REORGANIZATION OF MOSSY FIBER SYNAPSES IN MALE AND FEMALE HYPOTHYROIDRATS - A STEREOLOGICAL STUDY

Thyroid hormone deficiency has been reported to interfere with synapto genesis, particularly in those regions of the brain where the neurons display a late and protracted histogenesis, although the extent of the synaptic alterations remains unknown. To provide detailed quantitativ e data on the effects of hypothyroidism upon synapses, a link of the h ippocampal circuitry was selected: the contact between mossy fibers an d dendritic excrescences of CA3 pyramidal cells (MF-CA3 synapses). Gro ups of six male and six female rats aged 30 and 180 days were analyzed separately after being treated as follows: (1) hypothyroid from day 0 until day 30 (30 day old hypothyroid group); (2) hypothyroid from day 0 until day 180 (180 day old hypothyroid group); (3) hypothyroid unti l day 30 and thenceforth maintained euthyroid (recovery group); and (4 ) and (5) 30 and 180 day old control groups, respectively. Timm staini ng, Golgi impregnation, and electron microscopy were employed to estim ate the volume of the mossy fiber system, the number and size of mossy fiber boutons, and the number and related features of MF-CA3 synapses . The volume of the mossy fiber system and the number of synaptic bout ons were reduced in all experimental groups. The total number of synap ses was decreased in 30 day old hypothyroid rats, but did not differ a mong 180 day old animals. Postsynaptic densities were shorter in hypot hyroid and recovery groups than in controls, although the reduction wa s not as marked in recovery rats as it was in hypothyroid animals. Str uctural alterations were noted in the pre- and postsynaptic compartmen ts of MF-CA3 synapses of both 180 day old hypothyroid and recovery rat s. These changes can be regarded as mechanisms of reorganization as th ey underlie the compensation for the hypothyroid-induced numerical red uction of synapses observed in 30 day old animals and enable a complet e catch-up of their total number. However, synaptic reorganization was not fully achieved, as revealed by the reduction in the size of the s ynaptic sites in hypothyroid and recovery animals. Finally, we demonst rate that hypothyroidism did not interfere with the sex-related differ ences of MF-CA3 synapses described in normal rats. (C) 1993 Wiley-Liss , Inc.