Rapid release of substance P and LH-RH from synaptosomes prepared from the medial basal hypothalamus and substantia nigra.

We investigated Ca2+-dependent, depolarization-induced release of substance P (SP) and LH-RH from medial basal hypothalamic (MBH) and substantia nigra (SN) synaptosomes prepared from male rat brain. Depolarization of MBH synaptosomes evoked significant release of SP from 10.0 +/- 0.1 (5 mM K+) to 28.0 +/- 2.4 (75 mM K+) pg released/10 seconds. Fractional release was 1.0% and 2.7% respectively. In contrast, LH-RH was not released by depolarization of MBH synaptosomes: 11.6 +/- 0.9 (5 mM K+) to 11.0 +/- 0.7 (75 mM K+) pg released/10 seconds. Fractional release was 1.1 and 1.0% respectively. Depolarization-induced LH-RH release also did not occur in the presence of 10(-4) or 10(-6) M norepinephrine, 10(-7) M 12-O-tetradecanoylphorbol-13-acetate (TPA, PMA), 10(-5) M forskolin or in female rats. The inability of depolarizing concentrations of K+ to stimulate LH-RH release in physiological buffers remains an enigma. Significant depolarization-induced SP release was seen from MBH and SN synaptosomes at 20, 15, 10, 5 and only 1 second of release. Despite comparable basal release of SP from MBH and SN synaptosomes, the rate and magnitude of evoked release were much more pronounced in SN synaptosomes. The initial rate (0-1 second) of SP release was 4.5-fold greater from SN than from MBH synaptosomes [krel = 0.027(-1) (SN), krel = 0.006(-1) (MBH)]. The magnitude of SP release from SN synaptosomes was 2- to 3-fold greater at any given time interval compared with release from MBH synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)