Neurotransmitter synthesizing enzymes in experimental viral encephalitis

Stereotactic intracerebral inoculation of a non-neuroadapted strain of herpes simplex virus type 1 into the left neostriatum of Sprague-Dawley rats induced clinical acute encephalitis within 3 to 5 days postinoculation, with microscopic evidence of inflammation in brain parenchyma, but with no gross areas of tissue destruction. Viral presence in brain was unequivocally confirmed by tissue culture, immunofluorescence and electron microscopy. Levels of activity of neurotransmitter synthesizing enzymes tyrosine hydroxylase (TH), glutamate decarboxylase (GAD), and choline acetyltransferase (ChAT) in the substantia nigra, caudate-putamen and frontal cortex of acutely encephalitic animals were not significantly different from those of PBS-inoculated controls; neither were there significant differences between the inoculated and non-inoculated sides of the individual animals. Our results show that locally injected herpes simplex virus may spread in brain causing neurological symptoms and death without major local structural changes or loss of neurotransmitter synthesizing enzymes. The degree and distribution of cell dysfunction and cell loss in viral encephalitis basically determine any alterations of enzyme activities specific to the involved cell population. The literature on neurotransmitter enzymes and experimental viral encephalitis is reviewed.