Date Published: July 14, 2017
Publisher: Public Library of Science
Author(s): Diána G. Balázsfi, Dóra Zelena, Lívia Farkas, Kornél Demeter, István Barna, Csaba Cserép, Virág T. Takács, Gábor Nyíri, Flóra Gölöncsér, Beáta Sperlágh, Tamás F. Freund, József Haller, Alexandra Kavushansky.
The median raphe region (MRR) is believed to control the fear circuitry indirectly, by influencing the encoding and retrieval of fear memories by amygdala, hippocampus and prefrontal cortex. Here we show that in addition to this established role, MRR stimulation may alone elicit the emergence of remote but not recent fear memories. We substituted electric shocks with optic stimulation of MRR in C57BL/6N male mice in an optogenetic conditioning paradigm and found that stimulations produced agitation, but not fear, during the conditioning trial. Contextual fear, reflected by freezing was not present the next day, but appeared after a 7 days incubation. The optogenetic silencing of MRR during electric shocks ameliorated conditioned fear also seven, but not one day after conditioning. The optogenetic stimulation patterns (50Hz theta burst and 20Hz) used in our tests elicited serotonin release in vitro and lead to activation primarily in the periaqueductal gray examined by c-Fos immunohistochemistry. Earlier studies demonstrated that fear can be induced acutely by stimulation of several subcortical centers, which, however, do not generate persistent fear memories. Here we show that the MRR also elicits fear, but this develops slowly over time, likely by plastic changes induced by the area and its connections. These findings assign a specific role to the MRR in fear learning. Particularly, we suggest that this area is responsible for the durable sensitization of fear circuits towards aversive contexts, and by this, it contributes to the persistence of fear memories. This suggests the existence a bottom-up control of fear circuits by the MRR, which complements the top-down control exerted by the medial prefrontal cortex.
Due to its links to anxiety and post-traumatic stress disorder, the way in which adverse experience is transformed into persistent fear memories is a fundamental question in neuroscience and psychiatry. Preclinical studies making use of the conditioned fear model implicate effector brain regions, such as the amygdala and periaqueductal gray in the etiology of dysfunctional fear states [1, 2]. The stimulation of these brain areas can elicit fear-like behavior (e.g. freezing) even in the absence of traumatic stress or fear learning, and the very same regions undergo plastic changes in response to traumatic stress, which appear causally linked to the development of learned fear. While such studies identify a basic mechanism of fear learning, the neural mechanisms that induce neuronal plasticity in these regions are poorly understood.
The optogenetic activation of the MRR did not trigger freezing acutely, demonstrating that this brain region detaches from fear-effector systems, which elicit freezing when stimulated [1, 2]. Yet MRR stimulation induced agitation that likely reflected its unpleasantness as shown by reduced exploration and shock-runs. Despite the agitation produced, MRR activation did not evoke conditioned fear responses the next day, which indicates that MRR stimulation alone was not sufficient to form an associative memory that linked an adverse experience to a context. By contrast, MRR activation transformed a likely adverse, but not fearful experience into a long-term fear memory trace after a week. It has been demonstrated earlier that MRR has a large impact on the circuits that process stressful experiences, encode them as fear memories, and retrieve this information when the subject is re-exposed to contexts or cues reminiscent of fearful experiences, i.e. the general belief is that its role is indirect [10–12, 14, 15]. Here we show that in addition to this established role, MRR stimulation may per se elicit the emergence of remote fear memories ‒ surprisingly in the absence of recent ones. The main question raised by this finding is how fear emerged over time, if stimulation per se was not fearful nor did it elicit fear 24h after conditioning. In brief, how can fear memories develop “retrospectively” i.e. in the absence of fear learning? We propose that this phenomenon resulted from the sensitization of the fear circuitry to the recall of aversive memories.