Domoic Acid Lesions in Nucleus of the Solitary Tract: Time-Dependent Recovery of Hypoxic Ventilatory Response and Peripheral Afferent Axonal Plasticity

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Abstract
The nucleus of the solitary tract (NTS) plays a pivotal role in the ventilatory response to hypoxia (HVR). However, the effects of excitotoxic lesions and the potential for functional recovery and plasticity remain unknown. Domoic acid (DA) or vehicle were bilaterally injected within the NTS of adult male Sprague Dawley rats. HVR (10% O2) and anatomical changes were assessed at 5–90 d after surgery. DA induced dose-dependent HVR attenuations (∼70% at peak effect) that exhibited saturation at concentrations of 0.75–1.0 mm. However, although sodium cyanide-induced ventilatory responses were virtually abolished, DA did not modify baroreceptor gain. Consistent with ventilatory reductions, NTS neurons showed a significant degeneration 3 d after DA injection. In addition, the projection fields and the density of vagal afferent terminals to the NTS, and the motor neurons in the dorsal motor nucleus of the vagus were substantially reduced at 15 d. At 30 d, no functional or neural recovery were apparent. However, at day 60, the reduction in HVR was only ∼40% of control, and at 90 d, HVR returned to control levels, paralleling regeneration of vagal afferent terminals within the NTS. The regeneration was particularly prominent in the commissural and dorsomedial subnuclei in the absence of cellular recovery. Thus, the integrity of the NTS is critical for HVR, spontaneous HVR recovery occurs after excitotoxic lesions in the NTS, and vagal–glossopharyngeal terminal sprouting in the NTS may underlie the anatomical substrate for such spontaneous functional recovery. The adult brainstem/NTS has self-repairing capabilities and will compensate for functional losses after structural damage by rewiring of its neural circuitry.