Presynaptic functional trkB receptors mediate the release of excitatory neurotransmitters from primary afferent terminals in lamina II (substantia gelatinosa) of postnatal rat spinal cord

Abstract

A subset of primary sensory neurons produces BDNF, which is implicated in control of nociceptive neurotransmission. We previously localized full-length trkB receptors on their terminals within lamina II. To functionally study these receptors, we here employed patch-clamp recordings, calcium imaging and immunocytochemistry on slices from 8-12 days post-natal rats. In this preparation, BDNF (100-500 ng/mL) enhances the release of sensory neurotransmitters (glutamate, substance P, CGRP) in lamina II by acting on trkB receptors expressed by primary afferent fibers of the peptidergic nociceptive type (PN-PAFs). Effect was blocked by trk antagonist K252a or anti-trkB antibody clone 47. A pre-synaptic mechanism was demonstrated after (i) patch-clamp recordings where the neurotrophin induced a significant increase in frequency, but not amplitude, of AMPA-mediated mEPSCs, (ii) real time calcium imaging, where sustained application of BDNF evoked an intense response in up to 57% lamina II neurons with a significant frequency rise. Antagonists of ionotropic glutamate receptors and NK(1) receptors completely inhibited the calcium response to BDNF. Reduction of CGRP (a specific marker of PN-PAFs) and substance P content in dorsal horn following BDNF preincubation, and analysis of the calcium response after depletion with capsaicin, confirmed that the neurotrophin presynaptically enhanced neurotransmitter release from PN-PAFs. This is the first demonstration that trkB receptors expressed by PN-PAF terminals in lamina II are functional during postnatal development. Implications of this finding are discussed considering that BDNF can be released by these same terminals and microglia, a fraction of which (as shown here) contains BDNF also in unactivated state.

Neuromodulatory function of neuropeptides in the normal CNS

“…Therefore, by concurrent receptor localization (Salio et al, 2005;Merighi and Salio, 2010) and functional observations (Bardoni et al, 2007;Merighi et al, 2008a) one can start to envisage a more precise definition of the neurotransmitter role of neuropeptides and growth factors in this area of CNS (Fig. 1D).…”

BDNF contributes to the development of neuropathic pain by induction of spinal long-term potentiation via SHP2 associated GluN2B-containing NMDA receptors activation in rats with spinal nerve ligation

“…Phosphorylation of GluN2B by Fyn is suggested to contribute to the increase of glutamatergic synaptic transmission induced by BDNF (Alder et al, 2005;Xu et al, 2006), and tyrosine phosphorylation of GluN2B is also considered to underlie the induction of LTP in the hippocampus (Nakazawa et al, 2001(Nakazawa et al, , 2002 as well as the maintenance of neuropathic pain (Abe et al, 2005). In addition, the BDNF-mediated potentiation of excitatory transmission in the spinal dorsal horn appears to be associated with the activation of postsynaptic NMDA receptors (Garraway et al, 2003;Merighi et al, 2008a). The release of BDNF within the spinal cord results in phosphorylation and potentiation of NMDA receptors on the spinal cord neurons (Kerr et al, 1999;Obata and Noguchi, 2006;Slack et al, 2004;Slack and Thompson, 2002), and this effect represents a possible mechanism by which BDNF mediates LTP in the spinal dorsal horn (Zhou et al, 2008(Zhou et al, , 2011 as well as central sensitization in nociceptive pathways (Garraway et al, 2003;Malcangio and Lessmann, 2003;Ruscheweyh et al, 2011;Wang et al, 2009).…”

Targeting the glial-derived neurotrophic factor and related molecules for controlling normal and pathologic pain

“…These neurons and their terminals were also decorated by the isolectin B4 (IB4) produced by the plant Griffonia simplicifolia [12], a marker of the cell membrane of the DRG neurons supported by GDNF during development [13]. Very likely, a similar pattern occurs in rat considering the mirroring distribution of brain-derived neurotrophic factor (BDNF)/tyrosine kinase receptor B (TrkB) and GDNF in the two species [14,15].…”

“…Since at least some of these fibers express the GFRα1/RET complex they can be an additional potential target for the antinociceptive effects of GDNF. For details see [11,15].…”