Mechanisms intrinsic to 5-HT 2B receptor-induced potentiation of NMDA receptor responses in frog motoneurones

Abstract

1 In the presence of NMDA receptor open-channel blockers [Mg 2+; (+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten-5,10-imine maleate (MK-801); 1-amino-3,5-dimethyladamantane (memantine)] and TTX, high concentrations (30-100 μM) of either 5-hydroxytryptamine hydroxytryptailline (5-HT) or α-methyl-5-hydroxytryptamine (α-Me-5-HT) significantly potentiated NMDA-induced depolarizations of frog spinal cord motoneurones. 2 Potentiation was blocked by LY-53,857 (10-30 μM), SB 206553 (10 μM), and SB 204741 (30 μM), but not by spiroxatrine (10 μM), WAY 100,635 (1-30 μM), ketanserin (10 μM), RS 102221 (10 μM), or RS 39604 (10-20 μM). Therefore, α-Me-5-HT's facilitatory effects appear to involve 5-HT 2B receptors. 3 These effects were G-protein dependent as they were prevented by prior treatment with guanylyl-5′-imidodiphosphate (GMP-PNP, 100 μM) and H-Arg-Pro-Lys-Pro-Gln-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH 2 (GP antagonist 2A, 3-6 μM), but not by pertussis toxin (PTX, 3-6 ng ml -1, 48h preincubation). 4 This potentiation was not reduced by protein kinase C inhibition with staurosporine (2.0 μM), U73122 (10 μM) or N-(2-aminoethyl)-5-isoquinolinesulfonamide HCl (H9) (77 μM) or by intracellular Ca 2+ depletion with thapsigargin (0.1 μM) (which inhibits Ca 2+/ATPase). Exposure of the spinal cord to the L-type Ca 2+ channel blockers nifedipine (10 μM), KN-62 (5 μM) or gallopamil (100 μm) eliminated α-Me-5-HT's effects. 5 The calmodulin antagonist N-(6-aminohexyl)-5-chloro-l-naphtalenesulfonamide (W7) (100 μM) diminished the potentiation. However, the calcium/calmodulin-dependent protein kinase II (CaM Kinase II) blocker KN-93 (10 μM) did not block the 5-HT enhancement of the NMDA responses. 6 In summary, activation of 5-HT 2B receptors by α-Me-5-HT facilitates NMDA-depolarizations of frog motoneurones via a G-protein, a rise in [Ca 2+] i from the entry of extracellular Ca 2+ through L-type Ca 2+ channels, the binding of Ca 2+ to calmodulin and a lessening of the Mg 2+ -produced open-channel block of the NMDA receptor.