Melatonin ameliorates cognitive memory by regulation of cAMP-response element-binding protein expression and the anti-inflammatory response in a rat model of post-traumatic stress disorder

Abstract

Background: Post-traumatic stress disorder (PTSD) is an important psychological disease that can develop following the physical experience or witnessing of traumatic events. The psychopathological response to traumatic stressors increases inflammation in the hippocampus and induces memory deficits. Melatonin (MTG) plays critical roles in circadian rhythm disorders, Alzheimer's disease, and other neurological disorders. However, the cognitive efficiency of MTG and its mechanisms of action in the treatment of PTSD remain unclear. Thus, the present study investigated the effects of MTG on spatial cognitive impairments stimulated by single prolonged stress (SPS) in rats, an animal model of PTSD. Male rats received intraperitoneal (i.p.) administration of various doses of MTG for 21 consecutive days after the SPS procedure.

Results: SPS-stimulated cognitive impairments in the object recognition task and Morris water maze were reversed by MTG treatment (25 mg/kg, i.p). Additionally, MTG significantly increased cognitive memory-related decreases in cAMP-response element-binding (CREB) protein and mRNA levels in the hippocampus. Our results also demonstrate that MTG significantly inhibited SPS-stimulated cognitive memory impairments by inhibiting the expression of proinflammatory cytokines, including tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) in the rat brain.

Conclusion: The present results indicate that MTG can be beneficial for SPS-stimulated memory impairments via changes in CREB expression and proinflammatory mediators. Thus, MTG may be a prophylactic strategy for the prevention or mitigation of the progression of some features of the PTSD pathology.

Keywords: Melatonin; Memory; Post-traumatic stress disorder; Proinflammatory cytokines; cAMP-response element-binding protein.

Fig. 1

Experimental protocol for single-prolonged stress (SPS)-induced memory impairment and melatonin (MTG) treatment in rats. Groups of six or seven rats were used for each experimental condition. OFT open field test, ORT object recognition test, MWM Morris water maze test

Fig. 2

Effects of MTG on plasma corticosterone (CORT) levels in rats with SPS-induced memory impairments: assessed with an enzyme-linked immunosorbent assay (ELISA). Parameters were determined at the end of the experiments. Data are expressed as the mean ± SEM of 4 animals in each group. *p < 0.05 versus the CON group; ^#p < 0.05 versus the SPS group

Fig. 3

Effects of MTG on recognition memory assessed by the novel object recognition test (ORT) in which the time spent sniffing familiar and novel objects during a 3-min choice trial (a) and the ability to discriminate (b) between familiar and novel objects were measured. The Morris water maze (MWM) test was used to assess the effects of MTG on spatial learning and memory. Time to escape (latency) from the water onto a submerged platform during acquisition trials (c), percentages of time spent in the target quadrant (d), percentages of distance traversed in the target quadrant (e), and swimming speed (f) were used as outcome measures. The open field test (OFT) was used to assess the effect of MTG on locomotor activity (counts) and total number of rearing bouts (g). Six or seven rats were used per treatment group. Data are represented as the mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 versus the CON group; ^#p < 0.05, ^##p < 0.01, ^###p < 0.001 versus the SPS group

Fig. 4

Effects of MTG on brain-derived neurotrophic factor (BDNF) and cAMP-response element-binding (CREB) protein levels (a and b) and BDNF and CREB mRNA expression in the hippocampus of rats with SPS-induced memory impairments. Polymerase chain reaction (PCR) bands on agarose gels and relative intensities (c). BDNF and CREB mRNA expression was normalized to the expression of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA as an internal control. Parameters were determined at the end of the experiments. Data are expressed as the mean ± SEM of 4 animals in each group. *p<0.05, **p < 0.01 versus the CON group; ^#p < 0.05, ^##p < 0.01 versus the SPS group

Fig. 5

Effects of MTG on tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) protein levels (a and b) and TNF-α and IL-6 mRNA expression in the hippocampus of rats with SPS-induced memory impairments. PCR bands on agarose gels and relative intensities (c). TNF-α and IL-6 mRNA levels were normalized to GAPDH levels as an internal control. Parameters were determined at the end of the experiments. Data are expressed as the mean ± SEM of 4 animals in each group. *p < 0.05, **p < 0.01 versus the CON group; ^#p < 0.05 versus the SPS group

Fig. 6

Effects of MTG on the mean number of CREB-stained hippocampal areas after the MWM test. Representative photographs and relative percentages are shown in Fig. 6. The scale bar represents 100 μm. Representative images of immunoreactive neurons are shown above each graph. Their values were calculated as a percentage of the corresponding value of the control (CON) group. Data are expressed as the mean ± SEM of 3 animals in each group. *p < 0.05, **p < 0.01 versus the CON group; ^#p < 0.05 versus the SPS group