Background: An excitatory imbalance in the hypothalamus of rodents caused by local chemical stimulation elicits fear-related defensive reactions such as escape and freezing. In addition, these panic attack-like defensive reactions induced by hypothalamic neurons may cause antinociception. However, there is a shortage of studies showing the participation of the anterior hypothalamic nucleus in these adaptive defensive mechanisms. Nitric oxide (NO) donors have been shown to evoke fear-related defensive responses when microinjected into paralimbic and limbic structures, and this excitatory neuromodulation can recruit the glutamatergic system. Aims: The aim of this work was to investigate the influence of the glutamatergic system in the nitrergic effects on fear-related defensive responses organised by anterior hypothalamic neurons. Methods: The present study evaluates the effects of the molsidomine active metabolite SIN-1 NO donor administered into the anterior hypothalamus (AH) of mice at different concentrations (75, 150 and 300 nmol/0.1 μL). Then, we investigated the effects of pre-treatment of the AH with AP-7 (an N-methyl-d-aspartate (NMDA) receptor-selective antagonist; 0.02, 0.2 and 2 nmol/0.1 μL) on the behavioural and antinociceptive effects provoked by AH chemical stimulation with SIN-1 microinjections. Results: The 300 nmol dose of SIN-1 was the most effective at causing panic-like defensive behaviours followed by a significant antinociceptive response. In addition, both of these effects were attenuated or inhibited by AH pre-treatment with AP-7. Conclusions: These findings suggest that the panicogenic and antinociceptive effects evoked by intra-AH microinjections of SIN-1 depend on NMDA receptor activation.
