Noradrenergic Tone is Not Required for Neuronal Activity-Induced Rebound Sleep in Zebrafish

Sleep pressure builds during wakefulness, but the mechanisms underlying this homeostatic process are poorly understood. One zebrafish model suggests that sleep pressure increases as a function of global neuronal activity, such as during sleep deprivation or acute exposure to drugs that induce widespread brain activation. Given that the arousal-promoting noradrenergic system is important for maintaining heightened neuronal activity during wakefulness, we hypothesised that genetic and pharmacological reduction of noradrenergic tone during drug-induced neuronal activation would dampen subsequent rebound sleep in zebrafish larvae. Unexpectedly, dampening noradrenergic tone with the 2-adrenoceptor agonist clonidine during acute caffeine or pentylenetetrazol treatment enhanced subsequent rebound sleep, while stimulating noradrenergic signalling during caffeine exposure with a cocktail of 1- and {beta}-adrenoceptor agonists did not enhance sleep. Similarly, CRISPR/Cas9-mediated elimination of the dopamine {beta}-hydroxylase (dbh) gene, which encodes an enzyme required for noradrenalin synthesis, enhanced baseline sleep in larvae but did not prevent additional rebound sleep following acute induction of neuronal activity. Across all drug conditions, c-fos expression immediately after drug exposure varied inversely with noradrenergic tone and correlated strongly with the amount of induced rebound sleep. These results are consistent with a model in which increases in neuronal activity, as reflected by brain-wide levels of c-fos induction, drive a sleep pressure signal that promotes rebound sleep independently of noradrenergic tone.