br Introduction The dorsal periaqueductal gray matter
Introduction The dorsal periaqueductal gray matter (dPAG) is a midbrain site markedly involved in fear/anxiety-evoked responses as well as in nociception (see, for example, Bandler and Carrive, 1988, Deakin and Graeff, 1991, Fardin et al., 1984a, Fardin et al., 1984b, Litvin et al., 2007). Chemical or electrical stimulation of dPAG elicits defensive behavior such as freezing, flight and fight reaction, escalation of risk assessment behavior and arousal (e.g., Bandler and Carrive, 1988, Schenberg et al., 2005) and autonomic activation (e.g., tachycardia, hypertension, tachypnea — Bandler et al., 1991, Hayward et al., 2003, McDougall et al., 1985). These responses are generally accompanied by antinociception (e.g., Coimbra and Brandão, 1997, Fanselow, 1991). Among the various neurotransmitter systems pointed out to play a role in the mediation of defensive and antinociceptive responses elicited by environmentally aversive stimuli, the neuropeptide corticotropin-releasing factor or hormone (CRF or CRH) has attracted the interest of many researchers investigating its role in the modulation of defensive reactions (Baldwin et al., 1991, Berridge and Dunn, 1989, Carvalho-Netto et al., 2007, Litvin et al., 2007, Stenzel-Poore et al., 1994). CRF is a 41-amino XAV939 peptide that activates the hypothalamo–pituitary–adrenal (HPA) axis, releasing, at the end of a cascade, glucocorticoids from the adrenal gland. HPA axis hyperactivation has been related to several brain disturbances such as anxiety disorders, depression, epilepsy and drug addiction (Allen et al., 2011, Kanner, 2011, Lim et al., 2011, Pariante and Lightman, 2008). Besides its action on the HPA axis, CRF also acts in other brain areas such as the amygdala (Carrasco and Van de Kar, 2003, Shekhar et al., 2005), bed nucleus of stria terminalis (Sahuque et al., 2006), locus coeruleus (Chen et al., 1992), dorsal raphe nucleus (Carrasco and Van de Kar, 2003) and PAG (Borelli and Brandão, 2008, Martins et al., 1997) increasing anxiety-like responses in various animal tests. Regarding the involvement of CRF receptors located within the PAG on fear/anxiety-related responses, previous studies have shown that intra-dPAG infusion of h/rCRF (Martins et al., 2000) and ovine CRF (Borelli and Brandão, 2008) increase anxiety-like behaviors in rats exposed to the elevated plus maze (EPM), a widely used animal test of anxiety (Handley and Mithani, 1984, Lister, 1987, Pellow et al., 1985). In mice, Carvalho-Netto et al. (2007) have shown that intra-PAG infusion of ovine CRF increases avoidance behavior in two different anxiety tests, the mouse defense test battery (MDTB) and the rat exposure test (RET). It has been emphasized that CRF can activate Gs-protein-coupled CRF1 or CRF2 receptors triggering the cAMP-PKA cascade pathway (Chang et al., 1993, Chen et al., 1993, Lovenberg et al., 1995, Perrin et al., 1995, Vita et al., 1993). Moreover, both receptor subtypes have been found strongly expressed in different PAG columns, where a high density of CRFergic neurons have also been reported (Merchenthaler, 1984, Steckler and Holsboer, 1999, Swanson et al., 1983). In addition, Bowers et al. (2003) have related CRF microinjection into the PAG with increased neuron firing and excitatory activity. However, it remains unclear at which receptor subtypes (CRF1, CRF2 or both) CRF produces anxiety-like responses in rodents. Evidence showing that CRF1 receptors play an important role in the modulation of defensive responses has been reported in several studies. For instance, intra-PAG (Litvin et al., 2007) or intracerebroventricular (Tezval et al., 2004) infusions of CRF1 receptor agonist, cortagine, produce anxiogenic-like effects in various animal tests of anxiety. On the other hand, intracerebral injection of CRF2 receptor antagonist (e.g., antisauvagine-30) has been related to both anxiogenic and anxiolytic effects (Kishimoto et al., 2000, Takahashi et al., 2001). However, as far as we know there are no studies comparing the anxiogenic effects of CRF at CRF1 and CRF2 receptors within the PAG.