Time‐Dependent Changes in the Expression of Synapsin I mRNA in the Rat Hippocampus During Long‐Term Potentiation: Comparison with the Changes After Amygdaloid Kindled Seizures

Abstract

Purpose: Nonepileptogenic high‐frequency stimulation of the perforant path input to the dentate gyrus can result in a long‐lasting increase of synaptic efficacy known as long‐term potentiation (LTP). LTP is the most widely used paradigm for studying the cellular and molecular events underlying neuronal plasticity. The synapsins, a family of phosphoproteins associated with the cytoplasmic surface of the synaptic vesicular membrane, are involved in synaptic maturatioin and regulation of neurotransmitter release. During depolarization, activation of calmodulin kinase II leads to phosphorylation of synapsin I, which weakens the linkage between the cytoskeleton and synaptic vesicles, thereby promoting the availability of synaptic vesicles for exocytosis. To investigate the roles of synapsin 1 in neuronal plasticity, we studied the time‐dependent changes in synapsin 1 messenger RNA (mRNA) expression after hippocampal LTP and compared them with those after kindled seizures. Methods: LTP was induced in the right dentate gyrus of the hippocampus by stimulating the ipsilateral perforant path in 49 rats under urethane anesthesia. LTP was evaluated in terms of the precentage increases in the slope of the excitatory postsynaptic potential (EPSP) and height of the population spike (PS) before brain removal and before the application of LTP‐inducing trains. Rat brains were removed at various times [immediately (time 0), 15 and 30 min, and 1, 2, 4, and 8 h; n = 7 at each time] after the completion of LTP stimulation. For in situ hybridization (ISH), the frozen brains were cut into 10‐μm‐thick sections, and tissue sections were processed by using a 35S‐labeled oligonucleotide antisense probe for synapsin 1 mRNA. The ISH procedures were performed as described previously (Epilepsia 1996;37:6–14). One‐way analysis of variance was used for statistical analysis. Results: LTP‐inducing stimulations produced long‐lasting significant increases in the EPSP slope and PS height for ≤8 h after stimulation, compared with the values just before stimulation. After LTP induction, synapsin 1 mRNA expression in the granule cell layer of the dentate gyrus ipsilateral to the site of stimulation increased significantly in a time‐dependent manner. From 2 to 8 h after stimulation, the levels of synapsin 1 mRNA in the ipisilateral dentate gyrus were significantly higher than those in controls subjected to a sham procedure (n = 6). The synapsin 1 mRNA levels reached a maximum (157.4 ± 7.1% of the control level, mean ± SEM) 8 h after stimulation. The synapsin 1 mRNA levels in animals that received only test pulses (n = 5) did not increase significantly in comparison with the control levels. The time‐dependent increases in synapsin 1 mRNA during LTP were quite consistent with those after amygdaloid kindled seizures [Epilepsia 1997;38(suppl 6):59]. Conclusions: The increase in synaptic proteins was considered to indicate transsynaptic propagation of plasticity and modification of synaptic function in the neural networks, supporting the idea that the increase in synapsin 1 is a marker of enhanced synaptic activity. The findings suggest that the increased level of synapsin 1 mRNA is related to the persistent enhancement of synaptic activity within the neural networks, where the dentate granule cells participate in LTP and kindling.