Stimulated [S-35]GTP gamma S binding by 5-HT1A receptor agonists in recombinant cell lines - Modulation of apparent efficacy by G-protein activation state

G-protein activation by different 5-HT receptor ligands was investigated in h5-HT1A receptor-transfected C6-glial and HeLa cells using agonist-stimulated [S-35]GTP gamma S binding to membranes in the presence of excess GDP.5-HT (10 mu M) stimulated [S-35]GTP gamma S binding in the C6-glial membrane preparation to a larger extent than in the HeLa preparation; maximal responses with 30 mu M GDP were 490 +/- 99 and 68 +/- 12%, respectively. With the 5-HT receptor agonists that were being investigated, the two preparations displayed the same rank order of potency for stimulation of [S-35]GTP gamma S binding. In the C6-glial preparation at 0.3 mu M GDP, the rank order of maximal effects was: 5-HT (1.00) > 8-OH-DPAT (0.90) = R(+)-8-OH-DPAT (0.87)= 5-CT (0.86) = L694247 (0.84) > S(-)8-OH-DPAT (0.68) = buspirone (0.67) = spiroxatrine (0.67) = flesinoxan (0.64) > ipsapirone (0.53) = (-)pindolol (0.50) > SDZ216525 (0.25). However, differences in maximal response in the C6-glial preparation were magnified by increasing the GDP concentrations, indicating that the activity state of G-proteins can affect the maximal response. With the exception of 5-CT and L694247, increasing the amount of GDP to 30 mu M and higher concentrations resulted in an attenuation of both the ligand's maximal effect (24 to 56%) and apparent potency (6 to 24-fold). Each of the [S-35]GTP gamma S binding responses was mediated by a 5-HT1A receptor as indicated by the competitive blockade by WAY100635 and spiperone. Only 5-CT and L694247 in some conditions displayed an efficacy similar to that of 5-HT at the h5-HT1A receptor; the other agents with intrinsic activity are partial agonists at this receptor. The data also suggest that the activity state of the G-proteins is involved in the maximal effects that can be produced by activating the hS-HT1A receptor.