2.4. Mouse respiration experiments
The changes in respiratory function were assessed in awake, freely
moving mice using four parallel whole-body plethysmography chambers
connected to a pressure transducer with the accompanying controller and
software (Buxco, DSI Instruments). Experiments were conducted at room
temperature (23–25°C). A continuous flow of fresh air (2 L/min) through
the plethysmograph chamber was used to maintain constant temperature and
humidity. Breathing variables were determined by Ponemah Analysis
Modules (DSI). System software was used to calculate the respiratory
frequency, tidal volume, minute ventilation (respiratory frequency ×
tidal volume), inspiratory time, and expiratory time. To measure
respiratory functions, each mouse was allowed to acclimatize to the
chamber for 1 h. A baseline plethysmographic recording was performed for
30 min. Following the baseline recordings, the mice were removed from
the chamber, and a pharmacological agent or vehicle was administered.
The mice were then returned to the chamber, and respiratory functions
were recorded after morphine or fentanyl administration. The average f,
TI, TE, and VE were
calculated.