Augmented breath phase timing is altered by systemic administration of the serotonin precursor 5‐HTP conscious rats

Abstract

Augmented breaths (ABs) are a part of the normal repertoire of respiratory behavior. ABs consist of 4 distinct phases. The inspiratory phase of an AB includes an initial phase or AB-Ti-1 which represents a normal inspiratory effort. This is followed by a second inspiratory phase or AB-Ti-2 which reflects the augmented inspiratory effort. The beginning of the expiratory phase is characterized by a postinspiratory inspiratory phase (PIIA). The fourth phase or AB-Te-2 occurs from the end of PIIA to the onset of the next inspiratory burst (Fig. 1). The present study was undertaken to assess whether elevated serotonin could selectively change AB phase timing in the conscious rat. Six male Sprague-Dawley rats were chronically instrumented with EEG, neck and diaphragm EMG electrodes. Following surgery, rats received intraperitoneal injections of vehicle (3% ethanol in saline) or 5HTP (25 mg/kg) in random order on alternate days. Sleep stage and muscle activities were recorded for 6 hrs after injection. ABs was observed primarily during quiet wakefulness. AB-Ti-1, Ti-2 and PIIA durations were unchanged following 5HTP administration compared to vehicle. In contrast, AB-Te-2 phase was significantly decreased (0.43±0.1s) following 5HTP compared to control (2.04±0.3s). The results suggest that serotonin may have a selective effect on modulating the central neural circuitry involved in the timing of Te-2 of ABs.