Ca-stimulated adenylyl cyclases (ACs) transduce neuronal stimulation-evoked increase in calcium to the production of cAMP, which impinges on the regulation of many aspects of neuronal function. Type 1 and type 8 AC (AC1 and AC8) are the only ACs that are directly stimulated by Ca. Although AC1 function was implicated in regulating reference spatial memory, the function of AC8 in memory formation is not known. Because of the different biochemical properties of AC1 and AC8, these two enzymes may have distinct functions. For example, AC1 activity is regulated by both Ca and G-proteins. In contrast, AC8 is a pure Ca sensor. It is neither stimulated by G s nor inhibited by G i . Recent studies also suggested that AC1 and AC8 were differentially concentrated at different subcellular domains, implicating that Ca-stimulated signaling might be compartmentalized. In this study, we used AC8 knock-out (KO) mice and found behavioral deficits in memory retention for temporal dissociative passive avoidance and object recognition memory. When examined by Morris water maze, AC8 KO mice showed normal reference memory. However, the acquisition of newer spatial information was defective in AC8 KO mice. Furthermore, AC8 KO mice were severely impaired in hippocampus-dependent episodic-like memory when examined by the delayed matching-to-place task. Because AC8 is preferentially localized at the presynaptic active zone, our results suggest a novel role of presynaptic cAMP signaling in memory acquisition and retention, as well as distinct mechanisms underlying reference and working/episodic-like memory.

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Key pointsr Despite the clinical importance of pre-emptive analgesia, the mechanisms by which it attenuates pain associated with central sensitization are poorly understood.r We find that fentanyl and the α2-adrenoceptor agonist dexmedetomidine (Dex) differ significantly in their modulatory actions on noxious mechanical and noxious heat-evoked nociception in vivo.r Unlike fentanyl, Dex modified descending control of nociception by decreasing the threshold for descending inhibition and/or increasing the threshold for descending facilitation.r Dex exhibited after-actions on activities of thalamus in prolongation of noxious heat-evoked paw withdrawal latency that persisted for at least 7 days.r This study provides insight into the organization of thalamic modulation in pre-emptive analgesia.Abstract We investigated and compared the antinociceptive effects of intraperitoneal administration of fentanyl (2-60 μg kg −1 ) and dexmedetomidine (Dex, 1-10 μg kg −1 ; a highly selective α2-adrenoceptor agonist) in the regulation of nociception assessed by measuring noxious paw withdrawal reflexes in rats. Fentanyl elevated noxious mechanical paw withdrawal threshold and prolonged paw withdrawal heat latency within 1-1.5 h (P < 0.05). Dex failed to affect the mechanical paw withdrawal threshold, yet significantly prolonged the paw withdrawal heat latency in a bi-phasic manner; a short transient 1-1.5 h period followed by a second, slowly developing increase in latency that persisted for at least 7 days (P < 0.05). Lesion of the dorsolateral funiculus (DLF) did not influence fentanyl-induced antinociceptive effects, indicating peripheral and spinal antinociceptive mechanisms. By contrast, the Dex-induced second, but not the first, phase of the prolonged paw withdrawal heat latency was significantly blocked by the lesion of either DLF or thalamic ventromedial (VM) nuclei, and was attenuated by intracerebral administration of either atipamezole (α2-adrenoceptor antagonist) or WAY-100635 (5-HT 1A receptor antagonist) into the VM nuclei (P < 0.05). Upon intramuscular 5.8% saline-induced muscle nociception, pre-emptive injection of fentanyl enhanced mechanical hyperalgesia and blocked heat hypoalgesia, whereas Dex significantly prevented the occurrence of mechanical hyperalgesia and enhanced heat hypoalgesia. It is suggested that Dex, but not fentanyl, significantly enhances descending inhibition and/or decreases descending facilitation to modulate pain and nociception. The present study provides novel insight into thalamus-mediated mechanisms in pre-emptive analgesia.

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