Pineal transplants in oculo: limitations on the ability of collateral sprouts of foreign neurons to establish normal function.

The pineal gland is innervated by sympathetic neurons whose cell bodies are located in the superior cervical ganglia. This pathway, which carries information concerning environmental lighting to the gland, is important for maintaining a circadian rhythm in the activity of the pineal enzyme serotonin N-acetyltransferase (NAT). Lesioning this pathway blocks the normal nighttime increase in NAT activity. Following transplantation of the pineal gland to the anterior chamber of the eye, the gland becomes reinnervated by collateral sprouts of sympathetic neurons that innervate the iris. In addition, a day-night rhythm in NAT activity is restored to the transplanted glands (Moore, 1975; Bäckström et al., 1976). These findings raise the possibility that the neural regulation of pineal function may be restored to normal by collateral sprouts of foreign neurons. To determine whether this is the case, other aspects of the neural regulation of NAT activity were examined in transplanted and in situ pineal glands. When the dark period was extended into the normal light period, NAT activity decreased to daytime values in both groups, suggesting that, in both cases, darkness is only effective in maintaining high levels of NAT activity at certain times during the day-night cycle. In contrast to their similar responses to extended darkness, the 2 groups of pineal glands responded differently to a brief pulse of light during the dark period. While the light exposure caused a dramatic decrease in nighttime NAT activity in in situ pineal glands, it produced no change in this enzyme activity in transplanted glands.(ABSTRACT TRUNCATED AT 250 WORDS)