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Opioid Receptors on Bulbospinal Respiratory Neurons Are Not Activated During Neuronal Depression by Clinically Relevant Opioid Concentrations

¹Zablocki Veterans Affairs Medical Center; and ²Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisconsin

Submitted 28 May 2008; accepted in final form 12 September 2008

Opioids depress the activity of brain stem respiratory-related neurons, but it is not resolved whether the mechanism at clinical concentrations consists of direct neuronal effects or network effects. We performed extracellular recordings of discharge activity of single respiratory neurons in the caudal ventral respiratory group of decerebrate dogs, which were premotor neurons with a likelihood of 90%. We used multibarrel glass microelectrodes, which allowed concomitant highly localized picoejection of opioid receptor agonists or antagonists onto the neuron. Picoejection of the µ receptor agonist [D-Ala², N-Me-phe⁴, gly-ol⁵]-enkephalin (DAMGO, 1 mM) decreased the peak discharge frequency (mean ± SD) of expiratory neurons to 68 ± 22% (n = 12), the ₁ agonist D-Pen^2,5-enkephalin (DPDPE, 1 mM) to 95 ± 11% (n = 15), and ₂ receptor agonist [D-Ala²] deltorphin-II to 86 ± 17% (1 mM, n = 15). The corresponding values for inspiratory neurons were: 64 ± 12% (n = 11), 48 ± 30% (n = 12), and 75 ± 15% (n = 11), respectively. Naloxone fully reversed these effects. Picoejection of morphine (0.01–1 mM) depressed most neurons in a concentration dependent fashion to maximally 63% (n = 27). Picoejection of remifentanil (240–480 nM) did not cause any significant depression of inspiratory (n = 11) or expiratory neurons (n = 9). 4. Intravenous remifentanil (0.2–0.6 µg·kg^–1·min^–1) decreased neuronal peak discharge frequency to 60 ± 12% (inspiratory, n = 7) and 58 ± 11% (expiratory, n = 11). However, local picoejection of naloxone did not reverse the neuronal depression. Our data suggest that µ, ₁, and ₂ receptors are present on canine respiratory premotor neurons. Clinical concentrations of morphine and remifentanil caused no local depression. This lack of effect and the inability of local naloxone to reverse the neuronal depression by intravenous remifentanil suggest that clinical concentrations of opioids produce their depressive effects on mechanisms upstream from respiratory bulbospinal premotor neurons.