Interplay between neuromodulator-induced switching of short-term plasticity at sensorimotor synapses in the neonatal rat spinal cord.
The Journal of Physiology. 2008-03-31; 586(7): 1903-1920
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In the present study, we investigated the modulation of short-term depression
(STD) at synapses between sensory afferents and rat motoneurons by serotonin,
dopamine and noradrenaline. STD was elicited with trains of 15 stimuli at 1, 5
and 10 Hz and investigated using whole-cell voltage-clamp recordings from
identified motoneurons in the neonatal rat spinal cord in vitro. STD was
differentially modulated by the amines. Dopamine was effective at all stimulation
frequencies, whereas serotonin affected STD only during 5 and 10 Hz stimulus
trains and noradrenaline during 1 and 5 Hz trains. Dopamine and serotonin
homogenized the degree of depression observed with the different stimulation
modalities, in contrast to noradrenaline, which amplified the rate differences.
The different modulatory profiles observed with the amines were partly due to
GABAergic interneuron activity. In the presence of GABA(A) and GABA(B) receptor
antagonists, the rate and/or kinetics of STD did not vary with the stimulation
frequency in contrast to the control condition, and noradrenaline failed to alter
either synaptic amplitude or STD, suggesting indirect actions. Dopamine and
serotonin strongly decreased STD and converted depression to facilitation at 5
and 10 Hz during the blockade of the GABAergic receptors in 50% of the neurons
tested. Altogether, these results show that STD expressed at sensorimotor
synapses in the neonatal rat not only is a function of the frequency of afferent
firing but also closely depends on the neuromodulatory state of these
connections, with a major contribution from GABAergic transmission.