Whole-cell Patch-clamp Recording in Vivo

Abstract

In the last two decades, electrophysiological studies for understanding the mechanisms of central sensitization, which have been induced in many cases by overwhelming inputs from the periphery, have been extensively investigated in slice preparations with intracellular or patch-clamp recordings from spinal dorsal horn neurons. The main difficulty in using slice preparations is that there is no information about what modality of afferent fibers elicits a certain response in dorsal horn neurons. To overcome this problem an in vivo patch-clamp recording method has been developed. Under the in vivo condition, noxious or non-noxious stimuli applied to the skin elicits a barrage of EPSCs in substantia gelatinosa (SG) neurons which have no slow membrane currents, suggesting that the mechanical information is mediated by glutamate through the activation of AMPA type receptors. Unexpectedly, thermal stimuli elicit no response, in spite of the fact that the SG neurons receive abundant inputs from C afferents and express c-FOS in many neurons by thermal stimuli. Further study from deeper laminae shows that the noxious thermal information appears to be transmitted to laminae III – V. However, these responses are mediated by fast EPSCs but not by slow synaptic current. These observations are inconsistent in regard to the assumption that thermal sensation is carried by polymodal afferents, in particular C, which are known to contain various peptides, such as substance P or CGRP. The role of such peptides in sensory transmission is still largely unknown, although it is believed the peptidergic transmission would become obvious under extreme or pathological conditions. In addition to the excitatory responses, an inhibitory response is commonly observed in dorsal horn neurons. The IPSCs have two components: the first being mediated by GABA through GABAA receptors, the other is glycinergic. GABAergic IPSCs are dominant and have a much longer time course. Intriguingly, the IPSCs are elicited by non-noxious rather than noxious mechanical stimuli in SG neurons. After integration at the spinal cord, sensory information is finally ascended to the primary somatosensory cortex through the thalamus where further sensory processing will be made. Based on the accumulating evidence, pain research is now focusing on clarifying mechanisms of plastic changes in the sensory pathways under pathological conditions.