Anatomical connections of the periaqueductal gray: specific neural substrates for different kinds of fear

Abstract

The periaqueductal gray (PAG) has been traditionally considered to be an exit relay for defensive responses. Functional mapping of its subdivisions has advanced our knowledge of this structure, but synthesis remains difficult mainly because results from lesion and stimulation studies have not correlated perfectly. After using a strategy that combined both techniques and a reevaluation of the available literature on PAG function and connections, we propose here that freezing could be mediated by different PAG subdivisions depending on the presence of immediate danger or exposure to related signaling cues. These subdivisions are separate functional entities with distinct descending and ascending connections that are likely to play a role in different defensive responses. The existence of ascending connections also suggests that the PAG is not simply a final common path for defensive responses. For example, the possibility that indirect ascending connections to the cingulate cortex could play a role in the expression of freezing evoked by activation of the neural substrate of fear in the dorsal PAG has been considered.

Effects of intra-PAG infusion of ovine CRF on defensive behaviors in Swiss-Webster mice

“…The flight behavior or jumping that follows DPAG stimulation [17,40,48] as well as the reports from human patients undergoing neurosurgery [35] give support to this view. Present results with MDTB (Experiment 1) corroborate these evidences since intra-DPAG infusion of oCRF increased escape trials (jump escapes) during the predator approaches and contacts in the forced contact test.…”

Proposal for mechanisms of protection of supine sleep against sudden infant death syndrome: an integrated mechanism review

“…The more ventral and caudal column of the PAG (dorsolateral) evokes a freeze response (74), in which intense discharge of parasympathetic is balanced by equally intense sympathetic discharge. The first 2 mo are a "critical period of organization of the amygdalar-hypothalamic system" (75), the stronger this becomes the less risk for a purely parasympathetic AAE.…”

Genotype-dependent responsivity to inflammatory pain: A role for TRPV1 in the periaqueductal grey

“…The projections from the dorsal raphe nucleus to the DLPAG are serotonergic and known to modulate the activity of neurons involved in defensive behaviour (Lovick, 1994). The DLPAG receives direct neuronal input from the CeA (Rizvi et al, 1991;Vianna et al, 2003). Stimulation of the BLA, which projects to the CeA, produced sympathetic responses which were similar to responses produced by direct DLPAG activation (Soltis et al, 1998).…”