Functional properties of the uptake of amines in immortalised peptidergic neurones (transport‐P)

1 Most neurotransmitters are inactivated by uptake into presynaptic nerve terminals and into glial cells. We recently provided evidence for uptake of amines in postsynaptic neurones. Uptake was evident at nanomolar concentrations of prazosin, but at concentrations of unlabelled prazosin greater than 10−7 m, there was a further activation of uptake, manifested by a paradoxical increase in accumulation of the radioligand. We have now studied further characteristics of amine uptake in immortalised gonadotrophin‐releasing hormone (GnRH) neurones. Control cells included SK‐N‐SH neuroblastoma cells (which possess presynaptic type amine transporters) and non‐neuronal (COS‐7) cells. 2 [3H]‐prazosin bound to intact GnRH cells and was displaced by unlabelled prazosin in concentrations of 10−9 to 10−7 m. However, at higher concentrations of unlabelled prazosin, there was an increase in apparent [3H]‐prazosin binding, as we had previously described. This paradoxical increase in accumulation of the radioligand was abolished by desipramine. 3 Desipramine had no effect on the association of prazosin with COS‐7 cells. There was no paradoxical increase in accumulation of [3H]‐prazosin in COS‐7 cells, indicating that this effect requires the presence of a desipramine‐blockable uptake process. 4 The increase in binding of the radioligand that was observed in the GnRH cells is not a general property of neuronal transporters; in SK‐N‐SH cells, there was no increase in accumulation of (−)−[3H]‐noradrenaline in the presence of concentrations of unlabelled (−)−noradrenaline greater than 10−7 m. 5 The uptake of prazosin and the increase in accumulation of [3H]‐prazosin were abolished in the cold, indicating that this is an active, energy‐requiring process. 6 Desipramine‐sensitive uptake of prazosin was demonstrable in the GnRH cells in the absence of sodium. Further, the Na+/K+‐ATPase inhibitor, vanadate, abolished noradrenaline uptake in SK‐N‐SH cells but had no effect on prazosin uptake in GnRH cells. Thus, the uptake of prazosin does not derive its energy from the sodium pump. 7 Prazosin uptake was inhibited by the V‐ATPase inhibitor bafilomycin A1, the H+/Na+ ionophore, monensin and the organic base, chloroquine, indicating that uptake derives its energy from a proton pump. In contrast to other proton‐dependent amine transporters, the uptake of prazosin was unaffected by reserpine. 8 Increasing extracellular pH did not increase the uptake of prazosin into GnRH cells, indicating that it is unlikely to be due to non‐specific diffusion and concentration of a lysosomotropic drug into intracellular acidic particles. 9 The uptake of prazosin was unaffected by steroid hormones. 10 In COS‐7 cells transfected with α1‐adrenoceptor cDNA, [3H]‐prazosin was displaced by unlabelled prazosin without causing an increase in binding of the radioligand. This indicated that the increase in accumulation of the radioligand is unlikely to be due simply to some function of α1‐adrenoceptors. 11 Thus, peptidergic neurones possess an uptake process with properties that are distinguishable from known amine transporters.