The adult human typically exhibits a monophasic sleep-wake cycle, i.e., remains awake and alert for approximately 16 hours and then sleeps for 8 hours. Recent experiments have provided new insights in the role of the endogenous circadian pacemaker in this consolidation of sleep and wakefulness.

Sleep deprivation studies had shown previously that sleepiness and alertness are co-determined by a process which keeps track of the history of sleep and wakefulness and the circadian pacemaker, which keeps track of time. During every day life and during sleep deprivation both processes change simultaneously and their relative contribution to alertness and sleep propensity cannot be assessed under these conditions.

It has been demonstrated in juvenile rodents that the inhibitory neurons of the nucleus reticularis thalami (NRT) communicate with each other via connexin 36-based electrical synapses. However, whether functional electrical synapses persist into adulthood is not fully known. Here we show that in the presence of the metabotropic glutamate receptor agonists, either trans-ACPD (100 μM) or DHPG (100 μM), 15% of neurons in slices of the adult cat NRT maintained in vitro exhibit stereotypical spikelets with several properties that indicate that they reflect action potentials that have been communicated through an electrical synapse. In particular, these spikelets (1) display a conserved, all-or-nothing waveform with a pronounced after-hyperpolarization (AHP), (2) exhibit an amplitude and time to peak that are unaffected by changes in membrane potential, (3) always occur rhythmically with the precise frequency increasing with depolarization, and (4) are resistant to blockers of conventional, fast, chemical synaptic transmission. Thus, these results indicate that functional electrical synapses in the NRT persist into adulthood where they are likely to serve as an effective synchronizing mechanism for the wide variety of physiological and pathological rhythmic activities displayed by this nucleus.