4. Discussion
In the present study, a series of experiments were performed to explore
the neurochemical background of social buffering of conditioned fear
responses in rats. In Experiment 1, pretreatment with naloxone, but not
haloperidol, SR49059, or atosiban, increased freezing and decreased
walking and investigation behaviors. These results suggest that naloxone
blocks social buffering, whereas the other three antagonists do not. In
Experiment 2, naloxone did not affect walking steps during the test.
Therefore, the results of Experiment 1 are less likely due to decreased
activity of naloxone. In Experiment 3, Fos expression was increased in
the PVN, supporting the blockade of social buffering by naloxone. In
addition, Fos expression was decreased in the NAc shell, ACC, and IC.
Furthermore, Fos expression in the NAc core tended to be decreased.
Naloxone thus appears to affect these four regions and/or upstream
regions during the blockade of social buffering. From these collective
results, we conclude that naloxone blocks social buffering of
conditioned fear responses in rats.
Based on the present findings, we suggest that certain strains of rats
can induce social buffering because they stimulate opioid receptors.
Animals show distinct social behaviors when they evaluate an individual
as to whether it is a member of the same group and recognize social
similarity to that individual. In rats, the strain can serve as the
group with regard to recognizing social similarity (Ben-Ami Bartalet al. , 2014; Han et al. , 2019; Kogo et al. , 2021;
Kiyokawa et al. , 2022). We previously found that social buffering
is affected by the strain of associates (Nakamura et al. , 2016).
Specifically, social buffering in Wistar rats can be induced by
unfamiliar Wistar, Sprague-Dawley, Long-Evans, or Lewis rats, but not by
unfamiliar Fischer344 or Brown-Norway rats. In addition, autoradiography
data suggested that opioid receptors are stimulated when rats interact
with other rats of the same strain (Panksepp & Bishop, 1981).
Furthermore, the stimulation of opioid receptors is known to ameliorate
stress responses. For example, although rats show an augmented anxiety
response after undergoing acute restraint, this augmentation is blocked
by morphine, an opioid receptor agonist (Joshi et al. , 2014).
These findings suggest that the recognition of social similarity
stimulates opioid receptors, which in turn induces social buffering as a
part of ingroup favoritism. This conclusion is further supported by the
finding that naltrexone, an opioid receptor antagonist, reduces feelings
of social connection in humans (Inagaki et al. , 2016).
We also found in the present study that reduced Fos expression in the
Nac, ACC, and IC accompanied the blockade of social buffering by
naloxone. Based on these results, we hypothesize that opioid receptors
in the NAc play an important role in social buffering. It is known that
the benefit of social interaction is produced by the stimulation of
opioid receptors in the NAc. For example, rats prefer to stay in a
compartment where they had previously interacted with conspecifics
(Tzschentke, 1998; Thiel et al. , 2008). This socially conditioned
place preference was blocked by the administration of the mu-opioid
receptor antagonist CTAP into the NAc immediately before social
interaction (Trezza et al. , 2011). Given that social buffering is
one of the benefits of social interaction, it is possible that social
buffering is also induced by the stimulation of opioid receptors in the
NAc. However, the observed decrease in Fos expression was unexpected.
Stimulation of opioid receptors is known to decrease neuronal
excitability (Stein, 2016). Therefore, Fos expression would be expected
to increase when an antagonist binds to opioid receptors. An alternative
possibility is that opioid receptors responsible for social buffering
are located in brain regions upstream of the NAc. For example, the
ventral tegmental area (VTA) is a region that projects into the NAc and
expresses opioid receptors (Le Merrer et al. , 2009). In addition,
microinjection of the mu-opioid receptor agonist DAMGO into this region
was shown to increase Fos expression in the NAc (Bontempi & Sharp,
1997). Therefore, it is possible that opioid receptors in the VTA,
rather than the NAc, are responsible for social buffering. Furthermore,
it is also possible opioid receptors in the ACC and IC or their upstream
regions contribute to social buffering because the ACC and IC are known
to contribute to empathy (Apps et al. , 2016) and stress
identification (Rogers-Carter et al. , 2018; Djerdjaj et
al. , 2022), respectively, in rats. Further research is needed to
determine the location of the opioid receptors responsible for social
buffering.