Fig. 1.

Experimental protocols and synaptic circuitry. A: a pretest recording of the N-to-L or T-to-L synapse was made prior to low-frequency stimulation (LFS; 900 s, 1 Hz) of the T cell [or N cell if homosynaptic long-term depression (LTD) was being elicited]. In some experiments, the LFS was replaced by superfusion of 2-arachidonoyl glycerol (2AG), capsaicin, or resiniferatoxin for 900 s. Following a 60 min consolidation period, a posttest recording of the N-to-L or T-to-L synapse was carried out. B: during occlusion experiments, pretreatment with either 60 μM 2AG or 10 μM capsaicin was carried out prior to the pretest recordings of the N-to-L synapse. This was followed by LFS, 2AG (60 μM), or capsaicin (10 μM), depending on the experiment. Following a 60 min consolidation period, the posttest recordings of the N-to-L synapse were carried out. C: the nociceptive (N cell) sensory neuron has a monosynaptic chemical connection onto the longitudinal (L) motor neuron. The touch (T cell) sensory neuron has a monosynaptic electrical synapse and a polysynaptic chemical connection onto the L motor neuron; the interneuron(s) mediating the polysynaptic connection is unknown (?). D: 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX) abolished the N-to-L excitatory postsynaptic potential (EPSP), indicating a glutamatergic synapse. Representative traces of the N-to-L synapse in normal saline (black line), during application of 20 μM of CNQX (gray line) and after 30 min washout in normal saline (dark gray line). Similar results were observed in CNQX experiments performed on the T-to-L synapse (data not shown) indicating that the polysynaptic chemical component of this circuit is glutamatergic.