The pineal gland sits atop the brainstem and, in mammals, is overlaid by the cerebral hemispheres (see Figure 1a; see also Figure 8.1 in the textbook). Most brain structures are paired (with symmetrical left and right sides), but the pineal gland is a single structure. It is this unusual aspect of the pineal that led the seventeenth-century philosopher René Descartes to propose it as “the seat of the soul”; religious dogma of his day held that the soul, like the pineal, is indivisible.
Today we know that the pineal gland plays a crucial role in biological rhythms. Governed by the superior cervical ganglion—part of the sympathetic nervous system—the pineal releases an amine hormone called melatonin. The melatonin receptor is a G protein–coupled receptor residing in cell membranes and is similar to receptors for peptide hormones. Because melatonin is released almost exclusively at night (see Figure 1b), it provides a signal that tracks day length and, by extension, the seasons.
Melatonin secretion controls breeding condition in many seasonally breeding mammals. In hamsters, for example, the lengthening nights of autumn affect activity in the superior cervical ganglion, which in turn causes the pineal to prolong its nocturnal release of melatonin. The hypothalamus responds to the prolonged exposure to melatonin by becoming extremely sensitive to the negative feedback effects of gonadal steroids (Revel et al., 2009). Consequently, less and less gonadotropin-releasing hormone (GnRH) is released, resulting in less gonadotropin release, as well as atrophy of the gonads. In the spring, as days lengthen and the breeding season approaches, the process reverses: the hypothalamus becomes less sensitive to the negative feedback signal from sex steroids, the gonads swell, and the animal prepares to breed. Lesion of the pineal prevents seasonal gonadal regression, whereas chronic melatonin treatments induce regression.
In birds, light from the environment penetrates the thin skull and reaches the pineal gland directly. Photosensitive cells in the bird pineal gland monitor daily light durations (Foster and Soni, 1998). In several reptile species the pineal is close to the skull and even has an extension of photoreceptors providing a “third eye” in the back of the head. The pineal photoreceptors do not form images but simply monitor day length to regulate seasonal functions.
Humans are not, strictly speaking, seasonal breeders, but melatonin plays a role in our biological rhythms, especially the timing of sleep onset. Like other vertebrates, we release melatonin at night, and administering exogenous melatonin reportedly induces sleep sooner. This is why melatonin has been used to treat jet lag (Sack et al., 1992).
Foster, R. G., and Soni, B. G. (1998). Extraretinal photoreceptors and their regulation of temporal physiology. Reproduction 3: 145–150.
Reppert, S. M., and Weaver, D. R. (2002). Coordination of circadian timing in mammals. Nature 418: 935–941.
Revel, F. G., Masson-Pévet, M., Pévet, P., Mikkelsen, J. D., et al. (2009). Melatonin controls seasonal breeding by a network of hypothalamic targets. Neuroendocrinology 90: 1–14.
Sack, R. L., Blood, M. L., and Lewy, A. J. (1992). Melatonin rhythms in night shift workers. Sleep 15: 434–441.