4-Methylpyrazole alters phenobarbitone hypnotic concentrations in rats.

The liver alcohol dehydrogenase inhibitor, 4-methylpyrazole, has been tested for its ability to change the hypnotic concentrations of phenobarbitone (phenobarbital) in rats. Following a single dose of 1 mmol kg-1 i.v., administered 60 min before phenobarbitone, 4-methylpyrazole shortened the onset time and reduced the dose of phenobarbitone required to produce loss of righting reflex. Consistent with this, phenobarbitone concentrations in serum (both total and free), brain and in cerebrospinal fluid at onset of hypnosis were about half in 4-methylpyrazole compared with saline-treated rats. These results suggest that acute 4-methylpyrazole pretreatment increases the central nervous system sensitivity to phenobarbitone and presumably other barbiturates; an effect apparently distinct from its inhibition of liver alcohol dehydrogenase.