Pain is a central experience of most higher organisms. Despite its useful character as a warning symptom, that indicates tissue damage, it may become an independent disease on its own, when pain becomes chronic even in the absence of injury. Despite long-lasting efforts of investigation it still poses an unsolved problem in modern medicine.
The spinal cord dorsal horn, particularly laminae I and II of the spinal cord grey matter, is an important modulatory site of incoming nociceptive information to the central nervous system (CNS). Here, incoming nociceptive fibres relay onto second order neurons that further propagate the sensory information to the brain, where pain sensation is evoked and linked to emotional and autonomic regulatory brain regions.
But at that crucial step of pain processing, neuronal information is not simply propagated, but rather modulated by numerous mechanisms that either enhance or weaken the strength of the incoming signal. Among others, such as synaptic long term potentiation, descending inhibition, etc., spinal inhibitory interneurons play a pivotal role in that process. There is plenty of evidence from literature that a malfunction of this important regulatory system may cause exaggerated pain states as seen in patients with neuropathic pain. However, the underlying cellular mechanisms are yet not fully understood.
In the spinal cord dorsal horn, GABAergic neurons cannot be identified by simple inspection. Here we took advantage in the use of transgenic mice expressing EGFP in their spinal GABAergic inhibitory neurons. This allowed us to identify inhibitory spinal neurons via epifluorescence microscopy. Chronic constriction injury (CCI) was used as a model of peripheral neuropathy. Group comparisons between CCI-treated and sham-operated animals were performed. We asked the question, if the excitatory synaptic input to spinal GABAergic inhibitory neurons was impaired in neuropathy. In CCI-treated mice we found a significant reduction of excitatory drive towards these neurons indicated by a reduced frequency of miniature excitatory postsynaptic currents (mEPSCs). When introducing the Ca2+ chelator BAPTA intracellularly mEPSC-frequencies returned to normal indicating that this phenomenon was induced postsynaptically in GABAergic neurons via a rise of intracellular Ca2+ concentration. Paired-pulse recordings from GABAergic neurons following electrical dorsal root stimulation revealed an increase of paired-pulse ratio recorded in neuropathic animals. This displays a reduced release probability of neurotransmitters. Again, intracellular introduction of BAPTA in GABAergic neurons led to a normalization of paired-pulse ratios. On the other hand, no morphological changes of GABAergic neurons and no change of excitatory synapse count on GABAergic neurons could be detected when neurons were imaged via two-photon laser-scanning microscopy and confocal microscopy.
These findings indicate that neuropathy induces postsynaptically a decrease of excitatory drive to inhibitory GABAergic neurons, which is expressed presynaptically. The retrograde signal responsible for this phenomenon remains unknown, since none of the tested molecules (endocannabinoids, nitric oxide, GABA, cannabinoid receptor blocker) proved to mimic or block that effect. Further on we show via immunohistochemical c-Fos detection that a reduced excitatory drive to GABAergic neurons correlates with less activation of these neurons.
In this study we revealed a mechanism that may (along with other neuroplasticity changes) be responsible for exaggerated pain states following neuronal damage in animals and perhaps also human patients.
Improvements in understanding the nature of how chronic pain diseases arise, elucidating their cellular and molecular mechanisms, may help to find new treatment strategies against chronic pain.