Brain Research, 142 (1978) 53-68 © Elsevier/North-HollandBiomedicalPress
53
BIOCHEMICAL EVIDENCE FOR THE PRESENCE OF PRESYNAPTIC RECEPTORS ON DOPAMINERGIC NERVE TERMINALS
J.
DE BELLEROCHEand H.
F.
BRADFORD
Department of Biochemistry, Imperial College, London SW7 (England)
(Accepted June 2nd, 1977)
SUMMARY Two compartments of striatal synaptosome dopamine were identified by differential labelling with the isotopic precursors, L-tyrosine and DOPA, and from specific radioactivity measurement. Either, endogenous or exogenous e-tyrosine could provide a source for the dopamine pool synthesised and released in response to K + depolarization, whereas external DOPA did not enter this pool. Acetylcholine (0.1 mM) in the presence of neostigmine (0.1 mg/ml) increased dopamine turnover as shown by increased formation of [14C]dopamine and [14C]DOPAC from [14C]DOPA. Haloperidol (0.65 mM) did not affect the size of dopamine pools but increased the conversion of [14C]DOPA to [14C]dopamine and the formation of [14C]DOPAC. Acetylcholine stimulated the release of dopamine from synaptosomes, which effect could be modified by both muscarinic and nicotinic antagonists. In the presence of the muscarinic antagonist, atropine, acetylcholine stimulated dopamine release, whereas in the presence of the nicotinic antagonists, hexamethonium (0.2 mM) or a-bungarotoxin (0.188 /~M), acetylcholine inhibited dopamine release. This showed that presynaptic cholinergic receptors were operational, excitatory nicotinic receptors in the former case and inhibitory muscarinic in the latter. The nicotinic receptor was shown to be saturable and to bind specifically 11.2 fmoles of [3H]a-bungarotoxin per rag. protein which could be prevented by hexamethonium or D-tubocurarine.
INTRODUCTION A substantial interdependence between dopaminergic and cholinergic transmission in the neostriatum is suggested from a number of observations such as the beneficial effects of anticholinergic drugs (e.g., atropine and benztropine) in reducing rigidity and tremor in Parkinson's disease ~0. Anticholinergic drugs, e.g. atropine have been shown to decrease dopamine turnover in the striatum and limbic system47. On the
54 other hand, drugs which favour cholinergic systems such as physostigmine increase firing of dopamine sensitive caudate cells 6 and systemically administered physostigmine and oxotremorine increase striatal and limbic dopamine turnover:'. ~:. Effects of dopamine and agents affecting dopaminergic transmission, on the turnover and release of acetylcholine in the striatum have also been demonstrated l.'~',e
53
BIOCHEMICAL EVIDENCE FOR THE PRESENCE OF PRESYNAPTIC RECEPTORS ON DOPAMINERGIC NERVE TERMINALS
J.
DE BELLEROCHEand H.
F.
BRADFORD
Department of Biochemistry, Imperial College, London SW7 (England)
(Accepted June 2nd, 1977)
SUMMARY Two compartments of striatal synaptosome dopamine were identified by differential labelling with the isotopic precursors, L-tyrosine and DOPA, and from specific radioactivity measurement. Either, endogenous or exogenous e-tyrosine could provide a source for the dopamine pool synthesised and released in response to K + depolarization, whereas external DOPA did not enter this pool. Acetylcholine (0.1 mM) in the presence of neostigmine (0.1 mg/ml) increased dopamine turnover as shown by increased formation of [14C]dopamine and [14C]DOPAC from [14C]DOPA. Haloperidol (0.65 mM) did not affect the size of dopamine pools but increased the conversion of [14C]DOPA to [14C]dopamine and the formation of [14C]DOPAC. Acetylcholine stimulated the release of dopamine from synaptosomes, which effect could be modified by both muscarinic and nicotinic antagonists. In the presence of the muscarinic antagonist, atropine, acetylcholine stimulated dopamine release, whereas in the presence of the nicotinic antagonists, hexamethonium (0.2 mM) or a-bungarotoxin (0.188 /~M), acetylcholine inhibited dopamine release. This showed that presynaptic cholinergic receptors were operational, excitatory nicotinic receptors in the former case and inhibitory muscarinic in the latter. The nicotinic receptor was shown to be saturable and to bind specifically 11.2 fmoles of [3H]a-bungarotoxin per rag. protein which could be prevented by hexamethonium or D-tubocurarine.
INTRODUCTION A substantial interdependence between dopaminergic and cholinergic transmission in the neostriatum is suggested from a number of observations such as the beneficial effects of anticholinergic drugs (e.g., atropine and benztropine) in reducing rigidity and tremor in Parkinson's disease ~0. Anticholinergic drugs, e.g. atropine have been shown to decrease dopamine turnover in the striatum and limbic system47. On the
54 other hand, drugs which favour cholinergic systems such as physostigmine increase firing of dopamine sensitive caudate cells 6 and systemically administered physostigmine and oxotremorine increase striatal and limbic dopamine turnover:'. ~:. Effects of dopamine and agents affecting dopaminergic transmission, on the turnover and release of acetylcholine in the striatum have also been demonstrated l.'~',e
