Objective    To study the effect of propranolol on retention of fear memory and morphology of neurons in hippocampal CA3 area of rats with conditioned fear.     Methods    A rat model of conditioned fear was established. Twenty-four male adult SD rats were randomly divided into control group (which received no drug treatment with their fear level measured at the corresponding time points)

immediate drug treatment group (which received propranolol treatment 15 min after the model was established)

and 24 h drug treatment group (which received propranolol treatment 24 h after the model was established)

8 rats in each group. Conditioned fear memory levels in rats of drug treatment groups were measured daily for 3 d before the model was established and after propranolol was given. The rats were killed on day 20 after measurement of their fear memory levels. Morphology

number and ultra-structure of neurons in hippocampi were observed under a transmission electron microscope.     Results    No significant difference was observed in fear memory levels among the 3 groups 1 d after the model was established. The fear memory levels decreased gradually in two drug treatment groups 2

3 and 20 d after the model was established and were significantly different among the 3 groups at the same time point (P0.05 or P0.01). The fear memory level was the lowest in immediate drug treatment group and the highest in control group

was still rather high 20 d after the model was established

and decreased gradually in two drug treatment groups 2

3

and 20 d after the model was established. The number of pyramidal cells in hippocampal CA3 area was significantly higher in two drug treatment groups than in control group (4.94±2.67 and 4.65±1.98 vs 3.58±2.03

P0.05）. Abnormal changes of ultra-structure were observed in hippocampal CA3 area of control group under a transmission electron microscope after the model was established

but no significant difference was seen between the two drug treatment groups.     Conclusion    Conditioned fear memory level is related with the abnormal morphology of hippocampus. Early use of propranolol

a β-receptor blocker

can inhibit the activity of noradrenergic nervous system

decrease the conditioned fear memory level

and improve the impaired function of neurons in hippocampus.