Intranasal immune challenge induces sex-dependent depressive-like behavior and cytokine expression in the brain

Abstract

The association between activation of the immune system and mood disorders has been reported by several studies. However, the mechanisms by which the immune system affects mood are only partially understood. In the present study, we detected depressive-like behavior in a rat animal model which involves the induction of inflammation in the nasal cavities by intranasal (i.n.) instillation of bacterial lipopolysaccharides (LPS). Female rats showed depressive-like behavior as evidenced by the forced swim test after repeated i.n. administration of LPS. These responses were not paralleled by alterations in motor activity as measured by the open field test. In the same animals, corticosterone responses after the swimming sessions were the highest of all the groups evaluated. Real-time RT PCR was used to analyze the transcriptional regulation of the cytokines interleukin-1beta, tumor necrosis factor-alpha, and interleukin-6 in several brain regions. Increased tumor necrosis factor-alpha was detected in the hippocampus and brainstem of female rats challenged with i.n. LPS. These results suggest that peripheral inflammation in the upper respiratory tract is an immune challenge capable of inducing depressive-like behavior, promoting exaggerated glucocorticoid responses to stress, and increasing cytokine transcription in the brain. These results further our understanding of the role that the immune system may play in the pathophysiology of depression.

Figure 1

Immobility, swimming, and escape behaviors in inbred Fischer (F344/NHsd) rats in control conditions during 10 min of FST on day 1 (a) and during 5 min of re-exposure on day 2 (b). On day 1 (a), significant increases in immobility and decrease in swimming, and escape times were detected in females evaluated during the proestrus-estrus phase (n = 12) respect to females during diestrus 1 and 2 (n = 12) and to males (n = 21). **Significant proestrus/estrus differences p<0.01. On day 2 (b) significantly higher immobility time and reduced swimming time were detected in female with respect to male rats regardless the phase of the estrous cycle. Values are means±SEM. ^#Significant sex differences p<0.01.

Figure 2

Total motor horizontal activity during 5 min of test in an open field arena (a) and total immobility time during 10 min of FST (b) in male and female rats administered intraperitoneally (i.p.) with control saline solution (n = 24) or LPS (2 mg/kg). The animals were tested after 12 h (n = 24) or 24 h (n = 24) after administration. Values are means±SEM. *Significant differences with respect to control saline conditions (*p<0.05; **p<0.01).

Figure 3

Total motor horizontal activity, center crossings, and total rears during 5 min of open field test (a) and immobility, swimming, and escape time during 10 min of FST (b) on day 1 in female and male rats in control saline conditions (n = 24) or after 24 h (n = 24) of intraperitoneal (i.p.) administration of LPS (2 mg/kg). Values are means±SEM. *Significant differences with respect to control (*p<0.05; **p<0.01).

Figure 4

Total motor horizontal activity during 5 min of test in an open field arena (a) and total immobility time during 10 min of FST (b) in male and female rats administered with a single intranasal (i.n.) dose of LPS (100 μg/rat) (n = 24) or with 2 i.n. instillations of LPS (100 μg/rat each day) in 2 consecutive days (n = 24) or with respective volumes of control saline solution (n = 24). The animals were tested 24 h after the single dose or 24 h after the second dose respectively. Values are means±SEM. **Significant differences with respect to control saline conditions (**p<0.01).

Figure 5

Total motor horizontal activity, center crossings and total rears during 5 min of open field test (a) and immobility, swimming, and escape time during 10 min of FST in day 1 (b) in female and male rats in control saline conditions (n = 24) or after two doses of intranasal (i.n.) administration of LPS (100 μg/rat each day) (n = 24). Values are means±SEM. **Significant differences with respect to control **p<0.01. ^#Significant sex differences (^#p<0.05).

Figure 6

Plasma corticosterone concentrations (ng/ml) as determined by radioimmunoassay in F344/NHsd rats. Values are means±SEM (i.n. saline no-FST): animals administered i.n. with saline and not subjected to the forced swim test (n = 16). (i.n. saline FST): animals administered intranasally with saline and evaluated in the forced swim test (n = 24). (i.n. LPS no-FST): animals administered i.n. with LPS (two doses of 100 μg/rat) and not subjected to the FST (n = 16). (i.n. LPS FST): animals administered i.n. with LPS (two doses of 100 μg/rat) and evaluated in the FST (n = 24). *Significant differences with respect to control (*p<0.05; **p<0.001). ^#Significant sex differences (^#p<0.01; ^##p<0.001).

Figure 7

Relative mRNA cytokine expression in the hippocampus and brainstem of female and male F344/NHsd rats in control saline conditions (n = 12), 24 h after a single intranasal (i.n.) administration of LPS (100 μg/rat) (n = 12) or after two instillations of LPS (100 μg/rat each day) in 2 consecutive days (n = 12). The determinations for IL-1β (a, d), TNF-α (b, e), and IL-6 (c, f) were done by triplicates obtained from the same cDNA preparation for each individual brain region. Values are means±SEM. Values are fold induction with respect to control saline displayed in a logarithmic scale. Hip: hippocampus; Bnstm: brainstem. **Significant differences with respect to control p<0.01. ^#Significant sex-difference, p<0.05.