Excitatory synaptic currents in lumbosacral parasympathetic preganglionic neurons elicited from the lateral funiculus

Excitatory postsynaptic currents (EPSCs) in parasympathetic preganglionic n eurons (PGNs) were examined using the whole cell patch-clamp recording tech nique in L-6 and S-1 spinal cord slices from neonatal rats (6-16 days old). PGNs were identified by labeling with retrograde axonal transport of a flu orescent dye (Fast Blue) injected into the intraperitoneal space 3-7 days b efore the experiment. Synaptic responses were evoked in PGNs by field stimu lation of the lateral funiculus (LF) in the presence of bicuculline methiod ide (10 muM) and strychnine (1 muM). In approximately 40% of the cells (tot al, 100), single-shock electrical stimulation of the LF elicited short, rel atively constant latency [3.0 +/-0.1 (SE) ms] fast EPSCs consistent with a monosynaptic pathway. The remainder of the cells did not respond to stimula tion. At low intensities of stimulation, the EPSCs often occurred in an all -or-none manner, indicating that they were mediated by a single axonal inpu t. Most cells (n=33) exhibited only fast EPSCs (type 1), but some cells (n= 8) had fast EPSCs with longer, more variable latency polysynaptic EPSCs sup erimposed on a slow inward current (type 2). Type I fast synaptic EPSCs wer e pharmacologically dissected into two components: a transient component th at was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 muM), a non -NMDA glutamatergic antagonist, and a slow decaying component that was bloc ked by 2-amino-5-phosphonovalerate (APV, 50 muM), a NMDA antagonist. Type 2 polysynaptic currents were reduced by 5 AM CNQX and completely blocked by combined application of 5 muM CNQX and 50 muM APV. The fast monosynaptic co mponent of type I EPSCs had a linear current-voltage relationship and rever sed at a membrane potential of 5.0 +/-5.9 mV (n=5), whereas the slow compon ent exhibited a negative slope conductance at holding potentials greater th an -20 mV. The type 1, fast synaptic EPSCs had a time to peak of 1.4 +/-0.1 ms and exhibited a biexponential decay (time constants, 5.7 +/-0.6 and 38. 8 +/-4.0 ms). In the majority of PGNs (n=11 of 15 cells), EPSCs evoked by e lectrical stimulation of LF exhibited paired-pulse inhibition (range; 25-33 % depression) at interstimulus intervals ranging from 50 to 120 ms. These r esults indicate that PGNs receive monosynaptic and polysynaptic glutamaterg ic excitatory inputs from axons in the lateral funiculus.