EVIDENCE OF A SUFFOCATION ALARM SYSTEM WITHIN THE PERIAQUEDUCTAL GRAY MATTER OF THE RAT F. G. SCHIMITEL, a G. M. DE ALMEIDA, a D. N. PITOL, a R. S. ARMINI, a S. TUFIK b AND L. C. SCHENBERG a * a Department of Physiological Sciences, Federal University of Espírito Santo, Vitória, ES, Brazil b Department of Psychobiology, Federal University of São Paulo, São Paulo, SP, Brazil Abstract—Dyspnea, hunger for air, and urge to flee are the cardinal symptoms of panic attacks. Patients also show baseline respiratory abnormalities and a higher rate of co- morbid and antecedent respiratory diseases. Panic attacks are also precipitated by infusion of sodium lactate and inha- lation of 5% CO 2 in predisposed patients but not in healthy volunteers or patients without panic disorder. Accordingly, Klein [Klein (1993) Arch Gen Psychiatry 50:306 –317] sug- gested that clinical panic is the misfiring of an as-yet-uniden- tified suffocation alarm system. In rats, selective anoxia of chemoreceptor cells by potassium cyanide (KCN) and elec- trical and chemical stimulations of periaqueductal gray mat- ter (PAG) produce defensive behaviors, which resemble panic attacks. Thus, here we examined the effects of single or combined administrations of CO 2 (8% and 13%) and KCN (10 – 80 g, i.v.) on spontaneous and PAG-evoked behaviors of rats either intact or bearing electrolytic lesions of PAG. Exposure to CO 2 alone reduced grooming while increased exophthalmus, suggesting an arousal response to non-visual cues of environment. Unexpectedly, however, CO 2 attenu- ated PAG-evoked immobility, trotting, and galloping while facilitated defecation and micturition. Conversely, KCN pro- duced all defensive behaviors of the rat and facilitated PAG- evoked trotting, galloping, and defecation. There were also facilitatory trends in PAG-evoked exophthalmus, immobility, and jumping. Moreover, whereas the KCN-evoked defensive behaviors were attenuated or even suppressed by discrete lesions of PAG, they were markedly facilitated by CO 2 . Authors suggest that the PAG harbors an anoxia-sensitive suffocation alarm system which activation precipitates panic attacks and potentiates the subject responses to hypercapnia. © 2011 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: periaqueductal gray, defensive behaviors, panic disorder, hypoxia, hypercapnia, cyanide. Klein (1993) suggested that clinical panic is bound to the fear of suffocation produced by the misfiring of an as-yet- unidentified suffocation alarm system. This would result in a sudden dyspnea followed by a brief hyperventilation, panic, and the urge to flee. Klein (1993) argued that the false suffocation alarm hypothesis is a consistent explana- tion of the hypersensitivity of panic patients to sodium lactate and CO 2 , of panic attacks during relaxation and sleep, of the increased frequency of panic attacks in the late luteal phase dysphoric disorder and, conversely, its reduction in pregnancy, delivery, and lactation. Indeed, CO 2 - and lactate-induced panic attacks remain the best models of clinical panic insofar that they are not precipi- tated in healthy subjects (Pitts and McClure, 1967; Klein, 1993) or patients with obsessive-compulsive disorder (Griez and Schruers, 1998), social phobia (Liebowitz et al., 1985b) and, less certainly, generalized anxiety disorder (Lapierre et al., 1984). Moreover, whereas CO 2 - and lac- tate-induced panics are blocked by chronic treatment with tricyclic antidepressants (Rifkin et al., 1981; Liebowitz et al., 1985a; Woods et al., 1990; Yeragani et al., 1988; Gorman et al., 1997), -carboline and yohimbine precipi- tate panic attacks in healthy subjects that are blocked by low doses of diazepam but not tricyclics (Dorow et al., 1983; Klein, 1993). Recently, Preter and Klein (2008) ex- panded the suffocation false alarm hypothesis suggesting that the high comorbidity of panic disorder with childhood separation anxiety is related to a dysfunction of an endog- enous opioidergic system common to both conditions. In- deed, Preter and collaborators (2011) showed that nalox- one-treated healthy volunteers present panic-like respira- tory responses to intravenous infusions of sodium lactate. The presumptive link of suffocation alarm system and childhood separation anxiety was further supported by recent studies showing that rats subjected to neonatal mother separation present a sex-dependent facilitation of respiratory responses to both hypoxia (males) and hyper- capnia (females) in adult life (Genest et al., 2004, 2007a,b; Dumont et al., 2011). On the other hand, electrical and chemical stimulations of periaqueductal gray matter (PAG) produce defensive behaviors in animals (Bittencourt et al., 2004; Schenberg et al., 2005) and emotional feelings and symptoms in humans (Nashold et al., 1969; Young, 1989), which resem- ble panic attacks (Deakin and Graeff, 1991; Jenck et al., 1995; Schenberg et al., 2001). Interestingly, PAG stimula- tions in humans were also reported to produce smothering *Corresponding author. Tel: +55(27)3335-7332; fax: +55(27)3335-7330. E-mail address: schenber@npd.ufes.br (L. C. Schenberg). Abbreviations: Bar, Barrington’s nucleus; CeA, central amygdala; CUN, cuneiform nucleus; DEF, defecation; DLPAG, dorsolateral peri- aqueductal gray; DMH, dorsomedial hypothalamus; DMPAG, dorso- medial periaqueductal gray; DPAG, dorsal periaqueductal gray; DR, dorsal raphe nucleus; ED 50 , median effective dose; EXO, exophthal- mus; GLP, galloping; I 50 , median effective intensity; IMO, immobility; JMP, jumping; KCN, potassium cyanide; LC, locus coeruleus; LPAG, lateral periaqueductal gray; MIC, micturition; MPAG, medial periaque- ductal gray matter; NTS, nucleus of the solitary tract; PAG, periaque- ductal gray matter; PBA, parabrachial area; PET, positron emission tomography; PH, posterior hypothalamus; PMC, pontine micturition center (or M-region); PMD, premammillary dorsal nucleus; PPT, pe- dunculopontine tegmental nucleus; RA, nucleus retroambiguus; TRT, trotting; VLPAG, ventrolateral periaqueductal gray;  2 , chi-square. Neuroscience 200 (2012) 59–73 0306-4522/12 $36.00 © 2011 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2011.10.032 59