[Effects of repeated ultrasound on expression of NR1, NR2B subunit of NMDA receptor and synaptic structure in prenatal rats].

Objective: To explore the effects of different doses of repeated ultrasound on the expressions of NR1 and NR2B subunits of NMDA receptor and synaptic structure so as to understand the molecular mechanisms of ultrasound on learning and memory in prenatal rats.

Methods: The pregnant rats were randomly divided into 3 groups (n = 8, each) and exposed to ultrasound for 0, 4 and 20 minutes at Days 6, 12 and 18 of gestation respectively. Their offspring were tested by Morris water maze (MWM) for evaluating the learning and memory abilities at an age of 2 months. The hippocampal pyramidal cell was observed by hematoxylin and eosin staining, the expressions of NR1 and NR2B by immunohistochemistry and Western blot and the morphologies of synapse in CA1 by electron microscopy.

Results: (1) In the water maze test, compared with the control group, the escape latency of 4 min group was significantly shorter (P < 0.05) while that of 20 min group significantly longer in the 1-4 d latency (P < 0.01). The time in platform quadrant (TQ) and the frequency of crossing through original platform increased in 4 min group (P < 0.01).However, they decreased in the 20 min group (P < 0.01); (2) in 4 min group, the density of hippocampal CA1 pyramidal cells per unit area was higher than that of the control group (P < 0.05).Sparse pyramidal cells and low density were observed in the 20 min group (P < 0.01); (3) compared with the control group, the expressions of total NR1, NR2B proteins in hippocampal regions significantly increased in 4 min group and both significantly decreased in 20 min (P < 0.05); (4) the structure of synapses became damaged in 20min group with decreased number density and surface density.

Conclusion: High doses of repeated ultrasound may decrease the ability of learning and memory through a down-regulated expression of NR1 and NR2B subunits in hippocampal regions and damaging synaptic structure.However, low doses of repeated ultrasound may promote the expressions of NR1 and NR2B, maintain synaptic structure and further enhance cognitive ability in prenatal rats.