Substantia gelatinosa (SG) neurons receive synaptic inputs from primary afferent Aδ- and C-fibers, where nociceptive information is integrated and modulated by numerous neurotransmitters or neuromodulators. A number of studies were dedicated to the molecular mechanism underlying the modulation of excitability or synaptic plasticity in SG neurons and revealed that second messengers, such as cAMP and cGMP, play an important role. Recently, cAMP and cGMP were shown to downregulate each other in heart muscle cells. However, involvement of the crosstalk between cAMP and cGMP in neurons is yet to be addressed. Therefore, we investigated whether interaction between cAMP and cGMP modulates synaptic plasticity in SG neurons using slice patchclamp recording from rats. Synaptic activity was measured by excitatory post-synaptic currents (EPSCs) elicited by stimulation onto dorsal root entry zone. Application of 1 mM of 8- bromoadenosine 3,5-cyclic monophosphate (8-Br-cAMP) or 8-bromoguanosine 3,5-cyclic monophosphate (8-Br-cGMP) for 15 minutes increased EPSCs, which were maintained for 30 minutes. However, simultaneous application of 8-BrcAMP and 8-Br-cGMP failed to increase EPSCs, which suggested antagonistic cross-talk between two second messengers. Application of 3-isobutyl-1-methylxanthine (IBMX) that prevents degradation of cAMP and cGMP by blocking phosphodiesterase (PDE) increased EPSCs. Co-application of cAMP/cGMP along with IBMX induced additional increase in EPSCs. These results suggest that second messengers, cAMP and cGMP, might contribute to development of chronic pain through the mutual regulation of the signal transduction.