Thus, serotonergic signals relayed from the RVM in the brainstem can produce anti- or pro-nociceptive effects depending on the particular 5-HT receptor subtype recruited. It has been shown that activation of 5-HT3 receptors increases the open probability of voltage-gated calcium channels, which in turn enhances excitatory neurotransmitter release to exert a pronociceptive effect. However, Paul and colleagues reported that spinal injection of 5-HT3 receptor antisense attenuated intrathecally-administered 5-HT-mediated antinociception in the mouse tail flick model, which would suggest an antinociceptive effect from activation of spinal 5-HT3 receptors. The apparent discrepancy between these results and the present observations may reflect a differential contribution of spinal and supraspinal 5-HT3 receptors in the respective pain models. For example, a large body of literature from the rat formalin model supports the hypothesis that activation of both spinal and supraspinal 5-HT3 receptors is pronociceptive. In the present study, coadministration of the 5-HT3 receptor selective antagonist, ondansetron, potentiated the antinociceptive response to duloxetine and morphine at doses that failed to show activity alone. One interpretation of the present data is that activation of 5-HT3 receptors in the descending serotonergic facilitatory pathway prohibits synergistic antinociceptive interactions between monoamine reuptake inhibitors and morphine in the rat formalin model. However, as discussed above, the observation that the NET-selective reuptake inhibitor, esreboxetine, did not enhance the antinociceptive response to morphine suggests that a critical level of SERT inhibition, and consequent elevation of spinal and/ or supraspinal 5-HT levels, is necessary. We hypothesize that a modest level of SERT inhibition may be required to enhance descending Cannabinol inhibitory serotonergic pathways, while CIM 0216 near-maximal SERT inhibition enhances descending facilitatory serotonergic pathways which, through activation of 5-HT3 receptors, preclude a synergistic interaction between a monoamine reuptake inhibitor and morphine in the rat formalin model. When testing a pair of drugs for potential synergy, additivity or antagonism, both ����fixed-dose���� and ����fixed-ratio���� experimental designs are commonly used. A ����fixed-dose���� design tests the dose-response curve for drug A in the presence and absence of a fixed-dose of drug B, whereas a ����fixed-ratio���� design tests a series of dilutions of a constant ratio of drug A to B. The present study used both regimens to explore, and subsequently confirm, potential synergistic interactions between atomoxetine and morphine. Previous studies have highlighted how dose ratios can influence the apparent synergistic, additive or even antagonistic interactions between two drugs.