EXTRACELLULAR-SODIUM REMOVAL INCREASES RELEASE OF NEUROPEPTIDE Y-LIKEIMMUNOREACTIVITY FROM RAT-BRAIN HYPOTHALAMIC SYNAPTOSOMES - INVOLVEMENT OF INTRACELLULAR ACIDIFICATION

Rat hypothalamic synaptosomes were exposed via superfusion to various stimuli and the release of neuropeptide Y-like immunoreactivity (NPY-L I) was measured by means of radioimmunoassay procedures. High KCl (15- 50 md concentration dependently evoked NPY-LI release; the evoked over flow reached a plateau at 30 mM KCl and was abolished in the absence o f Ca2+ ions. Furthermore, a remarkable NPY-LI overflow was obtained wh en extracellular Na+ ions were removed. Low external Na+-evoked NPY-LI release was independent of the presence of Ca2+ ions from the superfu sion medium. It is well known that the reduction of external Nai ions activates the release of several neurotransmitters through an inversio n of the uptake-carrier working direction; but such mechanisms, involv ing Na+-dependent uptake, have never been described for neuropeptides. The alteration of the extracellular Nai concentration is able to modi fy the concentration of the intracellular Ca2+ and H+ ions. In fact, t he concentrations of these two ions are regulated through Na+-dependen t exchange mechanisms across the membrane. Amiloride, blocking the Na/H+ exchanger, was able to maintain low Naf-evoked NPY-LI release, und erlying that the blockade of the exchanger preserves the Hf accumulati on induced by the reduction of the external Na+ ions. NPY-LI release c ould also be stimulated by nigericine, a proton ionophore, showing tha t the intracellular acidification is responsible for NPY-LI release. I ntracellular acidification may stimulate Ca2+ ion release from intrace llular stores, as has been shown by other workers. Large dense-core ve sicles containing the peptide appear to be more sensitive to local int racellular Ca2+ release compared with extracellular Ca2+ ion entry thr ough voltage-dependent channels. (C) 1997 Wiley-Liss, Inc.