Raphe GABAergic neurons mediate the acquisition of avoidance after social defeat.

AUTHORS Challis C, Boulden J, Veerakumar A, Espallergues J, Vassoler FM, Pierce RC, Beck SG, Berton O.
J Neurosci. 2013 Aug 28;33(35):13978-88. PMID: 23986235

 

Objective:   The ascending serotonin (5-HT) pathway originates primarily in the dorsal raphe nucleus (DRN) and is known to play a role in aggression, avoidance, dominance, and a variety of other behaviors.  Though it is known that the DRN has a strong involvement in behavior, surprisingly little is known about the regulation of the heterogeneous cells in this area.  While much of the existing literature about this area has been about the serotonergic cells of the DRN, this paper examines the role of DRN GABAergic neurons in the negative regulation of the 5-ht neurons of the DRN.

Methods:    Mice were bred to express tdTomato in either GABAergic or serotonergic cells by mating floxed-stop reporter mice with mice expressing Cre under the control cell-type-specific promoters.  Mice expressing Cre in GABAergic neurons were injected with AAV that delivered a Cre-inducible archeorhodopsin in order to inhibit the neurons with photostimulation.  Mice were fitted with fiber optic cannula in to the DRN to allow for photoactivation of the inhibitory light-gated anion channel, archeorhodopsin.  After 6 weeks, mice were challenged with repeated social defeat, open field test, and elevated plus-maze.  In some mice, slices of brain containing the DRN were taken for whole-cell patch clamp recording.  Mice were transcardially perfused with 4% paraformaldehyde and the brains were processed for immunohistochemistry.  

Results:

·         Mice experiencing repeated social defeat showed an increase in cFos expression in the ventrolateral DRN, where 5-HT neurons are sparse.  This area corresponded to regions receiving input from the prefrontal cortex, which is involved in social cognition.  There was increased cFos in cells also expressing a marker for GABA synthesis.

·         GABAergic neurons were located by tdTomato expression and used for whole-cell recording.  They were found to have properties different from 5-HT cells, in terms of increased spontaneous activity, shorter action potential duration, and higher membrane resistance.  GABAergic neurons from mice susceptible to social defeat showed more excitatory post synaptic currents as well as more action potentials per current injection than controls or mice resistant to defeat.  5-HT neurons of mice susceptible to social defeat showed decreased excitability and more inhibitory post synaptic currents.

·         Stimulation of slices with 543nm light activated archeorhodopsin, resulting in silencing of spontaneous action potentials in GABA neurons.  Photoactivation of archeorhodopsin caused a significant decrease in the frequency of miniature inhibitory postsynaptic potentials in 5-HT neurons, demonstrating a local monosynaptic inhibitory connection between GABA producing neurons and serotonergic neurons within the dorsal raphe nucleus (for the first time in the literature).  When the GABAergic cells were silenced during the social defeat training protocol, mice showed significantly less defeat (time spent interacting) during the testing phase than mice that did not have photo-silencing of GABAergic neurons.  However, when GABAergic neurons were silenced during the testing phase (after the development of social defeat), photostimulation did not increase interaction time.  This indicates that GABAergic neurons of the DRN are involved in encoding of the defeat behavior, but not during the execution of pre-established defeat.

·         In contrast with suggestions in previous literature, optogenetic inhibition of the encoding of social defeat did not result in a decrease in more generalized anxiety as shown by the open field and eleveated plus maze tests.

 

-DH