Fig A shows a schematic representation of histological
Fig. 2A shows a schematic representation of histological results according to the Franklin and Paxinos Mouse Brain Atlas (2001). The black circles represent the sites of drug infusion that were on-target within the amygdala. Gray circles represent the animals that had infusion locations outside the amygdala. Fig. 2B shows a photomicrograph of a LY500307 coronal section of a representative subject showing an injection site within the amygdala.
Discussion The present findings show that while 5-HT1A receptors activation in the amygdala accentuated nociceptive behavior, increasing the number of writhes in the OA and EA, the 5-HT2C receptors activation intensified antinociception. Prior systemic treatment with fluoxetine, at a dose devoid of intrinsic activity on nociception (2.5 mg/kg), did not change the hypernociceptive effect provoked by intra-amygdala injection of 8-OH-DPAT (5-HT1A agonist), but this SSRI blocked the OAA enhancement induced by intra-amygdala activation of 5-HT2C. Importantly, the effects of intra-amygdala injection of MK-212 (5-HT2C agonist) were also antagonized by local injection of SB-242084, a selective 5-HT2C receptor antagonist. The present results corroborate previous studies wherein the confinement of mice to the OA of the EPM elicits antinociception, a phenomenon known as OAA (Nunes-de-Souza et al., 2000; Baptista et al., 2009). Nunes-de-Souza et al. (2000) demonstrated that intra-amygdala microinjections of 5.6 nmol of 8-OH-DPAT did not change OAA; however, this dose produced an anxiogenic effect in mice exposed to the EPM. Based on these findings, the authors suggested a possible dissociation between the mechanisms involved in the modulation of anxiety and nociception control in the amygdaloid complex. However, our results demonstrated that intra-amygdala injection of a higher dose of 8-OH-DPAT, i. e, 10 nmol, leads to an increasing in the number of writhes regardless of confinement type (OA or EA), suggesting a direct modulation of this serotonergic receptor subtype in the nociceptive pathways. Previous findings have been shown that systemic serotonin administration produces a dose-dependent hyperalgesic effect, and this effect would be the result of a direct excitatory effect of the primary afferent neuron (Taiwo and Levine, 1992). This increase in nociceptive behavior, observed in Sprague–Dawley rats subjected to the paw withdrawal test, was shown through the administration of 5-HT1A selective agonists, such as 8-OH-DPAT and DP-5-CT (Taiwo and Levine, 1992). Altogether, these evidence confirm the already described role of 5-HT1A receptors in facilitate the hypernociception (Sommer, 2006). The hypernociceptive role of 5-HT1A receptors have been shown in various experimental protocols. For instance, Canto-de-Souza and colleagues (1998) demonstrated the blockade of social-defeat analgesia through intra-dPAG injection of BAY-R 1531, a 5-HT1A full agonist, whereas local injection of the 5-HT1A antagonist, WAY-100135, enhanced this type of environmentally induced pain inhibition. This antagonist also blocked the increase in the viscero-motor response to colorectal distension induced by 8-OH-DPAT (Mickle et al., 2012). The pro-nociceptive property of 8-OH-DPAT and the antinociceptive effect of WAY-100135 have already been reported by Millan and colleagues (1995). Considering that the 5-HT1A receptor is coupled to an inhibitory Gi protein, it is probable the binding to 8-OH-DPAT causes an efflux of potassium, thereby promoting cell hyperpolarization, and consequently a neuronal inhibition (Fox and Sorenson, 1994; Raymond et al., 2001; Artigas, 2013) within the amygdaloid complex, which in turn would result in a pro-nociceptive effect. In this context, several studies have shown that amygdala lesions are able to increase pain-related responses in different animal models (Nakagawa et al., 2003; Tanimoto et al., 2003). Recently, our research group have observed that amygdala inhibition through local injection of cobalt chloride, a non-selective synaptic inhibitor (Canto-de-Souza et al., 2014), produced an increase in the number of writhes induced by abdominal injection of 0.6% acetic acid in mice.