Fast inhibitory synapses: targets for neuromodulation and development of vertebrate motor behaviour.
Locomotor networks must possess the inherent flexibility to adapt their output. In this review we discuss evidence from a simple vertebrate locomotor network that suggests fast inhibitory synapses are important targets for the forms of neuromodulation that afford this flexibility. Two important inhibitory transmitters, glycine and GABA, are present in the CNS of Xenopus tadpoles, where they each play distinct roles in the control of swimming. Glycine, but not GABA, contributes to the inhibitory mid-cycle component of each swim-cycle, the strength of which determines the frequency of swimming. Meanwhile, GABA release onto the swim network prematurely terminates swimming episodes. Hence, glycine controls how fast, whilst GABA controls how far the tadpole swims. Our work has focused on how the amines serotonin (5-HT) and noradrenaline (NA), and more recently the gas nitric oxide (NO), selectively target glycine and GABA release in the spinal cord to modulate swimming. In particular, we have identified three brainstem populations of nitrergic neurons, which suggests that nitric oxide may co-localise with 5-HT, NA and GABA. Here we review this work and suggest a hierarchy of brainstem modulatory systems, with NO acting as a metamodulator.
Sillar
Keith T
KT
School of Biology, Division of Biomedical Sciences, University of St Andrews, Bute Medical Buildings, St Andrews, KY16 9TS, Fife, UK. kts1@st-andrews.ac.uk
McLean
David L
DL
Fischer
Hanno
H
Merrywest
Simon D
SD
D000818:Animals; D004525:Efferent Pathways / Q000502:physiology; D004625:Embryo, Nonmammalian; D005998:Glycine / Q000502:physiology; D009068:Movement / Q000502:physiology*; D009415:Nerve Net / Q000502:physiology; D009569:Nitric Oxide / Q000502:physiology; D009638:Norepinephrine / Q000502:physiology; D012701:Serotonin / Q000502:physiology; D013550:Swimming / Q000502:physiology*; D013569:Synapses / Q000502:physiology*; D014714:Vertebrates / Q000502:physiology*; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000502:physiology
D009569:Nitric Oxide; D012701:Serotonin; D005680:gamma-Aminobutyric Acid; D005998:Glycine; D009638:Norepinephrine