INTRODUCTION
Psychological factors such as stressful events induce a coordinated set of behavioural and physiological changes (McEwen, 2000). In the short term, physiological adjustments are important adaptive responses that maintain homeostasis and ensure survival (Sterling, 2012). The physiological responses to stress are mainly characterized by alterations in the autonomic nervous system and cardiovascular system, increases in plasma catecholamine levels and activation of the hypothalamic-pituitary-adrenal (HPA) axis (Herman et al., 2016; Joels & Baram, 2009). Autonomic responses include increases in both blood pressure (BP) and heart rate (HR) (Campeau & Watson, 1997; R. A. Dampney, Horiuchi, & McDowall, 2008), a drop in tail skin temperature as a consequence of sympathetically mediated vasoconstriction in skin beds (Blessing & Seaman, 2003; Nakamura, 2015; Vianna & Carrive, 2005) and baroreflex activity modulation (Crestani, 2016; R. A. L. Dampney, 2017).
Studies using image analysis techniques and Fos protein measurement have demonstrated that stressful stimuli activate neurons in the insular cortex (IC) (Ahn et al., 2015; Imbe, Kimura, Donishi, & Kaneoke, 2014; Uematsu, Kitamura, Iwatsuki, Uneyama, & Tsurugizawa, 2015). In addition, nonselective synaptic blockade in the IC generated by local microinjection of CoCl2 decreased both the increased blood pressure and tachycardia evoked by restraint stress (Alves, Crestani, & Correa, 2010). Moreover, the inhibition of local IC neurotransmission by CoCl2 attenuated freezing and increased the mean arterial pressure and heart rate in the groups that received CoCl2 either immediately after conditioning or 10 min before re-exposure to the aversive context but not in the group that received CoCl2 before the conditioning session. Regarding the HPA axis response, cortical regions such as the prefrontal cortex are related to the control of the HPA axis (Ekstrand, Hellsten, & Tingstrom, 2008; Gjerstad, Lightman, & Spiga, 2018; Neigh, Owens, Taylor, & Nemeroff, 2010) and participate in negative feedback via intermediate synapses in the PVN (Herman, McKlveen, Solomon, Carvalho-Netto, & Myers, 2012; Ulrich-Lai & Herman, 2009). In limbic areas such as the hippocampus and prefrontal cortex, glutamatergic neurotransmission plays an inhibitory role in the HPA axis (Diorio, Viau, & Meaney, 1993; Figueiredo, Bodie, Tauchi, Dolgas, & Herman, 2003; Ulrich-Lai & Herman, 2009). IC seems to be related to the regulation of cortisol in women with depression (Ottowitz et al., 2004).
The presence of glutamatergic terminals has been demonstrated in the IC (Dori, Dinopoulos, Cavanagh, & Parnavelas, 1992). In addition, microinjection of glutamate into the IC caused cardiovascular responses (Butcher & Cechetto, 1995; Ranjbar, Hatam, & Nasimi, 2015; Ruggiero, Mraovitch, Granata, Anwar, & Reis, 1987), showing that IC glutamatergic neurotransmission participates in the modulation of cardiovascular activity. Glutamatergic neurotransmission via NMDA receptors in the IC also has a facilitatory influence on baroreflex activity (Alves, Crestani, Resstel, & Correa, 2009), suggesting that the control of stress-evoked cardiovascular changes might be mediated by modulation of this cardiovascular reflex mechanism. Thus, in the present study, we tested the hypothesis that cardiovascular, neuroendocrine and autonomic responses to an acute session of restraint stress are mediated by glutamatergic neurotransmission in the IC.