Figure 9.

Summary diagram illustrating the concerted metamodulation of spinal inhibition by nitric oxide. At larval stage 42 (top schematic), the major subdivisions of the CNS (middle schematic) are recognizable, including the forebrain (fb), midbrain (mb), hindbrain (hb), and spinal cord (sc). Brain regions are separated by dotted lines. The locus ceruleus (in black) is located just caudal to the midbrain-forebrain boundary and is the major source of spinal NA (González et al., 1994). The mid-hindbrain reticulospinal (Mhr) region (gray) is located dorsally in the caudal hindbrain, with axons that cross the midline (dashed line) and descend to spinal cord, providing a major source of spinal GABAergic inhibition (Roberts et al., 1987). Commissural interneurons (in light gray) cross the midline and provide the major source of spinal glycinergic inhibition (Dale et al., 1986). NO modulates swimming frequency by indirectly decreasing or increasing endogenous spinal NA release (in black), which in turn modulates spinal glycine release (in light gray) to motor neurons (MN). NO modulates episode durations by directly modulating GABA release (in gray) to motor neurons. However, the nitrergic modulation of GABA could also affect swimming frequency (see Discussion for details).