Effect of thyroid hormone level on the expression of synaptotagmin Ⅰ in adult rat hippocampus

Objective To observe the effect of different thyroid hormone level on the expression of synaptotagmin Ⅰ(Syt Ⅰ) in adult rat hippocampus. Methods All 28 adult male SD rats were assigned randomly into hypothyroid, hyperthyroid and control group, hypothyroid group was established by daily intraperitoneal injections with propylthiou raci(PTU, 10.0 mg/kg body weight) for 6 weeks and hyperthyroid group with L-Thyroxine (L-T4, 0.5 mg/kg body weight) for 3 weeks. Radioimmunity method was used to assay the levels of serum T3 and T4, immunohistochemical S-P technology to assay the levels of Syt Ⅰ protein in hippoeampus CA1, CA3 and dentate gyrus (DG). The layers analyzed in the different subfields 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 T3 and T4(0.34±0.12), (41.03± 11.37)nmol/L]in the hypothyroid rats were significantly lower than those in the control group(0.65±0.15), (55.20±10.68)nmol/L, P < 0.01 or < 0.05], and the positive granule of Syt Ⅰ was significantly lower in PCL and SR of CA1 and CA3, GL of DG. The average optical value responsible for Syt Ⅰ immunoreactivity was obviously reduced in SO(0.048±0.007), PCL(0.299±0.035), SR(0.042±0.007), SLM(0.038±0.006) of CA1, PCL(0.085± 0.019), SR(0.040±0.011), SLM (0.038±0.006) of CA3, GL (0.076±0.019) of DG than normal controls (0.068± 0.014, 0.376±0.053, 0.053±0.008,0.056±0.009,0.118±0.026,0.052±0.010,0.053±0.009,0.099±0.015; P< 0.01 or < 0.05). Serum T3 and T4 levels (1.43±0.30), (157.18±19.95)nmol/L]of hyperthyroid rats were significantly higher than those of control group(P < 0.01). The value was reduced in PCL(0.322±0.050), SR(0.039±0.006), SLM (0.042±0.006) of CA1, PCL(0.098±0.034), SR(0.046±0.013), SLM(0.046±0.010) of CA3 and GL(0.085± 0.024), ML (0.042±0.009) of DG (P < 0.05 or < 0.01). Conclusion Adult-onset of hypothyroidism and hyperthyroidism can reversibly decrease the expression of Syt Ⅰ in CA1, CA3 and DG regions of hippocampus.