Overexpression of neuropeptide Y induced by brain-derived neurotrophic factor in the rat hippocampus is long lasting.

Brain-derived neurotrophic factor (BDNF) plays an important role in hippocampal neuroplasticity. In particular, BDNF upregulation in the hippocampus by epileptic seizures suggests its involvement in the neuronal rearrangements accompanying epileptogenesis. We have shown previously that chronic infusion of BDNF in the hippocampus induces a long-term delay in hippocampal kindling progression. Although BDNF has been shown to enhance the excitability of this structure upon acute application, long-term transcriptional regulations leading to increased inhibition within the hippocampus may account for its suppressive effects on epileptogenesis. Therefore, the long-term consequences of a 7-day chronic intrahippocampal infusion of BDNF (12 microg/day) were investigated up to 2 weeks after the end of the infusion, on the expression of neurotransmitters contained in inhibitory hippocampal interneurons and which display anti-epileptic properties. Our results show that BDNF does not modify levels of immunostaining for glutamic acid decarboxylase, the rate-limiting enzyme for gamma-aminobutyric acid (GABA) synthesis, and somatostatin. Conversely, BDNF induces a long-lasting increase of neuropeptide Y (NPY) in the hippocampus, measured by immunohistochemistry and radioimmunoassay, outlasting the end of the infusion by at least 7 days. The distribution of BDNF-induced neuropeptide Y immunoreactivity is similar to the pattern observed in animals submitted to hippocampal kindling, with the exception of mossy fibres which only become immunoreactive following seizure activity. The enduring increase of neuropeptide Y expression induced by BDNF in the hippocampus suggests that this neurotrophin can trigger long-term genomic effects, which may contribute to the neuroplasticity of this structure, in particular during epileptogenesis.