Periaqueductal grey circuits mediating freezing and flight

PHILIP TOVOTE; MARIA SOLEDAD ESPOSITO; JONATHAN P. FADOK; PAOLO BOTTA; STEFFEN B. E. WOLFF; SILVIA ARBER; ANDREAS LÜTHI

Milan

Congreso; 9th Fens Forum of Neuroscience; 2014

Fens Forum of Neuroscience

The midbrain periaqueductal gray (PAG) mediates adaptive defensive responses to aversive stimuli. It plays a major role in expression of passive (e.g. freezing) and active (e.g. flight) fear responses. Via a strong projection to the PAG, the central nucleus of the amygdala (CeA) mediates behavioral fear responses. Little is known about circuit mechanisms within the PAG underlying those behaviors and rapid switching between them. We first identified a GABAergic projection from CeA to PAG using retrograde bead tracing in GAD67-GFP mice. We then combined anterograde tracing with AAV virus injected into PAG and retrograde tracing with mutated rabies virus injected into spinal cord or forelimb muscles to identify potential pre-motor areas involved in defensive behavior. Rabies-infected cells were found in medullary axonal projection areas of PAG neurons such as lateral paragigantocellular (LPGi) and gigantocellular nuclei. Cell-specific trans-synaptic rabies tracing revealed CeA input to a specific GABAergic cell cluster within vlPAG projecting to LPGi. We then took an optogenetic approach to identify cell types within ventrolateral (vlPAG), and dorsal/lateral (dl/lPAG) columns of PAG contributing to freezing versus flight responses. We conditionally expressed Channelrhodopsin-2 (ChR2) and halorhodopsin (NpHR) within specific subpopulations of neurons in vlPAG and dl/lPAG. Light-mediated inhibition of inhibitory GABAergic cells in vlPAG resulted in freezing whereas light activation of excitatory glutamatergic cells in dl/lPAG elicited marked flight responses. In vivo single-unit recordings revealed strong correlation between freezing and neuronal activity. Using in vivo calcium imaging through an optical fiber implanted into PAG, we observed correlation between cellular activity in glutamatergic cells and locomotor activity. Our findings confirm involvement of dl/lPAG neurons in flight behavior, whereas vlPAG cells contribute to freezing, and suggest specific roles of defined neuronal subpopulations and circuits within PAG for expression of passive versus active defensive behavior.