title: Neural Correlates of Fear in the Periaqueductal Gray.
abstract: The dorsal and ventral periaqueductal gray (dPAG and vPAG, respectively) are embedded in distinct survival networks that coordinate, respectively, innate and conditioned fear-evoked freezing. However, the information encoded by the PAG during these survival behaviors is poorly understood. Recordings in the dPAG and vPAG in rats revealed differences in neuronal activity associated with the two behaviors. During innate fear, neuronal responses were significantly greater in the dPAG compared with the vPAG. After associative fear conditioning and during early extinction (EE), when freezing was maximal, a field potential was evoked in the PAG by the auditory fear conditioned stimulus (CS). With repeated presentations of the unreinforced CS, animals displayed progressively less freezing accompanied by a reduction in event-related field potential amplitude. During EE, the majority of dPAG and vPAG units increased their firing frequency, but spike-triggered averaging showed that only ventral activity during the presentation of the CS was significantly coupled to EMG-related freezing behavior. This PAG-EMG coupling was only present for the onset of freezing activity during the CS in EE. During late extinction, a subpopulation of units in the dPAG and vPAG continued to show CS-evoked responses; that is, they were extinction resistant. Overall, these findings support roles for the dPAG in innate and conditioned fear and for the vPAG in initiating but not maintaining the drive to muscles to generate conditioned freezing. The existence of extinction-susceptible and extinction-resistant cells also suggests that the PAG plays a role in encoding fear memories.
  
  
  
  The periaqueductal gray (PAG) orchestrates survival behaviors, with the dorsal (dPAG) and ventral (vPAG) PAG concerned respectively with innate and learnt fear responses. We recorded neural activity from dPAG and vPAG in rats during the expression of innate fear and extinction of learned freezing. Cells in dPAG responded more robustly during innate fear, but dPAG and vPAG both encoded the time of the conditioned stimulus during early extinction and displayed extinction sensitive and resistant characteristics. Only vPAG discharge was correlated with muscle activity, but this was limited to the onset of conditioned freezing. The data suggest that the roles of dPAG and vPAG in fear behavior are more complex than previously thought, including a potential role in fear memory.
authors.affiliation: School of Physiology, Pharmacology and Neuroscience, Biomedical Sciences Building, University of Bristol, University Walk, Bristol BS8 1TD, United Kingdom, and Thomas.watson203@gmail.com.|Neuroscience Paris Seine, Cerebellum, Navigation, and Memory Team, Sorbonne Universities, Université Pierre et Marie Curie, University of Paris 06 Unité Mixte de Recherche Scientifique 8246, INSERM Unité Mixte de Recherche Scientifique 1130, and Centre National de la Recherche Scientifique Unité Mixte de Recherche 8246, F-75005 Paris, France.
authors.forename: Thomas C
authors.identifier: 0000-0002-5146-6981
authors.initials: TC
authors.lastname: Watson
authors.affiliation: School of Physiology, Pharmacology and Neuroscience, Biomedical Sciences Building, University of Bristol, University Walk, Bristol BS8 1TD, United Kingdom, and.
authors.forename: Nadia L
authors.identifier: 0000-0001-8207-5871
authors.initials: NL
authors.lastname: Cerminara
authors.affiliation: School of Physiology, Pharmacology and Neuroscience, Biomedical Sciences Building, University of Bristol, University Walk, Bristol BS8 1TD, United Kingdom, and.
authors.forename: Bridget M
authors.identifier: 0000-0002-0268-6419
authors.initials: BM
authors.lastname: Lumb
authors.affiliation: School of Physiology, Pharmacology and Neuroscience, Biomedical Sciences Building, University of Bristol, University Walk, Bristol BS8 1TD, United Kingdom, and.
authors.forename: Richard
authors.identifier: 0000-0002-2064-3359
authors.initials: R
authors.lastname: Apps
mesh_terms: D000161:Acoustic Stimulation; D000818:Animals; D003213:Conditioning, Psychological / Q000502:physiology; D004576:Electromyography; D005071:Evoked Potentials / Q000502:physiology; D005108:Extinction, Psychological / Q000502:physiology; D005239:Fear / Q000502:physiology*; D008297:Male; D010487:Periaqueductal Gray / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar
keywords: EMG; extinction; fear; fear conditioning; freezing; periaqueductal gray