Excitatory Synaptic Currents in Lumbosacral Parasympathetic Preganglionic Neurons Elicited From the Lateral Funiculus

Excitatory postsynaptic currents (EPSCs) in parasympathetic preganglionic neurons (PGNs) were examined using the whole cell patch-clamp recording technique in L6 and S1 spinal cord slices from neonatal rats (6–16 days old). PGNs were identified by labeling with retrograde axonal transport of a fluorescent dye (Fast Blue) injected into the intraperitoneal space 3–7 days before the experiment. Synaptic responses were evoked in PGNs by field stimulation of the lateral funiculus (LF) in the presence of bicuculline methiodide (10 μM) and strychnine (1 μM). In approximately 40% of the cells (total, 100), single-shock electrical stimulation of the LF elicited short, relatively constant latency [3.0 ± 0.1 (SE) ms] fast EPSCs consistent with a monosynaptic pathway. The remainder of the cells did not respond to stimulation. At low intensities of stimulation, the EPSCs often occurred in an all-or-none manner, indicating that they were mediated by a single axonal input. 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 superimposed on a slow inward current (type 2). Type 1 fast synaptic EPSCs were pharmacologically dissected into two components: a transient component that was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 μM), a non-NMDA glutamatergic antagonist, and a slow decaying component that was blocked by 2-amino-5-phosphonovalerate (APV, 50 μM), a NMDA antagonist. Type 2 polysynaptic currents were reduced by 5 μM CNQX and completely blocked by combined application of 5 μM CNQX and 50 μM APV. The fast monosynaptic component of type 1 EPSCs had a linear current-voltage relationship and reversed at a membrane potential of 5.0 ± 5.9 mV ( n = 5), whereas the slow component exhibited a negative slope conductance at holding potentials greater than −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 electrical stimulation of LF exhibited paired-pulse inhibition (range; 25–33% depression) at interstimulus intervals ranging from 50 to 120 ms. These results indicate that PGNs receive monosynaptic and polysynaptic glutamatergic excitatory inputs from axons in the lateral funiculus.

Position of Spinothalamic Tract Axons in Upper Cervical Spinal Cord of Monkeys

Percutaneous upper cervical cordotomy continues to be performed on patients suffering from several types of severe chronic pain. It is believed that the operation is effective because it cuts the spinothalamic tract (STT), a primary pathway carrying nociceptive information from the spinal cord to the brain in humans. In recent years, there has been controversy regarding the location of STT axons within the spinal cord. The aim of this study was to determine the locations of STT axons within the spinal cord white matter of C2 segment in monkeys using methods of antidromic activation. Twenty lumbar STT cells were isolated. Eleven were classified as wide dynamic range neurons, six as high-threshold cells, and three as low-threshold cells. Eleven STT neurons were recorded in the deep dorsal horn and nine in superficial dorsal horn. The axons of the examined neurons were located at antidromic low-threshold points (<30 μA) within the contralateral lateral funiculus of C2. All low-threshold points were located ventral to the denticulate ligament, within the lateral half of the ventral lateral funiculus (VLF). None were found in the dorsal half of the lateral funiculus. The present findings support our previous suggestion that STT axons migrate ventrally as they ascend the length of the spinal cord. Also, the present findings indicate that surgical cordotomies that interrupt the VLF in C2 likely disrupt the entire lumbar STT.

Locations of Spinothalamic Tract Axons in Cervical and Thoracic Spinal Cord White Matter in Monkeys

The spinothalamic tract (STT) is the primary pathway carrying nociceptive information from the spinal cord to the brain in humans. The aim of this study was to understand better the organization of STT axons within the spinal cord white matter of monkeys. The location of STT axons was determined using method of antidromic activation. Twenty-six lumbar STT cells were isolated. Nineteen were classified as wide dynamic range neurons and seven as high-threshold cells. Fifteen STT neurons were recorded in the deep dorsal horn (DDH) and 11 in superficial dorsal horn (SDH). The axons of 26 STT neurons were located at 73 low-threshold points (<30 μA) within the lateral funiculus from T9 to C6. STT neurons in the SDH were activated from 33 low-threshold points, neurons in the DDH from 40 low-threshold points. In lower thoracic segments, SDH neurons were antidromically activated from low-threshold points at the dorsal-ventral level of the denticulate ligament. Neurons in the DDH were activated from points located slightly ventral, within the ventral lateral funiculus. At higher segmental levels, axons from SDH neurons continued in a position dorsal to those of neurons in the DDH. However, axons from neurons in both areas of the gray matter were activated from points located in more ventral positions within the lateral funiculus. Unlike the suggestions in several previous reports, the present findings indicate that STT axons originating in the lumbar cord shift into increasingly ventral positions as they ascend the length of the spinal cord.

Excitatory postsynaptic currents and glutamate receptors in neonatal rat sympathetic preganglionic neurons in vitro

1. We obtained whole cell patch-clamp recordings from visually identified sympathetic preganglionic neurons (SPNs) in thin (200-300 microns) transverse spinal cord slices of neonatal rats (1-14 days postnatal). Exogenous application of glutamate (100 microM), N-methyl-D-aspartate (NMDA; 100 microM), kainate (100 microM), quisqualate (1 microM), and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 50 microM) induced inward currents at a holding potential of -30 mV. 2. Excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation either in the dorsal horn or the lateral funiculus. They reversed at 1.2 +/- 4.6 (SD) mV and could in most cases (49 of 51) be separated into two components. 3. In the presence of DL-2-amino-5-phosphonovalerate (10-40 microM) the current-voltage (I-V) relationship of the remaining EPSC was linear. When stimulated in the lateral funiculus, its rise time (10-90%) and the time constant of the monoexponential decay were 1.6 +/- 1.0 and 5.5 +/- 2.7 ms, respectively. By contrast, when stimulated in the dorsal horn, this component had a rise time (10-90%) of 3.0 +/- 0.8 ms and a decay time constant of 13.7 +/- 7.6 ms. 4. We studied the NMDA receptor-mediated component of the EPSCs after superfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (5 microM). The I-V relationship of this component had a region of negative slope conductance between -30 and -80 mV, which was abolished in Mg(2+)-free saline. The rise time (10-90%) ranged from 3.3 to 9.5 ms and the decay was biexponential. Both decay time constants increased with depolarization. Mg(2+)-free saline reduced this voltage sensitivity. 5. At a membrane potential of -80 mV and in 1 mM extracellular Mg2+, the NMDA receptor-mediated component represented 74.8 +/- 11.2% of the total charge carried by the EPSCs evoked by stimulation in the dorsal horn. In contrast, when stimulated from the lateral funiculus, 28.9 +/- 18.9% of the total charge carried during the EPSC was mediated by the NMDA receptor-mediated component. The contribution of the NMDA receptor-mediated component increased in both cases with depolarization. In addition, in 2 of 18 SPNs the EPSC evoked in the dorsal horn was exclusively carried by NMDA receptors. 6. We conclude that L-glutamate or a related substance mediates the fast excitatory input onto SPNs. Viscerosomatic and supraspinal inputs form synapses with different topographical locations on the SPN.(ABSTRACT TRUNCATED AT 400 WORDS)