Pharmacologically evoked apnoeas. Receptors and nervous pathways involved

This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT_1A serotonin receptors in overcoming opioid-induced respiratory inhibition.

DAMGO depresses inhibitory synaptic transmission via different downstream pathways of μ opioid receptors in ventral tegmental area and periaqueductal gray

Opioid-induced rewarding and motorstimulant effects are mediated by an increased activity of the ventral tegmental area (VTA) dopamine (DA) neurons. The excitatory mechanism of opioids on VTA-DA neurons has been proposed to be due to the depression of GABAergic synaptic transmission in VTA-DA neurons. However, how opioids depress GABAergic synaptic transmission in VTA-DA neurons remain to be studied. In the present study, we explored the mechanism of the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) on GABAergic synaptic transmission in VTA-DA neurons using multiple approaches and techniques. Our results showed that (1) DAMGO inhibits GABAergic inputs in VTA-DA neurons at presynaptic sites; (2) effect of DAMGO on GABAergic inputs in VTA-DA neurons is inhibited by potassium channel blocker 4-aminopyridine (4-AP) and Gi protein inhibitor N-ethylmaleimide (NEM); (3) phospholipase A2 (PLA2) does not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons, but mediates it in the periaqueductal gray (PAG); (4) multiple downstream signaling molecules of μ receptors do not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons. These results suggest that DAMGO depresses inhibitory synaptic transmission via μ receptor-Gi protein-Kv channel pathway in VTA-DA neurons, but via μ receptor-PLA2 pathway in PAG neurons.

Presynaptic mechanism for anti-analgesic and anti-hyperalgesic actions of kappa-opioid receptors

Glutamate neurotransmission plays an important role in the processing of pain and in chronic opioid-induced neural and behavioral plasticity, such as opioid withdrawal and opioid dependence. Kappa-opioid receptors also have been implicated in acute opioid modulation of pain and chronic opioid-induced plasticity, both of which are primarily mediated by mu-opioid receptors. Using whole-cell patch clamp recordings in brain slices in vitro and system analysis of pain behaviors in rats in vivo, this study investigated the functional role of glutamate synaptic transmission and kappa-opioid receptors in two behavioral pain conditions: m-opioid-induced analgesia (decreased pain) and mu-opioid withdrawal-induced hyperalgesia (increased pain). In the nucleus raphe magnus (NRM), a brainstem structure that controls spinal pain transmission, we found that kappa-receptor agonists presynaptically inhibited glutamate synaptic currents in both of the two cell types that are thought to respectively inhibit or facilitate spinal pain transmission. In rats, both glutamate receptor antagonists and the kappa agonist microinjected into the NRM attenuated mu-opioid-induced analgesia, which is most likely mediated through activation of such pain-inhibiting neurons. However, during opioid abstinence-induced withdrawal, the same doses of glutamate receptor antagonists and the kappa agonist administered in the NRM suppressed the withdrawal-induced hyperalgesia, which is thought to be mediated by activation of those pain-facilitating neurons during opioid withdrawal. These results demonstrate that kappa-opioid receptors antagonize mu-receptor-induced effects in both analgesic and hyperalgesic states, and suggest inhibition of glutamate synaptic transmission as a presynaptic mechanism for the kappa antagonism of these two mu receptor-mediated actions.