Pruriceptive spinothalamic tract neurons: physiological properties and projection targets in the primate

Abstract

Itch of peripheral origin requires information transfer from the spinal cord to the brain for perception. Here, primate spinothalamic tract (STT) neurons from lumbar spinal cord were functionally characterized by in vivo electrophysiology to determine the role of these cells in the transmission of pruriceptive information. One hundred eleven STT neurons were identified by antidromic stimulation and then recorded while histamine and cowhage (a nonhistaminergic pruritogen) were sequentially applied to the cutaneous receptive field of each cell. Twenty percent of STT neurons responded to histamine, 13% responded to cowhage, and 2% responded to both. All pruriceptive STT neurons were mechanically sensitive and additionally responded to heat, intradermal capsaicin, or both. STT neurons located in the superficial dorsal horn responded with greater discharge and longer duration to pruritogens than STT neurons located in the deep dorsal horn. Pruriceptive STT neurons discharged in a bursting pattern in response to the activating pruritogen and to capsaicin. Microantidromic mapping was used to determine the zone of termination for pruriceptive STT axons within the thalamus. Axons from histamine-responsive and cowhage-responsive STT neurons terminated in several thalamic nuclei including the ventral posterior lateral, ventral posterior inferior, and posterior nuclei. Axons from cowhage-responsive neurons were additionally found to terminate in the suprageniculate and medial geniculate nuclei. Histamine-responsive STT neurons were sensitized to gentle stroking of the receptive field after the response to histamine, suggesting a spinal mechanism for alloknesis. The results show that pruriceptive information is encoded by polymodal STT neurons in histaminergic or nonhistaminergic pathways and transmitted to the ventrobasal complex and posterior thalamus in primates.

Fig. 1.

Characterization of a histamine-responsive spinothalamic tract (STT) neuron with an axon termination in ventral posterior inferior nucleus (VPI). A: section of thalamus containing the lesion (arrow) marking the furthest rostral low threshold point (LTP) (+8.0 mm rostral of interaural plane). B: illustration of the section in A showing the location of each antidromic test site (gray dot). The minimum current amplitude required to elicit antidromic action potentials is shown by a color contour plot. Outside of the contour plot antidromic stimulation did not elicit action potentials. C: receptive field. D: lesion marking the recording site in the superficial dorsal horn (SDH) (arrow). E: antidromic action potentials (i) follow high-frequency triple stimulation (iii) and demonstrate collision (ii; absence of 1st spike of the triplicate, white arrow) when an ascending orthodromic action potential occurs (black arrow). F: responses to mechanical stimulation [brush (Br), pressure (Pr), pinch (Pi)] of the receptive field. G: application of cowhage produced no activation, and intradermal injection of vehicle produced some activation during the injection, but the cell returned quickly to baseline. Injection of histamine produced a large initial discharge and a long-lasting period of activity. Capsaicin also activated the cell. Arrows indicate time of injection/application. 3V, 3rd ventricle; CL centrolateral n.; CM, centromedian n.; LD, lateral dorsal n.; LG, lateral geniculate; LP, lateral posterior n.; LV, lateral ventricle; MD, mediodorsal n.; MG, medial geniculate; Pla, anterior pulvinar; PLi, inferior pulvinar; VMb, basal ventromedial n.; VPM, ventral posterior medial n.; VPL, ventral posterior lateral n.

Fig. 2.

Characterization of a cowhage-responsive STT neuron with an axon projection to VPL. A: section of thalamus containing the lesion marking the LTP (arrow). B: illustration of the same section showing antidromic thresholds in a color contour plot. C: receptive field. D: lesion marking the recording site in the SDH (arrow). E: mechanical sensitivity; the cell was classified as high threshold (HT). Sq, squeeze. F: application of inactivated cowhage produced little or no activation, but active cowhage evoked 2 min of activity after a latency period.

Fig. 3.

Functional characterization of histamine-responsive STT neurons. A: response to histamine (vehicle activity subtracted) is shown in 15-s bins (n = 15). Cells recorded in the SDH account for most of the robust initial response and all of the persistent activity [SDH: n = 10, deep dorsal horn (DDH): n = 5]. AP, action potential. B: response to capsaicin in histamine-responsive STT neurons (SDH: n = 7, DDH, n = 7). C: mean response to a 50°C heat ramp in heat-responsive/histamine-responsive STT neurons (n = 16). D: locations of each histamine-responsive STT neuron profiled by heat and capsaicin activation. Lesion sites marking cells to right of dashed line were unrecovered, and position was estimated by recording depth from the surface of the spinal cord.

Fig. 4.

Functional characterization of cowhage-responsive STT neurons. A: response to cowhage (inactive activity subtracted) is shown in 15-s bins (n = 11) and sorted by SDH (n = 6) and DDH (n = 5). B: mean capsaicin discharge over time (combined SDH and DDH, n = 4). C: mean response to a 50°C heat ramp in heat-responsive/cowhage-responsive STT neurons (n = 7). D: locations of each cowhage-responsive STT neuron and heat and capsaicin profiles.

Fig. 5.

Pruritogen-responsive STT neurons are a subset of nociceptive STT neurons. A: peristimulus time histogram showing time course of response to histamine (n = 15), cowhage (n = 11), and capsaicin (n = 18) in pruriceptive STT neurons. The discharge from capsaicin was significantly greater than the discharge from either histamine or cowhage (***P < 0.001, 2-ANOVA), which were not different from each other. B and C: area of each circle is proportional to the size of each STT subpopulation indicated. Of the total STT population nociceptive-only cells represent 67%, histamine+ represent 20%, and cowhage+ represent 13%. Pie charts indicate % of STT neurons from each subpopulation that were responsive to a 50°C heat ramp (B) or capsaicin (C) (**P < 0.01, Fisher's exact test).

Fig. 6.

Analysis of spike timing in pruritogen-responsive STT neurons. A: the level of the coefficient of variation (CV) of interspike intervals (ISIs) indicates an irregular discharge pattern for pruriceptive STT neurons. The variability is not different between histamine and cowhage or between pruritogen and capsaicin. B: sample of representative spike trains from 1 cowhage- and 1 histamine-responsive STT neuron showing the typical bursting pattern of AP firing. C: normalized ISI distribution histograms. Bimodal peaks indicate that histamine, cowhage, and capsaicin evoke discharges that exhibit bursting behavior. Capsaicin discharge possesses significantly shortened interburst duration compared with histamine (*P < 0.05, **P < 0.01, ***P < 0.001, 2-ANOVA, Tukey posttest).

Fig. 7.

Sensitivity to mechanical stimuli before and after a response to a pruritogen. Stroking the receptive field with a cotton-tipped swab evoked a greater discharge after a histamine response than before the histamine response (*P < 0.05, paired t-test). Other mechanical stimuli elicited no change in discharge after a response to histamine or cowhage.

Fig. 8.

Zones of termination of pruriceptive STT neurons: antidromically mapped and completely surrounded projection targets of 6 histamine-responsive and 6 cowhage-responsive subsets of STT neurons. Termination zones were located in the ventrobasal complex and posterior thalamus. Anatomical and functional classifications for each STT neuron are indicated. WDR, wide dynamic range.

Fig. 9.

LTPs (<30 μA) within the thalamus for antidromically activated pruritogen-responsive STT neurons. Projections from pruritogen-responsive STT neurons were found in VPL, VPI, and posterior nucleus (Po). Anatomical and mechanical classifications are indicated.

Fig. 10.

Receptive fields of each pruritogen-responsive STT neuron with mechanical and heat-response classification indicated. Projection target is indicated, and completely surrounded LTPs are circled.