Intranasal Administration of Oxytocin Attenuates Social Recognition Deficits and Increases Prefrontal Cortex Inhibitory Postsynaptic Currents following Traumatic Brain Injury

Abstract

Pediatric traumatic brain injury (TBI) results in heightened risk for social deficits that can emerge during adolescence and adulthood. A moderate TBI in male and female rats on postnatal day 11 (equivalent to children below the age of 4) resulted in impairments in social novelty recognition, defined as the preference for interacting with a novel rat compared with a familiar rat, but not sociability, defined as the preference for interacting with a rat compared with an object in the three-chamber test when tested at four weeks (adolescence) and eight weeks (adulthood) postinjury. The deficits in social recognition were not accompanied by deficits in novel object recognition memory and were associated with a decrease in the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from pyramidal neurons within Layer II/III of the medial prefrontal cortex (mPFC). Whereas TBI did not affect the expression of oxytocin (OXT) or the OXT receptor (OXTR) mRNAs in the hypothalamus and mPFC, respectively, intranasal administration of OXT before behavioral testing was found to reduce impairments in social novelty recognition and increase IPSC frequency in the mPFC in brain-injured animals. These results suggest that TBI-induced deficits in social behavior may be linked to increased excitability of neurons in the mPFC and suggests that the regulation of GABAergic neurotransmission in this region as a potential mechanism underlying these deficits.

Keywords: GABAergic neurotransmission; excitability; intranasal administration; oxytocin; pediatric TBI; social behavior.

Figure 1.

Timeline of experiments. Eleven-day-old rat pups were subjected to TB1 or sham injury. Behavioral experiments were conducted in three separate cohorts of animals beginning at either four weeks postinjury (adolescent group and OXT group) or eight weeks postinjury (adult group). At the conclusion of the behavioral testing in adolescence, animals were used to generate tissue for qRT-PCR experiments, and animals from the OXT group were used for electrophysiological experiments.

Figure 2.

Pediatric TB1 impaired social recognition but not sociability during adolescence. Eleven-day-old male and female rat pups were tested for sociability (stage 2; A, C) and social recognition (stage 3; B, D) at four weeks after injury (adolescent age) using the three-chamber test as described in Materials and Methods. Data are presented as time (in seconds) spent interacting with rat/object (A, B) or as DI (C, D). Open symbols represent sham animals, closed symbols represent injured animals. Bars represent group mean values, and the error bars represent SEM; *p < 0.05.

Figure 3.

Pediatric TB1 impaired social recognition but not sociability during adulthood. Eleven-day-old male and female rat pups were tested for sociability (stage 2; A, C) and social recognition (stage 3; B, D) at eight weeks after injury (adult age) using the three-chamber test as described in Materials and Methods. Data are presented as time (in seconds) spent interacting with rat/object (A, B) or as DI (C, D). Open symbols represent sham animals, closed symbols represent injured animals. Bars represent group mean values, and the error bars represent SEM; *p < 0.05.

Figure 4.

Brain-injured animals did not exhibit impairments in novel object recognition memory. At four weeks (adolescence, A) and eight weeks (adulthood, B) following injury, sham-injured and brain-injured animals were tested for novel object recognition memory as described in Materials and Methods. The DI was calculated as described in Materials and Methods. Open symbols represent sham animals, closed symbols represent injured animals. Bars represent group mean values, and the error bars represent SEM.

Figure 5.

Expression of mRNA for OXT and OXTR in adolescence was not affected following TB1 in 11-d-old rats. After behavioral testing at four to five weeks postinjury, a subset of animals was euthanized and the expression of OXT (A) and OXTR (B) was evaluated in the PVN and mPFC, respectively. Open symbols represent sham animals, closed symbols represent injured animals. Expression (ddCT) values were normalized to sham values. Bars represent group mean values, and the error bars represent SEM.

Figure 6.

Intranasal administration of OXT administration reversed social recognition deficits in brain-injured animals in adolescence. At 1 hr before testing animals in the three-chamber test at four after injury, sham-injured and brain-injured animals were administered with saline, OXT at 20 μg, or oxy at 60 μg as described in Materials and Methods. A, Seconds sniffing in stage 2. B, Seconds sniffing in stage 3. C, DI in stage 2. D, DI in stage 3. E, empty cup; R, rat cup; F, familiar rat; N, novel rat. Open symbols represent sham rats, filled symbols represent injured rats, triangles represent vehicle-treated, squares represent 20-μg oxy-treated rats, diamonds represent 60-μg oxy-treated rats; *p < 0.05 in all panels. In panel C, *p < 0.05 compared with brain-injured animals that received saline. Horizontal lines represent group mean values, and the error bars represent SEM.

Figure 7.

OXT administration did not affect novelty-induced locomotor activity. At five weeks after brain injury and following testing in the three-chamber test, sham-injured and brain-injured animals were tested for locomotor activity. Animals were administered either saline or OXT 60 μg as described in Materials and Methods.

Figure 8.

OXT did not affect membrane properties of Layer II/III pyramidal neurons within the mPFC. Following behavioral testing, slices containing the mPFC were obtained at six to seven weeks after injury and neuronal activity was measured using whole-cell patch clamp electrophysiology as described in Materials and Methods. A, Representative current clamp traces from sham and brain-injured neurons. B, Frequency of neuron firing in response to varying levels of current injection. C, Rheobase. D, Input resistance. E, Membrane voltage. F, Spike threshold. Bars represent mean group values, and error bars represent SEM. Open triangles represent sham cells bathed with aCSF (N = 12), open squares represent sham cells bathed with 1 μm OXT (N = 6), filled triangles represent injured cells bathed with aCSF (N = 13), and filled squares represent injured cells bathed with OXT (N = 10); *p < 0.05.

Figure 9.

OXT did not affect spontaneous EPSCs in Layer II/III neurons within the mPFC. A, Representative traces of spontaneous EPSCs recorded from Layer II/III pyramidal neurons within the mPFC. B, Frequency of spontaneous EPSCs. C, Amplitude of spontaneous EPSCs. Bars represent group mean value, and error bars represent SEM. Open triangles represent sham cells bathed with aCSF (N = 16), open squares represent sham cells bathed with 1 μm OXT (N = 11), filled triangles represent injured cells bathed with aCSF (N = 20), filled squares represent injured cells bathed with 1 μm OXT (N = 19).

Figure 10.

OXT increased the frequency of spontaneous IPSCs in Layer II/III neurons within the mPFC. A, Representative traces of spontaneous IPSCs recorded from Layer II/III pyramidal neurons within the mPFC. B, Frequency of spontaneous IPSCs. C, Amplitude of spontaneous IPSCs. Bars represent group mean value, and error bars represent SEM. Cumulative probability of small IPSC frequencies in sham (D) and injured cells (E). F, G, Cumulative probability of large IPSC frequencies in sham (F) and injured cells (G). Cumulative probability of small IPSC amplitudes in sham (H) and injured cells (I). Cumulative probability of large IPSC amplitudes in sham (J) and injured cells (K; Ryan et al., 2016; Douglas, 2020). Open triangles represent sham cells bathed with aCSF (N = 13), open squares represent sham cells bathed with 1 μm OXT (N = 12), filled triangles represent injured cells bathed with aCSF (N = 17), filled squares represent injured cells bathed with 1 μm OXT (N = 15); *p < 0.05.