Inhibition of trigeminal neurones after intravenous administration of naratriptan through an action at 5-hydroxytryptamine (5-HT1B/1D) receptors

1 The observation that 5-hydroxytryptamine (5-HT) is effective in treating acute attacks of migraine when administered intravenously resulted in a research effort that led to the discovery of the 5-HT1B/1D receptor agonist sumatriptan. 2 Clinical experience has shown sumatriptan to be an effective treatment with some limitations, such as relatively poor bioavailability, which naratriptan was developed to address. Increasing bioavailability has been achieved with greater lipophilicity and thus the potential for greater activity in the central nervous system. 3 In this study the increased access to central sites has been exploited in an attempt to characterize the pharmacology of those central receptors with the newer tools available. Trigeminovascular activation was examined in the model of superior sagittal sinus stimulation. 4 Cats were anaesthetized with alpha-chloralose (60 mg kg(-1), intraperitoneal), paralyzed (gallamine 6 mg kg(-1), intravenously) and ventilated. The superior sagittal sinus was accessed and isolated for electrical stimulation (250 mu s pulses, 0.3 Hz, 100 V) by a mid-line circular craniotomy. The region of the dorsal surface of C-2 spinal cord was exposed by a laminectomy and an electrode placed for recording evoked activity from sinus stimulation. 5 Stimulation of the superior sagittal sinus resulted in activation of cells in the dorsal horn of C-2 Cells fired with a probability of 0.69 +/- 0.1 at a latency of 9.2 +/- 0.2 ms. Intravenous (i.v.) administration of naratriptan at clinically relevant doses (30 and 100 mu g kg(-1)), inhibited neuronal activity in trigeminal neurones of the C-2 dorsal horn, reducing probability of firing without affecting latency. 6 The effect of naratriptan could be reversed by administration of the selective 5-HT1B/1D receptor antagonist GR127935 (100 mu g kg(-1), i.v.). 7 These data establish that naratriptan acts on central trigeminal neurones since sagittal sinus stimulation activates axons within the tentorial nerve and there are no inhibitory effects mediated within the trigeminal ganglion. Furthermore, given that this inhibition could be reversed by the relatively selective 5-HT1B/1D receptor antagonist GR127935, it is highly likely that the anti-migraine effects of drugs of this class with central nervous system access are mediated, at least in part, by 5-HT receptors within the trigeminal nucleus.