Depression of Spinal Sensory Transmission During REM Sleep: Dopaminergic Involvement and Insights Into Restless Legs Syndrome. Focus on “State-Dependent Changes in Glutamate, Glycine, GABA, and Dopamine Levels in Cat Lumbar Spinal Cord”

Abstract

Rapid-eye-movement (REM) sleep is also called paradoxical sleep because brain electroencephalographic (EEG) activity is similar to wakefulness. Aside from the obvious, one distinction between wakefulness and paradoxical sleep is the central suppression of both somatosensory and motor activity. The mechanisms that control these state-dependent changes appear to involve bulbospinal systems, while the contribution of diencephalospinal dopaminergic projections are not known. A recent article by Taepavarapruk et al. employed in vivo microdialysis and high-performance liquid chromatography (HPLC) analysis techniques to provide the first evidence of spinal cord amino acid transmitter and dopamine release related to these descending systems during naturally occurring sleep/wake cycles. The authors measured release in the vicinity of two ascending tracts, the dorsal spinocerebellar tract (DSCT; Clarke’s column) and the spinoreticular tract. Both tracts receive multiconvergent sensory input from numerous sensory qualities. The DSCT is a major relay point for the processing of spinal proprioceptive sensory information (Bosco and Poppele 2001) while the spinoreticular tract (SRT) also encodes nociceptive stimuli (Willis. and Coggeshall 1991). Naturally occurring release profiles were also compared with that seen during stimulation of the nucleus reticularis gigantocellularis (NRGc), an area known to inhibit both motoneurons and activity in the aforementioned ascending tracts. The authors tested whether changes in transmitter release during REM sleep are associated with the known reduction of sensory inflow to higher brain centers via the DSCT and SRT. When compared with wakefulness, whether during REM sleep or following NRGc stimulation, glutamate and glycine levels increased identically (48 or 69%, respectively) supporting release by common mechanisms. The neural pathways involved in this process therefore likely include reticulospinal and local interneuronal systems. In comparison, GABA levels increased only modestly during REM sleep and not at all following NRGc stimulation. More intriguingly, the authors also showed that dopamine levels significantly decreased during REM sleep when compared with wakefulness. Spinal dopamine is derived from the A11 dopaminergic nucleus of the dorsoposterior hypothalamus