Binding of herpes simplex virus to regional areas of the rodent brain

The Hippocampal Vulnerability to Herpes Simplex Virus Type I Infection: Relevance to Alzheimer's Disease and Memory Impairment

Herpes simplex virus type 1 (HSV-1) as a possible infectious etiology in Alzheimer’s disease (AD) has been proposed since the 1980s. The accumulating research thus far continues to support the association and a possible causal role of HSV-1 in the development of AD. HSV-1 has been shown to induce neuropathological and behavioral changes of AD, such as amyloid-beta accumulation, tau hyperphosphorylation, as well as memory and learning impairments in experimental settings. However, a neuroanatomical standpoint of HSV-1 tropism in the brain has not been emphasized in detail. In this review, we propose that the hippocampal vulnerability to HSV-1 infection plays a part in the development of AD and amnestic mild cognitive impairment (aMCI). Henceforth, this review draws on human studies to bridge HSV-1 to hippocampal-related brain disorders, namely AD and aMCI/MCI. Next, experimental models and clinical observations supporting the neurotropism or predilection of HSV-1 to infect the hippocampus are examined. Following this, factors and mechanisms predisposing the hippocampus to HSV-1 infection are discussed. In brief, the hippocampus has high levels of viral cellular receptors, neural stem or progenitor cells (NSCs/NPCs), glucocorticoid receptors (GRs) and amyloid precursor protein (APP) that support HSV-1 infectivity, as well as inadequate antiviral immunity against HSV-1. Currently, the established diseases HSV-1 causes are mucocutaneous lesions and encephalitis; however, this review revises that HSV-1 may also induce and/or contribute to hippocampal-related brain disorders, especially AD and aMCI/MCI.

Behavioral effects of neonatal herpes simplex type 1 infection of mice

Neonatal infection with a mutant herpes simplex type 1 virus produced hyperactivity in mice. Activity was measured throughout a 24 hour period during adulthood, and the elevation of activity occurred during the period of the day when mice are normally inactive. In a second experiment, infected mice showed deficits in learning to inhibit behavior in a passive avoidance task, but no deficit in learning a complex spatial task. Virus was detected in the brain by 5 days of age. The peak percentage of mice infected was reached at 10 days of age and declined thereafter. Mortality due to the virus declined with age at which the mouse was infected, but rates of hyperactivity were not different when injection occurred within the first 4 days of life. The viral infection produced no deficit in body weight in suckling mice. Thus we have shown that a mild neonatal virus infection can produce specific behavioral deficits.

Contrasting patterns of virus spread and neuropathology following microinjection of herpes simplex virus into the hippocampus or cerebellum of mice

Two strains of herpes simplex virus type 1 (HSV) were microinjected into either the hippocampus or cerebellum of two different mouse strains. Immunoperoxidase staining revealed rapid and extensive spread of virus within the hippocampus and certain of its afferent connections. In contrast, only scattered peroxidase positive cells were observed following cerebellar inoculation. Extensive lesions were observed only in animals surviving intrahippocampal inoculations. These results suggest that localization of the pathological process in HSV encephalitis is due in part to a selective vulnerability of telencephalic or limbic structures.