Role of suprachiasmatic nuclei in circadian and light-entrained behavioral rhythms of lizards

Neural activity in catecholaminergic populations following sexual and aggressive interactions in the brown anole, Anolis sagrei

Social behaviors in vertebrates are modulated by catecholamine (CA; dopamine, norepinephrine, epinephrine) release within the social behavior neural network. Few studies have examined activity across CA populations in relation to social behaviors. The involvement of CAs in social behavior regulation is especially underexplored in reptiles, relative to other amniotes. In this study, we mapped CA populations throughout the brain (excluding retina and olfactory bulb) of the male brown anole lizard, Anolis sagrei, via immunofluorescent visualization of the rate-limiting enzyme for CA synthesis, tyrosine hydroxylase (TH). Colocalization of TH with the immediate early gene product Fos, an indirect marker of neural activity, also enabled us to relate activity in TH-immunoreactive (TH-ir) neurons to appetitive and consummatory sexual and aggressive behaviors. We detected most major TH-ir cell populations that are present in other amniotes (within the hypothalamus, midbrain, and hindbrain), although the A15 population was entirely absent. We also detected a few novel or rare cell clusters within the amygdala, medial septum, and inferior raphe. Many CA populations, especially dopaminergic groups, showed increased TH-Fos colocalization in association with appetitive and consummatory sexual behavior expression, while a small number of regions showed increased colocalization in relation to solely consummatory aggression (biting of an opponent). In conclusion, we here map CA populations throughout the brown anole brain and demonstrate evidence for catecholaminergic involvement in appetitive and consummatory sexual behaviors and consummatory aggressive behaviors in this species.

Circadian rhythms in the pineal organ persist in zebrafish larvae that lack ventral brain

Background

The mammalian suprachiasmatic nucleus (SCN), located in the ventral hypothalamus, is a major regulator of circadian rhythms in mammals and birds. However, the role of the SCN in lower vertebrates remains poorly understood. Zebrafish cyclops (cyc) mutants lack ventral brain, including the region that gives rise to the SCN. We have used cyc embryos to define the function of the zebrafish SCN in regulating circadian rhythms in the developing pineal organ. The pineal organ is the major source of the circadian hormone melatonin, which regulates rhythms such as daily rest/activity cycles. Mammalian pineal rhythms are controlled almost exclusively by the SCN. In zebrafish and many other lower vertebrates, the pineal has an endogenous clock that is responsible in part for cyclic melatonin biosynthesis and gene expression.

Results

We find that pineal rhythms are present in cyc mutants despite the absence of an SCN. The arginine vasopressin-like protein (Avpl, formerly called Vasotocin) is a peptide hormone expressed in and around the SCN. We find avpl mRNA is absent in cyc mutants, supporting previous work suggesting the SCN is missing. In contrast, expression of the putative circadian clock genes, cryptochrome 1b (cry1b) and cryptochrome 3 (cry3), in the brain of the developing fish is unaltered. Expression of two pineal rhythmic genes, exo-rhodopsin (exorh) and serotonin-N-acetyltransferase (aanat2), involved in photoreception and melatonin synthesis, respectively, is also similar between cyc embryos and their wildtype (WT) siblings. The timing of the peaks and troughs of expression are the same, although the amplitude of expression is slightly decreased in the mutants. Cyclic gene expression persists for two days in cyc embryos transferred to constant light or constant dark, suggesting a circadian clock is driving the rhythms. However, the amplitude of rhythms in cyc mutants kept in constant conditions decreased more quickly than in their WT siblings.

Conclusion

Our data suggests that circadian rhythms can be initiated and maintained in the absence of SCN and other tissues in the ventral brain. However, the SCN may have a role in regulating the amplitude of rhythms when environmental cues are absent. This provides some of the first evidence that the SCN of teleosts is not essential for establishing circadian rhythms during development. Several SCN-independent circadian rhythms have also been found in mammalian species. Thus, zebrafish may serve as a model system for understanding how vertebrate embryos coordinate rhythms that are controlled by different circadian clocks.

Interseasonal variation in the circadian rhythms of locomotor activity and temperature selection in sleepy lizards, Tiliqua rugosa

Few studies in non-mammalian vertebrates have examined how various effectors of the circadian system interact. To determine if the daily locomotor and behavioural thermoregulatory rhythms of Tiliqua rugosa are both controlled by the circadian system in different seasons, lizards were tested in laboratory thermal gradients in four seasons and in constant darkness. Circadian rhythmicity for both rhythms was present in each season, being most pronounced in spring and summer and least evident in autumn. Most lizards displayed a unimodal locomotor activity pattern across all seasons. However, some individuals presented a bimodal locomotor activity pattern in spring and summer. Seasonal variations in the phase relationships of both rhythms to the light:dark (LD) cycle were demonstrated. No seasonal differences in the free-running period lengths of either rhythm were detected, raising the possibility that a single circadian pacemaker drives both rhythms in this species. Our present results demonstrate that both rhythms are similarly controlled by the circadian system in each season. Although seasonal variations in the thermal preferences of reptiles both in the field and laboratory have previously been well documented, this study is the first to demonstrate circadian rhythms of temperature selection in a reptile species in each season.

Hypothalamic activity during altered salt and water balance in the snake Bothrops jararaca

The effects of water and salt overload on the activities of the supraoptic and paraventricular nuclei and the adjacent periventricular zone of the hypothalamus of the snake Bothrops jararaca were investigated by measurements of Fos-like immunoreactivity (Fos-ir). Both water and salt overload resulted in changes in body mass, plasma osmolality, and plasma concentrations of sodium, potassium, and chloride. Hyper-osmolality increased Fos immunoreactivity in the rostral supraoptic nucleus (SON), the paraventricular nucleus (PVN), and adjacent periventricular areas. Both hyper- and hypo-osmolality increased Fos immunoreactivity in the intermediate SON, but not in other areas of the hypothalamus. Immunostaining was abundant in cerebrospinal fluid (CSF)-contacting tanycyte-like cells in the ependymal layer of the third ventricle. These data highlight some features of regional distribution of Fos immunoreactivity that are consistent with vasotocin functioning as a hormone, and support the role of hypothalamic structures in the response to disruption of salt and water balance in this snake.

Effects of season, testosterone and female exposure on c-fos expression in the preoptic area and amygdala of male green anoles

Expression of the immediate early gene, c-fos, was used to investigate changes in neuronal activity in forebrain regions involved in male sexual behavior following social, hormonal and/or seasonal manipulations in the male green anole. These factors all influence behavior, yet it is unclear how they interact to modify neuronal activity in forebrain regions, including the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY). These regions are involved in the display of sexual behaviors in male green anoles as in many other vertebrates. To determine the effects of seasonal, hormonal and social cues on these brain areas, we investigated c-fos under environmental conditions typical of the breeding or non-breeding season in adult male green anoles that were castrated and implanted with either testosterone (T) or blank (Bl) capsules. We also manipulated social cues by exposing only half of the animals in each group to females. T enhanced courtship and copulatory behaviors, but decreased c-fos expression in the AMY. A similar, although not statistically significant, pattern was observed in the POA, and the density of c-fos+ cells was negatively correlated in that region with the number of extensions of a throat fan (dewlap) used during courtship. Therefore, it appears that in the male green anole, T may diminish c-fos expression (likely in inhibitory neurons) in the POA and AMY to create a permissive environment in which the appropriate behavioral response can be displayed.

Hypothermia modulates circadian clock gene expression in lizard peripheral tissues

The molecular mechanisms whereby the circadian clock responds to temperature changes are poorly understood. The ruin lizard Podarcis sicula has historically proven to be a valuable vertebrate model for exploring the influence of temperature on circadian physiology. It is an ectotherm that naturally experiences an impressive range of temperatures during the course of the year. However, no tools have been available to dissect the molecular basis of the clock in this organism. Here, we report the cloning of three lizard clock gene homologs (Period2, Cryptochrome1, and Clock) that have a close phylogenetic relationship with avian clock genes. These genes are expressed in many tissues and show a rhythmic expression profile at 29 degrees C in light-dark and constant darkness lighting conditions, with phases comparable to their mammalian and avian counterparts. Interestingly, we show that at low temperatures (6 degrees C), cycling clock gene expression is attenuated in peripheral clocks with a characteristic increase in basal expression levels. We speculate that this represents a conserved vertebrate clock gene response to low temperatures. Furthermore, these results bring new insight into the issue of whether circadian clock function is compatible with hypothermia.

Neural Interaction of Gonadotropin-regulating Hormone Immunoreactive Neurons and the Suprachiasmatic Nucleus with the Paraventricular Organ in the Japanese Grass Lizard (Takydromus tachydromoides)

Our previous study demonstrated that the paraventricular organ (PVO) in the hypothalamus of the Japanese grass lizard (Takydromus tachydromoides) showed immunoreactivity against the light signal-transducing G-protein, transducin. This finding suggested that the PVO was a candidate for the deep-brain photoreceptor in this species. To understand functions of the PVO, we investigated distributions of transducin, serotonin, gonadotropin-releasing hormone (GnRH), and gonadotropin-inhibitory hormone (GnIH) in the lizard's brain. We immunohistochemically confirmed co-localization of transducin and serotonin in PVO neurons that showed structural characteristics of cerebrospinal fluid (CSF)-contacting neurons. GnRH-immunoreactive (ir) cells were localized in the posterior commissure and lateral hypothalamic area. Some of the serotonin-ir fibers extending from the PVO to the lateral hypothalamic area contacted the GnRH-ir cell bodies. GnIH-ir cells were localized in the nucleus accumbens, paraventricular nucleus, and upper medulla, and GnIH-ir fibers from the paraventricular nucleus contacted the lateral processes of serotonin-ir neurons in the PVO. In addition, we found that serotonin-ir fibers from the PVO extended to the suprachiasmatic nucleus (SCN), and the retrograde transport method confirmed the PVO projections to the SCN. These findings suggest that the PVO, by means of innervation mediated by serotonin, plays an important role in the regulation of pituitary function and the biological clock in the Japanese grass lizard.

Period and phase control in a multioscillatory circadian system (Iguana iguana)

The circadian system of the lizard Iguana iguana is composed of several independent pacemakers that work in concert: the pineal gland, retinae of the lateral eyes, and a fourth oscillator presumed to be located in the hypothalamus. These pacemakers govern the circadian expression of multiple behaviors and physiological processes, including rhythms in locomotor activity, endogenous body temperature, electroretinogram, and melatonin synthesis. The numerous, easily measurable rhythmic outputs make the iguana an ideal organism for examining the contributions of individual oscillators and their interactions in governing the expression of overt circadian rhythms. The authors have examined the effects of pinealectomy and enucleation on the endogenous body temperature rhythm (BTR) and locomotor activity rhythm (LAR) of juvenile iguanas at constant temperature both in LD cycles and in constant darkness (DD). They measured the periods (tau) of the circadian rhythms of LAR and BTR, the phase relationships between them in DD (psiAT), and the phase relationship between each rhythm and the light cycle (psiRL). Pinealectomy lengthened tau of locomotor activity in all animals tested and abolished the BTR in two-thirds of the animals. In those animals in which the BTR did persist following pinealectomy, tau lengthened to the same extent as that of locomotor activity. Pinealectomy also delayed the onset of activity with respect to its normal phase relationship with body temperature in DD. Enucleation alone had no significant effect on tau of LAR or BTR; however, after enucleation, BTR became 180 degrees out of phase from LAR in DD. After both pinealectomy and enucleation, 4 of 16 animals became arrhythmic in both activity and body temperature. Their data suggest that rhythmicity, period, and phase of overt circadian behaviors are regulated through the combined output of multiple endogenous circadian oscillators.

Photoperiod affects amplitude but not duration of in vitro melatonin production in the ruin lizard (Podarcis sicula)

The pineal gland and its major output signal melatonin have been demonstrated to play a central role in the seasonal organization of the ruin lizard Podarcis sicula. Seasonal variations in the amplitude of the nocturnal melatonin signal, with high values in spring as compared to low values in summer and autumn, have been found in vivo. The authors examined whether the pineal gland of the ruin lizard contains autonomous circadian oscillators controlling melatonin synthesis and whether previously described seasonal variations of in vivo melatonin production can also be found in isolated cultured pineal glands obtained from ruin lizards in summer and winter. In vitro melatonin release from isolated pineal glands of the ruin lizard persisted for 4 days in constant conditions. Cultured explanted pineal glands obtained from animals in winter and summer showed similar circadian rhythms of melatonin release, characterized by damping of the amplitude of the melatonin rhythm. Although different photoperiodic conditions were imposed on ruin lizards before explantation of pineal glands, the authors did not find any indication for corresponding differences in the duration of elevated melatonin in vitro. Differences were found in the amplitude of in vitro melatonin production in light/dark conditions and, to a lesser degree, in constant conditions. The presence of a circadian melatonin rhythm in vitro in winter, although such a rhythm is absent in vivo in winter, suggests that pineal melatonin production is influenced by an extrapineal oscillator in the intact animal that may either positively or negatively modulate melatonin production in summer and winter, respectively.

Temperature effect on entrainment, phase shifting, and amplitude of circadian clocks and its molecular bases

Effects of temperature and temperature changes on circadian clocks in cyanobacteria, unicellular algae, and plants, as well as fungi, arthropods, and vertebrates are reviewed. Periodic temperature with periods around 24 h even in the low range of 1-2 degrees C (strong Zeitgeber effect) can entrain all ectothermic (poikilothermic) organisms. This is also reflected by the phase shifts-recorded by phase response curves (PRCs)-that are elicited by step- or pulsewise changes in the temperature. The amount of phase shift (weak or strong type of PRC) depends on the amplitude of the temperature change and on its duration when applied as a pulse. Form and position of the PRC to temperature pulses are similar to those of the PRC to light pulses. A combined high/low temperature and light/dark cycle leads to a stabile phase and maximal amplitude of the circadian rhythm-when applied in phase (i.e., warm/light and cold/dark). When the two Zeitgeber cycles are phase-shifted against each other the phase of the circadian rhythm is determined by either Zeitgeber or by both, depending on the relative strength (amplitude) of both Zeitgeber signals and the sensitivity of the species/individual toward them. A phase jump of the circadian rhythm has been observed in several organisms at a certain phase relationship of the two Zeitgeber cycles. Ectothermic organisms show inter- and intraspecies plus seasonal variations in the temperature limits for the expression of the clock, either of the basic molecular mechanism, and/or the dependent variables. A step-down from higher temperatures or a step-up from lower temperatures to moderate temperatures often results in initiation of oscillations from phase positions that are about 180 degrees different. This may be explained by holding the clock at different phase positions (maximum or minimum of a clock component) or by significantly different levels of clock components at the higher or lower temperatures. Different permissive temperatures result in different circadian amplitudes, that usually show a species-specific optimum. In endothermic (homeothermic) organisms periodic temperature changes of about 24 h often cause entrainment, although with considerable individual differences, only if they are of rather high amplitudes (weak Zeitgeber effects). The same applies to the phase-shifting effects of temperature pulses. Isolated bird pineals and rat suprachiasmatic nuclei tissues on the other hand, respond to medium high temperature pulses and reveal PRCs similar to that of light signals. Therefore, one may speculate that the self-selected circadian rhythm of body temperature in reptiles or the endogenously controlled body temperature in homeotherms (some of which show temperature differences of more than 2 degrees C) may, in itself, serve as an internal entraining system. The so-called heterothermic mammals (undergoing low body temperature states in a daily or seasonal pattern) may be more sensitive to temperature changes. Effects of temperature elevation on the molecular clock mechanisms have been shown in Neurospora (induction of the frequency (FRQ) protein) and in Drosophila (degradation of the period (PER) and timeless (TIM) protein) and can explain observed phase shifts of rhythms in conidiation and locomotor activity, respectively. Temperature changes probably act directly on all processes of the clock mechanism some being more sensitive than the others. Temperature changes affect membrane properties, ion homeostasis, calcium influx, and other signal cascades (cAMP, cGMP, and the protein kinases A and C) (indirect effects) and may thus influence, in particular, protein phosphorylation processes of the clock mechanism. The temperature effects resemble to some degree those induced by light or by light-transducing neurons and their transmitters. In ectothermic vertebrates temperature changes significantly affect the melatonin rhythm, which in turn exerts entraining (phase shifting) functions.