Neurons in the frontal cortex of the rat carry multiple opiate receptors

Acute desensitization to the inhibitory action of iontophoretically applied opiate alkaloids and opioid peptides was used to investigate the possibility of multiple opiate receptors located on single neurons in the frontal cortex of rats. Such short term exposure resulted in adaptive processes which were similar to tolerance to and dependence on opiate agonists occurring after chronic treatment. Neurons desensitized to methionine-enkephalin (ME) or D-Ala², D-Leu⁵-enkephalin (DADL) became subsensitive to morphine, whereas cells desensitized to morphine remain sensitive to the inhibitory action of the opioid peptides. This lack of cross-desensitization may suggest the existence of multiple opiate receptors on the same cell.