ATP can be released from the peripheral endings of sensory nerves, and thus may be a neurotransmitter of primary afferent fibres [12]. We have found that ATP is excitatory when applied to single sensory neurones in the rat caudal trigeminal nucleus, and this action is consistent with a neurotransmitter role. However, adenosine monophosphate and pyrophosphate, two metabolites of ATP, were respectively found to be inhibitory and excitatory, and thus it remains unclear whether the actions of ATP are direct or are due to actions of its metabolites.