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 dor sal horn during post-natal development, synaptic responses evoked by large A beta and fine A delta afferents were recorded from substantia gelatinosa (SG) neurons in slices obtained from immature (post-natal days 21-23) and m ature rats (post-natal days 56-60). Threshold stimulus intensities and cond uction velocities (CVs) of A beta and A delta afferents were determined by intracellular recordings of the antidromic action potentials from dorsal ro ot ganglion (DRG) neurons isolated from immature and mature rats. In immatu re rats, excitatory postsynaptic currents (EPSCs) were elicited by stimulat ion sufficient to activate A beta afferents in the majority of SG neurons ( 64.9%, 24 of 37 neurons), while most EPSCs observed in mature rats were eli cited by stimulation of A delta afferents (62.5%, 25 of 40 neurons). These observations suggest that the primary afferents innervating SG neurons were reorganized following maturation; A beta afferents were the predominant in puts to the SG neurons in the immature state, thereafter A delta afferents were substituted for the A beta afferents to convey sensory information to the SG neurons. This relatively slow reorganization of the sensory circuitr y may correlate with slow maturation of the SG neurons and with a delay in the functional connections of C afferents to the SG neurons. (C) 1999 Elsev ier Science B.V. All rights reserved.