Cynthia L. Darlington

The neuroprotective properties of the Ginkgo biloba leaf : a review of the possible relationship to platelet-activating factor (PAF)

Ginkgo biloba (Ginkgoaceae) is an ancient Chinese tree which has been cultivated and held sacred for its health-promoting properties. There is substantial experimental evidence to support the view that Ginkgo biloba extracts have neuroprotective properties under conditions such as hypoxia/ischemia, seizure activity and peripheral nerve damage. Research on the biochemical effects of Ginkgo biloba extracts is still at a very early stage. One of the components of Ginkgo biloba, ginkgolide B, is a potent platelet-activating factor (PAF) antagonist. Although the terpene fraction of Ginkgo biloba, which contains the ginkgolides, may contribute to the neuroprotective properties of the Ginkgo biloba leaf, it is also likely that the flavonoid fraction, containing free radical scavengers, is important in this respect. Taken together, the evidence suggests that Ginkgo biloba extracts are worthy of further investigation as potential neuroprotectant agents.

The CNS effects of Ginkgo biloba extracts and ginkgolide B

Ginkgo biloba extracts such as EGb-761 have been suggested to have a multitude of beneficial effects on CNS function, from enhancing cognitive function in dementia to facilitating recovery from acute forms of neural damage such as hypoxia/ischemia. Ginkgolide B, one of the major components of EGb-761, is a potent platelet-activating factor (PAF) receptor antagonist, which is also regarded as having neuroprotective effects in the CNS. The aim of this review is to summarise and to critically evaluate the current evidence on the CNS effects of EGb-761 and ginkgolide B, with particular emphasis on the data relating to their neuroprotective effects.

Expression of the cannabinoid CB2 receptor in the rat cerebellum: An immunohistochemical study

Reports of cannabinoid CB2 receptor protein in the brain have been ambiguous. We therefore tested for CB2 immunoreactivity in the rat brain using immunofluorescence. We detected CB2 labeling in fine fibers in the granule layer. This CB2 labeling did not co-localise with the astrocyte marker glial fibrillary acidic protein (GFAP) and, therefore, the CB2-positive fibers were not astrocytes and were possibly microglial or neuronal. Additionally, strong CB2 labeling was detected in capillary endothelia in the granule, Purkinje cell, and molecular layers. Our results suggest that the role of CB2 receptors in the brain may have been previously underestimated.

Molecular mechanisms of recovery from vestibular damage in mammals: recent advances

The aim of this review is to summarise and critically evaluate studies of vestibular compensation published over the last 2 years, with emphasis on those concerned with the molecular mechanisms of this process of lesion-induced plasticity. Recent studies of vestibular compensation have confirmed and extended the previous findings that: (i) compensation of the static ocular motor and postural symptoms occurs relatively rapidly and completely compared to the dynamic symptoms, many of which either do not compensate substantially or else compensate variably due to sensory substitution and the development of sensori-motor strategies which suppress or minimize symptoms; (ii) static compensation is associated with, and may be at least partially caused by a substantial recovery of resting activity in the ipsilateral vestibular nucleus complex (VNC), which starts to develop very quickly following the unilateral vestibular deafferentation (UVD) but does not correlate perfectly with the development of some aspects of static compensation (e.g., postural compensation); and (iii) many complex biochemical changes are occurring in the VNC, cerebellum and even areas of the central nervous system like the hippocampus, following UVD. However, despite many recent studies which suggest the importance of excitatory amino acid receptors such as the N-methyl-D-aspartate receptor, expression of immediate early gene proteins, glucocorticoids, neurotrophins and nitric oxide in the vestibular compensation process, how these various factors are linked and which of them may have a causal relationship with the physiological changes underlying compensation, remains to be determined.

Neurochemical mechanisms of recovery from peripheral vestibular lesions (vestibular compensation)

This paper reviews the literature relating to the neurochemical basis of vestibular compensation, a process of behavioral recovery which occurs following the removal of afferent input from one labyrinth (unilateral labyrinthectomy, UL). Although vestibular compensation is known to be correlated with a return of resting activity to the vestibular nucleus (VN) ipsilateral to the UL (the deafferented VN), the neurochemical mechanisms by which this neuronal recovery occurs, are unknown. At present, there is little evidence to support the hypothesis that denervation supersensitivity of excitatory amino acid, dopamine, norepinephrine or acetylcholine receptors in the deafferented VN, is responsible for vestibular compensation: binding studies for glutamate or acetylcholine do not support an upregulation of these receptor types. However, changes in the affinity or efficacy of these receptor complexes cannot be ruled out. There are still many neurotransmitter systems, such as serotonergic and histaminergic systems, which have not been investigated in relation to vestibular compensation. In several species it has been shown that treatment with adrenocorticotropic hormone, fragment 4-10 (ACTH-(4-10], can accelerate vestibular compensation. It is not clear how these drugs exert their effects. In vitro electrophysiological studies have shown that VN neurons are capable of generating resting activity in the absence of their normal afferent inputs and it is possible that these neurons have pacemaker-like membrane characteristics which contribute to the regeneration of activity following UL. Recent biochemical studies have revealed changes in the phosphorylation patterns of a number of proteins during compensation. The possible relationship between these phosphorproteins and the synaptic or membrane changes which are responsible for vestibular compensation remains to be determined.

The contribution of the intrinsic excitability of vestibular nucleus neurons to recovery from vestibular damage

Damage to the peripheral vestibular system results in a syndrome of ocular motor and postural abnormalities that partially and gradually abate over time in a process known as ‘vestibular compensation’. The first, rapid, phase of compensation has been associated with a recovery of spontaneous resting activity in the ipsilateral vestibular nucleus complex (VNC), as a consequence of neuronal and synaptic plasticity. Increasing evidence suggests that normal VNC neurons in labyrinthine‐intact animals, as well as ipsilateral VNC neurons following unilateral vestibular deafferentation (UVD), rely to some extent on intrinsic pacemaker activity provided by voltage‐dependent conductances for their resting activity. Modification of this intrinsic pacemaker activity may underlie the recovery of resting activity that occurs in ipsilateral VNC neurons following UVD. This review summarizes and critically evaluates the ‘intrinsic mechanism hypothesis’, identifying discrepancies amongst the current evidence and suggesting experiments that may test it further.

Regional variations and age-related changes in nitric oxide synthase and arginase in the sub-regions of the hippocampus

L-arginine can be metabolised by nitric oxide synthase (NOS) with the formation of L-citrulline and nitric oxide (NO), or arginase with the production of L-ornithine and urea. In contrast to studies showing a potential involvement of NOS/NO in the aging process, the role of arginase has not been well documented. The present study investigates for the first time the regional variations and age-related changes in both NOS and arginase in sub-regions of the hippocampus. In young adult rats, although the total NOS activity was not significantly different across the hippocampal CA1, CA2/3 and the dentate gyrus (DG) sub-regions, the total arginase activity showed a clear regional variation with the highest level in DG. Western blotting revealed that the highest levels of neuronal NOS (nNOS) and endothelial NOS (eNOS) proteins were located in CA1. Arginase I is expressed at a very low level in the brain (the whole hippocampus) as compared with the liver. By contrast, arginase II protein shows an extremely high expression in the brain with little or no expression in the liver. There was no regional variation in arginase I or arginase II protein expression across the sub-regions of the hippocampus. When a comparison was made between young (4-month-old) and aged (24-month-old) rats, a significant increase in total NOS activity was found in DG and significant decreases in arginase activity were observed in the CA1 and CA2/3 regions in the aged animals. Western blotting further revealed a dramatic decrease in eNOS protein expression in aged CA2/3 with no age-associated changes in nNOS, arginase I and II protein expression in any region examined. Interestingly, evidence of activity or protein expression of the inducible isoform of NOS (iNOS) was not detected in any tissue from either group. The present results, in conjunction with previous findings, support the contribution of NOS/NO to aging but question the involvement of iNOS in the normal aging process. Region-specific changes in arginase suggest that this enzyme may also contribute to aging.

PLATELET-ACTIVATING FACTOR IN THE CNS

Platelet-activating factor (PAF) is a phospholipid synthesized in a variety of cells throughout the body. Platelet-activating factor has been identified in the CNS and has a number of diverse physiological and pathological functions. It has been shown to be a modulator of many CNS processes, ranging from long-term potentiation (LTP) to neuronal differentiation. Excessive levels of PAF appear to play an important role in neuronal cell injury, such as that resulting from ischaemia, inflammation, human immunodeficiency syndrome (HIV) and meningitis. The beneficial effects of PAF receptor antagonists are many and give rise to possible therapeutic strategies for neurotrauma.

The NMDA antagonists MK801 and CPP disrupt compensation for unilateral labyrinthectomy in the guinea pig

Unilateral labyrinthectomy results in eye movement and postural disorders which diminish over time in a process of behavioral recovery called vestibular compensation. This compensation process is due to CNS plasticity which generates a renewal of spontaneous activity in vestibular nucleus neurons ipsilateral to the labyrinthectomy. However, the mechanisms responsible for the induction and maintenance of this neural recovery are unknown. The present results show that i.p. injections of the N-methyl-D-aspartate (NMDA) receptor antagonists MK801 [( (+)-5-methyl-10,11-dihydro-5H- dibenzo[a,d]-cyclohepten-5,10-imine maleate]) and CPP (3-[+/- )-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) disrupt the maintenance of compensation following unilateral labyrinthectomy, producing a reappearance of eye movement symptoms after compensation has been attained. These results suggest that NMDA receptors may be involved in the maintenance of the neural changes responsible for vestibular compensation.

Molecular mechanisms of brainstem plasticity. The vestibular compensation model.

Vestibular compensation is the process of behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (unilateral labyrinthectomy, UL). Since UL results in a permanent loss of vestibular input from the ipsilateral vestibular (VIIIth) nerve, vestibular compensation is attributed to CNS plasticity and has been used as a general model of lesion-induced CNS plasticity. Behavioral recovery from the ocular motor and postural symptoms of UL is correlated with a partial return of resting activity to neurons in the vestibular nucleus (VN) on the deafferented side (the “deafferented VN”), and lesions to the deafferented VN prevent compensation; therefore, the regeneration of resting activity within the deafferented VN is believed to have a causal role in vestibular compensation. The biochemical mechanisms responsible for the adaptive neuronal changes within the deafferented VN are poorly understood. Neuropeptide hormone fragments, such as adrenocorticotrophic hormone (ACTH)-4–10, have been shown to accelerate vestibular compensation and can act directly on some VN neurons in vitro. Antagonists for theN-methyl-D-aspartate (NMDA) receptor have been shown to inhibit vestibular compensation if administered early in the compensation process. Biochemical studies in frog indicate marked alterations in the phosphorylation patterns of several proteins during compensation, and the in vitro phosphorylation of some of these proteins is modulated by ACTH-(1–24), calcium (Ca2+), and calmodulin or protein kinase C. It is therefore possible that ACTH fragments and NMDA antagonists (via their effects on NMDA receptor-mediated Ca2+ channels) modulate vestibular compensation through their action on Ca2+-dependent pathways within VN neurons. Recent studies have shown that some Ca2+-channel antagonists and the Ca2+-dependent enzyme inhibitor calmidazolium chloride facilitate vestibular compensation. How the regulation of Ca2+ may be related to the neuronal changes responsible for vestibular compensation is unclear at present.