Actions of acetylcholinesterase in the guinea-pig cerebellar cortex in vitro.
Acetylcholinesterase is released in a calcium-dependent manner when afferents of the cerebellar cortex are stimulated. Since cholinergic transmission is probably insignificant in the cerebellar cortex, the esterase itself might serve as a transmitter or modulator. Therefore, the effect of acetylcholinesterase in the cerebellum was investigated in slices of guinea-pig cerebella during intracellular recording from Purkinje cell somata or dendrites. Addition of acetylcholinesterase (20 U/ml) to the superfusion medium did not change the membrane potential or the input resistance of the Purkinje cells. Thus, esterase does not act like a classical transmitter. The threshold for Na+ spikes generated by intracellular current injection was unaffected, but the threshold for Ca2+ spikes was increased. This increase was abolished by tetrodotoxin (1 microM). Furthermore, when Ca2+ currents were blocked by substituting Mn2+ for Ca2+ (2 mM) a decrease in a Na+ plateau potential was seen in the presence of esterase. The effect of acetylcholinesterase of Ca2+ spikes is therefore most likely due to a reduction of the non-inactivating Na+ current of the Purkinje cell membrane. When present this current contributes to activation of Ca2+ spikes in dendrites. Acetylcholinesterase also enhanced the response of Purkinje cells to the excitatory amino acids glutamate and aspartate thought to be transmitters in the cerebellar cortex. The responses became larger and faster in the presence of esterase. Responses to climbing fibre stimulation were also enhanced by acetylcholinesterase. The late part of this synaptic response was increased. The potentiation by esterase of responses of Purkinje cells to excitatory amino acids and to climbing fibre stimulation may be mediated through interference with transmitter uptake, because it was prevented by treatment with DL-2-amino-4-phosphonobutyric acid (0.5 mM) and di-hydrokainate (0.1 mM). None of the effects of esterase was due to hydrolysis of acetylcholine because irreversible inhibition of the catalytic site of the enzyme with soman did not prevent the actions. The observations were specific for acetylcholinesterase. Butyrylcholinesterase (20-40 U/ml) showed none of the effects. It is concluded that acetylcholinesterase in the cerebellar cortex seems to mediate a novel type of modulation by two separate mechanisms. Esterase reduces the tendency towards Ca2+ spike generation in Purkinje cells. Ca2+ spikes are followed by afterhyperpolarizations and in their absence firing of Na+ spikes at higher frequencies is possible. Secondly, there is an enhancement of the action of excitatory transmitters so that the extended operating range can be utilized.[1]References
- Actions of acetylcholinesterase in the guinea-pig cerebellar cortex in vitro. Appleyard, M., Jahnsen, H. Neuroscience (1992) [Pubmed]
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