A sky polarization compass in lizards: the central role of the parietal eye.

The present study first examined whether ruin lizards Podarcis sicula are able to orientate using the e-vector direction of polarized light. Ruin lizards were trained and tested indoors, inside a hexagonal Morris water maze, positioned under an artificial light source producing plane polarized light with a single e-vector, which provided an axial cue. Lizards were subjected to axial training by positioning two identical goals in contact with the centre of two opposite side walls of the Morris water maze. Goals were invisible because they were placed just beneath the water surface, and water was rendered opaque. The results showed that the directional choices of lizards meeting learning criteria were bimodally distributed along the training axis, and that after 90 deg rotation of the e-vector direction of polarized light the lizards directional choices rotated correspondingly, producing a bimodal distribution which was perpendicular to the training axis. The present results confirm in ruin lizards results previously obtained in other lizard species showing that these reptiles can use the e-vector direction of polarized light in the form of a sky polarization compass. The second step of the study aimed at answering the still open question of whether functioning of a sky polarization compass would be mediated by the lizard parietal eye. To test this, ruin lizards meeting learning criteria were tested inside the Morris water maze under polarized light after their parietal eyes were painted black. Lizards with black-painted parietal eyes were completely disoriented. Thus, the present data show for the first time that the parietal eye plays a central role in mediating the functioning of a putative sky polarization compass of lizards.

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.

The circadian system of reptiles: a multioscillatory and multiphotoreceptive system.

Many parameters exhibited by organisms show daily fluctuations that may persist when the organisms are held in constant environmental conditions. Rhythms that persist in constant conditions with a period close to 24 h are called circadian. Although nowadays most research in this field is focused on the molecular and genetic aspects--and therefore mostly on two animal models (Drosophila and mouse)--the study of alternative animal models still represent a useful approach to understanding how the vertebrate circadian system is organized, and how this fascinating time-keeping system has changed throughout the evolution of vertebrates. The present paper summarizes the current knowledge of the circadian organization of Reptiles. The circadian organization of reptiles is multioscillatory in nature. The retinas, the pineal, and the parietal eye (and, possibly, the suprachiasmatic nuclei of the hypothalamus, SCN) contain circadian clocks. Of particular interest is the observation that the role these structures play in the circadian organization varies considerably among species and within the same species in different seasons. Another remarkable feature of this class is the redundancy of circadian photoreceptors: retinas of the lateral eyes, pineal, parietal eye, and the brain all contain photoreceptors.

Temperature cycles induce a bimodal activity pattern in ruin lizards: masking or clock-controlled event? A seasonal problem.

The daily locomotor activity pattern of Ruin lizards in the field is mainly unimodal, except for summer months when soil temperatures exceed 40 degrees C to 42 degrees C around midday. In such a situation, lizards reduce their locomotor activity around midday to avoid overheating, and thus their daily activity pattern becomes bimodal. The bimodal pattern expressed in the field is usually retained in the free-running rhythm under constant temperature and DD for a couple of weeks, after which the bimodal pattern changes into a unimodal pattern. In the present study, the authors examined whether 24-h temperature cycles (TCs) would change lizard activity from a unimodal to a bimodal pattern. Administration of TCs to unimodal lizards free-running in DD is able to entrain locomotor rhythms and to induce a bimodal pattern both in summer and autumn-winter. There are, however, striking seasonal differences in the effectiveness with which TCs achieve bimodality: (a) Numbers of lizards rendered bimodal are significantly higher in summer than in autumn-winter; (b) TCs require less time to achieve bimodality in summer than autumn-winter; (c) bimodality is retained as an aftereffect in the postentrainment free-run in summer, but not in autumn-winter; (d) TCs change activity duration in summer, but not in autumn-winter. All this demonstrates the existence of seasonal changes in responsiveness of the circadian oscillators controlling activity to the external factors inducing bimodality. Oscillators' responsiveness is high in summer, when bimodality is the survival strategy of Ruin lizards to avoid overheating around midday in open fields, and low in autumn-winter, when bimodality has no recognizable adaptive significance.

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

To establish whether the suprachiasmatic nuclei (SCN) of the Ruin lizard (Podarcis sicula) play a role in entrainment of circadian rhythms to light, we examined the effects of exposure to 24-h light-dark (LD) cycles on the locomotor behavior of lizards with SCN lesions. Lizards became arrhythmic in response to complete SCN lesion under constant temperature and constant darkness (DD), and they remained arrhythmic after exposure to LD cycles. Remnants of SCN tissue in other lesioned lizards were sufficient to warrant entrainment to LD cycles. Hence, the SCN of Ruin lizards are essential both to maintain locomotor rhythmicity and to mediate entrainment of these rhythms to light. We also asked whether light causes expression of Fos-like immunoreactivity (Fos-LI) in the SCN. Under LD cycles, the SCN express a daily rhythm in Fos-LI. Because Fos-LI is undetectable in DD, the rhythm seen in LD cycles is caused by light. We further showed that unilateral SCN lesions in DD induce dramatic period changes. Altogether, the present data support the existence of a strong functional similarity between the SCN of lizards and the SCN of mammals.

Seasonality and role of SCN in entrainment of lizard circadian rhythms to daily melatonin injections.

To establish whether the capability of daily melatonin injections to entrain circadian rhythms varies with season, we examined in constant conditions the locomotor behavior of lizards Podarcis sicula collected and subjected to daily melatonin injections at different times of the year. Although in summer locomotor rhythms of both pineal-intact and pinealectomized lizards became entrained to the 24-h injection period, in the other seasons their rhythms did not entrain to the injection period. To establish whether the suprachiasmatic nuclei (SCN) mediate summer entrainment of locomotor rhythms to melatonin, we examined the behavioral effects of daily melatonin injections in lizards subjected to either bilateral (SCN-X) or unilateral (USCN-X) ablation of the SCN. SCN-X lizards became behaviorally arrhythmic, and daily melatonin injections did not restore rhythmicity. USCN-X lizards remained rhythmic, and their locomotor rhythms did entrain to the injections. Besides demonstrating for the first time in a vertebrate that daily melatonin injections are capable of entraining circadian rhythmicity in only one season (summer), the present results support the view that the SCN (and not the pineal gland) are the primary target sites of melatonin in the circadian system of P. sicula.

Pineal transplantation to the brain of pinealectomized lizards: effects on circadian rhythms of locomotor activity.

Pinealectomized lizards (Podarcis sicula) whose locomotor rhythms were recorded in constant temperature (29 degrees C) and darkness were subdivided into 2 groups of hosts: Each belonging to the 1st group (experimentals) received from a donor a pineal gland, and each belonging to the 2nd one (controls) received a piece of cerebellum. Pineal transplantation induced drastic changes in the free-running period (tau) of locomotor rhythms, which were significantly greater than the tau changes induced by cerebellum transplantation. Either application or removal of melatonin implants left the locomotor rhythms of the controls completely undisturbed, showing that in absence of melatonin rhythms (pinealectomy alone abolishes blood-borne melatonin rhythms) melatonin implants are ineffective. Melatonin rhythms, however, had to be present in the experimentals, because the circadian system reacts to melatonin implants by changing tau (as if melatonin rhythms had been suppressed) and to removal of the implants by again changing tau (as if melatonin rhythms had been restored).

Electrolytic lesions to the optic chiasm affect circadian locomotor rhythms in lizards.

Electrolytic lesions to 85-95% of both optic nerves at the level of the optic chiasm (OC-X) induce a significant shortening of the free-running period (tau) of locomotor rhythms in Podarcis sicula held in constant temperature (29 degrees C) and constant darkness. Together with previous data in P. sicula, showing that retinalectomy (RET-X) in constant darkness also mostly induces a shortening of tau, the present results demonstrate that the retinae play a central role in the control of behavioural circadian rhythmicity independently of light perception. The fact that OC-X and RET-X affect locomotor rhythms in the same way (mostly by shortening tau), strongly supports the contention that the influence of the retinae on the circadian system is neurally mediated.

The pineal and circadian rhythms of temperature selection and locomotion in lizards.

The existence of a circadian rhythm of behavioral temperature selection has been demonstrated in lizards (Podarcis sicula) held on a thermal gradient in constant darkness. This rhythm becomes temporarily abolished during 1 week following parietalectomy and 2-3 weeks following pinealectomy. Parietalectomy does not affect the locomotor rhythm, while pinealectomy invariably lengthens the freerunning period of this rhythm. These results support the contention of separate control systems for the temperature selection rhythm and the locomotor rhythm. As neither rhythm is definitively abolished by parietalectomy and pinealectomy, other pacemaking components exist elsewhere in the circadian system of Podarcis sicula which can control both rhythms.

Pattern of organization of primary visual pathways in the European lizard Podarcis sicula Rafinesque.

Central visual pathways are similarly organized in all vertebrates, although differences are also evident in the distribution of retinofugal axons in mammals, birds and reptiles. We traced the retinofugal projections in the European lizard Podarcis sicula Rafinesque and compared our observations with previous findings in other reptilian species, as well as in mammals and in birds, in order to contribute to the understanding of similarities and differences in the pattern of organization of visual pathways in these vertebrate classes. Either HRP or 3H-proline injections were placed into one eye. No differences in staining pattern were observed between HRP and 3H-proline experiments. Prominent retinofugal projections were observed to the thalamic nuclei nucleus dorsolateralis, nucleus geniculatus lateralis dorsalis, nucleus geniculatus lateralis ventralis and nucleus geniculatus lateralis ventralis, pars ventralis. In the hypothalamus the suprachiasmatic nucleus, and in the pretectum nuclei geniculatus pretectalis, lentiformis mesencephali and posterodorsalis contained detectable amounts of labeled terminals. Retinal fibers were distributed to layers 14 and 12 of the optic tectum and terminated in layers 13, 11, and 8. Labeled fibers were also present in the basal optic tract, and terminals were located in the nucleus of the basal optic tract. Ipsilateral retinofugal fibers were detected in the optic tract, and terminals were observed in the same thalamic and pretectal nuclei which receive crossed projections. No terminals in the ipsilateral tectal layers could be demonstrated. Compared to the crossed projection, the ipsilateral retinofugal contingent was very small. These results confirm and extend data from previous studies in reptiles and are consistent with the distribution of retinofugal fibers found in other vertebrates. The presence of ipsilateral retinofugal projections represents a common feature of the organization of visual pathways in reptiles, although its importance for binocular interactions remains to be elucidated.

Circadian rhythms of plasma melatonin in the ruin lizard Podarcis sicula: effects of pinealectomy.

Plasma melatonin was measured in lizards (Podarcis sicula) at six different times of day under conditions of constant temperature and darkness. Intact animals showed a circadian rhythm of melatonin with a peak in the subjective night of 207 pg/ml (median) and a trough during the subjective day that was below the minimum detection level of the assay (50 pg/ml). Pinealectomy abolished the circadian rhythm of plasma melatonin; median levels were near or below the minimum detection level at all times sampled. The data suggest that the pineal is the only source of rhythmic blood-borne melatonin in Podarcis sicula, and are consistent with the hypothesis that changes in the free-running period of the locomotor rhythm induced by pinealectomy in this species are due to withdrawal of rhythmic melatonin from the blood.

Effects of exogenous melatonin on sun-compass orientation of homing pigeons.

Experiments were performed to test whether melatonin plays a role in sun-compass orientation of homing pigeons. Birds were kept for a period of time in dim continuous light (LL) or in artificial light-dark (LD) cycles and then released under the sun from unfamiliar sites. Control birds in dim LL were oriented homeward in all cases. Birds with melatonin implants in LD were capable of a correct use of the sun compass at release. Birds with melatonin implants in dim LL, on the contrary, performed very poorly in orientation. The present results demonstrate for the first time that melatonin is involved in the control of the circadian rhythms underlying sun-compass orientation in birds.

Homing pigeons subjected to section of the anterior commissure can build up two olfactory maps in the deflector lofts.

Pigeons kept in two cages with screens which deflect the wind clockwise (CW) or counter-clockwise (CCW) show corresponding deflections in their initial orientation. In order to determine the nature of this phenomenon, experimental birds were treated as follows: The anterior commissure of the forebrain (AC), which mediates the interhemispheric transfer of olfactory input was sectioned; After surgery, each experimental bird was kept alternately in a CW cage with its right nostril plugged, and in a CCW cage with its left nostril plugged; the two treatments were alternated every 3 days for 69 days before test releases began. In 23 out of 28 cases the experimentals showed CW deflections when released with the right nostril plugged and CCW deflections when released with the left nostril plugged. The controls were intact, and their nostrils were free in each phase of the experiment. They were subdivided into two groups: one group was kept in the CW cage when the experimentals were in the same cage, the other group in the CCW cage when the experimentals stayed there. In the remaining time each group was kept in a fenced loft. The behaviour of controls demonstrated that the time the experimentals had spent in each kind of deflector cage had been long enough to produce the corresponding deflections in initial orientation. Control experiments were then performed on pigeons with the AC sectioned (2 series) and on intact birds (1 series), both maintained in lofts which did not deflect the wind, and released with one nostril plugged. They did not show deflections similar to those of the experimentals.(ABSTRACT TRUNCATED AT 250 WORDS)