Electric stimulation at sciatic nerve evokes long-term potentiation of cornu dorsale medullae spinalis field potential in rats at various developmental phases

Abstract

Background Long-term potentiation of cornu dorsale medullae spinalis field potential in adult rats has already been reported; however, there is lack of correlated researches on naenonate, infant and adult rats which have different responses to pain conduction information. Objective To observe the various effects of electric stimulation at sciatic nerve on long-term potentiation of evoked field potential at superficial layer of cornu dorsale medullae spinalis of rats at various developmental phases and analyze manifestations of pain conduction information at superficial layers (I___II) of cornu dorsale medullae spinalis in immature rats. Design Grouping controlled study. Setting Department of Physiology, Medical College of Wuhan University. Materials The experiment was carried out in the Laboratory of Physiology (provincial laboratory), Medical College of Wuhan University from March 2006 to May 2007. A total of 27 healthy male Sprague-Dawley (SD) rats, 17–90 days old, SPF grade, weighing 41–200 g, were provided by Experimental Animal Center, Medical College of Wuhan University. Methods Based on their birthdays, rats were divided into naenonate group (17–20 days old, weighing 41–52 g, n =10), infant group (35–50 days old, weighing 87–125 g, n =10) and adult group (60–90 days old, weighing 180–200 g, n =7). Left sciatic nerve was separated and stimulated with single square wave (15 V, 0.5 ms). Meanwhile, evoked field potential was recorded at superficial layers of lateral T 13–L 1 cornu dorsale medullae spinalis and then stimulated with high-frequent and high-intensive tetanizing current (30–40 V, 0.5 ms, 100 Hz, 1 s per bundle, 10 s in bundle interval) four times. After the operation, onset of long-term potentiation was observed; meanwhile, amplitude changes and latency of field potential were analyzed. Main outcome measures Amplitude and latency changes of field potential at superficial layers of cornu dorsale medullae spinalis of rats in the three groups. Results A total of 27 accepted rats were involved in the final analysis. ▪ Amplitude changes: Electric stimulation at sciatic nerve with high-frequent and high-intensive tetanizing current could induce evoked field potential at superficial layers (I___II) of cornu dorsale medullae spinalis in the three groups. Long-term potentiation in the naenonate group manifested that amplitude of A-kind never fiber was raised and there was significant difference ( P < 0.05). In addition, average amplitude was increased and there was obviously significant difference ( P < 0.01). Long-term potentiation in the infant group manifested that amplitude of C-kind never fiber was raised and there was significant difference ( P < 0.01); while, long-term potentiation in the adult group manifested that amplitude of C-kind never fiber was raised and there was significant difference ( P < 0.01). Otherwise, latencies in the three groups were all shortened. ▪ Latency changes: Average latency of A-kind nerve fiber in the naenonate group was shortened and there was significant difference ( P < 0.01); in addition, evoked potential of C-kind nerve fiber was low and latency was immovable. There was no significant difference before and after high-frequent and high-intensive electric stimulation ( P > 0.05). Average latency of C-kind nerve fiber in the infant group was shortened and there was significant difference ( P < 0.01); in addition, evoked potential of A-kind nerve fiber was stable and latency was immovable. There was no significant difference before and after high-frequent and high-intensive electric stimulation ( P > 0.05). Average latency of C-kind nerve fiber in the adult group was shortened and there was significant difference ( P < 0.01); in addition, evoked potential of A-kind nerve fiber was stable and latency was immovable. There was no significant difference before and after high-frequent and high-intensive electric stimulation. Conclusion Evoked field potential at superficial layer of cornu dorsale medullae spinalis can be recorded through electric stimulation at sciatic nerve. Single stimulation and tetanizing electric stimulation can cause different characteristics of evoked field potential in rats at various developmental phases. Superficial layer of cornu dorsale medullae spinalis of naenonate rats is mainly caused by A-kind nerve fiber which participants in pain conduction and formation of pain sensitivity; however, that of infant and adult rats mainly depends on C-kind nerve fiber.