Localization of Mu and delta opioid receptors to anterior cingulate afferents and projection neurons and input/output model of Mu regulation.

Anterior cingulate cortex (ACC) has one of the highest densities of opioid receptors in the CNS and it has been implicated in acute and chronic pain responses. Little is known, however, about which neurons express opioid receptors in their dendrites and axon terminals. The present studies employed experimental techniques to remove afferent axons or classes of projection neurons from rat ACC area 24 followed by coverslip autoradiography to localize changes in binding of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO) to mu receptors and 2-[3H]D-penicillamine-5-D-penicillamine-enkephalin (DPDPE) to delta receptors. Removal of all afferents to area 24 with undercut lesions did not alter DPDPE binding, but significantly reduced binding of DAMGO in layers I, III, and V. In contrast, removal of all cortical neurons with the excitotoxin ibotenic acid almost abolished DPDPE binding in all layers. The same lesions reduced DAMGO binding in most layers; however, there was a postlesion bimodal distribution in binding with high levels of binding in layer I and moderate levels in layer VI. These data suggest that delta receptors are expressed by cortical neurons, while mu receptors are expressed by both cortical neurons and afferent axons. To explore the distribution of postsynaptic receptors, immunotoxin lesions were made in area 24 by injection of OX7-saporin into the caudate and/or thalamic nuclei. Almost complete removal of projection neurons to these targets in layers Vb and VIa did not alter DPDPE binding, while the lesions reduced DAMGO binding in all but layer II. Removal of layer Vb corticostriatal projection neurons with caudate OX7-saporin injections reduced binding only in this layer. It is proposed that opioidergic circuits in area 24 are organized according to an input/output model for mu opioid regulation. In this model mu receptors regulate axon terminal activity from the thalamus in layer Ia and the locus coeruleus in layers Ic and II, whereas cortical outputs to the thalamus are modulated via postsynaptic receptors expressed in all layers by thalamocortical projection neurons with somata in layer VI. These opioidergic circuits in ACC are of particular importance because they may regulate responses to chronic nociceptive activity and associated pain perceptions.