Type 4 phosphodiesterase (PDE4) inhibitors mimic the pharmacological actions of alpha2-adrenoceptor antagonists. This has been postulated as the mechanism by which PDE4 inhibitors induce emesis and was also demonstrated by their ability to reverse xylazine/ketamine-induced anaesthesia. We further characterized this latter effect since it appears to reflect the emetic potential of PDE4 inhibitors.
Selective inhibitors of PDE 1, 2, 3, 4 and 5 were studied in rats, on the duration of anaesthesia induced by the combination of xylazine (10 mg kg−1, i.m.) and ketamine (10 mg kg−1, i.m.) PMNPQ (i.e. 6-(4-pyridylmethyl)-8-(3-nitrophenyl)quinoline) – PDE4 inhibitor: 0.01 – 3 mg kg−1), like MK-912 (alpha2-adrenoceptor antagonist: 0.01 – 3 mg kg−1), dose-dependently reduced the duration of anaesthesia. In contrast, vinpocetine (PDE1 inhibitor), EHNA (PDE2 inhibitor), milrinone (PDE3 inhibitor) and zaprinast (PDE5 inhibitor) had no significant effect at the doses tested (1 – 10 mg kg−1). Analysis of plasma and cerebrospinal fluid (CSF) of treated animals confirmed the absorption and distribution to the brain of the inactive inhibitors.
Neither MK-912 (3 mg kg−1) nor PMNPQ (0.1 – 1 mg kg−1) altered the duration of anaesthesia induced via a non-alpha2-adrenoceptor pathway (sodium pentobarbitone 50 mg kg−1, i.p.)
Central NK1 receptors are involved in PDE4 inhibitor-induced emesis. Consistently, [sar9, Met(O2)11]-substance P (NK1 receptor agonist, 6 μg i.c.v.) reduced the duration of anaesthesia induced by xylazine/ketamine.
In summary, this model is functionally coupled to PDE4, specific to alpha2-adrenoceptors and relevant to PDE4 inhibitor-induced emesis. It therefore provides a novel way of evaluating the emetic potential of PDE4 inhibitors in rats.
British Journal of Pharmacology (2002) 135, 113–118; doi:10.1038/sj.bjp.0704457