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Volume 17, Number 5,
Issue of March 1, 1997
pp. 1786-1794
Copyright ©1997 Society for Neuroscience
5-HT Inhibits Calcium Current and Synaptic Transmission from
Sensory Neurons in Lamprey
Received Sept. 9, 1996; revised Dec. 9, 1996; accepted Dec. 11, 1996.
Abdeljabbar El Manira,
Weiqi Zhang,
Erik Svensson, and
Nathalie Bussières
The Nobel Institute for Neurophysiology, Department of
Neuroscience, Karolinska Institutet, S-171 77 Stockholm, Sweden
In the lamprey spinal cord, 5-hydroxytryptamine (5-HT)
immunoreactivity (ir) is present in the ventromedial plexus originating from intraspinal neurons, ventrolateral column arising from the brainstem, and dorsal column. The latter 5-HT system originates from
small dorsal root ganglion neurons. Combined Lucifer yellow intracellular labeling of the intraspinal sensory neurons, dorsal cells, and 5-HT immunohistochemistry showed close appositions between
5-HT-ir fibers and dorsal cell axons. Application of 5-HT depressed
monosynaptic EPSPs evoked in giant interneurons by stimulation of
single dorsal cells, dorsal roots, or dorsal column without any
detectable change in the input resistance of postsynaptic neurons.
Furthermore, the amplitude of AMPA-evoked depolarizations in giant
interneurons was unaffected by 5-HT. The lack of postsynaptic effects
of 5-HT indicates that the decrease of the amplitude of sensory
monosynaptic EPSPs by 5-HT is mediated by presynaptic mechanisms. The
inhibition of monosynaptic EPSPs by 5-HT was not counteracted by an
antagonist of 5-HT1A receptors. 5-HT also reduced the
amplitude of the calcium current recorded in isolated dorsal cells and
slowed down its kinetics. The inhibition of calcium channels could
represent the mechanism mediating the depression of synaptic
transmission at the axonal level. These results show that activation of
5-HT receptors on dorsal cell axons as well as on other sensory neurons
mediates inhibition of sensory synaptic transmission to giant
interneurons. In intact animals, 5-HT could be released from small 5-HT
neurons in dorsal root ganglia, which thus may underlie direct
sensory-sensory interactions.
Key words:
synaptic transmission;
5-HT;
presynaptic inhibition;
modulation;
calcium channels;
spinal cord;
locomotion
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