating local rhythms in metabolism, immune function, and other conserved pathways.

A Low-Intensity, Hybrid Design between a "Traditional" and a "Course-Based" Research Experience Yields Positive Outcomes for Science Undergraduate Freshmen and Shows Potential for Large-Scale Application.

Based on positive student outcomes, providing research experiences from early undergraduate years is recommended for science, technology, engineering, and mathematics (STEM) majors. To this end, we designed a novel research experience called the "STEMCats Research Experience" (SRE) for a cohort of 119 second-semester freshmen with diverse college preparatory levels, demographics, and academic majors. The SRE targeted student outcomes of enhancing retention in STEM majors, STEM competency development, and STEM academic performance. It was designed as a hybrid of features from apprenticeship-based traditional undergraduate research experience and course-based undergraduate research experience designs, considering five factors: 1) an authentic research experience, 2) a supportive environment, 3) current and future needs for scale, 4) student characteristics and circumstances, and 5) availability and sustainability of institutional resources. Emerging concepts for facilitating and assessing student success and STEM curriculum effectiveness were integrated into the SRE design and outcomes evaluation. Here, we report the efficient and broadly applicable SRE design and, based on the analysis of institutional data and student perceptions, promising student outcomes from its first iteration. Potential improvements for the SRE design and future research directions are discussed.

The premammillary nucleus of the hypothalamus is not necessary for photoperiodic timekeeping in female turkeys (Meleagris gallopavo).

In birds, seasonal reproduction is regulated by day length, with long days in the spring activating the hypothalamic-pituitary-gonadal axis and reproductive behaviors. The photoreceptors mediating this process remain unknown, but recently, the premammillary nucleus (PMM) of the hypothalamus has been implicated as the site of photoperiodic signaling in turkeys. We performed electrolytic lesions of the PMM to elucidate its role in the photoactivation and maintenance of egg production in female turkeys. Our results show that ablation of the PMM does not alter the normal lay cycle. No differences were found between lesioned birds and sham controls in the latency to lay following photostimulation, nor in subsequent egg production over a period of 29 weeks. No differences in the incidence of gonadal regression were found, indicating that the PMM is not essential for the termination of breeding. We conclude that any role of the PMM in photoperiodic regulation, if it exists, is redundant with other components of the system.