Postnatal development of excitation propagation in the trigeminal subnucleus caudalis evoked by afferent stimulation in mice

The postnatal development of nociceptive afferent activity expansion and its modulation features were examined in mice using an optical imaging technique. Developing mice (1-2 weeks old (N1-2w), 3-4 weeks old (N3-4w), 5-6 weeks old (N5-6w) and 7-8 weeks old (N7-8w)) and neonatally capsaicin-treated mice were used. The propagation of neuronal excitation was measured by changes in fluorescent intensity in horizontal brain stem slices evoked by electrical stimulation to the trigeminal spinal tract. A single-pulse stimulation evoked excitation propagation in the trigeminal caudalis (Vc). The propagation area was larger in N1-2w than in N7-8w, and no differences were observed between capsaicin-treated and naive mice in the same age groups. Repetitive stimulation (100 Hz, 30 pulses) elicited long-lasting and widespread excitation propagation. The excitation propagation area was significantly larger in N7-8w than in N1-2w, N3-4w and N5-6w. This propagation was suppressed by 5 μM L-703.606, an NK1-receptor antagonist, suggesting that the repetitive stimulation-elicited excitation may require substance-P releases. Morphological observations demonstrated that the neural network in the Vc had grown by postnatal week 5. These results suggest that nociceptive afferent activity co-operatively matures with development of the network structure in the Vc, and that a mechanism for prolonged increase in central excitability is established during a later postnatal period.