Presynaptic functional trkB receptors mediate the release of excitatory neurotransmitters from primary afferent terminals in lamina II (substantia gelatinosa) of postnatal rat spinal cord.
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.
Merighi
Adalberto
A
Dipartimento di Morfofisiologia Veterinaria, Università di Torino, Via Leonardo da Vinci 44, 10095 Grugliasco, Italy. adalberto.merighi@unito.it
Bardoni
Rita
R
Salio
Chiara
C
Lossi
Laura
L
Ferrini
Francesco
F
Prandini
Massimiliano
M
Zonta
Micaela
M
Gustincich
Stefano
S
Carmignoto
Giorgio
G
D000818:Animals; D000831:Animals, Newborn; D019208:Brain-Derived Neurotrophic Factor / Q000378:metabolism*; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D005455:Fluorescent Antibody Technique; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D046529:Microscopy, Electron, Transmission; D009475:Neurons, Afferent / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D017729:Presynaptic Terminals / Q000378:metabolism*; D051381:Rats; D020813:Receptor, trkB / Q000378:metabolism*; D017661:Receptors, Presynaptic / Q000378:metabolism*; D013376:Substantia Gelatinosa / Q000378:metabolism*
D019208:Brain-Derived Neurotrophic Factor; D018377:Neurotransmitter Agents; D017661:Receptors, Presynaptic; D020813:Receptor, trkB