GABAB receptor-mediated presynaptic potentiation of ATP ionotropic receptors in rat midbrain synaptosomes.
Nucleotides can activate ionotropic P2X receptors that induce calcium-responses in rat midbrain synaptosomes. In this report, we show that ATP elicits Ca(2+) responses producing a monophasic dose-response curve with an EC(50) value of 24.24+/-1.42 micro M. In the presence of gamma-aminobutyric acid (GABA), the ATP dose-response curve becomes biphasic with EC(50) values of 3.69+/-0.44 nM and 59.65+/-8.32 micro M. Moreover, the maximal calcium response induced by ATP is 52.1% higher than the control. This effect is mimicked or blocked by the specific GABA(B) receptor agonist and antagonist, baclofen and saclofen, respectively. Presynaptic GABA(B) receptors, identified by immunocytochemistry are present in 62% of the total synaptosomal population. Adenylate cyclase and protein kinase A cascades are involved in the potentiatory effects mediated by baclofen and their activation or inhibition modifies calcium signalling and synaptosomal cAMP levels. The potentiatory action of baclofen was confirmed by microfluorimetry performed on single synaptic terminals. In its presence, 86% of the terminals responding to 100 micro M ATP, are also able to respond to nanomolar concentrations (100 nM) of this nucleotide. This potentiatory effect is reduced to 32% in the presence of pertussis toxin. Our data suggest that the activity of P2X receptors is modulated by GABA(B) receptors in midbrain synaptosomes.
Gómez-Villafuertes
R
R
Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, Av. Puerta de Hierro s/n, 28040, Madrid, Spain.
Pintor
J
J
Gualix
J
J
Miras-Portugal
M T
MT
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001418:Baclofen / Q000031:analogs & derivatives* / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D003164:Competitive Bidding / Q000379:methods; D000242:Cyclic AMP / Q000819:agonists / Q000031:analogs & derivatives / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D016257:Fura-2 / Q000378:metabolism; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D008636:Mesencephalon / Q000187:drug effects / Q000378:metabolism*; D009118:Muscimol / Q000494:pharmacology; D009419:Nerve Tissue Proteins / Q000378:metabolism; D010852:Picrotoxin / Q000494:pharmacology; D051381:Rats; D018080:Receptors, GABA-B / Q000187:drug effects / Q000378:metabolism*; D018048:Receptors, Purinergic P2 / Q000187:drug effects / Q000378:metabolism*; D058469:Receptors, Purinergic P2X; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D013997:Time Factors; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D000255:Adenosine Triphosphate; D001418:Baclofen; D002118:Calcium; D005576:Colforsin; D000242:Cyclic AMP; D016257:Fura-2; D018755:GABA Agonists; D018756:GABA Antagonists; D005904:Glial Fibrillary Acidic Protein; D009118:Muscimol; D009419:Nerve Tissue Proteins; D010852:Picrotoxin; D018080:Receptors, GABA-B; D018048:Receptors, Purinergic P2; D058469:Receptors, Purinergic P2X; D005680:gamma-Aminobutyric Acid; C033652:postsynaptic density proteins; C080629:saclofen; C002630:alpha,beta-methyleneadenosine 5'-triphosphate