Afferent projections to the hamster intergeniculate leaflet demonstrated by retrograde and anterograde tracing.

The intergeniculate leaflet (IGL) is considered involved in nonphotic shifting of the circadian clock through a direct connection, the geniculo-hypothalamic tract. The brain areas mediating nonphotic arousal to the hamster IGL have not been thoroughly investigated by both retrograde and anterograde tracing. We, therefore, reinvestigated the IGL afferent connections with the retrograde tracer Cholera toxin B and subsequently verified the results with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We also defined a subset of neurons projecting to the IGL that were activated by arousal using c-Fos immunocytochemistry. Apart from a dense afferent projection from the retina- and the contralateral leaflet, there were ipsilateral projections from other structures: layer V and VI of the prefrontal cortex, the zona incerta, the magnocellular part of the subparafascicular nucleus, the dorsal raphe nucleus, the locus coeruleus, and the cuneiform nucleus. Dense bilateral projections to the leaflet from the pretectal nuclei were found. Hypothalamic afferents were observed dorsal to the suprachiasmatic nuclei, in the retrochiasmatic area (RCh) and in the ventromedial hypothalamic nuclei. All of these projections were confirmed by anterograde tracing. Furthermore, arousal (wheel-running) induced c-Fos in neurons projecting to the IGL (prefrontal cortex, RCh, pretectum). Taken together, the data strengthen the view that the IGL integrates photic and nonphotic information.

Behavioral inhibition of circadian responses to light.

Circadian locomotor rhythms in rodents may be synchronized by either photic or nonphotic events that produce phase shifts of the rhythm. Little is known, however, about how these two types of stimuli interact to produce entrainment. The well-characterized circadian photic response of the golden hamster was examined in situations where a short light pulse and locomotor activity, a nonphotic event, occurred simultaneously. Light-induced phase advances were attenuated when animals were active during light exposure. The results show that circadian responses to light depend upon the environmental situation in which the light is given, and call into question the implicit assumption in circadian rhythm research that phase shifting and entrainment to light-dark cycles depend simply on photic activation of well-known retinofugal pathways. Moreover, since light therapy is becoming an important component in the treatment of circadian-based disorders in humans, the results emphasize the need for evaluation of the behavioral aspects of light therapy protocols.

Body fat: what is regulated?

This paper assumes that body fat is regulated and then reviews our ignorance about how this is accomplished. It concentrates on the challenge posed by site differences between different depots, and discusses a variety of experimental approaches that may be helpful.

The amplitude and period of circannual cycles of body weight in golden-mantled ground squirrels with medial hypothalamic lesions.

Golden-mantled ground squirrels, Citellus lateralis, were kept for several years in conditions of constant temperature and photoperiod. Control animals showed the expected circannual cycles of body weight, becoming fat in the autumn and thin in the spring. Animals with lesions in the ventromedial region of the hypothalamus also had circannual cycles of weight but in most animals the amplitude was increased. At one time of year the lesioned ground squirrels were grossly obese while often at other times their weight was close to that of control animals or their own preoperative weights. There were unusually early peaks in weight after the lesion in some animals and it is probable that a phase shift occurred. Observations on hibernation, food intake, transient weight gains and delayed weight gains following the lesions are also reported. Evidently during the winter phase of the annual cycle in these ground squirrels there exist mechanisms that lead not only to a loss of weight gained naturally before the time for hibernation but also to a temporary attenuation of additional obesity produced by ventromedial hypothalamic lesions.