BDNF and NT-4 differentiate two pathways in the modulation of neuropeptide protein levels in postnatal hippocampal interneurons.

Neuropeptide protein levels in hippocampal interneurons exhibit a considerable maturation in postnatal animals. This study characterizes the role of neuronal activity in determining neuropeptide protein levels in postnatal hippocampal interneurons, and the involvement of neurotrophins. In hippocampal slices from 7-day-old rats cultured for 2 weeks, treatment with the gamma-aminobutyric acidA (GABAA) receptor antagonist bicuculline increased the staining intensity and the number of neurons immunoreactive for neuropeptide Y (NPY). An opposite effect was observed when non-N-methyl-d-aspartate (non-NMDA) excitatory transmission was blocked. The effects of either treatment were reversed after return to control medium. These findings were similar to those previously obtained on the effects of activity on somatostatin immunostaining. Blockade of endogenous tyrosine kinase neurotrophin receptors using K252a prevented the effects of bicuculline on NPY- and somatostatin-immunoreactive neurons. Application of exogenous neurotrophin-3 (NT-3) increased NPY and somatostatin protein levels in long-term but not short-term cultures, while nerve growth factor (NGF) had no effect. In contrast, brain-derived neurotrophic factor (BDNF) or neurotrophin-4 (NT-4) did not affect equally NPY and somatostatin immunoreactivity: they mimicked the effects of bicuculline treatment on NPY-immunoreactive neurons, but exerted no conspicuous effect on somatostatin immunostaining. These results indicate that although neuronal activity plays a major role in determining neuropeptide protein levels in postnatal hippocampal interneurons, its effects on different neuropeptides might be exerted through different mechanisms, with or without the mediation of BDNF or NT-4.