Structural properties of phenylethylamine derivatives which inhibit transport‐P in peptidergic neurones

Abstract

1 Transport-P is an antidepressant-sensitive, proton-dependent, V-ATPase-linked uptake process for amines in peptidergic neurones of the hypothalamus. It is unusual in its anatomical location in postsynaptic neurones and in that it is activated by its substrate (prazosin). This study examined the structural properties of phenylethylamine derivatives which are substrates for transport-P, as judged by competitive inhibition of the uptake of prazosin 10 76 M in immortalized hypothalamic peptidergic neurones. 2 A basic amine was essential for activity; absence of the amine or neutralization with a carboxyl group abolished activity. Primary, secondary and tertiary amines were active but quaternary and guanyl amines were inactive. 3 A phenyl group was essential for activity at transport-P. Potency at transport-P was reduced by phenolic hydroxyl groups and enhanced by phenolic halogens. Thus, for maximal potency, the phenyl group should be hydrophobic. Phenolic methoxyl groups had no eect on potency at transport-P. 4 A side chain was necessary for activity at transport-P. Potency at transport-P was reduced by bhydroxyl and enhanced by a-methyl groups. 5 These ®ndings further distinguish transport-P from other amine uptake processes in the brain.

α1B adrenergic receptors in gonadotrophin‐releasing hormone neurones: relation to Transport‐P

“…The paradoxical increase in binding of [ 3 H]‐prazosin in the presence of greater concentrations of unlabelled prazosin is likely to be due, instead, to an unusual uptake process whose functional properties are distinct from those of α 1 adrenoceptors. Further, the structural properties of ligands for Transport‐P are different from those of ligands for α 1 adrenoceptors ( Al‐Damluji & Kopin, 1998 ).…”

“…It is therefore possible that the GT1‐1 and the GT1‐7 cell lines may have been derived from subsets of GnRH neurones which differ in the expression of α 2 adrenoceptors. In any case, presence of α 2 adrenoceptors does not account for the functional properties of Transport‐P for the following reasons: (a) The structural properties of ligands for Transport‐P are different from the properties of ligands for α 2 adrenoceptors ( Al‐Damluji & Kopin, 1998 ); (b) The α 2 adrenergic agonist clonidine does not cause a paradoxical increase in accumulation of [ 3 H]‐prazosin in the GnRH neurones (Figure 5B). The displacement of [ 3 H]‐prazosin by clonidine is attributable to the known agonist action of clonidine at α 1 adrenoceptors ( Nichols & Ruffolo, 1991 ).…”

“…Thus, there was no antidepressant‐blockable prazosin paradox in COS‐7 kidney cells or in DDT 1 MF‐2 smooth muscle cells (Figure 4B). Other features which distinguish Transport‐P from Uptake 1 , Uptake 2 and from vesicular uptake have been discussed ( Al‐Damluji & Kopin, 1998 ).…”

“…A considerable amount of evidence supported this second part of the hypothesis. Thus: (a) the paradoxical increase in binding of [ 3 H]‐prazosin is inhibited competitively by antidepressants which are known to inhibit the pre‐synaptic re‐uptake of amines ( Al‐Damluji et al ., 1993 ; Al‐Damluji & Kopin, 1996b ); (b) the paradoxical increase in accumulation of [ 3 H]‐prazosin was not seen in membrane preparations, indicating that it requires intact cells or storage organelles ( Al‐Damluji et al ., 1993 ); (c) the prazosin paradox is an energy‐dependent process, the source of energy being the electrochemical proton gradient which is generated by vacuolar‐type ATPase (V‐ATPase; Al‐Damluji & Kopin, 1996a ); (d) ligands for Transport‐P must possess a specific chemical structure, consisting of a hydrophobic phenyl group, an alkyl side chain and a basic amine ( Al‐Damluji & Kopin, 1998 ); (e) a fluorescent analogue of prazosin accumulated in intracellular vesicles, providing visual evidence for uptake ( Al‐Damluji et al ., 1997 ); (f) following accumulation by Transport‐P, the amines can be released by an energy‐dependent process ( Shen & Al‐Damluji, 1997 ).…”

Release of amines from acidified stores following accumulation by Transport‐P

“…These plasma membrane transporters can be blocked by antidepressants ( Glowinski & Axelrod, 1964 ; Giros et al ., 1994 ; Barker et al ., 1994 ). Transport‐P resembles pre‐synaptic plasma membrane transporters in its sensitivity to antidepressants but it differs as follows: Transport‐P is located in post‐synaptic peptidergic neurones ( Al‐Damluji et al ., 1997 ); in pre‐synaptic neurones, increasing concentrations of unlabelled neurotransmitter amine do not cause a paradoxical increase in accumulation of the radiolabelled ligand ( Al‐Damluji & Kopin, 1996a ); pre‐synaptic re‐uptake is linked to a P‐ATPase (phosphorylated‐type ATPase, the Na + /K + ATPase) whereas Transport‐P is linked to V‐ATPase ( Al‐Damluji & Kopin, 1996a ); Transport‐P is independent of sodium which is required for pre‐synaptic plasma membrane uptake ( Al‐Damluji & Kopin, 1996a ); efficacy at Transport‐P is reduced by the presence of phenolic hydroxyl groups in the ligands, whereas these phenolic hydroxyl groups increase efficacy at pre‐synaptic plasma membrane uptake ( Al‐Damluji & Kopin, 1998 ); efficacy at Transport‐P is unaffected by the presence of phenolic methoxyl groups in the ligands, whereas these phenolic methoxyl groups reduce efficacy at pre‐synaptic plasma membrane uptake ( Al‐Damluji & Kopin, 1998 ).…”