Excitatory cardiovascular and respiratory effects of baclofen in intact rats

The rat has become the preferred animal model for study of the central control of the cardiorespiratory system. However, data in the literature suggest that the role of GABAB receptors in control of the respiratory timing may be different in rats from that in other species. Therefore in this study we investigated cardiorespiratory effects of repeated injections of 0.5 nM baclofen and 50 nM CGP35348, GABAB receptor agonist and antagonist, respectively, applied into the fourth ventricle of urethane-anesthetized intact, spontaneously breathing Wistar rats. Baclofen increased arterial blood pressure (ABP), heart rate, inspiratory time (Ti), and the rate of rise (Di/t) and peak amplitude of the integrated diaphragmatic EMG (Di). Expiration (Te) was unaffected. CGP35348 reversed the ABP and Di effects of baclofen and decreased Di/t and Te below their control values, whereas Ti remained prolonged. These excitatory effects of baclofen are consistent with previously reported effects of low i.v. doses of baclofen in cats, and suggest that GABAB receptors may modulate the depth and duration of inspiration. We conclude that the intact rat represents a suitable animal model for investigations of the integrated control of cardiorespiratory functions. To our knowledge this is the first investigation reporting excitatory effects of GABAB receptor stimulation on inspiratory activity in rats. The depressant effects of baclofen on the respiratory rhythm reported in the rat "isolated" brain in situ, and the neonatal rat brainstem--spinal cord in vitro seem to be unique for those specific preparations.