Date Published: January 24, 2019
Publisher: Public Library of Science
Author(s): Laura Berg, Josephine Eckardt, Olivia Andrea Masseck, Alexandra Kavushansky.
We show that in an animal model of anxiety the overall excitation, particularly in the infralimbic region of the medial prefrontal cortex (IL), is increased and that the activity ratio between excitatory pyramidal neurons and inhibitory interneurons (AR PN/IN) is shifted towards excitation. The same change in AR PN/IN is evident for wildtype mice, which have been exposed to an anxiety stimulus. We hypothesize, that an elevated activity and the imbalance of excitation (PN) and inhibition (IN) within the neuronal microcircuitry of the prefrontal cortex is responsible for anxiety behaviour and employed optogenetic methods in freely moving mice to verify our findings. Consistent with our hypothesis elevation of pyramidal neuron activity in the infralimbic region of the prefrontal cortex significantly enhanced anxiety levels in several behavioural tasks by shifting the AR PN/IN to excitation, without affecting motor behaviour, thus revealing a novel mechanism by which anxiety is facilitated.
Anxiety disorders belong to the most common mental illnesses within the western world affecting a total amount of about 20% of the human population, a lifetime prevalence of nearly 30% and with a steadily increasing occurrence [1–3]. One of the key areas involved in the expression of anxiety and fear, beside the hippocampus and the amygdala, is the infalimbic region (IL) of the medial prefrontal cortex (mPFC) [4–9], which is the rodent homolog to the anterior cingulate cortex of humans. In contrast to fear, which is a specific in time limited response to a threatening stimulus, often investigated with fear conditioning, anxiety is a sustained negative emotional state, in which unknown or uncertain situations are encountered with defensive behaviour .
All experiments in this study were approved by the Institutional Animal Research Facility (Landesamt für Natur, Umwelt und Verbraucherschutz NRW). Mice were housed in standardized cages and maintained on a 12:12 (h) light-dark cycle (lights on from10.00 pm-10.00am) in a temperature and humidity controlled scantainer. Food and water were available ad libitum. All behavioral experiments were performed in the main activity phase of the animals. Experiments were performed in C56Bl/6J mice (Jackson Laboratory) and in two different transgenic mousselines with a C56Bl/6J background obtained from collaborators. For characterizing the activity pattern of different neuron types during anxiety 5-HT1A(-/-) mice (kindly provided by L. Tecott, University of California San Fransisco) were used. To specifically express ChR2 or a fluorescent control plasmid (td-tomato) in pyramidal neurons Nex-Cre(-/-) mice were used (kindly provided by K. Nave and S. Goebbels, Max Planck Institute für Experimentelle Medizin) .
By combining behavioural and immunohistochemistry studies we could show, that in a model of anxiety an imbalance of excitatory pyramidal neurons and inhibitiory interneuron activity in the IL is evident and that optogenetic elevation of the pyramidal neuron activity in the IL is sufficient to resemble this phenotype and drives anxiety behaviour. In conjunction with current models of anxiety our study provides further evidence that a functional decrease of serotonergic neurotransmission at postsynaptic sites and subsequent disinhibition of IL pyramidal neurons may be a pathogenic mechanism of anxiety [9,26].