With the increasing prevalence [1], comorbidity with other diseases [3–5], and a lack of tolerable and effective treatment options [2, 6], OSA is a significant health problem. An impediment to effective treatments of OSA is insufficient knowledge of peripheral and central neural mechanisms of respiratory control, especially during sleep. Though we have shown previously that modulation of vagal afferents via activation of CB receptors located on nodose ganglion neurons can attenuate reflex apneas [14, 15], it was uncertain what role central CB receptors play in respiratory pattern control. Here we show that ICV injection of dronabinol, a non-specific agonist of CB₁ and CB₂ receptors, does not attenuate peripherally-induced reflex apneas.

Vagal afferent neurons, which relay important information about respiratory drive and upper airway muscle tone [16, 17, 36], provide input to the excitatory or inhibitory neurons located in nucleus of the solitary tract (NTS) [37, 38]. The NTS projects to the respiratory centers of the brainstem, including the hypoglossal nucleus (XII) and the rostral ventrolateral medulla (RVLM) [36, 39]. The NTS contains a variety of neuronal populations that fire at distinct points in the expiratory-inspiratory phases of breathing [40]. Reflex apneas are induced peripherally via the vagus nerve by pharmacologically activating excitatory receptors located on nodose ganglia, or by mechanically activating stretch receptors located in the lung [16, 41]. Reflex apneas are also induced “downstream” by activation of glutamatergic receptors in the NTS [42–45]. Conversely, vagally-induced apnea can be reversed by microinjection of a GABA agonist [46], or a glutamatergic NMDA antagonist [45], into the NTS. Vagal afferents also synapse in regions of the NTS that modulate sympathetic activity, with activation of vagal afferents not only inducing apnea, but decreasing heart rate and blood pressure [41]. This trifecta of responses, known as the Bezold-Jarisch reflex, is modulated by different regions of the NTS, and these regions, when activated or inhibited, induce variable responses in blood pressure, heart rate, and breathing [45–47]. The NTS contains both CB₁ and CB₂ receptors that, when activated, inhibit or promote NTS activity, and attenuate other vagally-mediated behaviors [26, 27, 48–51]. CB₁ activation decreases glutamate release in the NTS [51], thus CB₁ activation would have the effect of suppressing apneas. CB₁ activation also disinhibits second order NTS neurons by pre-synaptically decreasing release of GABA [51–53], thus CB₁ activation would have the opposite effect of potentiating apneas. It might be plausible that the dual effects of CB₁ activation occurred, with the consequence of neither suppressing nor potentiating apneas.

Moreover, the XII, which modulates the phasic and tonic activation of the genioglossus muscle [36], also contains CB₁ receptors [23, 25, 28, 54, 55], of which the physiologic role is unclear. CB₁ activation in the XII is known to disinhibit XII by preventing release of glycine, an inhibitory neurotransmitter [25, 54]. In fact, a CB agonist microinjected in the XII activated the genioglossus in awake, but not sleeping, rats [55]. A recent report showed cardiorespiratory anomalies, including unstable breathing and apneas, in CB₁ knockout mice [56]. We hypothesized that the activation of CB receptors located at crucial respiratory centers in the brainstem would modulate reflex apneas and increase genioglossus activity; however, we saw no effect of a centrally-administered CB agonist, dronabinol.

The present work focused on global activation of CB receptors located in the brain, which mimicked patients undergoing dronabinol treatment [12], and did not elucidate any specific local effects of the respiratory centers of the brainstem. Due to the variability of Bezold-Jarisch reflex responses to NTS activation or inhibition, and the location of CB receptors on both excitatory and inhibitory neurons of the NTS, it is possible that non-specific and global activation of central CB receptors via ICV injection of dronabinol led to multiple and potentially opposing responses, masking any specific local effects. Therefore no physiologic response was observed. For example, Padley et al. induced apnea by microinjecting a CB₁ agonist centrally into the RVLM, which sends projections to inhibitory neurons located in the respiratory centers of the brainstem [39]. Also, injection of CBs peripherally in the nodose ganglia increased genioglossus activity, presumably through disinhibition of parasympathetic input into respiratory centers of the brain [15]. Carley et al. showed respiratory stability with intraperitoneal injections of dronabinol, which would involve both peripheral and central CB receptors [13]. Moreover, low and high doses of CBs are known to have biphasic effects [31, 57]; however, we saw no changes in apnea response at low or high doses of dronabinol. Since CB receptors are widely distributed in the brain, on both excitatory and inhibitory neurons, and are activated differentially by different concentrations of CBs, specific microinjections in the NTS or XII will need to be completed to see if CBs have any effect on breathing and genioglossus activity in rats.