We also found that females had a greater density of MOR+ neurons compared with males in both areas. Our results suggest a differential contribution of MOR-mediated modulation to ACC and S1 outputs. While MOR+ somatostatin interneurons are more prominent in ACC, MOR+ excitatory neurons and MOR+ parvalbumin interneurons are more prominent in S1. We found that the electrophysiological signatures of MOR+ neurons differ significantly between these 2 areas, with interneuron-like firing patterns more frequent in ACC.

Using the Oprm1-mCherry mouse line, we performed selective patch clamp recordings of MOR+ neurons, as well as immunohistochemistry with validated neuronal markers, to determine the identity and laminar distribution of MOR+ neurons in ACC and S1. Mu opioid receptors (MORs) are expressed in both ACC and S1 however, the identity of MOR-expressing cortical neurons remains unknown. Injection of morphine in the ACC has been reported to be analgesic, and endogenous opioids in this area are required for pain relief. The anterior cingulate cortex (ACC) processes the affective component of pain, whereas the primary somatosensory cortex (S1) is involved in its sensory-discriminative component.

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