Thermal nociception is decreased by hypocretin-1 and an adenosine A1 receptor agonist microinjected into the pontine reticular formation of Sprague Dawley rat

Abstract

Clinical and preclinical data concur that sleep disruption causes hyperalgesia, but the brain mechanisms through which sleep and pain interact remain poorly understood. Evidence that pontine components of the ascending reticular activating system modulate sleep and nociception encouraged the present study testing the hypothesis that hypocretin-1 (orexin-A) and an adenosine receptor agonist administered into the pontine reticular nucleus, oral part (PnO) each alter thermal nociception. Adult male rats (n = 23) were implanted with microinjection guide tubes aimed for the PnO. The PnO was microinjected with saline (control), hypocretin-1, the adenosine A(1) receptor agonist N(6)-p-sulfophenyladenosine (SPA), the hypocretin receptor-1 antagonist N-(2-Methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl-urea (SB-334867), and hypocretin-1 plus SB-334867. As an index of antinociceptive behavior, the latency (in seconds) to paw withdrawal away from a thermal stimulus was measured following each microinjection. Compared to control, antinociception was significantly increased by hypocretin-1 and by SPA. SB-334867 increased nociceptive responsiveness, and administration of hypocretin-1 plus SB-334867 blocked the antinociception caused by hypocretin-1. These results suggest for the first time that hypocretin receptors in rat PnO modulate nociception.

Perspective: Widely distributed and overlapping neural networks regulate states of sleep and pain. Specifying the brain regions and neurotransmitters through which pain and sleep interact is an essential step for developing adjunctive therapies that diminish pain without disrupting states of sleep and wakefulness.

Figure 1

Histological localization of microinjection sites. A. The coronal diagram modified from a rat brain atlas schematizes the placement of a microinjector into the PnO. The diameter of the microinjector has been drawn to scale. B. The cresyl violet-stained coronal section illustrates a representative microinjection site (arrow) in the PnO. C. Schematic coronal diagrams were modified from a rat brain atlas. These diagrams span from 7.30 mm to 8.72 mm posterior to bregma and show the location of each microinjection site (black circles). D. The anterior to posterior range of all microinjection sites used in this study is indicated by the vertical lines in the sagittal diagram modified from a rat brain atlas. The 1 mm calibration bar applies to parts A-C.

Figure 2

Microinjection of hypocretin-1 and SPA into the PnO increased antinociceptive behavior. A. Percent maximum possible effect (%MPE) on paw withdrawal latency as a function of time and drug. Graphs summarize measures obtained during the two hours following microinjection of saline (n = 12 rats), hypocretin-1 (Hcrt-1, n = 12), and the adenosine A₁ receptor agonist SPA (n = 12) into the pontine reticular formation. B. The %MPE averaged over 2 h after PnO microinjection of hypocretin-1 and SPA was significantly (*, p < 0.01) increased compared to saline.

Figure 3

The hypocretin receptor-1 antagonist SB-334867 blocked the hypocretin-1-induced increase in antinociception. A. The time course for the mean percent maximum possible effect (%MPE) measures of PWL following microinjection of saline (n = 11 rats), hypocretin-1 (Hcrt-1, n = 11), the hypocretin receptor-1 antagonist SB-334867 (SB, n = 9), and hypocretin-1 plus SB-334867 (Hcrt-1+SB, n = 8). B. Percent maximum possible effect (%MPE) on paw withdrawal latency averaged over 2-h after PnO microinjection of saline, hypocretin-1, hypocretin-1 plus SB-334867, and SB-334867 alone. PnO microinjection of hypocretin-1 increased average %MPE as compared to saline. The increase in antinociception due to PnO microinjection of hypocretin-1 was blocked when hypocretin-1 was co-administered with the hypocretin receptor-1 antagonist SB-334867. SB-334867 increased nociceptive responses when administered alone. Asterisks indicate a significant difference in %MPE (p < 0.01) compared to saline.