Effects of LHRH, progesterone, estradiol-17 beta and dexamethasone in vitro on pineal synaptic ribbons and serotonin N-acetyltransferase activity in diestrous rats

Abstract

Pineal glands of regularly cycling Sprague Dawley rats (180-220 g) killed on the diestrous morning (between 0900-1000 h) were incubated in appropriate media for six hours with LHRH (8.5 µM), progesterone (3.2µ M), estradiol-17β (370 nM) or dexamethasone (250 nM). Pineals incubated in hormone-free medium and unincubated glands served as controls. Six rats were used in each group. After incubation the glands were divided into two parts. One part was used to estimate serotonin N-acetyltransferase (NAT) activity. The other part was processed for electron microscopy to quantify synaptic ribbons (SR). The SR numbers were computed to 20,000 m² area of pineal tissue. The number and distribution pattern of SR were identical in incubated as well as in the unincubated controls. In both these groups the SR located close to the cell membrane were more (23 ± 1) than those that lay away from it (9 ± 2). LHRH had no effect on the number of SR located close, to or distant from, the cell membrane. Incubation of pineals with progesterone significantly (p ≤ 0.05) depressed the number of SR present close to membranes (23 ± 1 in controls vs 11 ± 2 in treated group), total SR (34 ± 3 in controls vs 21 ± 2 in treated group) and synaptic fields (26 ± 2 in controls vs 17 ± 2 in treated group). Likewise, in the estradiol-17 βgroup also membrane-associated SR decreased significantly. The effect of progesterone was more severe than estrogen on the SR possibly due to the differences in the doses used. The SR situated distant from the membranes were unaffected in both progesterone and estrogen groups. In pineals incubated with dexamethasone there was a depressive trend in the number of SR but with no statistical significance. The NAT activity was undetectable in control and all experimental groups. The above findings demonstrate that ovarian sex steroids in vitro are capable of specifically depressing the number of SR located close to the cell membrane while those lying distant from it remain unaffected. Whether or not this differential response is also present in vivo remains to be elucidated.