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  • Given these findings above we hypothesized that hypernocicep

    2024-04-22

    Given these findings above, we hypothesized that hypernociception produced by intra-amygdala 8-OH-DPAT is result of hyperpolarization in this area. This neuronal inhibition of the amygdaloid complex might result in an inactivation of crucial neurotransmitter systems of the descending inhibitory pain circuitry (Fields, 2004; McGaraughty et al., 2004), triggering an exacerbation of the sensory nociceptive response. In the other hand, some studies have suggested that the therapeutic effects of fluoxetine treatment are result of this SSRI interaction with the subtypes of serotonergic receptors, mainly 5-HT1A and 5-HT2C (Chen et al., 1995; De Vry et al., 2004). In order to investigate this possible interaction, we choose a dose of fluoxetine devoid of intrinsic effect on nociceptive response (2.5 mg/kg), as previously observed by our group (Baptista-de-Souza et al., 2014), to investigate its potential effects to alter the OAA when combined with intra-amygdala injections of 5-HT receptor ligands. However, in experiment 2, we observed that fluoxetine (2.5 mg/kg) did not alter the effects of 5-HT1A agonist in the amygdaloid complex. In experiment 3, we evaluated the effect of the intra-amygdala injection of MK-212 on nociceptive response. Different to that observed with the 5-HT1A agonist, intra-amygdala injection of MK-212 (0.63 nmol) intensified OAA. A similar result was previously demonstrated with the activation of 5-HT2C receptors in the midbrain periaqueductal gray HOE 33187 (PAG) (Baptista et al., 2012). Despite this apparent function convergences, in that the 5-HT2C receptors of the amygdaloid complex and PAG play similar roles in the modulation of nociceptive response, their activations seem to produce quite different effects in the control of anxiety-like behaviors (Deakin and Graeff, 1991; Graeff et al., 1997). In brief, previous studies have reported that while intra-PAG injections of 5-HT2C agonists exert anxiolytic-like effect, stimulation of the same receptor subtype in the amygdaloid complex seems to elicit anxiogenesis (Cornélio and Nunes-de-Souza et al., 2007; de Mello Cruz et al., 2005; Gomes and Nunes-De-Souza, 2009). Also with regard the role of PAG 5-HT2 receptors in the modulation of nociception, microinjections of ritanserin, a 5HT2A/2C antagonist, into the superior colliculus and dorsolateral PAG decreased the innate fear-induced antinociception (de Oliveira et al., 2017). Similar to the results found by Coimbra and Brandão (1997), de Oliveira et al. (2017) demonstrated that fear-induced analgesia was inhibited by microinjections of a 5-HT2 blocker ketanserin, in the midbrain tectum. Thereby, we suggest that the increase in OAA observed in response to the highest dose of MK-212 is directly related to exacerbation of anxiety produced by the activation of 5-HT2C receptors in this prosencephalic structure. Notably, this relationship is distinct from that observed in the PAG, which performs a different modulation in the pain and anxiety responses (Baptista et al., 2012). Interestingly, systemic treatment with fluoxetine prevented OAA enhancement induced by intra-amygdala injection of MK-212 (experiment 4). However, these results are different from those reported in previous studies wherein have been suggested that SSRIs provoke an exacerbation of anxiolytic and panicolytic responses induced by 5-HT2 agonists in the dorsal PAG (dPAG) (de Bortoli et al., 2006; Zanoveli et al., 2007, 2010). Considering the accentuation of antinociception induced by MK-212, we expected that an increase in synaptic serotonin levels triggered by the acute treatment of fluoxetine (Carlsson, 1970) would cause an exacerbation of antinociception. However, we observed the attenuation the effect of the 5-HT2C agonist induced by inactive dose of fluoxetine, and thus did not accentuate antinociception, as we have hypothesized. In this context, previous studies have suggested that fluoxetine can also act as an antagonist of 5-HT2C receptors (Ni and Miledi, 1997; Palvimaki et al., 1999). Through electrophysiological techniques, Ni and Miledi (1997) demonstrated that fluoxetine inhibits the binding of serotonin to 5-HT2C receptors expressed in the membranes of cortical cells in rats. This observation suggests that fluoxetine can act as a reversible and competitive antagonist of 5-HT2C receptors.