Role of Central Hypercapnia in Rhythmic Spontaneous Neural Activity and Breathing Rhythms in the Developing Avian Medulla

Abstract

The execution of rhythmic behaviors such as breathing requires proper construction of neural circuits during embryogenesis and throughout the perinatal period. A unique feature of developing circuits that may support neuronal circuit organization is the appearance and maintenance of rhythmic spontaneous neural activity (rSNA). Although rSNA has been studied extensively using a variety of in vitro preparations, rarely has rSNA been observed continuously from its onset until it contributes to mature behaviors. Little is known about how rSNA is maintained throughout the embryonic and perinatal periods due to difficulties accessing mammalian embryos and the delicate nature of embryonic tissue. To better understand these details, we used the zebra finch embryo brainstem preparation, and recorded rSNA and subsequent breathing rhythms continuously from embryonic day (E) 4 through hatching on E14. Our objective was to test whether breathing‐related rhythms during the aqueous embryonic period and aerial breathing rhythms during the perinatal period are sensitive to changes in CO2 (hypercapnia or hypocapnia). We hypothesized that hypercapnic and hypocapnic stimuli would alter rSNA frequency according to embryonic age. We predicted prenatal embryos (<E11) would show changes in rSNA that corresponded with generalized dampening or strengthening of neural activity based on whether pH was more acidic or alkaline, respectively. In non‐respiratory neurons, acidic stimuli typically lower activity and alkaline stimuli raise it. In contrast, we predicted the most mature, and continuously air breathing, embryos (>E11) would show respiratory‐related directional alterations in breathing‐related rhythm frequency similar to neonatal mammals exposed to hypercapnia or hypocapnia. To test our hypotheses, we superfused zebra finch brainstems with artificial cerebrospinal fluid equilibrated with control (90% O2, 5% CO2), hypercapnic (90% O2, 8% CO2), or hypocapnic (90% O2, 2% CO2) gases. During the hour‐long treatments, rSNA (<E11) or breathing‐related rhythms (>E11) were recorded from the spinal accessory nerve. Results suggest that CO2 modifies rSNA in younger embryos as well as breathing rhythms in older embryos, however the polarity of the response reversed near E11. Specifically, hypercapnia decreased rSNA frequency up to approximately 66 ± 14% of control frequency in prenatal embryos (<E11) and increased respiratory‐related neural activity frequency to approximately 112 ± 10% of control in perinatal embryos (>E11). Moreover, preliminary data from experiments where [NaHCO3] was raised such that pH was 7.7 show that hypocapnia increased the frequency of rSNA in E4–E11 embryos and leads to slow, irregular breathing‐related rhythms in E11–E14 embryos. These data indicate that the undifferentiated future respiratory circuits of the developing embryo are capable of sensing changes in CO2 and that this signal may serve to regulate rhythmic oscillations prior to air breathing. As predicted, near E11, when air breathing begins, the response to CO2 switches from the embryonic condition to the mature condition—consistent with CO2 effects in perinatal and newborn rodents and humans.Support or Funding InformationIdaho IDeA Network of Biomedical Research and Excellence (INBRE) Program Grant # P20 GM103408 (National Institute of General Medical Sciences) to JRW and CWL, and the National Institute of Neurological Disorders and Stroke via the Health Academic Research Enhancement Award (AREA) Program Grant # 1R15NS087521‐01A1 to JQP.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.