EFFECTS OF UNILATERAL LESIONS OF RETROTRAPEZOID NUCLEUS ON BREATHING IN AWAKE RATS

Effects of unilateral lesions of retrotrapezoid nucleus on breathing i n awake rats. J. Appl. Physiol. 82(2): 469-479, 1997.-In anesthetized rats, unilateral retrotrapezoid nucleus (RTN) lesions markedly decreas ed baseline phrenic activity and the response to CO2 (E. E. Nattie and A. Li. Respir. Physiol. 97: 63-77, 1994). Here we evaluate the effect s of such lesions on resting breathing and on the response to hypercap nia and hypoxia in unanesthetized awake rats. We made unilateral injec tions [24 +/- 7 (SE) nl] of ibotenic acid (IA; 50 mM), an excitatory a mino acid neurotoxin, in the RTN region (n = 7) located by stereotaxic coordinates and by field potentials induced by facial nerve stimulati on. Controls (n = 6) received RTN injections (80 +/- 30 nl) of mock ce rebrospinal fluid. A second control consisted of four animals with IA injections (24 +/- 12 nl) outside the RTN region. Injected fluorescent beads allowed anatomic identification of lesion location. Using whole body plethysmography, we measured ventilation in the awake state duri ng room air, 7% CO2 in air, and 10% O-2 breathing before and for 3 wk after the RTN injections. There was no statistically significant effec t of the IA injections on resting room air breathing in the lesion gro up compared with the control groups. We observed no apnea. The respons e to 7% CO2 in the lesion group compared with the control groups was s ignificantly decreased, by 39% on average, for the final portion of th e 3-wk study period. There was no lesion effect on the ventilatory res ponse to 10% O-2. In this unanesthetized model, other areas suppressed by anesthesia, e.g., the reticular activating system, hypothalamus, a nd perhaps the contralateral RTN, may provide tonic input to the respi ratory centers that counters the loss of RTN activity.