Neuronal responses to purinoceptor agonists in the leech central nervous system

Extracellular nucleotides like ATP and its derivatives are possible chemical messengers in vertebrate nervous systems. In invertebrate nervous system, however, little is known about their role in neurotransmission. We have studied the response of identified neurones of the leech Hirudo medicinalis to the purinoceptor agonist ATP, ADP, AMP, and adenosine using conventional intracellular microelectrodes and whole-cell patch-clamp recording. Bath application of the agonists depolarized the different neurons, but not the neuropil glial cells. The most effective responses (up to 10 mV) were observed with ATP (100 microM) or ADP (100 microM) in the noxious and touch cells. In most neurons the nonhydrolyzable ATP derivative ATP-gamma-S (5 microM) induced larger depolarizations than 100 microM ATP, indicating that most of the potency of ATP is lost presumably due to its degradation by ectonucleotidases. In medial noxious cells, ATP (100 microM) induced an inward current of 1.7 +/- 1.1 nA at a holding potential of -60 mV. The ATP-induced current-voltage relationship showed an inward rectification and a reversal potential close to 0 mV. In a Na+-free extracellular solution, the ATP-induced inward current decreased and in a Na+- and Ca(2+)-free saline only a small residual current persisted. The possible P2 purinoceptor antagonist suramin did not antagonize the ATP-induced current, but itself evoked an inward current and a conductance increase. We conclude that ATP activates nonselective cation channels in medial noxious cells of the leech with the order of potency of purinoceptor agonists ATP > or = ADP > AMP. The results suggest that these cells express purinoceptors of the P2 type.