Nitrergic and glutamatergic neuronal mechanisms at the trigeminovascular first-order synapse

Nitric oxide (NO) donors such as glyceryl trinitrate cause headache, which suggests involvement of NO in trigeminovascular sensory processing. Sensory transmission at first-order synapses is believed to involve glutamate and the question arises as to whether it is also involved in trigeminovascular sensation and whether it might interact with nitrergic mechanisms. We investigated these questions at the first central synapse in the trigeminovascular sensory system of the cat. Neuronal action potentials in the trigeminal nucleus were recorded while the superior sagittal sinus (SSS) or facial receptive field (RF) were stimulated electrically. Drugs, including the neuronal excitant glutamate, were applied to neurons via microiontophoresis. Results were obtained from 152 neurons activated with A-delta latencies by SSS stimulation and by glutamate. The NO donor S-nitrosoglutathione (SNOG, 50 nA) was applied iontophoretically to 41 neurons during SSS stimulation and 13 neurons during pulsatile glutamate ejection. Responses to both modes of stimulation were enhanced by SNOG; the proportion of neurons enhanced was 56% to SSS stimulation and 59% to glutamate. The inhibitor of nitric oxide synthase (NOS), N(omega)-propyl-L-arginine (p-ARG, 50 nA) was applied iontophoretically to 17 neurons during stimulation of SSS and to 10 neurons during pulsatile glutamate ejection. Responses to both stimuli were suppressed by p-ARG: The proportion of neurons suppressed were: to SSS stimulation 59% and to glutamate 80%. Microiontophoretic ejection of eletriptan (50 nA) reversibly suppressed responses of neurons to SSS stimulation, to RF electrical stimulation and to pulsatile iontophoretic application of glutamate. This suppression of responses was antagonised by the concurrent local iontophoretic application of the 5-HT1B/1D receptor antagonist GR127935 or by concurrent iontophoretic application of the selective 5-HT1D receptor antagonist BRL155732. These results suggest that glutamatergic mechanisms are important in sensory transmission in the trigeminovascular system and that they can be modulated by nitrergic and serotonergic mechanisms.