In vivo release of acetylcholinesterase in cat substantia nigra and caudate nucleus.

Abstract

Acetylcholinesterase (AChE), which is known to inactivate acetylcholine (ACh), is present in great abundance in the substantia nigra, although ACh levels and choline acetylase activity in this region are relatively low1. Nigral dopaminergic cell bodies and their dendrites also contain AChE2–4. The functional significance of this enzyme in nigro-striatal dopaminergic neurones has been questioned1,4,5. Earlier studies demonstrated an evoked release of AChE from unidentified central neurones into cerebrospinal fluid (CSF) in cats6, rabbits7,8 and dogs9. Later experiments have provided indirect evidence that the substantia nigra may contribute to a substantial amount of AChE detected: a unilateral nigral lesion in rabbits reduced AChE levels in the CSF, whereas electrical stimulation of the substantia nigra induced the opposite effect10. To investigate the possible release of AChE from dopaminergic dendrites and terminals we measured the in vivo release of this enzyme from the substantiae nigrae and caudate nuclei of cats implanted with four push–pull cannulae and compared it with that of dopamine (DA). DA is released from dendrites in the substantia nigra11,12 as well as nerve terminals in the caudate nucleus. Spontaneous AChE release was observed in the substantia nigra and in the caudate nucleus. Moreover, the application of potassium (30 mM) in one substantia nigra increased the local release of AChE. This was accompanied by remote changes in the enzyme release from the other three structures which differed from that seen for DA. The different patterns of responses observed for AChE and DA suggest that AChE may also originate from other neurones in both the substantia nigra and the caudate nucleus.