The analgesic properties of the opium poppy Papever somniferum were first mentioned by Hippocrates around 400 BC, and opioid analgesics remain the mainstay of pain management today. These drugs can cause the serious side-effect of respiratory depression that can be lethal with overdose, however the critical brain sites and neurochemical identity of the neurons mediating this depression are unknown. By locally manipulating neurotransmission in the adult rat, we identify the critical site of the medulla, the preBötzinger complex, that mediates opioid-induced respiratory depression in vivo. Here we show that opioids at the preBötzinger complex cause respiratory depression or fatal apnea, with anesthesia and deep-sleep being particularly vulnerable states for opioid-induced respiratory depression. Importantly, we establish that the preBötzinger complex is fully responsible for respiratory rate suppression following systemic administration of opioid analgesics. The site in the medulla most sensitive to opioids corresponds to a region expressing neurokinin-1 receptors, and we show in rhythmically active brainstem section in vitro that neurokinin-1 receptor-expressing preBötz-inger complex neurons are selectively inhibited by opioids. In summary, neurokinin-1 receptor-expressing preBötzinger complex neurons constitute the critical site mediating opioid-induced respiratory rate depression, and the key therapeutic target for its prevention or reversal.

Identification of an endogenous noradrenergic drive contributing to GG activation in wakefulness and non-REM sleep, but not REM sleep, is important given the prevalence and clinical significance of sleep-induced hypoventilation and obstructive sleep apnea in humans and the potential for pharmacologic treatment.

The genioglossus (GG) muscle of the tongue contributes to effective lung ventilation by maintaining an open pharyngeal airway. Decreased GG activity in sleep, especially REM sleep (Sauerland & Harper, 1976) can lead to airway narrowing, increased upper airway resistance and hypoventilation (Henke et al. 1992). In individuals with already anatomically narrow upper airways, such GG suppression can produce airway occlusion and obstructive sleep apnoea (Remmers et al. 1978), a serious sleep-related breathing disorder affecting approximately 4 % of adults (Young et al. 1993). However, despite increased knowledge of the major effects of sleep on GG activity, it is still not known which brainstem neural circuits and neurotransmitters modulate hypoglossal (XII) motor output to GG muscle in wakefulness and natural sleep.In vitro studies using neonatal tissue slices have shown that 5-HT depolarizes and increases the excitability of XII motoneurons (Berger et al. 1992). 5-HT also facilitates XII motoneurons in decerebrate cats (Kubin et al. 1992;Douse & White, 1996). Medullary raphe neurons provide the 5-HT inputs to XII motor nucleus (Manaker & Tischler, 1993) and show decreasing discharge from wakefulness to non-REM and REM sleep (Jacobs & Azmitia, 1992). There is also decreased discharge of medullary raphe neurons projecting to XII motor nucleus in a pharmacological model of REM sleep evoked by carbachol microinjection into the pontine reticular formation of decerebrate cats (Woch et al. 1996). This pharmacological REM-like state in decerebrate cats is also associated with reduced 5-HT at the XII motor nucleus (Kubin et al. 1994). 1. Serotonin (5-hydroxytryptamine, 5-HT) excites hypoglossal (XII) motoneurons in reduced preparations, and it has been suggested that withdrawal of 5-HT may underlie reduced genioglossus (GG) muscle activity in sleep. However, systemic administration of 5-HT agents in humans has limited effects on GG activity. Whether 5-HT applied directly to the XII motor nucleus increases GG activity in an intact preparation either awake or asleep has not been tested.2. The aim of this study was to develop a novel freely behaving animal model for in vivo microdialysis of the XII motor nucleus across sleep-wake states, and test the hypothesis that 5-HT application will increase GG activity.3. Eighteen rats were implanted with electroencephalogram and neck muscle electrodes to record sleep-wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the XII motor nucleus and perfused with artificial cerebrospinal fluid (ACSF) or 10 mM 5-HT.4. Normal decreases in GG activity occurred from wakefulness to non-rapid eye movement (non-REM) and REM sleep with ACSF (P < 0.01). Compared to ACSF, 5-HT caused marked GG activation across all sleep-wake states (increases of 91-251 %, P < 0.015). Importantly, 5-HT increased sleeping GG activity to normal waking levels for as long as 5-HT was applied (3-5 h). Despite tonic stimulation by 5-HT, periods of phasic GG ...

The effects of sleep on the ventilatory responses to hypercapnia have been well described in animals and in humans. In contrast, there is little information for genioglossus (GG) responses to a range of CO(2) stimuli across all sleep-wake states. Given the notion that sleep, especially rapid eye movement (REM) sleep, may cause greater suppression of muscles with both respiratory and nonrespiratory functions, this study tests the hypothesis that GG activity will be differentially affected by sleep-wake states with major suppression in REM sleep despite excitation by CO(2). Seven rats were chronically implanted with electroencephalogram, neck, GG, and diaphragm electrodes, and responses to 0, 1, 3, 5, 7, and 9% CO(2) were recorded. Diaphragm activity and respiratory rate increased with CO(2) (P < 0.001) across sleep-wake states with significant increases at 3-5% CO(2) compared with 0% CO(2) controls (P < 0.05). Phasic GG activity also increased in hypercapnia but required higher CO(2) (7-9%) for significant activation (P < 0.05). Further studies in 15 urethane-anesthetized rats with the vagi intact (n = 6) and cut (n = 9) showed that intact vagi delayed GG recruitment with hypercapnia but did not affect diaphragm responses. In the naturally sleeping rats, we also showed that GG activity was significantly reduced in non-REM and REM sleep (P < 0.04) and was almost abolished in REM even with stimulation by 9% CO(2) (decrease = 80.4% vs. wakefulness). Such major suppression of GG activity in REM, even with significant respiratory stimulation, may explain why obstructive apneas are more common in REM sleep.

The results show a minimal endogenous serotonergic drive at the HMN modulating GG activity across sleep-wake states, unless augmented by reflex inputs. This result has implications for pharmacologic strategies aiming to increase GG activity by manipulating endogenous serotonin in patients with obstructive sleep apnea.