Nitric oxide modulates the hyperalgesic response to mechanical noxious stimuli in sleep-deprived rats.

BACKGROUND: Sleep restriction alters pain perception in animals and humans, and many studies have indicated that paradoxical sleep deprivation (PSD) promotes hyperalgesia. The hyperalgesia observed after mechanical nociceptive stimulus is reversed through nitric oxide synthase (NOS) inhibition. Both nitric oxide (NO) and the dorsolateral periaqueductal gray matter (dlPAG) area of the brainstem are involved in hyperalgesia. Thus, in this work, we investigated the pain-related behavior response after mechanical noxious stimuli (electronic von Frey test), and the activity of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), an indicator of NOS activity, within the dlPAG of paradoxical sleep-deprived rats. We also evaluated the effects of pre-treatment with L-NAME on these parameters. RESULTS: These data revealed that PSD reduced the hindpaw withdrawal threshold (-47%, p < 0.0001) confirming the hyperalgesic effect of this condition. In addition, there were more NADPH-d positive cells in dlPAG after PSD than in control rats (+ 59%, p < 0.0001). L-NAME treatment prevented the reduction in the hindpaw withdrawal threshold (+ 93%, p < 0.0001) and the increase in the NADPH-d positive cells number in the dlPAG of PSD-treated rats (-36%, p < 0.0001). CONCLUSION: These data suggest that the hyperalgesic response to mechanical noxious stimuli in paradoxical sleep-deprived rats is associated with increased NOS activity in the dlPAG, which presumably influences the descending antinociceptive pathway.

Increased pain perception and attenuated opioid antinociception in paradoxical sleep-deprived rats are associated with reduced tyrosine hydroxylase staining in the periaqueductal gray matter and are reversed by L-dopa.

Paradoxical sleep deprivation (PSD) increases pain sensitivity and reduces morphine antinociception. Because dopaminergic neurons in the periaqueductal gray matter (PAG) participate in pain modulation and opioid-induced antinociception, we evaluated the effects of PSD on thermal pain sensitivity, morphine- and L-DOPA-induced antinociception and dopaminergic functionality in the PAG by assessing tyrosine hydroxylase (TH) immunoreactivity. Rats that were subjected to 96h of PSD received vehicle, morphine (2.5, 5 or 10mg/kg), L-DOPA (50 or 100mg/kg) or L-DOPA (50mg/kg)+morphine (2.5 and 5mg/kg) and were tested with a 46°C hot plate 1h after. The paw withdrawal latency responses to the hot plate were decreased in PSD rats and were modified by the highest dose of morphine, L-DOPA and L-DOPA+morphine. Analgesic effects were observed in control groups for all of the morphine doses as well as 100mg/kg of L-DOPA and L-DOPA (50mg/kg)+morphine (5mg/kg). The number of cell bodies that were immunopositive for TH in the PAG was reduced in PSD rats. In conclusion, increased thermal sensitivity was reversed by L-DOPA and could be caused by a reduction TH levels in the PAG. Our data also suggest a relationship between central dopaminergic networks and opiate-induced analgesia in rats.

Systemic amitriptyline administration does not prevent the increased thermal response induced by paradoxical sleep deprivation.

Sleep deprivation has been associated with hyperalgesia in humans and in animal models. The tricyclic antidepressant amitriptyline is used as an analgesic drug in patients and in animal models of chronic pain, including that associated with spinal nerve injury. Pain hypersensitivity following paradoxical sleep deprivation (PSD) and that following peripheral nerve injury seem to share common spinal mechanisms. Accordingly, we evaluated the effects of amitriptyline (acutely and chronically administered) on the increased thermal response observed in PSD rats (72 or 96 h). Rats were evaluated for thermal sensitivity using a hot plate (52 degrees C or 46 degrees C) at 1 or 24 h after the last administration of the drug. Following the hot plate test, motor behavior was analyzed in an open field arena for a period of 5 min. Paw withdrawal latency response to temperatures of 46 degrees C and 52 degrees C was significantly lower in PSD and in 24-hour post-PSD rats than in controls and it was not modified by amitriptyline (3, 10 and 30 mg/kg). Analgesic effects and reduced motor behavior were only observed in control groups. Overall, these findings indicate that a period of PSD can influence pain modulatory mechanisms, and that amitriptyline action is insufficient to reduce PSD-enhanced thermal sensitivity.