Title:
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Neurotransmission in the rodent anococcygeus: High resolution studies of autonomic neuroeffector mechanisms
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The mechanisms of autonomic neuroeffector transmission were investigated in the rat
anococcygeus using fluorescent Ca2+ imaging, electrophysiology with sharp
microelectrodes and contraction studies. The postjunctional effects of electricallystimulated
release of neurotransmitters and exogenous agonists on the smooth muscle
tissue were the particular focus of this study. Possible mechanisms underlying the
relationship between Ca2+ signals and electrical activity, and synchronisation of
activity between the smooth muscle cells in the smooth muscle cells, are discussed.
A summary ofthe key experimental results follows:
Brief periods of electrical field stimulation ~EFS) of the nerves produced transient
changes in intracellular cytosolic [Ca2l ([Ca li) in the smooth muscles. These took
the fonn of focal transients, propagating intracellular waves and global transients that
were synchronised between many contiguous smooth muscle cells. The waves and
global transients were abolished by the a-adrenoceptor antagonist prazosin, though
some focal events persisted. Mono- and bi-phasic depolarisations of the smooth
muscle membrane potential, tenned excitatory junction potentials (EJPs), were also
evoked by electrical stimulation. The a-adrenoceptor agonist, phenylephrine, at
100 nM produced asynchronous Ca2+ waves, while brief periods of EFS in the
presence of 100 nM phenylephrine produced synchronised global oscillations in
[Ca2+]j, which were accompanied by oscillations in membrane potential, indicating
that a coupling mechanism exists between the smooth muscle cells.
In intact tissue, Ca2+ waves and biphasic EJPs were not observed to be coupled at the
level of the individual cell. While the rapid phase (first component) of the biphasic
EJP in response to very short periods ofEFS is sensitive to caffeine and appears to be
mediated by release of Ca2+ from intracellular stores, acting on Ca2+-activated cr
channels, the depolarisation that is measured seems to be the sum of events in a pool
of electrically-coupled smooth muscle cells. Synchronised global transients were
associated with EJPs with a large first component, and were reduced in amplitude but
not abolished by nifedipine, though they were not observed in the presence of a
combination of caffeine and nifedipine, suggesting that L-type Ca2+ channels
contribute to these events but are not essential for their generation.
Guanethidine or amphetamine, which displace noradrenaline from sympathetic nerve
terminals, produced Ca2+ waves and synchronised global oscillations in [Ca2li. In
the presence of guanethidine, which also prevents evoked neurotransmitter release
from these tenninals, EFS produced temporary inhibition of the oscillation, stopping
it or slowing its rate and reducing its amplitude, though after the end of EFS the
oscillation usually returned with a slightly raised amplitude. These effects were
mediated largely by inhibitory nitrergic neurotransmission, because they were greatly
reduced by the inhibitor ofnitric oxide synthase, L_NG-nitro-arginine-methyl-ester.
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