Effect of Morphine in Mice Lacking the µ-Opioid Receptor (–/– = Homozygous µ-Opioid Receptor Knockout Mice) and Mice with Intact Receptors (+/+ = Wild-Type Mice) on Analgesic Responses (A, Tail Flick Test in Two Mice) and Respiratory Responses (B, the Hypercapnic Ventilatory Response [HCVR] in Two Mice)

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