Transient, activity dependent inhibition of transmitter release from low threshold afferents mediated by GABA A receptors in spinal cord lamina III/IV

Background Presynaptic GABA A receptors (GABA A Rs) located on central terminals of low threshold afferent fibers are thought to be involved in the processing of touch and possibly in the generation of tactile allodynia in chronic pain. These GABA A Rs mediate primary afferent depolarization (PAD) and modulate transmitter release. The objective of this study was to expand our understanding of the presynaptic inhibitory action of GABA released onto primary afferent central terminals following afferent stimulation. Results We recorded evoked postsynaptic excitatory responses (eEPSCs and eEPSPs) from lamina III/IV neurons in spinal cord slices from juvenile rats (P17–P23, either sex), while stimulating dorsal roots. We investigated time and activity dependent changes in glutamate release from low threshold A fibers and the impact of these changes on excitatory drive. Blockade of GABA A Rs by gabazine potentiated the second eEPSC during a train of four afferent stimuli in a large subset of synapses. This resulted in a corresponding increase of action potential firing after the second stimulus. The potentiating effect of gabazine was due to inhibition of endogenously activated presynaptic GABA A Rs, because it was not prevented by the blockade of postsynaptic GABA A Rs through intracellular perfusion of CsF. Exogenous activation of presynaptic GABA A Rs by muscimol depressed evoked glutamate release at all synapses and increased paired pulse ratio (PPR). Conclusions These observations suggest that afferent driven release of GABA onto low threshold afferent terminals is most effective following the first action potential in a train and serves to suppress the initial strong excitatory drive onto dorsal horn circuitry

GABAB receptors-mediated tonic inhibition of glutamate release from A\u3b2 fibers in rat laminae III/IV of the spinal cord dorsal horn.

Presynaptic GABAB receptors (GABABRs) are highly expressed in dorsal root ganglion neurons and spinal cord dorsal horn. GABABRs located in superficial dorsal horn play an important antinociceptive role, by acting at both pre- and postsynaptic sites. GABABRs expressed in deep dorsal horn could be involved in the processing of touch sensation and possibly in the generation of tactile allodynia in chronic pain. The objective of this study was to characterize the morphological and functional properties of GABABRs expressed on A\u3b2 fibers projecting to lamina III/IV and to understand their role in modulating excitatory synaptic transmission. We performed high-resolution electron microscopic analysis, showing that GABAB2 subunit is expressed on 71.9% of terminals in rat lamina III-IV. These terminals were engaged in axodendritic synapses and, for the 46%, also expressed glutamate immunoreactivity. Monosynaptic excitatory postsynaptic currents, evoked by A\u3b2 fiber stimulation and recorded from lamina III/IV neurons in spinal cord slices, were strongly depressed by application of baclofen (0.1-2.5\u2009\ub5M), acting as a presynaptic modulator. Application of the GABABR antagonist CGP 55845 caused, in a subpopulation of neurons, the potentiation of the first of two excitatory postsynaptic currents recorded with the paired-pulse protocol, showing that GABABRs are endogenously activated. A decrease in the paired-pulse ratio accompanied the effect of CGP 55845, implying the involvement of presynaptic GABABRs. CGP 55845 facilitated only the first excitatory postsynaptic current also during a train of four consecutive stimuli applied to A\u3b2 fibers. These results suggest that GABABRs tonically inhibit glutamate release from A\u3b2 fibers at a subset of synapses in deep dorsal horn. This modulation specifically affects only the early phase of synaptic excitation in lamina III-IV neuron

Osteoporosis in Light of a New Mechanism Theory of Delayed Onset Muscle Soreness and Non-Contact Anterior Cruciate Ligament Injury

Osteoporosis is a disorder, with a largely unknown pathomechanism, that is often marked as a "silent thief", because it usually only becomes undisguised when fractures occur. This implies that the pathological damage occurs earlier than the sensation of pain. The current authors put forward a non-contact injury model in which the chronic overloading of an earlier autologously microinjured Piezo2 ion channel of the spinal proprioceptor terminals could lead the way to re-injury and earlier aging in a dose-limiting and threshold-driven way. As a result, the aging process could eventually lead the way to the metabolic imbalance of primary osteoporosis in a quad-phasic non-contact injury pathway. Furthermore, it is emphasised that delayed onset muscle soreness, non-contact anterior cruciate injury and osteoporosis could have the same initiating proprioceptive non-contact Piezo2 channelopathy, at different locations, however, with different environmental risk factors and a different genetic predisposition, therefore producing different outcomes longitudinally. The current injury model does not intend to challenge any running pathogenic theories or findings, but rather to highlight a principal injury mechanism

5-HT7 Receptors Regulate Excitatory-Inhibitory Balance in Mouse Spinal Cord Dorsal Horn

Serotonergic receptors of the 5-HT7 type (5-HT7Rs) are widely expressed in the central nervous system (CNS), where they modulate several functions, such as pain. Behavioral experiments in vivo have shown both anti- and pro-nociceptive actions of 5-HT7Rs, although an analgesic effect seems to be prevalent. In the spinal cord dorsal horn, the mechanisms involved in 5-HT7R-mediated synaptic modulation are still poorly understood, especially those regarding the control of synaptic inhibition. The present study investigated the modulation exerted by 5-HT7Rs on dorsal horn excitatory and inhibitory synaptic circuits, by performing patch-clamp recordings from lamina II neurons in mouse spinal cord slices. Our results show that applying the selective 5-HT7 agonist LP-211 facilitates glutamatergic release by enhancing the frequency of spontaneous postsynaptic currents (sEPSCs) and increasing the peak amplitude of excitatory postsynaptic currents (EPSCs) evoked by dorsal root stimulation. The effects on sEPSCs were still observed in the presence of the 5-HT1A antagonist WAY-100635, while the 5-HT7 antagonist SB-269970 blocked them. LP-211 was also able to increase the release of gamma-aminobutyric acid (GABA) and glycine, as shown by the increase of spontaneous inhibitory currents (sIPSC) frequency and evoked inhibitory postsynaptic currents (IPSC) amplitude. LP-211 was proved to be more effective in potentiating synaptic inhibition as compared to excitation: consistently, 5-HT7R activation significantly enhanced the excitability of tonic firing neurons, mainly corresponding to inhibitory interneurons. Our data bring new insights into the mechanisms of synaptic modulation mediated by 5-HT7Rs in the dorsal horn. Stronger impact on synaptic inhibition supports the hypothesis that these receptors may play an anti-nociceptive role in the spinal cord of naïve animals

Development of nociceptive synaptic inputs to the neonatal rat dorsal horn: glutamate release by capsaicin and menthol

To study the postnatal development of nociceptive synaptic inputs in the superficial dorsal horn of the neonatal rat spinal cord, we examined the effect of capsaicin and menthol on glutamatergic mEPSCs in postnatal day (P) 0-1, P5-6 and P9-11 slices of spinal cord. Capsaicin (100 nM to 2 muM) increased the mEPSC frequency in a concentration-dependent manner at all ages tested, with a significant enhancement of the effect between P5 and P10. This effect was sensitive to vanilloid receptor (VR) antagonists. The elevation in mEPSC frequency occurred at concentrations of capsaicin (100 nM) that did not alter the distribution of mEPSC amplitudes and was abolished by a dorsal rhizotomy, demonstrating that capsaicin acts via presynaptic VR1 receptors localized on primary afferents. Menthol significantly increased the mEPSC frequency with a similar developmental pattern to capsaicin without consistently affecting mEPSC amplitude. The increase in mEPSC frequency following capsaicin did not depend on transmembrane calcium influx since it persisted in zero [Ca2+](o). The facilitation of spontaneous glutamate release by capsaicin was sufficient to evoke action potentials in neonatal dorsal horn neurons but was accompanied by a block of EPSCs evoked by electrical stimulation of the dorsal root. These results indicate that VR1-expressing nociceptive primary afferents; form functional synaptic connections in the superficial dorsal horn from birth and that activation of the VR1 receptor increases spontaneous glutamate release via an undetermined mechanism. In addition, the data suggest that immature primary afferents express functional menthol receptors that are capable of modulating transmitter release. These results have important functional implications for infant pain processing

Distinct roles of NMB and GRP in itch transmission

A key question in our understanding of itch coding mechanisms is whether itch is relayed by dedicated molecular and neuronal pathways. Previous studies suggested that gastrin-releasing peptide (GRP) is an itch-specific neurotransmitter. Neuromedin B (NMB) is a mammalian member of the bombesin family of peptides closely related to GRP, but its role in itch is unclear. Here, we show that itch deficits in mice lacking NMB or GRP are non-redundant and Nmb/Grp double KO (DKO) mice displayed additive deficits. Furthermore, both Nmb/Grp and Nmbr/Grpr DKO mice responded normally to a wide array of noxious stimuli. Ablation of NMBR neurons partially attenuated peripherally induced itch without compromising nociceptive processing. Importantly, electrophysiological studies suggested that GRPR neurons receive glutamatergic input from NMBR neurons. Thus, we propose that NMB and GRP may transmit discrete itch information and NMBR neurons are an integral part of neural circuits for itch in the spinal cord

Excitatory synaptic transmission in neonatal dorsal horn: NMDA and ATP receptors

Postnatal development sees a strong synaptogenesis in rat superficial dorsal born. My studies show that synapses mediated by two excitatory neurotransmitters, glutamate and ATP, are functional since the very first postnatal days. Using an electrophysiological approach, the functional properties of two receptors activated by these neurotransmitters, glutamatergic NMDA and ATP ionotropic receptors, are described

Modulazione della trasmissione sinaptica inibitoria da parte della neurotrofina BDNF nelle corna dorsali del midollo spinale di ratto

Studi compiuti in diverse aree del sistema nervoso centrale hanno mostrato che la neurotrofina BDNF è in grado di controllare la trasmissione sinaptica, modulando sia il rilascio del neurotrasmettitore che la funzionalità dei recettori postsinaptici. E’ noto che nelle corna dorsali del midollo spinale il BDNF, rilasciato dalle fibre afferenti primarie, modula la trasmissione sinaptica glutammatergica. Studi comportamentali hanno suggerito che il BDNF abbia un effetto pro-nocicettivo, anche se sono state osservate azioni analgesiche della neurotrofina in determinate condizioni. Effetti del BDNF sul rilascio di GABA e glicina nelle corna dorsali non erano ancora stati studiati in dettaglio. Il nostro studio si è avvalso sia di tecniche elettrofisiologiche che immunocitochimiche applicate alla microscopia elettronica. Tramite registrazioni in patch-clamp da fettine di midollo spinale di ratti neonati abbiamo osservato che il BDNF provoca un aumento della frequenza delle correnti postsinaptiche in miniatura, mediate sia da GABA che da glicina, senza alterarne l’ampiezza. La neurotrofina provoca anche una depressione delle correnti sinaptiche inibitorie evocate, causando una variazione significativa della variabilità delle risposte sinaptiche e del “paired pulse ratio”. Queste osservazioni suggeriscono che il BDNF possa modulare le sinapsi inibitorie delle corna dorsali, agendo a livello presinaptico. A supporto di ciò, gli studi ultrastrutturali mostrano la co-localizzazione di vescicole di GABA con i recettori per la neurotrofina (recettori trkB) a livello dei glomeruli sinaptici nella lamina II. I nostri dati suggeriscono quindi un ruolo del BDNF nel controllare la trasmissione nocicettiva a livello spinale, tramite la modulazione del rilascio dei due neurotrasmettitori inibitori