Human brain somatostatin release from isolated cortical nerve endings and its modulation through GABAB receptors.
1. The release of somatostatin-like immunoreactivity (SRIF-LI) in the human brain was studied in synaptosomal preparations from fresh neocortical specimens obtained from patients undergoing neurosurgery to remove deeply sited tumours. 2. The basal outflow of SRIF-LI from superfused synaptosomes was increased about 3 fold during exposure to a depolarizing medium containing 15 mM KCl. The K(+)-evoked overflow of SRIF-LI was almost totally dependent on the presence of Ca2+ in the superfusion medium. 3. The GABAB receptor agonist, (-)-baclofen (0.3 - 100 microM), inhibited the overflow of SRIF-LI in a concentration-dependent manner (EC50 = 1.84 +/- 0.20 microM; maximal effect: about 50%). The novel GABAB receptor ligand, 3-aminopropyl(difluoromethyl)phosphinic acid (CGP 47656) mimicked (-)-baclofen in inhibiting the SRIF-LI overflow (EC50 = 3.06 +/- 0.52 microM; maximal effect: about 50%), whereas the GABAA receptor agonist, muscimol, was ineffective up to 100 microM. 4. The inhibition by 10 microM (-)-baclofen of the K(+)-evoked SRIF-LI overflow was concentration-dependently prevented by two selective GABAB receptor antagonists, 3-amino-propyl (diethoxymethyl)-phosphinic acid (CGP 35348) (IC50 = 24.40 +/- 2.52 microM) and [3-[[(3,4-dichlorophenyl) methyl]amino]propyl] (diethoxymethyl) phosphinic acid (CGP 52432) (IC50 = 0.06 +/- 0.005 microM). 5. The inhibition of SRIF-LI overflow caused by 10 microM CGP 47656 was abolished by 1 microM CGP 52432. 6. When human synaptosomes were labelled with [3H]-GABA and depolarized in superfusion with 15 mM KCl, the inhibition by 10 microM (-)-baclofen of the depolarization-evoked [3H]-GABA overflow was largely prevented by 10 microM CGP 47656 which therefore behaved as an autoreceptor antagonist. 7.


CONCLUSIONS
(a) the characteristics of SRIF-LI release from synaptosomal preparations of human neocortex are compatible with a neuronal origin; (b) the nerve terminals releasing the neuropeptide possess inhibitory receptors of the GABAB type; (c) these receptors differ pharmacologically from the GABAB autoreceptors present on human neocortex nerve terminals since the latter have been shown to be CGP 35348-insensitive but can be blocked by CGP 47656.
Bonanno
G
G
Institute of Pharmacology and Pharmacognosy, University of Genova, Italy.
Gemignani
A
A
Schmid
G
G
Severi
P
P
Cavazzani
P
P
Raiteri
M
M
D000328:Adult; D000368:Aged; D001923:Brain Chemistry / Q000187:drug effects / Q000502:physiology*; D002118:Calcium / Q000502:physiology; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D018755:GABA Agonists / Q000494:pharmacology; D058786:GABA-B Receptor Agonists; D058788:GABA-B Receptor Antagonists; D006801:Humans; D066298:In Vitro Techniques; D008875:Middle Aged; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism*; D018080:Receptors, GABA-B / Q000378:metabolism*; D013004:Somatostatin / Q000378:metabolism*; D013574:Synaptosomes / Q000378:metabolism
D018755:GABA Agonists; D058786:GABA-B Receptor Agonists; D058788:GABA-B Receptor Antagonists; D018080:Receptors, GABA-B; D013004:Somatostatin; D002118:Calcium