CHRONIC N-METHYL-D-ASPARTATE ADMINISTRATION PREVENTS MELATONIN-ASSOCIATED CHANGES IN CELL-DIFFERENTIATION IN THE HARDERIAN GLANDS OF MALE HAMSTERS

The daily administration of 25 mug of melatonin for 10 weeks resulted in an increase in the percentage of Type II cells in the Harderian gla nds of male Syrian hamsters. Harderian glands of melatonin injected an imals consisted of 65-70% Type II cells while control animals which we re injected with saline had 40% Type II secretory cells. The daily adm inistration of 3 mg of the glutamate receptor agonist N-methyl-D-aspar tate (NMDA) prevented the effects of melatonin on cell differentiation but was without effect when administered to saline treated hamsters a lone. Both the relative number of mitoses and the number of total cell s, estimated by counting the nuclei, was not affected. Thus, a convers ion from Type I to Type II cells seems possible. The effects of melato nin and NMDA administration were independent of the serum levels of te stosterone, luteinizing hormone and thyroxine, hormones which have bee n implicated in Type II cell differentiation. However, prolactin level s, which were affected by melatonin and NMDA administration, might be involved in the differentiation of Harderian land secretory cells.