The Journal of Neuroscience, January 1, 2003, 23(1):332-338
Modulatory Effects of 
1-, 2-, and
-Receptor Agonists on Feline Spinal Interneurons with
Monosynaptic Input from Group I Muscle Afferents
Ingela
Hammar and
Elzbieta
Jankowska
Department of Physiology, Göteborg University, 405 30 Göteborg, Sweden
Previous studies have shown that monoamines may modulate operation
of spinal neuronal networks by depressing or facilitating responses of
the involved neurons. Recently, activation of interneurons mediating
reciprocal inhibition from muscle spindle (Ia) afferents and
nonreciprocal inhibition from muscle spindle and tendon organ (Ia/Ib)
afferents in the cat was found to be facilitated by noradrenaline (NA).
However, which subclass membrane receptors are involved in mediating
this facilitation was not established; the aim of the present
experiments was to investigate this. Individual Ia- and
Ia/Ib-inhibitory interneurons were identified in the cat lumbar spinal
cord, and NA agonists were applied close to these neurons by
ionophoresis. The agonists included the 
1-receptor agonist phenylephrine, the 2-receptor agonists clonidine and tizanidine, and
the 
-receptor agonist isoproterenol. Effects were measured by
comparing changes in the number of extracellularly recorded spike
potentials evoked by electrical stimulation of muscle nerves and
changes in the latency of these potentials before, during, and after
application of the tested compounds. Results show that the facilitatory
effect of phenylephrine is as strong as that of NA, whereas the
facilitatory effect of isoproterenol is weaker. Clonidine depressed
activity of both Ia- and Ia/Ib-inhibitory interneurons, whereas
tizanidine had no effect. These findings lead to the conclusion that
beneficial antispastic effects of clonidine and tizanidine in humans
are unlikely to be associated with an enhancement of the actions of Ia-
and Ia/Ib-inhibitory interneurons, and the findings also support
previous proposals that these compounds exert their antispastic actions
via effects on other neuronal populations.
Key words:
spinal cord; spinal reflexes; cat; group I afferents; spasticity; noradrenaline; clonidine; tizanidine; phenylephrine; isoproterenol
Copyright © 2003 Society for Neuroscience 0270-6474/03/231332-07$05.00/0
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