Lateralization of opioid receptors in turtle visual cortex

The opioid μ-agonist morphine, the δ-agonist [-Ala², -Leu⁵]enkephalin (DADL) and the κ-agonist bremazocine locally applied to the surface of turtle visual cortex inhibited the orthodromic evoked potential (EP; fast negative component N₁). After application of the σ-agonist SKF 10.047 the inhibition was followed by facilitation of EP. The lack of cross-desensitization to the inhibitory action of opioids upon EP indicates that the drugs exert their effects via different opioid receptors. Bremazocine and morphine predominantly inhibited the left cortical EP, whereas DADL was a potent inhibitor of the right cortical EP. Thus, the opioid receptors which modulate evoked electrical activity of the left visual cortex (LVC) apparently belong mostly to the κ- and μ-type, while σ-receptors were predominantly responsible for the modulation of electrical activity in the right visual cortex (RVC). Besides, we have studied in vivo binding of the κ-agonist [³H]ethylketocyclazocine ([³H]EKC) and the σ-agonist [³H]DADL to LVC and RVC. The binding was specific and could be accounted for by the interaction with membrane opioid receptors and/or specific uptake of these drugs by cortex cells. [³H]EKC and [³H]DADL exhibited a more effective binding to LVC and RVC membranes, respectively. The results obtained in vitro apparently indicate that LVC and RVC differ in the number of κ- and σ-binding sites. Thus, turtle brain may have a side-specific mechanism for selective neurochemical regulation of neuron activity in LVC and RVC realized predominantly via κ- and σ-receptor, respectively.