P-CHLOROPHENYLALANINE TREATMENT DEPRESSES THE NUMBER OF SYNAPTIC RIBBON PROFILES IN THE RAT PINEAL-GLAND, BUT DOES NOT ABOLISH THEIR DAY-NIGHT RHYTHM

It is largely unknown as to how the day/night rhythm of pineal synapti c ribbon number is regulated. Transcriptional events related to the no cturnal formation of new synaptic ribbons occur early in the morning, when pineal serotonin levels begin to increase. The present study was carried out in order to elucidate as to how altered serotonin levels a ffect the day/night changes in the number of synaptic ribbon profiles. To this end, male Sprague-Dawley rats received a single dose of p-chl orophenylalanine (pCPA, 300 mg/kg body weight, 72 hours before sacrifi ce), which depresses tryptophan hydroxylase activity and hence pineal serotonin levels. Control animals received saline injections. Experime ntal and control animals were killed in the middle of the light period and in the middle of the dark period, respectively. The pineal glands were removed and bisected. One half was used for electron microscopic quantification of synaptic ribbon profile numbers, and the other half for determination of several biochemical parameters (see below). pCPA decreased pineal serotonin levels to c. 30% of the controls and synap tic ribbon profile numbers by c. 30%, both at day and night. The day/n ight rhythm of the two parameters did not appear to be affected. The r ate-limiting enzyme of melatonin synthesis, serotonin N-acetyltransfer ase, which is only measurable at night, and pineal melatonin levels we re not affected at night. The intermediary product of melatonin format ion, N-acetylserotonin, was significantly depressed at night. It is co ncluded that serotonin plays a more important modulatory role on pinea l synaptic ribbons than does melatonin. These results in conjunction w ith data from the literature suggest that in acute experiments seroton in appears to exert an inhibitory influence on synaptic ribbon numbers , whereas in chronic experiments it has a stimulatory effect.