The pineal melatonin rhythm and its regulation by light in a subterranean rodent, the valley pocket gopher (Thomomys bottae).

The daytime and nighttime levels of pineal N-acetyltransferase (NAT) activity, hydroxyindole-O-methyltransferase (HIOMT) activity, and melatonin were measured in adult male and female valley pocket gophers, Thomomys bottae. This species was chosen for study because it is a subterranean rodent that inhabits burrows whose openings to the surface are closed. Therefore, under field conditions it is estimated that the pocket gopher spends roughly 99% of its time in absolute darkness in underground burrows. When wild captured pocket gophers were maintained under a light:dark cycle (light intensity during the day of roughly 140 mu W/cm(2)), nighttime levels of pineal NAT activity and melatonin content were higher than values measured during the day; on the other hand, HIOMT activity in the pineal gland was similar in the day and at night. When pocket gophers were exposed to an extended light period (220 mu W/cm(2)) 4 hr into the night, the rise in melatonin synthesis normally associated with darkness onset was not inhibited. Also, when gophers were acutely exposed to a light intensity of 400 mu W/cm(2) for 1 hr beginning 4 hr after darkness onset, neither high nocturnal levels of pineal NAT nor pineal melatonin contents were reduced. Finally, when pocket gophers were exposed to a 600 mu W/cm(2) light intensity at either 4 hr or 8 hr into the dark period, pineal melatonin synthesis remained elevated at a level comparable to that measured in dark-exposed controls. The results show that under controlled laboratory conditions the pineal gland of the valley pocket gopher, a species that in its natural habitat spends about 99% of its time in absolute darkness, exhibits higher melatonin synthesis during night than during the day. While the rhythm in pineal melatonin production in the pocket gopher is clearly synchronized by the prevailing light:dark cycle, high nighttime pineal melatonin synthesis is not readily inhibited by light in the intensity range of 220 to 600 mu W/cm(2). In terms of its relative insensitivity to light at night, the pineal gland of the valley pocket gopher resembles that of other diurnally active rodents.