Life-threatening side effects such as profound bradypnea or apnea and variable upper airway obstruction limit the use of opioids for analgesia. It is yet unclear which sites containing μ-opioid receptors (μORs) within the intact in vivo mammalian respiratory control network are responsible. The purpose of this study was 1) to define the pontine region in which μOR agonists produce bradypnea and 2) to determine whether antagonism of those μORs reverses bradypnea produced by intravenous remifentanil (remi; 0.1-1.0 μg·kg(-1)·min(-1)). The effects of microinjections of agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO; 100 μM) and antagonist naloxone (NAL; 100 μM) into the dorsal rostral pons on the phrenic neurogram were studied in a decerebrate, vagotomized, ventilated, paralyzed canine preparation during hyperoxia. A 1-mm grid pattern of microinjections was used. The DAMGO-sensitive region extended from 5 to 7 mm lateral of midline and from 0 to 2 mm caudal of the inferior colliculus at a depth of 3-4 mm. During remi-induced bradypnea (~72% reduction in fictive breathing rate) NAL microinjections (~500 nl each) within the region defined by the DAMGO protocol were able to reverse bradypnea by 47% (SD 48.0%) per microinjection, with 13 of 84 microinjections producing complete reversal. Histological examination of fluorescent microsphere injections shows that the sensitive region corresponds to the parabrachial/Kölliker-Fuse complex.

Cannabinoids have been shown to modulate central autonomic regulation and baroreflex control of blood pressure (BP). The presence of cannabinoid CB(1) receptors on fibers in the nucleus tractus solitarius (NTS) suggests that some presynaptic modulation of transmitter release could occur in this region, which receives direct afferent projections from arterial baroreceptors and cardiac mechanoreceptors. This study, therefore, was performed to determine the mechanism(s) of effects of microinjection of an endocannabinoid, arachidonylethanolamide (anandamide, AEA), into the NTS on baroreflex sympathetic nerve responses produced by phenylephrine-induced pressure changes in anesthetized rats. AEA prolonged reflex inhibition of renal sympathetic nerve activity (RSNA), suggesting an increase in baroreflex sensitivity. This effect of AEA was blocked by prior microinjection of SR-141716 to block cannabinoid CB(1) receptors. To determine whether this baroreflex enhancement by AEA involved a GABA(A) mechanism, the baroreflex response to AEA was tested after prior blockade of postsynaptic GABA(A) receptors by bicuculline, which would eliminate any effects due to modulation of GABA activity. After bicuculline, which alone prolonged the baroreflex inhibition of RSNA, AEA shortened the duration of RSNA inhibition, suggesting a possible presynaptic inhibition of glutamate release previously obscured by a more dominant GABA(A) effect. To support a possible physiological role for AEA, AEA concentration in the NTS was measured after a phenylephrine-induced increase in BP. AEA content in the NTS was increased significantly over that in normotensive animals. These results support the hypothesis that AEA content is increased by brief periods of hypertension and suggest that AEA can modulate the baroreflex through activation of CB(1) receptors within the NTS, possibly modulating effectiveness of GABA and/or glutamate neurotransmission.