Neuropeptide Y increases in vivo hippocampal extracellular glutamate levels through Y1 receptor activation.

Neuropeptide Y's (NPY) anticonvulsant effect is generally attributed to its inhibitory effect on glutamate release from presynaptic nerve terminals, which is nicely demonstrated in in vitro settings. To date no study has attempted to investigate the effect of NPY in vivo on extracellular (EC) glutamate levels thus, via intracerebral microdialysis, we determined NPY's effect on hippocampal glutamate concentrations in vivo, and consequently the involvement of Y(1) receptors to this effect. NPY or the Y(1) agonist D-His26-NPY was intrahippocampally administered in rats for 2h, during which the hippocampal glutamate dialysate levels were monitored. Pilocarpine was subsequently co-administered with NPY or D-His26-NPY to determine their effect on pilocarpine-induced limbic seizures. Unexpectedly we noted that intrahippocampal administration of NPY or D-His26-NPY increased glutamate dialysate levels in a reproducible manner. NPY attenuated pilocarpine induced seizures, whereas D-His26-NPY did not. To clarify the role of Y(1) receptors in NPY's glutamatergic effect, NPY was co-administered with the selective Y(1) antagonist BVD10. Hippocampal Y(1) receptor blockade prevented the NPY-induced increase in hippocampal glutamate, proving that this induced glutamate increase is clearly Y(1) receptor mediated. This is the first evidence that NPY enhances hippocampal EC glutamate overflow in vivo via hippocampal Y(1) receptors without interfering with or contributing to NPY's anticonvulsant effect. Whilst this finding contrasts with the supposed glutamatergic hypothesis for NPY in the hippocampus, it is of significance to further assist in deciphering NPY's mechanisms of action in in vivo settings.