Abstract
This study examined the analgesic effect of cocaine in Spontaneously Hypertensive Rats (SHR), which are considered a suitable model for the study of attention deficit hyperactivity disorder (ADHD), and in Wistar (WIS) rats of both sexes using the hot-plate test. In addition, we tested whether habituation to the unheated hot-plate apparatus, that "normalizes" the basal hypoalgesic phenotype of SHR, alters the subsequent cocaine-induced analgesia (CIA) in this strain. SHR of both sexes were hypoalgesic compared to WIS rats in the hot-plate test and showed higher sensitivity to CIA. Habituation to the unheated hot-plate reduced the basal nociceptive latency of SHR, suggesting cognitive/emotional modulation of pain in this strain, but did not alter the magnitude of CIA. The present study shows increased sensitivity to CIA in SHR, which may be related to abnormalities in the mesocorticolimbic dopaminergic system. Further studies using SHR strain may reveal new information on the neurobiological mechanisms underlying ADHD and its co-morbidity with drug addiction.
Highlights
Pain is a complex and subjective experience that involves the transduction of noxious stimuli by nociceptive fibers, and cognitive and emotional aspects [1]
In the hot-plate test, Spontaneously Hypertensive Rats (SHR) are hypoalgesic when compared to rats of other strains [4,5,7,8], but they show normal properties of nociceptive fibers [9]
We have recently reported that hypoalgesia was no longer observed in SHR rats after habituation to the unheated hot-plate apparatus, suggesting that their hypoalgesic phenotype may involve cognitive processes [8]
Summary
Pain is a complex and subjective experience that involves the transduction of noxious stimuli by nociceptive fibers, and cognitive and emotional aspects [1]. We have recently reported that hypoalgesia was no longer observed in SHR rats after habituation to the unheated hot-plate apparatus, suggesting that their hypoalgesic phenotype may involve cognitive processes (e.g. distraction) [8] This is consistent with the fact that SHR have been considered an animal model for the study of attention deficit hyperactivity disorder (ADHD), since they show inattention and impulsivity/hyperactivity [10,11]. Confirming our previous study [8], habituation of SHR to the unheated hot-plate resulted in a striking reduction of the basal nociceptive latency These findings, taken together with the fact that the SHR strain is considered a suitable model of ADHD [10,11], emphasizes that SHR hypoalgesia probably involves cognitive/emotional processes. Further studies using SHR strain may reveal new information on the neurobiological mechanisms underlying ADHD and its co-morbidity with drug addiction
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