Effect Of Thyroid Hormone Level On The Expression Of Synaptotagmin-Ⅰ In Adult Rat's Hippocampus

Objective Synaptotagmin I (syt I), an abundant integral membrane protein of synaptic vesicles, which may act as a calcium sensor to mediate stimulus-coupled fast chemical synaptic transmission, This study aimed to observe the effect of adult-onset hypothyroidism and hyperthyroidism on the expression of syt I in hippocampus.Methods All adult male Sprague-Dawley rats were devided into three groups,hypothyroid (n=10),hyperthyroid(n=9),control(n=9)。Hypothyroidism was induced by daily intraperitoneal injecting PTU (1mg /100 g body weight) for 6 weeks,hyperthyroidism by injecting L-Thyroxine(L-T4)(50 ug/100g body weight)for 3 weeks and the control group by injecting stroke-physiological saline solution for 6 weeks.Rats chow and tap water were provided ad libitum. 24 h after the last dose delivered, all rats were anesthetized by chloral hydrate injected intraperitoneally (3ml/kg BW). Approximately 5 ml blood was collected from abdominal aorta, using radioimmunity method to assay the levels of serum T3,T4. After blood collection, the animals were decapitated and their brains were rapidly removed from the skull. On dry ice, the whole right-side of the brain was dissected, the levels of syt I in hippocampus CA1,CA3 and dentate gyrus(DG) were assayed by immunohistochemistry. The layers analyzed in the different subfields of the dorsal hippocampus include the polymorphic cell layer (the stratum oriens,SO),pyramidal cell layer(PCL),stratum radiatum (SR),lacunosum-molecular layer (SLM) in CA1 and CA3, granular cell layer (GL) and molecular layer (ML) in DG..Results The levels of serum T(3P= 0.002),T(4P= 0.038) in the hypothyroid rats were significantly lower than those in the control group rats, the positive grana of syt I was reduced differently in different groups,significantly lower in PCL and SR of CA1 and CA3, GL of DG. The average optical ( OD ) value responsible for syt I immunoreactivity was significantly reduced in four layers of CA1( SO(P= 0.001),PCL (P= 0.001),SR (P= 0.003),SLM (P= 0.000) ),obviously reduced in three layers of CA3(PCL (P= 0.011),SR (P= 0.036),SLM (P= 0.001) ) and GL(P= 0.019) of DG,but no difference was found in SO of CA3 and ML of DG. Serum T3(P=0.000),T4(P=0.000) levels of hyperthyroid rats were significantly higher than the control group. The OD value was significantly reduced in three layers of CA1(PCL (P= 0.019),SR (P= 0.001),SLM (P=0.000) ), reduced in three layers of CA3(PCL (P= 0.034),SR (P= 0.036),SLM (P= 0.014) )and in GL(P= 0.034) and ML (P= 0.048) of DG,but no difference was found in SO(P= 0.163,0.878) of CA1 and CA3.Overall,change was more obvious in hypothyroidism than hyperthyroidism.Conclusion Adult-onset hypothyroidism and hyperthyroidism can reverserbly decrease the expression of sytⅠin CA1,CA3 and DG regions of hippocampus.This change maybe connected with cognition damage which caused by dysthyroid.