BACKGROUND: The liver enzyme system and blood brain barrier function of newb rabbits are incomplete.High level of bilirubin can lead to cerebral palsy (CP) of newb rabbits. The pathological and ultrastructural changes of brains of immature rabbits may differ from those of mature rabbits.OBJECTIVE: To observe the changes in dopaminergic neuron amount and ultrastructure in the extracorticospinal tract of animal models of CP induced by hyperbilirubinemia.DESIGN: A randomized controlled observation.SETTING: Rehabilitation Medical College of Jiamusi University; Laboratory for Pediatric Neurology Rehabilitation of Jiamusi University.MATERIALS: Thirty newb 2- to 5- day inbred Japanese rabbits, weighing about 50 g, of either gender, were provided by Experimental Animal Center, Jilin University. Bilirubin and tyrosine hydroxylase (TH) antibody were purchased from Sigma Corporation, USA.METHODS: This experiment was carried out in the laboratory for children neuro-rehabilitation, Jiamusi University between September 2002 and December 2005. ①Thirty rabbits were randomly divided into 2groups: control group (n =10) and model group (n =20). ②Bilirubin of 100 mg/kg was given to the rabbits of model group through intraperitoneal injection, once every other day, 3 times totally; The same amount of normal saline was given to the rabbits of control group. After injection, the newb rabbits had been fed by matal milk for 45 days. ③TH-positive neurons in substantia nigra and corpora striatum in each area of0.15 mm ×0.15 mm were counted. Uitrastructures of substantia nigra and corpora striatum were observed under the electron miscroscope. The concentration of bilirubin in brain tissue of CP rabbits of model group was measured with high performance liquid chromatogram.MAIN OUTCOME MEASURES: Quantitative analysis results and ultrastructure of TH-positive neurons in substantia nigra and corpora striatum in two groups.RESULTS: Thirty newb rabbits were involved, 6 rabbits from model group died and 4 were failed in modeling, finally, 20 rabbits were involved in the result analysis. ① The concentration of bilirubin in brain tissue of newb rabbits of model group was (59.9±17.04) mg/L, which was significantly higher than that of control group [(42.79±3.57) mg/L, P ＜ 0.01]. ② TH positive neurons amount in the substantia nigra and corpora striatum of newb rabbits in the model group was 30.30±9.58, 4,70±2.79, which was significantly less than that in the control group respectively (45.20±12.74, 11±2.45, P＜ 0.01). ③The concentration of bilirubin in brain tissue was significantly negatively correlated with TH-positive neurons in substantia nigra (r =-0.69, P ＜0.01 ); The concentration of bilirubin in brain tissue was also significantly negatively correlated with TH-positive neurons in corpora striatum (r =-0.59, P ＜ 0.01 ). TH-positive neurons in substantia nigra were positively correlated with those in corpora striatum (r =0.77, P ＜ 0.01 ).④ Under the electron microscope, the nerve cells in substantia nigra and corpora striatum of newb rabbits of model group were significantly changed in ultrastructure as compared with control group.CONCLUSION: Ultrastructure of extracorticospinal tract of newb rabbits with CP is significantly changed,presenting the injury of dopaminergic neuron and decrease of neuronal amount.