Differential Maturation of GABA Action and Anion Reversal Potential in Spinal Lamina I Neurons: Impact of Chloride Extrusion Capacity

Abstract

A deficit in inhibition in the spinal dorsal horn has been proposed to be an underlying cause of the exaggerated cutaneous sensory reflexes observed in newborn rats. However, the developmental shift in transmembrane anion gradient, potentially affecting the outcome of GABA A transmission, was shown to be completed within 1 week after birth in the spinal cord, an apparent disparity with the observation that reflex hypersensitivity persists throughout the first 2-3 postnatal weeks.To further investigate this issue, we used several approaches to assess the action of GABA throughout development in spinal lamina I (LI) neurons. GABA induced an entry of extracellular calcium in LI neurons from postnatal day 0 (P0) to P21 rats, which involved T-and N-type voltage-gated calcium channels. Gramicidin perforated-patch recordings revealed that the shift in anion gradient was completed by P7 in LI neurons. However, high chloride pipette recordings demonstrated that these neurons had not reached their adult chloride extrusion capacity by P10 -P11. Simultaneous patch-clamp recordings and calcium imaging revealed that biphasic responses to GABA, consisting of a primary hyperpolarization followed by a rebound depolarization, produced a rise in [Ca 2ϩ ] i . Thus, even if E anion predicts GABA A -induced hyperpolarization from rest, a low chloride extrusion capacity can cause a rebound depolarization and an ensuing rise in [Ca 2ϩ ] i . We demonstrate that GABA action in LI neurons matures throughout the first 3 postnatal weeks, therefore matching the time course of maturation of withdrawal reflexes. Immature spinal GABA signaling may thus contribute to the nociceptive hypersensitivity in infant rats.

Synaptic Inhibition and Disinhibition in the Spinal Dorsal Horn

“…First, NKCC1 is strongly expressed early in development while KCC2 is only weakly expressed, but a developmental switch occurs that leads to the inverse pattern in adulthood. 38,39 In the rat spinal dorsal horn, E anion appears to reach its mature value around 2 weeks after birth, 40 but full chloride extrusion capacity is not reached until 3-4 weeks after birth 41 ; in other words, chloride loads more readily overwhelm KCC2-mediated chloride extrusion in young neurons. Second, the developmental switch does not occur in primary afferent neurons, which means NKCC1 levels remain high, resulting in high intracellular chloride concentrations in those cells.…”

Morphological and functional characterization of cholinergic interneurons in the dorsal horn of the mouse spinal cord

“…Alexa 555 (0.5 mM; Invitrogen) was added to the pipette solution before the experiment. Wholecell voltage-or current-clamp recordings were made from neurons as previously described (Cordero-Erausquin et al, 2005). Signals were recorded with a patch-clamp amplifier (Axopatch 200B), filtered at 5 kHz, digitized at 10 kHz, and acquired using the Strathclyde electrophysiology software WinWCP (courtesy of Dr. J. Dempster, University of Strathclyde, Glasgow, United Kingdom).…”

C-fiber activity-dependent maturation of glycinergic inhibition in the spinal dorsal horn of the postnatal rat

“…Whereas some aspects of GABAergic neurotransmission are postnatally regulated in the dorsal horn (7)(8)(9), the robust GABAergic inhibition of spinal sensory circuits observed from birth makes this an unlikely explanation for the excitability of neonatal cutaneous spinal sensory circuits (6,10). Glycinergic transmission is also postnatally regulated (6) but remains to be investigated in neonatal dorsal horn circuits.…”