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Chapter 1: Neural Sciences
absence of seasonal cues with endogenous periods of approxi-
mately 1 year.
Melatonin and Seasonality
The most reliable environmental parameter providing a faith-
ful representation of the solar day is the day–night cycle. Simi-
larly, the most reliable environmental parameter reflecting the
progression through the seasons is the change in day length,
the fraction of the 24-hour day between sunrise and sunset.
In seasonally breeding animals, day length is physiologically
encoded through the melatonin profile. As described previ-
ously, melatonin levels are elevated during the night. A long
night, such as that experienced during the short day lengths of
winter, results in an elevated melatonin profile of a relatively
long duration. A short summer night, by contrast, results in a
short duration of elevated melatonin. This seasonal signal is
interpreted by the reproductive axis, resulting in an appropri-
ate reproductive response. Melatonin’s role in transducing day
length was elucidated by pinealectomizing seasonally breeding
animals, thereby removing the primary endogenous source of
melatonin. Melatonin was then infused in profiles mimicking
long days or short days. The duration of elevated melatonin was
the primary determinant of seasonal reproductive status, even
when the infused profile was administered under a conflicting
day length. Variations in other parameters, such as the amplitude
of the melatonin profile, the amount of total melatonin synthe-
sized, or the phase relationship of the profile to the light–dark
cycle, are of limited importance in producing a humoral signal
that transduces day length.
Reproductive responses to changing day length can be dra-
matic. A male Siberian hamster (
Phodopus sungorus
) main-
tained in long days is reproductively competent and typically
has a testicular weight of approximately 250 mg per testis.
Under short days, however, the testes regress to approximately
15 mg per testis, representing a 94 percent decrease in testicular
mass. The same degree of regression is observed in response to
melatonin infusions that mimic short days. Communication of
the hormonally transduced day length to the reproductive axis
is likely to be mediated, at least partially, through melatonin
receptors in the
pars tuberalis
of the pituitary gland. The exact
mechanism remains unknown, but activation of these recep-
tors is hypothesized to indirectly regulate an unidentified fac-
tor putatively named
tuberalin.
Tuberalin, in turn, controls gene
expression and prolactin release from lactotrophs in the adeno-
hypophysis of the pituitary.
Seasonality in Humans
Whether humans are truly seasonal is still a point of consider-
able debate. Several lines of evidence exist that suggest the pres-
ence of a residual tendency toward seasonality. A peak in the
rate of suicide occurs in the summer; this peak is cross-cultural.
Birth rates also tend to show a seasonal variation; a small but
distinguishable peak in the rate of births occurs in spring and
summer. This pattern, however, is itself variable and is heav-
ily influenced by unknown cultural and geographic factors. Of
interest, the amplitude of the spring–summer birth rate peak has
diminished as societies have become industrialized.
Figure 1.9-2
Change in sleep structure in response to artificial lighting. Total
sleep time is reduced, and periods of quiet wakefulness are abol-
ished by extending daytime into nighttime through artificial light-
ing. (From Wehr TA, Moul DE, Barbato G, et al. Conservation of
photoperiod-responsive mechanisms in humans.
Am J Physiol
.
1993;265:R846, with permission.)
from the plane of its own orbit around the sun (the ecliptic). As a
result, the relative proportion of daytime to nighttime within the
24-hour astronomical day varies as the Earth proceeds through
its orbit of the sun. Many organisms are capable of synchro-
nizing physiology to the seasonal cycle to maximize survival.
For example, precise seasonal cycles in reproduction are seen
throughout the plant and animal kingdoms. Large mammals that
typically have long gestation periods, such as sheep, conceive
in the fall when the nights are long and the days are short, so
birth occurs during the relatively mild season of spring. These
animals are referred to as
short-day breeders.
Conversely, mam-
mals with gestation periods of only a few weeks, such as ham-
sters, conceive and give birth during spring and summer, when
the days are long and the nights are short. Hence, these animals
are referred to as
long-day breeders.
Like circadian rhythms,
many of these yearly (circannual) rhythms tend to persist in the
