In vivo patch-clamp analysis of response properties of rat primary somatosensory cortical neurons responding to noxious stimulation of the facial skin

Abstract

Background: Although it has been widely accepted that the primary somatosensory (SI) cortex plays an important role in pain perception, it still remains unclear how the nociceptive mechanisms of synaptic transmission occur at the single neuron level. The aim of the present study was to examine whether noxious stimulation applied to the orofacial area evokes the synaptic response of SI neurons in urethane-anesthetized rats using an in vivo patch-clamp technique.

Results: In vivo whole-cell current-clamp recordings were performed in rat SI neurons (layers III-IV). Twenty-seven out of 63 neurons were identified in the mechanical receptive field of the orofacial area (36 neurons showed no receptive field) and they were classified as non-nociceptive (low-threshold mechanoreceptive; 6/27, 22%) and nociceptive neurons. Nociceptive neurons were further divided into wide-dynamic range neurons (3/27, 11%) and nociceptive-specific neurons (18/27, 67%). In the majority of these neurons, a proportion of the excitatory postsynaptic potentials (EPSPs) reached the threshold, and then generated random discharges of action potentials. Noxious mechanical stimuli applied to the receptive field elicited a discharge of action potentials on the barrage of EPSPs. In the case of noxious chemical stimulation applied as mustard oil to the orofacial area, the membrane potential shifted depolarization and the rate of spontaneous discharges gradually increased as did the noxious pinch-evoked discharge rates, which were usually associated with potentiated EPSP amplitudes.

Conclusions: The present study provides evidence that SI neurons in deep layers III-V respond to the temporal summation of EPSPs due to noxious mechanical and chemical stimulation applied to the orofacial area and that these neurons may contribute to the processing of nociceptive information, including hyperalgesia.

Figure 1

In vivo patch-clamp recording from rat SI neurons. A: Typical example of spontaneous discharges of SI neurons. The excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are shown by fluctuations above and below the resting membrane potential. Arrow head shows resting membrane potential. B: Location of each type of SI neurons successfully identified by biocytin injection. LTM: low-threshold mechanoreceptive, WDR: wide-dynamic range, NS: nociceptive specific. Values in parentheses are the number of neurons. C: Typical example of a biocytin-injected noxious specific SI neuron in layer IV with spontaneous discharges. Right panel, higher-power photomicrograph of left.

Figure 2

Response properties of SI neurons responding to noxious mechanical stimulation. A: Low-threshold mechanoreceptive (LTM) neurons only responding to non-noxious stimulation (brush). B: Example of wide-dynamic range (WDR) SI neurons responding to both noxious and non-noxious stimulation (blackened area). Arrow heads and broken horizontal line show resting membrane potential. C: Example of nociceptive-specific (NS) neurons - noxious pinch stimuli applied to the orofacial skin (blackened area) produced a barrage of excitatory postsynaptic potentials (EPSPs) accompanied by action potentials in a SI neuron under current-clamp conditions. This neuron did not respond to non-noxious stimulation (brush). Arrow heads and broken horizontal line show resting membrane potential.

Figure 3

Effect of noxious chemical stimulation of the receptive field on the spontaneous discharges and noxious pinch-evoked responses. A: Noxious pinch stimuli applied to the orofacial skin (blackened area) produced a barrage of excitatory postsynaptic potentials (EPSPs) accompanied by action potentials in SI neurons (NS-type). Blackened area indicates the location and size of the receptive field responding to noxious pinch stimulation. Arrow heads show resting membrane potential. B: Example of responses of spontaneous discharges and of noxious pinch stimulation after subcutaneous injection of mustard oil (MO) into the receptive field area. Note that after MO injection (5 min), spontaneous discharges of SI neurons increased, lasting for 10-15 min. Noxious pinch-evoked discharge rate was increased and the response was accompanied by augmented EPSP amplitudes. C: Change in the mean membrane potential of SI neurons after MO administration (10 min). *, P < 0.05. D: Change in the mean noxious pinch evoked discharge of SI neurons after MO administration (10 min). *, P < 0.05. E: Change in the mean noxious pinch evoked EPSP amplitude of SI neurons after MO administration (10 min). *, P < 0.05.