Vasoactive intestinal polypeptide (VIP) stimulated adenosine 3':5'-cyclic monophosphate (cyclic AMP) production by cultured GH4ZD10 cells with an EC50 value of about 7 nM. The extracellularly recovered cyclic AMP predominated, and was reduced by co-incubation with 8-hydroxy-2-(di-n-propyl-amino) tetralin (8-OH-DPAT) and 5-hydroxytryptamine (5-HT), whereas dopamine (0.1-30 microM) did not reduce VIP-stimulated cyclic AMP production. 2. The responses to 5-HT and 8-OH-DPAT were blocked by (-)-alprenolol and NAN 190. The antagonism by (-)-alprenolol was competitive in nature with a pA2 value of 7.0. 3. The responsiveness of the cells to 5-HT agonists was highly dependent upon the culturing conditions used. Thus, 8-OH-DPAT inhibition of VIP (30 nM)-stimulated cyclic AMP production decreased with increasing passage number of the cells. Reduction of the zinc concentration used to promote expression of the 5-HT1A receptor gene produced a greater sensitivity of the cells to 5-HT agonists. 4. Under such conditions, the following efficacies (5-HT = 100) were found: lisuride 106, (+)-lysergic-acid diethylamide 100, 5-methoxy-N,N-dimethyltryptamine 98, RU 24949 98, 5-carboxamidotryptamine 97, (+/-)-8-OH-DPAT 90, (+)-8-OH-DPAT 87, 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)-piperazine 86, flesinoxan 79/88, (-)-8-OH-DPAT 62, buspirone 43/50, ipsapirone 46. Spiroxatrine and spiperone had a low intrinsic activity, but reduced the response to 5-HT. These efficacies are similar to those reported in the literature for post-synaptically localized 5-HT1A receptors in the rat hippocampus. Thus, the GH4ZD10 cells serve as a useful in vitro model system for these receptors.
