Administration of which of the following will allete respiratory depression caused by opioids, without blocking their analgesic effect?

Neurons in the pre-Botzinger complex (medulla) are said to be the pacemaker cells for spontaneous breathing. There are NK1 receptors (for neurokinin 1) and mu-opioid receptors on these neurons. Substance P is known to stimulate these neurons (and hence stimulate respiration; opioids inhibit respiration their action on the mu-receptors. Depression of respiration becomes a limitation for the use of opioids in the management of pain. (mu-receptor antagonist would block the effect of respiratory depression, but it will also block the analgesic effect of opioids.) 5-HT4 receptors have been shown to be present on the Pre-Botzinger neurons. Treatment with 5-HT4 agonists will block the respiratory depressant effect of opioids, without inhibiting their analgesic effect.