ILLUMINATION PREFERENCE OF ALBINO RATS IN A CLOSED HEXAGONAL MAZE

Photic preference of the short-tailed opossum (Monodelphis domestica)

The gray short-tailed opossum (Monodelphis domestica) is a nocturnal South American marsupial that has been gaining popularity as a laboratory animal. However, compared to traditional laboratory animals like rats, very little is known about its behavior, either in the wild or in a laboratory setting. Here we investigated the photic preference of the short-tailed opossum. Opossums were placed in a circular testing arena and allowed to move freely between dark (0 lux) and light (∼1.4, 40, or 400 lux) sides of the arena. In each of these conditions opossums spent significantly more time in the dark than in the illuminated side and a greater proportion of time in the dark than would be expected by chance. In the high-contrast (∼400 lux) illumination condition, the mean bout length (i.e., duration of one trip on the light or dark side) was significantly longer on the dark side than on the light side. When we examined the number of bouts greater than 30 and 60s in duration, we found a significant difference between the light and dark sides in all light contrast conditions. These data indicate that the short-tailed opossum prefers the dark to the light, and can also detect very slight differences in light intensity. We conclude that although rats and opossums share many similar characteristics, including ecological niche, their divergent evolutionary heritage results in vastly different behavioral capabilities. Only by observing the behavioral capabilities and preferences of opossums will we be able to manipulate the experimental environment to best elicit and elucidate their behavior and alterations in behavior that can arise from experimental manipulations.

An improved method for precise control of light exposure at a known circadian time during an animal's subjective night

The paper describes an electronic device that improves the feedback lighting (LDFB) developed previously. LDFB links environmental lighting conditions to locomotor or other monitored behavior. Subjective Night Light (SNL) has the following advantages over LDFB: it eliminates the multiple transitions between light and dark; it allows for precise control over lighting so that a light signal of variable duration can be phase locked to any phase of the subjective night; it dissociates to a much greater extent any potential cognitive perception of the link between locomotor activity and lighting; and it can be programmed easily. Finally, SNL retains the significant advantage of LDFB in its ability to maintain phase relationship with the endogenous biologic rhythms even in circumstances of phase-shifting or free-running conditions. The SNL system is made from components that can be purchased at most electronics outlets for less than US$100.

Nocturnal illumination does not necessarily stimulate the photoperiodic response, despite mimicking the effects of constant light on the circadian system in the male Syrian hamster

In an effort to determine the inductive component(s) of photic input in long day seasonal breeders, adult male Syrian hamsters (Mesocricetus auratus) were exposed to one of nine lighting conditions for a duration of 10 weeks: a light-dark cycle of 14 hours of light followed by 10 hours of dark (LD 14:10, a long photoperiod); LD 10:14 (a short photoperiod); a high frequency light-dark cycle of 1 hour of light and 1 hour of dark (LD 1:1); a higher frequency light-dark cycle of 1 minute of light and 1 minute of dark (LD 1m:1m); constant light (LL); constant dark (DD); feedback lighting (LDFB; a condition that illuminates the cage in response to locomotor activity); a feedback lighting neighbor control (LDFB NC; the animal receives the same light pattern as a paired animal in LDFB, but has no control over it); or reverse feedback lighting (rLDFB; a condition that darkens an illuminated cage in response to locomotor activity). Exposure to LL, LD 1:1, LD 1m:1m, LDFB and rLDFB significantly and similarly lengthened the free-running period of the locomotor rhythm when compared to the period of animals in DD. The paired tests and accessory reproductive glands weights, spermiogenesis, seminiferous tubule diameter and serum concentrations of testosterone, prolactin, LH and FSH, suggest that LD 14:10, LL, LD 1:1, rLDFB and LDFB NC maintain reproductive function in the Syrian hamster, while LD 10:14, DD, LD 1m:1m and LDFB do not. It is known that as little as two 1-second pulses of light are stimulatory if coincident with the subjective night (17.22). Thus, it is not surprising that LD 1:1 is stimulatory. LD 1m:1m is not stimulatory, however, despite an identical quanta of light per 24 hours and similar phase relationship. It appears that mere light exposure during the subjective night is not necessarily reproductively inductive. It would also appear that behaviorally generated light-dark cycles can be (i.e., LDFB), but are not necessarily (i.e., rLDFB) inhibitory to the maintenance of the reproductive system in long day breeders. Furthermore, the lighting pattern derived from LDFB is stimulatory if given exogenously (i.e., LDFB NC). Although it is not understood why light exposure that is coincident with the subjective night (i.e., LD 1m:1m and LDFB) is not stimulatory in long day breeders, a possible hypothesis is that an internal coincidence model is involved in the photoperiodic response and that multiple transitions during the subjective night may cause a dissociation of internal oscillations which must be in phase for light to be stimulatory.

Minimum duration of light signals capable of producing the Aschoff effect

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

Responses of albino and hooded rats to various illumination choices in a six-chambered alleyway

To determine preferred illumination when multiple choices were simultaneously available, 10 different levels of illuminance (including darkness) were grouped into three overlapping ranges, each composed of six different choices. Subjects were permitted to move freely among six chambers in an enclosed alleyway for 48 continuous hours. Entries and duration in each of the chambers served as the dependent variables. Five groups of seven animals each were tested. Hooded rats preferred darkness, 0.01 ft-c (0.11 lx) and 20 ft-c (215.2 lx) about equally; each albino group preferred the dimmest available illuminated choice, viz., 0.01 ft-c (0.11 lx), 0.1 ft-c (1.08 lx), and 1.0 ft-c (10.76 lx); control rats distributed their time equally among all chambers. Activity of all groups reflected a typical circadian rhythm. These relative preferences for illumination contrast with absolute preference previously reported and perhaps reflect methodological differences.

Control of cage lighting by locomotor activity through feedback circuits

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

Illumination preference of albino rats in a tilt box as a function of age and illumination intensity

54 rats were tested for preference at 3 intensities (.01, 1.0, and 10.0 ft-c.) at 3 ages (30, 78, and 142 days). Results show that (a) age has no effect on either illumination preference or number of crossings in the tilt box; (b) darkness is preferred over 1.0 and 10.0 ft-c., but about equally preferred to .01 ft-c. The findings are discussed in terms of the reinforcing effect of stimulus change.