Opioidergic interactions between striatal projection neurons.

Medium spiny striatal projection neurons (MSNs) release opioid neuropeptides, but the role of these neurotransmitters is still poorly understood. While presynaptic inhibition of corticostriatal axons by opioid receptors has been demonstrated using exogenous ligands, the action of synaptically released opioids in the striatum has not been investigated. We performed single and paired whole-cell recordings from rat MSNs while corticostriatal fibers were electrically activated. In single recording experiments, we also activated antidromically the axons of a population of MSNs. Corticostriatal fibers were stimulated once every 10 s and every other stimulation was preceded by 5 antidromic spikes (at 100 Hz). This burst of antidromic spikes produced robust inhibition of evoked corticostriatal responses. This inhibition was not affected by the δ-opioid receptor antagonist SDM25N, but was completely abolished by the μ-opioid receptor antagonist CTOP. Inhibitory effects were maximal (on average 29.6 ± 11.4%) when the burst preceded the corticostriatal stimulation by 500 ms and became undetectable for intervals >2 s. Paired recordings from MSNs located <100 μm apart revealed that, in 30 of 56 (54%) pairs, a burst of five action potentials in one of the MSNs caused significant inhibition (17.1 ± 5.7%) of evoked glutamatergic responses in the other MSN. In 5 of these pairs, reciprocal inhibition of corticostriatal inputs was present. These effects were maximal 500 ms after the burst and were completely blocked by CTOP. Thus, these results reveal a novel, strong opioid-mediated communication between MSNs and provide a new cellular substrate for competitive dynamics in the striatum.