Neurosteroid; Endogenous, Analgesics
Peripheral sensitisation and central sensitisation are implicated in the development of neuropathic pain with neuroplasticity occurring at multiple levels of the pain pathway. Hypersensitivity of the spinothalamic tract has been described in neuropathic animal models of diabetes. Spinal dorsal horn neurones of diabetic rats exhibit abnormally high spontaneous firing, suggesting an imbalance between excitatory and inhibitory signals converging within this structure. GABAergic neurones within the spinal cord and thalamus are crucial for the transmission of painful stimuli to higher centres of the brain that are involved in pain perception. GABAA receptors (GABAARs) are an important target for many clinical drugs, and certain endogenous neurosteroids act as potent allosteric modulators of these receptors. A developmental change in the rate of exponential decay of GABAergic synaptic events has been observed in other types of neurones and this may be related to fluctuations in endogenous neurosteroid tone. The objective of this study was to investigate changes to inhibitory neurotransmission with development in three levels of the pain pathway and to explore potential mechanisms underlying diabetic neuropathy.
The whole-cell patch-clamp technique was used on slices of neural tissue. Electrophysiological recordings were obtained from wild type mice between the ages of 6 and 80 days in lamina II of the spinal cord, the nucleus reticularis (nRT) of the thalamus and the cerebral cortex. Recordings were also obtained from ob/ob mice between the ages of 60 and 80 days. Neurosteroids and their precursors were employed along with compounds that prevented their activity at the GABAAR
such as γ-cyclodextrin, which is a barrel-shaped cyclic oligosaccharide with a lipophilic interior that sequesters neurosteroids. Behavioural experiments were also performed using von Frey filaments and the tail flick test to examine mechanical and thermal nociception.
Recordings from the spinal cord, the thalamus and the cerebral cortex revealed that the decay time of miniature inhibitory postsynaptic currents are significantly reduced with development. The neurosteroids allopregnanolone and ganaxolone were significantly more effective in neurones from the older mice. In contrast, γ-cyclodextrin had significantly less effect in neurones from the older mice. In mature diabetic mice (ob/ob mice), the endogenous neurosteroid tone is reduced compared to control mice, but certain neurosteroid compounds have a greater effect on the GABAARs of these diabetic mice. In addition, the diabetic mice exhibit mechanical allodynia and hyperalgesia, which is responsive to exogenously applied neurosteroids.