Isolated in vitro brainstem-spinal cord preparations remain important tools in respiratory neurobiology

Abstract

Isolated in vitro brainstem-spinal cord preparations are used extensively in respiratory neurobiology because the respiratory network in the pons and medulla is intact, monosynaptic descending inputs to spinal motoneurons can be activated, brainstem and spinal cord tissue can be bathed with different solutions, and the responses of cervical, thoracic, and lumbar spinal motoneurons to experimental perturbations can be compared. The caveats and limitations of in vitro brainstem-spinal cord preparations are well-documented. However, isolated brainstem-spinal cords are still valuable experimental preparations that can be used to study neuronal connectivity within the brainstem, development of motor networks with lethal genetic mutations, deleterious effects of pathological drugs and conditions, respiratory spinal motor plasticity, and interactions with other motor behaviors. Our goal is to show how isolated brainstem-spinal cord preparations still have a lot to offer scientifically and experimentally to address questions within and outside the field of respiratory neurobiology.

Fig. 1. Advantages of brainstem-spinal cord preparations

A drawing of an isolated brainstem-spinal cord preparation illustrates several experimental advantages of this preparation. The circled numbers within the figure schematically highlight the advantages listed in the box below. ADVANTAGES OF BRAINSTEM-SPINAL CORD PREPARATIONS 1. Central respiratory network in the pons and medulla is intact. 2. Monosynaptic descending bulbospinal connections can be electrically activated. 3. Brainstem and spinal cord can be bathed separately. 4. Phrenic, intercostal, and abdominal motoneuron physiology can be compared. 5. Interaction with other motor behaviors (locomotion) can be studied in vitro. 6. Fictive spinal respiratory motor output is correlated with ventilation. 7. Respiratory spinal plasticity can be studied in vitro.

Fig. 2. Expiratory and inspiratory spinal motor activity produced by an adult vertebrate brainstem-spinal cord preparation

(A) A drawing of an isolated brainstem-spinal cord preparation from an adult turtle shows that respiratory-related motor activity is produced on pectoralis (expiratory) and serratus (inspiratory) nerves. (B) Rhythmic expiratory and inspiratory motor activity on nerves is correlated with respiratory activity on hypoglossal nerve roots in the brainstem. (C) Expiratory activity on pectoralis is typically bell-shaped while inspiratory activity on serratus is slowly-incrementing and rapidly decrementing. The resistance of turtle brain and spinal cord to hypoxia allows this preparation to produce spinal respiratory activity similar to intact adult turtles (discussed in Johnson et al., 1998). In contrast, the nature of respiratory-related motor output produced by neonatal rodent brainstem-spinal cord preparations is controversial. Abbreviations: KF = Kölliker-Fuse nucleus; PB = parabrachial nucleus; pFRG = para-facial group; preBötC = pre-Bötzinger Complex; VRC = ventral respiratory column.