alpha(1B) adrenergic receptors in gonadotrophin-releasing hormone neurones: relation to Transport-P.
1. Peptidergic neurones accumulate amines via an unusual uptake process, designated Transport-P. [(3)H]-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 [(3)H]-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 alpha(1b) adrenoceptor cDNA and in DDT(1) MF-2 cells which express native alpha(1B) adrenoceptors, [(3)H]-prazosin was displaced by unlabelled prazosin in a normal equilibrium process, with no prazosin paradox in concentrations up to 10(-6) M. In DDT(1) MF-2 cells, [(3)H]-prazosin was displaced likewise by a series of alpha(1) adrenergic agonists, none of which increased the binding of [(3)H]-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, alpha(1) adrenoceptors and Transport-P mediate distinct functions in peptidergic neurones.
Al-Damluji
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Division of Endocrinology, Royal Free and University College Medical School, London. damluji@rfhsm.ac.uk
Shen
W B
WB
White
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S
Barnard
E A
EA
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism / Q000502:physiology*; D002460:Cell Line; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D009474:Neurons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D009638:Norepinephrine / Q000378:metabolism; D016133:Polymerase Chain Reaction; D011224:Prazosin / Q000378:metabolism / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis; D018340:Receptors, Adrenergic, alpha-1 / Q000235:genetics / Q000378:metabolism / Q000502:physiology
D000317:Adrenergic alpha-Antagonists; D002352:Carrier Proteins; D012333:RNA, Messenger; D018340:Receptors, Adrenergic, alpha-1; D007987:Gonadotropin-Releasing Hormone; D004247:DNA; D009638:Norepinephrine; D011224:Prazosin