Thermally identified subgroups of marginal zone neurons project to distinct regions of the ventral posterior lateral nucleus in rats

Abstract

Spinal marginal zone (MZ) neurons play a crucial role in the transmission of nociceptive and thermoreceptive information to the brain. The precise areas to which physiologically characterized MZ neurons project in the ventral posterior lateral (VPL) nucleus of the thalamus have not been clearly established. Here, we examine this projection in rats using the method of antidromic activation to map the axon terminals of neurons recorded from the MZ. Thirty-three neurons were antidromically activated using pulses of < or =30 microA in the contralateral VPL. In every case, the most rostral point from which the MZ neuron could be antidromically activated was surrounded by stimulating tracks in which large-amplitude current pulses failed to activate the examined neuron, indicating the termination of the spinothalamic tract (STT) axon. Each of 30 examined neurons responded to noxious but not innocuous mechanical stimuli applied to their cutaneous receptive fields, which ranged in size from two digits to the entire limb. Of 17 thermally tested neurons, 16 responded to innocuous or noxious thermal stimuli. Among STT neurons that responded to thermal stimuli, 50% responded to innocuous cooling as well as noxious heat and cold, 31% responded to noxious heat and cold, and 19% responded only to noxious heat. Axons from cells responsive to innocuous cooling terminated in the core region of VPL, significantly dorsal and medial relative to other thermally responsive subgroups. In rats, thermally responsive subgroups of MZ neurons project directly to distinct regions of VPL.

Figure 1.

Example of a nociceptive-specific neuron in C7 spinal cord with an axon that projects to the contralateral VPL of the thalamus. A, A dorsal representation of stimulating electrode penetrations into the diencephalon at multiple rostrocaudal levels. This region is indicated by the black square in the whole-brain inset. The minimum current required for antidromic activation in each penetration is represented by a symbol (right inset). Note that the low-threshold point was surrounded medially, laterally, and rostrally by ineffective stimulation tracks, indicating the axon terminal area. CL, Centrolateral nucleus; MD, mediodorsal thalamic nucleus. B, Coronal section showing a lesion marking the low-threshold point in contralateral VPL. C, Lesion marking the recording site in the marginal zone. D, Receptive field on ipsilateral forepaw. E, Response of this neuron to increasingly intense cutaneous mechanical stimuli. Mean firing frequency (mean frequency during stimulation − mean background frequency) is indicated in parentheses. F, The antidromic response had a stable latency (3 overlapping traces), followed a high-frequency (333 Hz) train and collided with an orthodromic action potential within the critical window. Note that the stimulus artifact has been reduced for clarity here and also in Figure 4F. G, H, The neuron responded to noxious heat stimuli applied to its cutaneous receptive field. The peculiar brief discharge during the cooling stimulus offset was unique to this particular cell. I, Line graph showing the response to a range of thermal stimuli.

Figure 2.

Photomicrographs of representative lesions. A, Lesion marking a completely surrounded low-threshold point for antidromic activation in the contralateral VPL of the thalamus. CL, Centrolateral thalamic nucleus; IC, internal capsule; LD, laterodorsal thalamic nucleus; MD, mediodorsal thalamic nucleus; RT, reticular thalamic nucleus; VM, ventromedial thalamic nucleus; ZI, zona incerta. B, Lesion marking the recording site in the marginal zone of the cervical enlargement.

Figure 3.

Example of a COOL neuron in the C7 segment with an axon that projected to contralateral VPL. A, Schematic representation of dorsal view of the brain indicating locations of stimulating tracks. The low-threshold point was surrounded medially, laterally, and rostrally by tracks in which 500 μA current pulses did not antidromically activate this axon. B, Cross section showing the low-threshold point located in contralateral VPL. IC, Internal capsule; LD, laterodorsal thalamic nucleus; MD, mediodorsal thalamic nucleus. C, Recording site in the marginal zone of C7. D, Receptive field covered the entire ipsilateral forelimb. E, This cell responded to only high-intensity mechanical stimuli applied to its cutaneous receptive field. F, G, Innocuous cool, noxious cold, and heat stimulation elicit increased activity from this unit. H, Line graph shows the response of this unit to thermal stimuli across a range of temperatures.

Figure 4.

Example of a polymodal nociceptive neuron in the C7 that projected to contralateral VPL. A, Schematic representation of the dorsal view of the brain indicating locations of stimulating tracks. The current amplitude required to activate the neuron antidromically is indicated. B, The lesion marking the low-threshold point was located in contralateral VPL. CL, Centrolateral thalamic nucleus; IC, internal capsule; LD, laterodorsal thalamic nucleus; MD, mediodorsal thalamic nucleus; VL, ventrolateral thalamic nucleus. This neuron was recorded in C7 (C) and had a relatively small cutaneous receptive field on the forepaw (D). The neuron responded only to noxious mechanical stimulation and was classified as an HT neuron (E). F, The antidromic response had a stable latency (3 overlapping traces), followed a high-frequency (333 Hz) train, and collided with an orthodromic action potential. G, H, The neuron responded to both noxious heat and cold stimuli applied to its cutaneous receptive field in a graded manner. I, Line graph showing the response of this cell to thermal stimuli.

Figure 5.

Stimulus–response plots for thermally identified subgroups of marginal zone neurons. A, Individual responses of COOL-type neurons to noxious heat, cool, and noxious cold stimuli. B, Polymodal nociceptive neurons responded in a graded manner to noxious heat and cold stimuli. C, Nociceptive-specific neurons responded only to noxious heat. Mean ± SE stimulus–response curves for COOL (D), polymodal (E), and nociceptive-specific-type (F) cells.

Figure 6.

Characteristics of thermally defined subgroups of marginal zone neurons. A, At room temperature (23°C), COOL-type neurons had a greater level of spontaneous activity (10.8 ± 1.8 Hz) than the polymodal neurons (POLY) (0.6 ± 0.4 Hz) or the nociceptive-specific neurons (NS) (0.1 ± 0.1 Hz; p < 0.01). B, Double-logarithmic plot describing the stimulus–response functions of thermally tested subgroups within their temperature coding range. The abscissa represents the change in stimulus temperature from the holding temperature, and the ordinate represents the neural discharge frequency. The threshold for a response (≥1.5 spikes/s) is illustrated with a horizontal dotted line, and the threshold temperatures are indicated for each subgroup (arrows). The slope, x, of the regression line is equal to the exponent in Stevens' power law. A regression analysis was not completed for COOL and NS cells in the heating range because these subgroups responded to only two test temperatures.

Figure 7.

Cutaneous receptive fields of each examined MZ neuron with a projection to VPL. Receptive fields of COOL cells always included parts of the entire limb, including digits, paw, and limb. Polymodal and nociceptive-specific cells rarely had receptive fields as large. Only noxious mechanical stimulation applied to the receptive fields were adequate to activate MZ STT neurons projecting to VPL.

Figure 8.

Conduction velocities of examined axons. A, Conduction velocities of marginal zone STT neurons were measured from the recording site in the cervical enlargement to the low-threshold point in the contralateral VPL of the thalamus. B, Comparison of conduction velocities of the three different groups of thermally responsive marginal zone neurons.

Figure 9.

Summary of the locations of lesions in the thalamus and spinal cord marking 36 low-threshold points and 33 recording points. A, Locations of lesions marking 33 surrounded low-threshold points were in VPL, and three surrounded low-threshold points were located in other thalamic nuclei. The distance from bregma of each plane is indicated to the left of each section. MD, Mediodorsal thalamic nucleus; RT, reticular thalamic nucleus; VM, ventromedial thalamic nucleus; ZI, zona incerta; PF, parafascicular thalamic nucleus; VL, ventrolateral thalamic nucleus. B, Locations of lesions marking low-threshold points of the three functionally identified subgroups of thermally responsive MZ neurons. C, Recording sites of 26 neurons in the cervical enlargement and three neurons in the lumbar enlargement. D, Illustration of the recording sites from 16 units divided into thermally responsive subgroups. Cool, COOL-type neurons; Poly, polymodal neurons; NS, nociceptive-specific neurons.