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Anatomical Connections of the Periaqueductal Gray: Specific Neural Substrates for Different Kinds of Fear

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Anatomical Connections of the Periaqueductal Gray: Specific Neural Substrates for Different Kinds of Fear PeriaqueductalBrazilian Journal gray of Medical hodology and and Biological function Research (2003) 36: 557-566 557 ISSN 0100-879X Review Anatomical connections of the periaqueductal gray: specific neural substrates for different kinds of fear D.M.L. Vianna Laboratório de Psicobiologia, Faculdade de Filosofia, and M.L. Brandão Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil Abstract Correspondence The periaqueductal gray (PAG) has been traditionally considered to be Key words D.M.L. Vianna an exit relay for defensive responses. Functional mapping of its · Periaqueductal gray Laboratório de Psicobiologia subdivisions has advanced our knowledge of this structure, but syn- · Defense reaction FFCLRP, USP thesis remains difficult mainly because results from lesion and stimu- · Fear Av. Bandeirantes, 3900 lation studies have not correlated perfectly. After using a strategy that · Escape 14040-901 Ribeirão Preto, SP combined both techniques and a reevaluation of the available litera- · Freezing Brasil ture on PAG function and connections, we propose here that freezing · Periaqueductal gray Fax: +55-16-602-3632 columns E-mail: [email protected] could be mediated by different PAG subdivisions depending on the presence of immediate danger or exposure to related signaling cues. Research supported by FAPESP These subdivisions are separate functional entities with distinct de- (Nos. 97/00363-1 and 98/11187-2) scending and ascending connections that are likely to play a role in and CNPq (No. 140164/1999-5). different defensive responses. The existence of ascending connections D.M.L. Vianna is the recipient also suggests that the PAG is not simply a final common path for of a fellowship from CNPq. defensive responses. For example, the possibility that indirect ascend- ing connections to the cingulate cortex could play a role in the expression of freezing evoked by activation of the neural substrate of Received August 5, 2002 fear in the dorsal PAG has been considered. Accepted February 14, 2003 Introduction reductions of both innate (4) and learned (5) defensive behaviors. These findings were Since Hunsperger (1), the periaqueductal later supplemented by the discovery of the gray (PAG) has been viewed as the final organization of the PAG into longitudinal common path for all defensive responses. columns, which serve as distinct anatomical Chemical or electrical stimulation of some modules for the specific functions associ- structures located more rostrally, like the ated with this structure (6). Presently, four amygdala or medial hypothalamus, can in- main longitudinal cell-rich subdivisions are duce defense reactions (2), which are perma- accepted by consensus to exist in the PAG nently abolished after lesion of the PAG (1). (7), namely the dorsomedial (dmPAG), dor- The reverse is not true, because neither telen- solateral (dlPAG), lateral and ventrolateral cephalic ablation (3) nor medial or posterior (vlPAG) subdivisions. In this review, we hypothalamic lesions (1) blocks defense re- examine the special role played by these actions elicited by PAG stimulation. PAG distinct columns in the coordination of dif- lesions can also abolish or cause dramatic ferent types of fear in an attempt to present Braz J Med Biol Res 36(5) 2003 558 D.M.L. Vianna and M.L. Brandão an overview of the anatomical organization Some additional support for this view (8) of the PAG involved in the expression of came from the fact that lateral PAG stimula- emotional behavior. tion with excitatory amino acids provoked vigorous escape, while vlPAG stimulation Periaqueductal gray subdivisions: produced strong immobility (Figure 1A). functional differentiation The findings described above also raised some points for discussion. Microinjection It was only in the late eighties that func- of glutamate into the vlPAG causes an im- tional studies started to draw attention to mobile behavior (10) that is very different regional dissimilarities within the PAG. from the defensive freezing attenuated after LeDoux et al. (5) observed a reduction of lesioning the same region (9). While the conditioned freezing behavior after excito- former is accompanied by hypotension, toxic lesion of the caudal, but not the rostral bradycardia, and hyporeactivity (10), freez- PAG. In further studies of the rostrocaudal ing is followed by hypertension (11) and axis, stimulation of the caudal PAG pro- hypervigilance (12). duced forward escape, while rostral stimula- Recently, Morgan and Carrive (13) ar- tion provoked threatening reactions and back- gued against the assumption that vlPAG stim- ward avoidance (8), as seen in Figure 1A. ulation causes both immobility and hypoten- On the dorsoventral axis, electrical stim- sion, and also against the suggestion (10) ulation of the dorsal and lateral PAG of the that this immobility was part of a recupera- rat provoked vigorous motor reactions simi- tive response after an agonistic encounter lar to those induced by foot shock, and short- (quiescence). After microinjection of d,l- ly after the end of the stimulus the rat froze homocysteic acid into the vlPAG of awake, (9). In contrast, only freezing was observed freely moving rats, the resulting immobility after vlPAG stimulation (9). Ventral PAG and bradycardia were not accompanied by a lesions reduced conditioned freezing, and fall in blood pressure. These investigators dorsal PAG lesions reduced the activity concluded that, in nature, generation of be- burst that appears during foot shock (9). In havior usually requires the activation of vari- short, it was argued that the dorsal PAG ous brain regions and that the vlPAG could mediated escape, while the ventral PAG was be recruited in both defensive freezing and responsible for the freezing response (9). quiescence. This co-activation hypothesis Figure 1. Functional studies. A, Sites of excitatory amino acid mi- croinjections into the periaque- ductal gray and behavioral reac- tions thus elicited in the freely moving rat. B, Periaqueductal 1 mm gray neurons showing condi- EW A 3.2 tioned fear-related Fos expres- sion. III, oculomotor nucleus; A 2.7 EW, Edinger-Westphal nucleus; III A 2.2 MLF, medial longitudinal fascicu- lus. Reprinted respectively from A 1.7 Carrive (8) and Carrive et al. (18), MLF A 1.2 Ó 1993, 1997, with permission Backward defense of Elsevier Science. Forward avoidance A 0.7 Hyporeactive immobility A 0.2 AB Braz J Med Biol Res 36(5) 2003 Periaqueductal gray hodology and function 559 receives support from various correlation the stimulus propagation from dorsal PAG to studies. Fos immunoreactivity expressed vlPAG. This hypothesis has not received em- during neuronal activation increases in all pirical support. After vlPAG stimulation with PAG subdivisions after stimulation of the intensities high enough to provoke escape, dorsal PAG (14) and medial hypothalamus animals did not freeze beyond control levels, (15), after exposure to a predator (16) or to its as occurred with dlPAG stimulation (26). smell (17), and after re-exposure to a condi- Finally, the vlPAG does not seem to be tioned aversive context (18). However, it is critical to all kinds of freezing (27). While noteworthy that the effect is much more vlPAG-lesioned animals did freeze less after pronounced in the dlPAG after predator ex- foot shock conditioning, electrical or chem- posure, and in the vlPAG after re-exposure to ical stimulation of the dlPAG in vlPAG- the conditioned aversive context (Figure 1B). lesioned animals caused as much freezing Yet, there remains a second difficulty and escape as that shown by sham-lesioned, with the anatomical dissociation theory. dlPAG-stimulated animals. Based on this, it Freezing occurs during electrical (19,20) and has been suggested that the vlPAG is part of chemical (21,22) stimulation of the same a circuit that mediates conditioned fear, while PAG sites where escape is elicited, often unconditioned fear would use an alternate before it. Freezing induced by stimulation is route that may include the dlPAG. behaviorally similar to conditioned freezing, as both occur concomitantly with muscular Connections of periaqueductal gray tension, hypervigilance and hyperreactivity subdivisions (22). Therefore, these findings suggest that the neural substrates for freezing and escape The connections between PAG and other may overlap. structures of the central nervous system are Fanselow (9) suggested that freezing ob- not homogeneous throughout, suggesting served after dorsal PAG stimulation is simi- some regional specialization. A careful anal- lar to that observed after foot shock. This ysis of these connections may indicate the would imply that dorsal PAG stimulation, functions performed by each subregion. We which is known to be aversive (23) and to describe below the structures that maintain cause conditioning (24), could be respon- connections with the PAG, selected accord- sible for freezing by means of contextual ing to three criteria: a) having strong or conditioning, i.e., stimulation of PAG sites moderate monosynaptic connections with the that give rise to escape would function as an PAG; b) being implicated in the mediation of unconditioned aversive stimulus, that would somatic defense reactions, and c) whose con- become associated with the contextual cues nections are unequally distributed between present during the experimental session. Once PAG subregions. It should be pointed out associated with the stimulation, these cues that data obtained from radioligand assays would provoke
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