Fig. 2. Repeated exendin-4 treatment suppressed the acquisition of aggressive behaviors in male mice.

A schematic illustration of the resident intruder experiment (Panel A) with repeated exendin-4 (Ex4) treatment. At day 1, the mice were trained in the resident intruder paradigm and the time to first attack was scored. This allows stratification (#; with similar attack score between future treatment groups) into Ex4 (2.4 µg/kg, IP, N = 7) or vehicle (Veh, IP, n = 7) injections throughout training (day 2–5) and at the test day (day 6). During the training, the latency to attack was recorded (Panel B), and aggressive behaviors (attack, threat, Panel C) and non-aggressive behaviors (social and non-social, Panel D) were scored for 10 min at the test day. Directly after the test the mice were euthanized, and the brains and plasma from each mouse were collected for ex-vivo analyses (Panel E). A In these mice the baseline latency to attack was similar between future treatment groups (t(12) = 0.04, P = 0.9687). A Repeated treatment with Ex4 during training increased the latency to attack (treatment F(1,12) = 8.10, P = 0.0148, time F(3,36) = 4.09, P = 0.0134, interaction F(3,36) = 2.11, P = 0.1163; n = 7 per group). Post-hoc analysis reveal that this difference is evident at day 5 (P = 0.0023). B This is further evident as the area under the curve for attack latency during training is higher in Ex4 treated mice compared to those treated with vehicle (t(12) = 2.20, P = 0.0455). At test day, Ex4 reduced aggressive behaviors (Panel C), and enhanced social behaviors (Panel D). Compared to vehicle, Ex4 (C) did not affect the attack duration (t(12) = 1.71, P = 0.1131), but (D) reduced attack frequency (t(12) = 2.64, P = 0.0218) and (E) increased attack latency (t(12) = 2.49, P = 0.0286). Ex4 treated mice had no alterations in threat (F) duration (t(12) = 0.15, P = 0.8824), (G) frequency (t(12) = 0.98, P = 0.3448), (H) latency, (t(12) = 0.50, P = 0.6291) compared to vehicle-treated mice. Ex4 did not alter (I) social behavior duration (t(12) = 1.75, P = 0.1065), but (J) increased social behavior frequency (t(12) = 2.89, P = 0.0136) and (K) tended to reduce the social behavior latency (t(12) = 2.07, P = 0.0607). Ex4 did not affect (L) non-social behavior duration (t(12) = 0.33, P = 0.7482). In an attempt to define underlying mechanisms, the levels of monoaminergic neurotransmission in various aggression-related areas and the plasma levels of testosterone or corticosterone were investigated in these mice (Panel E). Compared to vehicle, repeated Ex4 injections to mice in the resident intruder test decreased (M) noradrenalin (NA) (t(12) = 2.78, P = 0.0168), (N) serotonin (5-HT) (t(12) = 2.91, P = 0.0132) and (O) 5-HIAA (t(12) = 2.83, P = 0.0152) levels in nucleus accumbens (NAc). Contrarily, there were no differences between treatments when it comes to (P) 5-HT turnover (t(12) = 0.53, P = 0.6027), (Q) dopamine (DA) (t(12) = 0.66, P = 0.5236), (R) DOPAC (t(12) = 0.14, P = 0.8898), (S) 3-MT (t(12) = 0.72, P = 0.4849) levels or (T) DA turnover (t(12) = 1.36, P = 0.1977) in NAc.In these mice, Ex4 (n = 6, one sample excluded due to contamination) does not alter the levels of (U) corticosterone or (V) testosterone in plasma compared to vehicle (n = 7). The plasma concentrations of (U) corticosterone (t(11) = 1.52, P = 0.1571) and (V) testosterone (t(11) = 0.35, P = 0.7300) did not differ between mice treated with Ex4 (n = 6, one sample contaminated following handling) or vehicle (n = 7) from these experiments. Data are presented as mean ± SEM; significant data are illustrated by *P < 0.05, **P < 0.01.