Primary afferent fibre input to the spinal dorsal horn plays a crucial role in the induction and maintenance of pain hypersensitivity.
It is, however, presently unknown in which pattern the different types of sensory afferents contribute to the symptoms of various aetiological forms of neuropathic and inflammatory pain conditions. Vesicular glutamate transporter 3 (VGluT3) -positive primary afferents have been implicated in mechanical hypersensitivity during peripheral inflammation, but their contribution to neuropathic pain after nerve lesions remains under discussion. The aim of this thesis was to investigate a possible aetiology-dependent contribution of VGluT3+ fibres to mechanical and cold hypersensitivity in in a variety of inflammatory and neuropathic pain models. In addition to VGluT3-/- mice, used to study the role of VGluT3 in chronic pain conditions, VGluT3 - Channelrhodopsin 2 (ChR2) mice were used to directly assess the contribution of VGluT3+ sensory afferents to hypersensitivities in chronic pain models. In these mice, VGluT3+ sensory fibres were activated by blue light, and light-evoked behaviour was assessed in freely moving animals with the use of a novel light stimulation paradigm. This work shows that VGluT3-/- mice display an impaired development of mechanical hypersensitivity upon carrageenan injection.
Mechanical and cold hypersensitivity were significantly reduced in VGluT3-/- mice in neuropathic pain evoked by the chemotherapeutic drug oxaliplatin, but not by the chronic constriction injury (CCI) model.
Further, we provide direct evidence that, despite not mediating painful stimuli in naïve mice, activation of VGluT3+ sensory fibres by light elicits pain behaviour in the oxaliplatin- but not the CCI model. In addition, we analyzed the expression of transient receptor potential melastatin 8 (TRPM8), the main detector of environmental cold, in VGluT3+ fibres. Immunohistochemical and electrophysiological data implicate a role of TRPM8-mediated facilitation of synaptic strength at the level of the dorsal horn as underlying mechanism. In sum, this thesis shows that VGluT3+ fibres contribute in an aetiology-dependent manner to the development of mechanical and cold hypersensitivity.