Urea enhances the nerve growth factor-induced neuroprotective effect on cholinergic neurons in organotypic rat brain slices.

Cholinergic neurons degenerate in Alzheimer's disease and dementia and neuroprotective substances are of high interest to counteract this cell death. The aim of the present study was to test the effect of urea and the nitric oxide synthetase inhibitor l-thiocitrulline on the survival of cholinergic neurons. Organotypic brain slices of the basal nucleus of Meynert were cultured for 2 weeks in the presence of 1-100 microM urea with or without NGF or other growth factors or with or without 1-10 microM of the NOS inhibitor L-thiocitrulline. A high number of cholinergic neurons survived in the presence of 0.1-100 ng/ml NGF. Urea or L-thiocitrulline alone did not exhibit neuroprotective activity; however, when brain slices were incubated with urea or L-thiocitrulline together with NGF there was a significant potentiating survival effect. Incubation of brain slices with NGF + urea + L-thiocitrulline did not further enhance the number of cholinergic neurons. NGF as well as urea did not stimulate expression of the enzyme choline acetyltransferase pointing to survival promoting effects. Urea did not modulate the NGF binding in PC12 cells indicating that this effect was indirect. It is concluded that urea may play a role as an indirect survival promoting molecule possibly involving the nitric oxide pathway.