P2X purinoceptor‐mediated excitation of trigeminal lingual nerve terminals in an in vitro intra‐arterially perfused rat tongue preparation

1 A novel in vitro intra‐arterially perfused adult rat tongue‐nerve preparation was used to explore the possible actions of P2X purinoceptor agonists (ATP and α,β‐methylene ATP (α,β‐meATP)) on sensory nerve terminals innervating the rat tongue. We made whole‐nerve recordings of the trigeminal branch of the lingual nerve (LN), which conducts general sensory information (pain, temperature, touch, etc.), and the chorda tympani (CT), which conducts taste information. Changes in LN and CT activity following intra‐arterial application of P2X agonists were compared. 2 In seven preparations, bolus close‐arterial injection of ATP (30–3000 μM, 0.1 ml) or α,β‐meATP (10–300 μM, 0.1 ml) induced a rapid (< 1 s after injection), dose‐related increase in LN activity that decayed within a few seconds. The minimal concentration of ATP (100 μM) required to elicit a response was about 10‐fold higher than that of α,β‐meATP (10 μM). Bolus injection of ATP or α,β‐meATP induced a moderate decrease in firing frequency in three of seven CT preparations. 3 LN responses to P2X agonists showed signs of rapid desensitisation with the peak frequency of discharge being smaller when the agonists were applied at short intervals. Suramin (200 μM) or PPADS (200 μM) applied by intra‐arterial perfusion each antagonised the rapid increase in LN activity following application of α,β‐meATP (100 μM). 4 Capsaicin (10 μM, 0.1 ml, n = 5 preparations) was injected intra‐arterially to desensitise nociceptive fibres. This was found to block ( n = 2 ) or greatly reduce ( n = 3 ) the excitatory effects of α,β‐meATP (100 μM, 0.1 ml) on LN activity, implying that only capsaicin‐sensitive nociceptive fibres in LN were responsive to P2X agonists. 5 In contrast to the consistent excitatory responses in LN activity following fast application of P2X agonists as bolus, a variable and moderate change in discharge rate of LN and no change in CT activity ( n = 5 ) was observed after applying ATP (100–300 μM, n = 21 ) or α,β‐meATP (100–300 μM, n = 14 ) by intra‐arterial perfusion. The variable responses in LN activity to slow perfusion in contrast to close‐arterial bolus injection are consistent with activation of the rapidly desensitising P2X 3 receptors. 6 In summary, ATP and α,β‐meATP preferentially activate general sensory afferent fibres (LN) but not taste fibres (CT). We suggest that the increase in whole‐nerve activity of LN following application of P2X agonists represents activation of nociceptive fibres which possess P2X 3 receptors. Our data indicate that ATP and P2X 3 receptors may play a role in nociception, rather than taste sensation in the tongue.