Cerebellar development and very preterm birth interactions with neonatal events

Sammanfattning: AbstractThe cerebellum is an important part of the brain and in very preterm infants the cerebellum is smaller at term equivalent age compared to that of infants born at term.Hypothesis and aims: The general hypothesis is that preterm birth per se causes cerebellar underdevelopment and that certain events following preterm birth, either complications or treatments, can aggravate the underdevelopment. The general aim is to evaluate how preterm birth in association with potentially damaging events i.e., intraventricular hemorrhage (IVH), the neurovascular disease retinopathy of prematurity (ROP) and the widely used adenosine antagonist caffeine, can affect the cerebellar development.Methods: Paper I. Postnatal growth, cerebellar external granular layer (EGL) development and circulating IGF-1 levels were evaluated following preterm birth in rabbit pups and compared to that of term pups to evaluate if the preterm rabbit pup model is suitable for future cerebellar studies. Paper II. The distribution and effect of cell-free hemoglobin (Hb) on cerebellar development was evaluated following IVH in preterm rabbit pups and compared to preterm pups without an IVH. Additionally the effect of the cell-free Hb scavenger Haptoglobin (Hp) was evaluated. Paper III. The relationship between any stage of ROP, brain volumes and developmental outcome was evaluated in very preterm infants. Paper IV. The effect of caffeine on cerebellar development and the Insulin-like growth factor 1 (IGF-1) system was evaluated in preterm rabbit pups given enterally administered caffeine and compared to control preterm pups.Results: Paper I. Mean weight and circulating levels of IGF-1 were significantly lower in preterm pups at all time- points (all p<0.05). Postnatal weight correlated with circulating IGF-1 (r2=0.89) independantly of gestational age at birth and postnatal age. Proliferation was reduced in preterm pups at postnatal day 2 (P2) compared to term pups (p=0.01). Purkinje cells in the preterm pups exhibited reduced calbindin staining at P0 (p=0.003), P2 (p=0.004) and P5 (p=0.04) compared term group. Paper II. IVH was associated with a decreased proliferative (Ki67 positive) portion of the EGL, delayed Purkinje cell maturation, and activated microglia in cerebellar white matter. Immunolabeling at P0 in pups with IVH, demonstrated a widespread presence of cell-free Hb in all cerebellar layers. Intraventricular injection of Hp resulted in a distribution of Hp corresponding to that of cell-free Hb in the cerebellar layers and a partial reversion of the damaging effects observed following IVH. Paper III. Infants with any stage of ROP had lower unmyelinated white matter volume (UWMV) (p<0.001), cerebellar volume (p<0.001), mental developmental index (p<0.001) and psychomotor developmental index (p=0.002) compared to infants without ROP. Paper IV. Caffeine treatment did not affect weight development, serum IGF-1 or hepatic expression of mRNA for IGF-1 at any given time-point. Cerebellar densities of IGF-1R, calbindin positive Purkinje cells and Ki67 positive proliferating cells were not affected by caffeine exposure.Conclusions: The preterm rabbit pup model exhibits characteristics relevant to human preterm birth and is appropriate for further studies on preterm birth and cerebellar development. IVH in the preterm rabbit pups is followed by extensive deposition of cell-free Hb in cerebellar layers and associated with microglial activation, reduced neuronal proliferation and Purkinje cell maturation. Intraventricular injection of the scavenger Hp partially blocked these effects suggesting that cell-free Hb is causal in cerebellar impairment following IVH. Development of any stage of ROP was associated with reduced cerebellar volume and UWMV and impaired neurodevelopmental outcome at 2 years corrected age. Enteral administration of caffeine to preterm rabbit pups did not reveal effect on growth or the closely related IGF-1 system in preterm rabbit pups and did not affect key neuronal maturation in the cerebellar EGL. It did increase survival.