Functional reorganization of the spinal pain pathways in developmental and pathological conditions

Following inflammation, a subpopulation of Aβ afferents that terminates preferentially in deeper laminae have been shown to extend their axons to the superficial dorsal horn, particularly substantia gelatinosa (SG). Similarly, SG neurons in immature spinal cord receive mainly Aβ afferent inputs. To clarify whether the reorganized sensory pathway in the inflamed rats has a functional similarity with that in the developmental state, we compared synaptic inputs from primary afferents using in vitro and in vivo patch-clamp recordings from SG neurons. SG neurons in the mature state had monosynaptic inputs from Aδ and C afferents, while only a few neurons received inputs from Aβ afferents. Following inflammation, the Aβ afferents extended their axons to SG and established functional monosynaptic transmission. Meanwhile, SG neurons in the immature state received preferentially Aβ as well as Aδ afferent inputs, and the majority of Aα afferent inputs were monosynaptic. These observations support the idea that the sprouting of the large afferent fibres observed in inflamed rats is, at least in part, a regeneration process. However, the process, maybe distinct at some point from the process during development, therefore, produces pathological pain. Though the idea that the regeneration mimics the developmental process has been widely accepted, other possibilities cannot be excluded.