The pineal gland: photoreception and coupling of behavioral, metabolic, and cardiovascular circadian outputs

Although removal of the pineal gland has been shown to have very little effect on the mammalian circadian system in constant darkness (DD), several recent reports have suggested that the mammalian pineal gland may be more important for circadian organization in nocturnal rodents than was previously believed. Removal of the pineal gland (PINX) facilitates the disruptive effects of constant bright light on wheel-running rhythmicity. This suggests at least two possibilities for the role of the pineal gland in the mammalian circadian system. First, pinealectomized rats may perceive ambient light intensity to be brighter than do sham-operated (SHAM) rats. Second, the pineal gland, probably via its secretion of melatonin, may also be involved in coupling components of the circadian system. Coupling, as we see it, may occur at several levels of organization: (1) between retinohypothalamic afferents and suprachiasmatic nuclei (SCN) oscillatory neurons, (2) among multiple SCN oscillators, (3) between the SCN and their multiple outputs, and/or (4) among the multiple circadian outputs themselves. In this study we show that PINX rats free-run with a longer period in four different light intensities than do SHAM rats. Moreover, the rate of increase of tau is greater among PINX rats than among SHAM rats. This supports the first hypothesis. We also show that in PINX rats the circadian rhythms of wheel running, general activity, body temperature, and heart rate are all more disrupted in constant bright light than are those of SHAM rats, and each rhythmic output is disrupted in parallel. This supports the second hypothesis. Melatonin is probably not involved in coupling presynaptic elements of SCN afferents in the retinohypothalamic tract to pacemaking cells within the SCN, since enucleation has no effect on SCN 2-[125I]iodomelatonin (IMEL) binding. Together the data do not discount either of the two hypotheses but do restrict the possible levels at which the pineal gland is involved in coupling. These data also further support a growing body of literature indicating that the pineal gland and its hormone melatonin play a role in mammalian circadian organization.