Reorganization of the primary afferent termination in the rat spinal dorsal horn during post-natal development

To study the reorganization of the primary afferent input in the spinal dorsal horn during post-natal development, synaptic responses evoked by large Abeta and fine Adelta afferents were recorded from substantia gelatinosa (SG) neurons in slices obtained from immature (post-natal days 21-23) and mature rats (post-natal days 56-60). Threshold stimulus intensities and conduction velocities (CVs) of Abeta and Adelta afferents were determined by intracellular recordings of the antidromic action potentials from dorsal root ganglion (DRG) neurons isolated from immature and mature rats. In immature rats, excitatory postsynaptic currents (EPSCs) were elicited by stimulation sufficient to activate Abeta afferents in the majority of SG neurons (64.9%, 24 of 37 neurons), while most EPSCs observed in mature rats were elicited by stimulation of Adelta afferents (62.5%, 25 of 40 neurons). These observations suggest that the primary afferents innervating SG neurons were reorganized following maturation; Abeta afferents were the predominant inputs to the SG neurons in the immature state, thereafter Adelta afferents were substituted for the Abeta afferents to convey sensory information to the SG neurons. This relatively slow reorganization of the sensory circuitry may correlate with slow maturation of the SG neurons and with a delay in the functional connections of C afferents to the SG neurons.