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

1 Peptidergic neurones accumulate amines via an unusual uptake process, designated Transport-P. [H-3]-prazosin binds to alpha (1) adrenoceptors on these cells and is displaceable by unlabelled prazosin in concentrations up to 10(-7) M. However, at greater concentrations of prazosin, there is a paradoxical accumulation of [H-3]-prazosin which we have attributed to Transport-P. Uptake of prazosin via Transport-P is detectable at 10(-10) M prazosin concentration, is linear up to 10(-7) M and at greater concentrations becomes non-linear. In contrast, in noradrenergic neurones, noradrenaline uptake is linear and saturates above 10(-7) M. In noradrenergic neurones and in non-neuronal cells, there is no uptake of prazosin in concentrations up to 10(-6) M, suggesting that Transport-P is a specialised function of peptidergic neurones. 2 Using a mouse peptidergic (gonadotrophin-releasing hormone, GnRH) neuronal cell line which possesses Transport-P, we have studied the interaction of alpha (1) adrenoceptors with Transport-P. Polymerase chain reactions and DNA sequencing of the products demonstrated that only the alpha (1B) sub-type of adrenoceptors is present in GnRH cells. 3 In COS cells transfected with dib adrenoceptor cDNA and in DDT1 MF-2 cells which express native alpha (1B) adrenoceptors, [H-3]-prazosin was displaced by unlabelled prazosin in a normal equilibrium process, with no prazosin paradox in concentrations up to 10(-6) M. In DDT1 MF-2 cells: [H-3]-prazosin was displaced likewise by a series of alpha (1) adrenergic agonists, none of which increased the binding of [H-3]-prazosin. Hence, the prazosin paradox is not due to some function of alpha (1) adrenoceptors, such as internalization of ligand-receptor complexes. 4 In neurones which possess Transport-P, transfection with alpha (1b) adrenoceptor cDNA resulted in over-expression of alpha (1B) adrenoceptors, but the prazosin paradox was unaltered. Thus, al adrenoceptors and Transport-P mediate distinct functions in peptidergic neurones.