Vacant postsynaptic densities on supraoptic dendrites of adult rats diminish in number with chronic stimuli

Summary

In earlier ultrastructural studies of the supraoptic nucleus in adult rats we noted “free” and incompletely covered postsynaptic densities (collectively referred to here as vacant postsynaptic densities) on dendritic shafts. Free postsynaptic densities have been reported in other parts of the central nervous system of normal rodents. We investigated the possibility that physiological activation of the supraoptic cells, which produces changes in many aspects of their morphology, would alter the incidence of the free or incompletely covered postsynaptic densities on dendrites in the supraoptic basal dendritic zone. The cells of the supraoptic nucleus are activated to increase cell firing and secretion of oxytocin and/or vasopressin in response to dehydration, gestation, and lactation. We have examined: (i) untreated virgin females; (ii) untreated males; (iii) 24 h water-deprived males; (iv) prepartum (21st day of gestation) females; (v) postpartum females (on the day of parturition); (vi) lactating females (14 days of suckling); (vii) mothers 10 days after weaning their pups; (viii) females given 2% saline to drink (dehydrated) for 10 days; and females or males given 2% saline to drink for 10 days, then given tap water for (ix) 2 or (x) 5 weeks to allow rehydration. Only long-term activation of the supraoptic nucleus by lactation or by drinking saline for 10 days brought about significant decreases in the percentage of dendrites with vacant postsynaptic densities. These densities did not reappear in saline treated rats which had been rehydrated for 2 weeks, but did return in both the 5-week rehydration and the 10-day postweaning groups. Short-term activation of the supraoptic nucleus, such as occurs at parturition or in acute dehydration, did not affect the vacant postsynaptic densities. Analysis of semiserial thin sections indicated that presynaptic elements facing the incompletely covered postsynaptic densities contain predominantly clear round vesicles and also that apparently free postsynaptic densities were usually at least partially contacted by a presynaptic ending in adjacent sections.