CaMKIIa may modulate fentanyl-induced hyperalgesia via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway in rats

Abstract

Each of the lateral capsular division of central nucleus of amygdala(CeLC), periaqueductal gray (PAG), rostral ventromedial medulla(RVM) and spinal cord has been proved to contribute to the development of opioid-induced hyperalgesia(OIH). Especially, Ca2+/calmodulin-dependent protein kinase IIa (CaMKIIa) in CeLC and spinal cord seems to play a key role in OIH modulation. However, the pain pathway through which CaMKIIa modulates OIH is not clear. The pathway from CeLC to spinal cord for this modulation was explored in the present study. Mechanical and thermal hyperalgesia were tested by von Frey test or Hargreaves test, respectively. CaMKIIa activity (phospho-CaMKIIa, p-CaMKIIa) was evaluated by western blot analysis. CaMKIIa antagonist (KN93) was micro-infused into Ce LC, spinal cord or PAG, respectively, to evaluate its effect on behavioral hyperalgesia and p-CaMKIIa expression in Ce LC, PAG, RVM and spinal cord. Then the underlying synaptic mechanism was explored by recording miniature excitatory postsynaptic currents (mEPSCs) on PAG slices using whole-cell voltage-clamp methods. Results showed that inhibition of Ce LC, PAG or spinal CaMKIIa activity respectively by KN93, reversed both mechanical and thermal hyperalgesia. Microinjection of KN93 into CeLC decreased p-CaMKIIa expression in Ce LC, PAG, RVM and spinal cord; while intrathecal KN93 can only block spinal but not CeLC CaMKIIa activity. KN93 injected into PAG just decreased p-CaMKIIa expression in PAG, RVM and spinal cord, but not in the Ce LC. Similarly, whole-cell voltage-clamp recording found the frequency and amplitude of mEPSCs in PAG cells were decreased by KN93 added in PAG slice or micro-infused into CeLC in vivo. These results together with previous findings suggest that CaMKIIa may modulate OIH via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway.