Effects of methylphenidate on attention in Wistar rats treated with the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)

Theory of visual attention (TVA) applied to rats performing the 5-choice serial reaction time task: differential effects of dopaminergic and noradrenergic manipulations

RATIONALE

Attention is compromised in many psychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). While dopamine and noradrenaline systems have been implicated in ADHD, their exact role in attentional processing is yet unknown.

OBJECTIVES

We applied the theory of visual attention (TVA) model, adapted from human research, to the rat 5-choice serial reaction time task (5CSRTT) to investigate catecholaminergic modulation of visual attentional processing in healthy subjects of high- and low-attention phenotypes.

METHODS

Rats trained on the standard 5CSRTT and tested with variable stimulus durations were treated systemically with noradrenergic and/or dopaminergic agents (atomoxetine, methylphenidate, amphetamine, phenylephrine and atipamezole). TVA modelling was applied to estimate visual processing speed for correct and incorrect visual perceptual categorisations, independent of motor reaction times, as measures of attentional capacity.

RESULTS

Atomoxetine and phenylephrine decreased response frequencies, including premature responses, increased omissions and slowed responding. In contrast, methylphenidate, amphetamine and atipamezole sped up responding and increased premature responses. Visual processing speed was also affected differentially. Atomoxetine and phenylephrine slowed, whereas methylphenidate and atipamezole sped up, visual processing, both for correct and incorrect categorisations. Amphetamine selectively improved visual processing for correct, though not incorrect, responses in high-attention rats only, possibly reflecting improved attention.

CONCLUSIONS

These data indicate that the application of TVA to the 5CSRTT provides an enhanced sensitivity to capturing attentional effects. Unexpectedly, we found overall slowing effects, including impaired visual processing, following drugs either increasing extracellular noradrenaline (atomoxetine) or activating the α1-adrenoceptor (phenylephrine), while also ameliorating premature responses (impulsivity). In contrast, amphetamine had potential pro-attentional effects by enhancing visual processing, probably due to central dopamine upregulation.

The locus coeruleus noradrenergic system gates deficits in visual attention induced by chronic pain

Despite years of research on pain comorbidity with affective disorders and cognitive deficits, it is still unclear how deficit in attention co-occurs with chronic pain. It is likely that altered neuroplasticity and or dysregulated neurotransmitters induced by chronic pain, at which pain and cognitive processing systems overlap, may have a negative effect on cognitive processing such as attention. One of the main common networks involved in attentional and pain processing is the noradrenergic system originating from the locus coeruleus (LC). We hypothesized that heightened noradrenaline release from LC induced by chronic pain could cause a deficit in visual attention. For this purpose, performance on the 5-choice serial reaction time test (5-CSRTT) was tested in animals with and without a chronic constriction injury and a selective depletion of noradrenaline in the LC. In addition, pain sensitivity was measured via mechanical allodynia and thermal hyperalgesia. We found that the increase in pain sensitivity following chronic pain correlates with a decline in executive functions as measured by 5-CSRTT. This was true in conditions of both low and high attentional demand. Interestingly, a selective depletion of noradrenaline in LC improved the attentional deficits caused by chronic pain. We argue that changes to the noradrenergic system originating in LC can improve deficits in visual attention induced by chronic pain. Deficit in attention is a common comorbidity among patients with chronic pain which adversely affects them in their family and work lives. Patients struggle with functional impairment due to pain, and deficite in attention adds to this dysfunction. Our findings identify the NE-LC system as a key mediator between chronic pain and the attentional deficits associated with this. This finding calls for further investigations concerning treatments related to the noradrenergic system to reduce the malicious effects of chronic pain.