Loss of awareness, anesthesia, memory replay, opioid-induced respiratory depression, and slow-wave sleep are caused by activation of the medial habenula–interpeduncular nucleus (MHb-IPN) circuit. Mu-opioids and anesthetics activate this circuit, slowing respiration and inducing unawareness. The MHb-IPN circuit promotes slow-wave sleep, memory replay, sharp-wave ripples, spindles, hippocampo-cortical replay of temporally, spatially, and relationally bound information, serotonin-BDNF-linked growth, synaptogenesis, rest, and recovery by activating median raphe serotonin and inhibiting the theta state circuit, new memory encoding, awareness, arousal, alert wakefulness, and REM sleep. Ketamine, nitrous oxide, and phencyclidine activate the IPN→median raphe→claustrum→cortical slow-wave activity circuit via 5-HT2a receptors, explaining their anxiolytic and antidepressant effects.
Mu-opioid activation of the medial habenula-interpeduncular nucleus circuit slows respiration and suppresses arousal, linking opioid-induced respiratory depression and prenatal nicotine exposure to sudden infant death syndrome. The same circuit, when activated during slow wave sleep, promotes memory replay, synaptogenesis, and immune defense while inhibiting theta states and REM sleep. Ketamine and hallucinogens may produce their anesthetic, anxiolytic, and antidepressant effects by activating this circuit via 5-HT2a receptors in the interpeduncular nucleus, claustrum, and infralimbic cortex, increasing safety signaling and cognitive flexibility.
The author proposes a neural model in which the medial habenula (MHb) to interpeduncular nucleus (IPN) circuit promotes slow-wave sleep, memory replay, and rest, while inhibiting awareness, arousal, and REM sleep. Activation of this circuit by mu-opioids, anesthetics, and other substances slows respiration and heart rate. The model also explains how ketamine and psychedelics may produce anxiolytic and antidepressant effects by activating 5-HT2a receptors in the IPN, claustrum, and infralimbic cortex, increasing feelings of safety and cognitive flexibility while attenuating fear and impulsivity.