Evaluating the control: Mini-pump implantation and breathing behavior in the neonatal rat

We evaluated genioglossus (GG) gross motoneuron morphology, electromyographic (EMG) activities and respiratory patterning in rat pups allowed to develop without interference (unexposed) and pups born to dams subjected to osmotic mini-pump implantation in utero (saline exposed). In experiment one, forty eight Sprague-Dawley rat pups (Charles-River Laboratories) ages post-natal day 7 (P7) through post-natal day 10 (P10) were drawn from two experimental groups: saline-exposed (N=24) and unexposed (N=24) and studied on post-natal day P7, P8, P9, or P10. Pups in both groups were sedated (Inactin hydrate, 70 mg kg^-1) and fine-wire electrodes inserted into the genioglossus (GG) muscle of the tongue and intercostal muscles to record EMG activities during breathing in air and at three levels of normoxic hypercapnia (F_ICO2: 0.03, 0.06, 0.09). Using this approach, we assessed breathing frequency, heart rate, apnea type, respiratory event types and respiratory stability. In experiment two, sixteen rat pups were drawn from the same experimental groups: saline exposed (N=9) and unexposed (N=7) and used in motoneuron labelling studies. In these pups a retrograde dye was injected into the GG muscle, and the brainstems subsequently harvested and sliced. Labeled GG motoneurons were identified with microscopy, and impaled and filled with Lucifer yellow. Double-labeled motoneurons were reconstructed and the number of primary projections and soma volumes calculated. Whereas pups in each group exhibited the same number (p=0.226) and duration (p=0.093) of respiratory event types and comparable motoneuron morphologies, pups in the implant group exhibited more central apneas and respiratory instability relative to pups allowed to develop without interference.

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