Abstract
Chronic stress impairs auditory attention in rats and monoamines regulate neurotransmission in the primary auditory cortex (A1), a brain area that modulates auditory attention. In this context, we hypothesized that norepinephrine (NE) levels in A1 correlate with the auditory attention performance of chronically stressed rats. The first objective of this research was to evaluate whether chronic stress affects monoamines levels in A1. Male Sprague–Dawley rats were subjected to chronic stress (restraint stress) and monoamines levels were measured by high performance liquid chromatographer (HPLC)-electrochemical detection. Chronically stressed rats had lower levels of NE in A1 than did controls, while chronic stress did not affect serotonin (5-HT) and dopamine (DA) levels. The second aim was to determine the effects of reboxetine (a selective inhibitor of NE reuptake) on auditory attention and NE levels in A1. Rats were trained to discriminate between two tones of different frequencies in a two-alternative choice task (2-ACT), a behavioral paradigm to study auditory attention in rats. Trained animals that reached a performance of ≥80% correct trials in the 2-ACT were randomly assigned to control and stress experimental groups. To analyze the effects of chronic stress on the auditory task, trained rats of both groups were subjected to 50 2-ACT trials 1 day before and 1 day after of the chronic stress period. A difference score (DS) was determined by subtracting the number of correct trials after the chronic stress protocol from those before. An unexpected result was that vehicle-treated control rats and vehicle-treated chronically stressed rats had similar performances in the attentional task, suggesting that repeated injections with vehicle were stressful for control animals and deteriorated their auditory attention. In this regard, both auditory attention and NE levels in A1 were higher in chronically stressed rats treated with reboxetine than in vehicle-treated animals. These results indicate that NE has a key role in A1 and attention of stressed rats during tone discrimination.
Highlights
Stress is a non-specific biological response of an organism to environment demands that affect their homeostasis
In both control and chronically stressed rats, the 5-Hydroxyindoleacetic acid (5-HIAA) concentrations were significantly higher than 5-HT levels (F(1,10) = 37.63, p < 0.001; Figure 1B), while the 5-HIAA/5-HT ratio was higher than the dopac/DA ratio (F(1,10) = 62.55, p < 0.0001; Figure 1D)
Chronically stressed rats treated with reboxetine that were subjected to acute swimming had lower levels of corticosterone than chronically stressed rats treated with vehicle (p < 0.001; Figure 2B)
Summary
Stress is a non-specific biological response of an organism to environment demands that affect their homeostasis. When the stressors are too intense and persistent (chronic stress), stress responses may become maladaptive, affecting brain structure and function (Tafet and Bernardini, 2003) In this sense, we have shown that the rat auditory system is sensitive to the effects of chronic stress (Dagnino-Subiabre, 2013). Chronic stress and chronic corticosterone treatment induce dendritic atrophy in the central auditory pathways, such as the pyramidal neurons from primary auditory cortex (A1; Dagnino-Subiabre et al, 2005, 2009, 2012; Bose et al, 2010) These morphological alterations correlate with lower inhibitory synaptic efficacy in A1 compared to controls not exposed to chronic stress, as well as impairments in the auditory attention (Pérez et al, 2013)
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