Potentiating effect of diltiazem on pentobarbital-induced hypnosis is augmented by serotonergic system: The TMN and VLPO as key elements in the pathway

To investigate the mechanism by which L-type Ca2+ channel blockers exerted potentiating effects on pentobarbital-induced hypnosis, the present study was undertaken to determine if the interaction of diltiazem and serotonergic system influences the architecture of pentobarbital sleep in rats and examined c-Fos expression in the ventrolateral preoptic nucleus (VLPO) and the tuberomammillary nucleus (TMN). The polysomnogram consisting of EEG and EMG was recorded for analyzing sleep architecture. The results showed that diltiazem (2.0 and 5.0 mg/kg, p.o.) increased both total pentobarbital sleep and slow wave sleep (SWS), but decreased rapid eye movement (REM) sleep. These effects were potentiated by 5-hydroxytryptophan (5-HTP), a precursor of serotonin, but abolished by p-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase. Diltiazem (1 mg/kg, p.o.) or 5-HTP (2 mg/kg, i.p.) alone did not change the architecture of pentobarbital sleep and pentobarbital-induced c-Fos expression in the VLPO and the TMN, but co-administration of them significantly increased both total pentobarbital sleep and SWS, whereas decreased REM sleep, with increasing c-Fos expression in the VLPO and concomitantly decreasing c-Fos expression in the TMN. These findings indicate that the serotonergic system may be involved in the augmentative effect of diltiazem on pentobarbital sleep and the VLPO–TMN neuronal circuit may play a key role.