Contingent Social Interaction Does Not Prevent Habituation towards Playback of Pro-Social 50-kHz Calls: Behavioral Responses and Brain Activation Patterns.

Rats, which are highly social animals, are known to communicate using ultrasonic vocalizations (USV) in different frequency ranges. Calls around 50 kHz are related to positive affective states and promote social interactions. Our previous work has shown that the playback of natural 50-kHz USV leads to a strong social approach response toward the sound source, which is related to activation in the nucleus accumbens. In male Wistar rats, the behavioral response habituates, that is, becomes weaker or is even absent, when such playback is repeated several days later, an outcome found to be memory-dependent. Here, we asked whether such habituation is due to the lack of a contingent social consequence after playback in the initial test and whether activation of the nucleus accumbens, as measured by c-fos immunohistochemistry, can still be observed in a retest. To this end, groups of young male Wistar rats underwent an initial 50-kHz USV playback test, immediately after which they were either (1) kept temporarily alone, (2) exposed to a same-sex juvenile, or (3) to their own housing group. One week later, they underwent a retest with playback; this time not followed by social consequences but by brain removal for c-fos immunohistochemistry. Consistent with previous reports, behavioral changes evoked by the initial exposure to 50-kHz USV playback included a strong approach response. In the retest, no such response was found, irrespective of whether rats had experienced a contingent social consequence after the initial test or not. At the neural level, no substantial c-fos activation was found in the nucleus accumbens, but unexpected strong activation was detected in the anterior cingulate cortex, with some of it in GABAergic cells. The c-fos patterns did not differ between groups but cell numbers were individually correlated with behavior, i.e., rats that still approached in response to playback in the retest showed more activation. Together, these data do not provide substantial evidence that the lack of a contingent social consequence after 50-kHz USV playback accounts for approach habituation in the retest. Additionally, there is apparently no substantial activation of the nucleus accumbens in the retest, whereas the exploratory findings in the anterior cingulate cortex indicate that this brain area might be involved when individual rats still approach 50-kHz USV playback.

High trait anxiety in mice is associated with impaired extinction in the contextual fear conditioning paradigm.

Trait anxiety is a form of chronic state anxiety that can be part of the individual's personality and one of the individual factors that increase the risk of developing PTSD. To be able to distinguish between trait and state anxiety in animal models might be challenging, since the tests themselves are often anxiogenic. One possible approach is the use of the free exploratory paradigm (FEP), a model that consists of providing the animal the choice to explore both a familiar and a novel environment. High trait anxiety animals choose to explore more the familiar environment. Thus, the objective of this study was to evaluate if differences in trait anxiety could lead to impairments in acquisition and extinction of fear memory in a contextual fear conditioning protocol. First, Swiss mice were divided into high trait anxiety (HTA) or low trait anxiety (LTA) based on their exploration of the novel environment (%TNS) in the FEP. We observed that the %TNS was stable across three testing sessions. Also, we observed that freezing behavior was not different between HTA and LTA mice in a retrieval session 1 day after the conditioning, which indicates that acquisition was not impaired. However, HTA presented higher freezing time during extinction training and test. Also, HTA presented higher freezing in reinstatement test, but this might be related to the poor extinction learning. Moreover, diazepam (0.25 or 1.0 mg/kg, i.p.) did not prevent the differences in extinction training and test when administer 30 min before conditioning training or 30 min before extinction training, even though diazepam (1.0 mg/kg) had anxiolytic effect in the elevated plus-maze. These results demonstrate that HTA mice presented increase in freezing during extinction training and test, as well as during reinstatement. These results indicate that increased freezing time in HTA mice is not explained by high state anxiety in a specific phase of the fear conditioning, but more likely, the overall high trait anxiety throughout the entire experiment.

Limited generalizability, pharmacological modulation, and state-dependency of habituation towards pro-social 50-kHz calls in rats.

Communication constitutes a fundamental component of mammalian social behavior. Rats are highly social animals and emit 50-kHz ultrasonic vocalizations (USV), which function as social contact calls. Playback of 50-kHz USV leads to strong and immediate social approach responses in receiver rats, but this response is weak or even absent during repeated 50-kHz USV playback. Given the important role of 50-kHz USV in initiating social contact and coordinating social interactions, the occurrence of habituation is highly unexpected. It is not clear why a social signal characterized by significant incentive salience loses its power to change the behavior of the receiver so rapidly. Here, we show that the habituation phenomenon displayed by rats in response to repeated playback of 50-kHz USV (1) is characterized by limited generalizability because it is present in Wistar but not Sprague-Dawley rats, (2) can be overcome by amphetamine treatment, and (3) depends on the subject's internal state.