Paula C. Bickford

Reversals of Age-Related Declines in Neuronal Signal Transduction, Cognitive, and Motor Behavioral Deficits with Blueberry, Spinach, or Strawberry Dietary Supplementation

Ample research indicates that age-related neuronal–behavioral decrements are the result of oxidative stress that may be ameliorated by antioxidants. Our previous study had shown that rats given dietary supplements of fruit and vegetable extracts with high antioxidant activity for 8 months beginning at 6 months of age retarded age-related declines in neuronal and cognitive function. The present study showed that such supplements (strawberry, spinach, or blueberry at 14.8, 9.1, or 18.6 gm of dried aqueous extract per kilogram of diet, respectively) fed for 8 weeks to 19-month-old Fischer 344 rats were also effective in reversing age-related deficits in several neuronal and behavioral parameters including: oxotremorine enhancement of K+-evoked release of dopamine from striatal slices, carbachol-stimulated GTPase activity, striatal Ca45buffering in striatal synaptosomes, motor behavioral performance on the rod walking and accelerod tasks, and Morris water maze performance. These findings suggest that, in addition to their known beneficial effects on cancer and heart disease, phytochemicals present in antioxidant-rich foods may be beneficial in reversing the course of neuronal and behavioral aging.

Long-Term Dietary Strawberry, Spinach, or Vitamin E Supplementation Retards the Onset of Age-Related Neuronal Signal-Transduction and Cognitive Behavioral Deficits

Recent research has indicated that increased vulnerability to oxidative stress may be the major factor involved in CNS functional declines in aging and age-related neurodegenerative diseases, and that antioxidants, e.g., vitamin E, may ameliorate or prevent these declines. Present studies examined whether long-term feeding of Fischer 344 rats, beginning when the rats were 6 months of age and continuing for 8 months, with diets supplemented with a fruit or vegetable extract identified as being high in antioxidant activity, could prevent the age-related induction of receptor-mediated signal transduction deficits that might have a behavioral component. Thus, the following parameters were examined: (1) oxotremorine-enhanced striatal dopamine release (OX-K+-ERDA), (2) cerebellar β receptor augmentation of GABA responding, (3) striatal synaptosomal45Ca2+ clearance, (4) carbachol-stimulated GTPase activity, and (5) Morris water maze performance. The rats were given control diets or those supplemented with strawberry extracts (SE), 9.5 gm/kg dried aqueous extract (DAE), spinach (SPN 6.4 gm/kg DAE), or vitamin E (500 IU/kg). Results indicated that SPN-fed rats demonstrated the greatest retardation of age-effects on all parameters except GTPase activity, on which SE had the greatest effect, whereas SE and vitamin E showed significant but equal protection against these age-induced deficits on the other parameters. For example, OX-K+-ERDA enhancement was four times greater in the SPN group than in controls. Thus, phytochemicals present in antioxidant-rich foods such as spinach may be beneficial in retarding functional age-related CNS and cognitive behavioral deficits and, perhaps, may have some benefit in neurodegenerative disease.

Antioxidant-rich diets improve cerebellar physiology and motor learning in aged rats.

The free radical theory of aging predicts that reactive oxygen species are involved in the decline in function associated with aging. The present paper reports that diets supplemented with either spinach, strawberries or blueberries, nutritional sources of antioxidants, reverse age-induced declines in beta-adrenergic receptor function in cerebellar Purkinje neurons measured using electrophysiological techniques. In addition the spinach diet improved learning on a runway motor task, previously shown to be modulated by cerebellar norepinephrine. Motor learning is important for adaptation to changes in the environment and is thus critical for rehabilitation following stroke, spinal cord injury, and the onset of some neurodegenerative diseases. These data are the first to indicate that age-related deficits in motor learning and memory can be reversed with nutritional interventions.

Tissue plasminogen activator induction in Purkinje neurons after cerebellar motor learning

The cerebellar cortex is implicated in the learning of complex motor skills. This learning may require synaptic remodeling of Purkinje cell inputs. An extracellular serine protease, tissue plasminogen activator (tPA), is involved in remodeling various nonneural tissues and is associated with developing and regenerating neurons. In situ hybridization showed that expression of tPA messenger RNA was increased in the Purkinje neurons of rats within an hour of their being trained for a complex motor task. Antibody to tPA also showed the induction of tPA protein associated with cerebellar Purkinje cells. Thus, the induction of tPA during motor learning may play a role in activity-dependent synaptic plasticity.

CX3CR1 Deficiency Leads to Impairment of Hippocampal Cognitive Function and Synaptic Plasticity

The protective/neurotoxic role of fractalkine (CX3CL1) and its receptor CX3C chemokine receptor 1 (CX3CR1) signaling in neurodegenerative disease is an intricate and highly debated research topic and it is becoming even more complicated as new studies reveal discordant results. It appears that the CX3CL1/CX3CR1 axis plays a direct role in neurodegeneration and/or neuroprotection depending on the CNS insult. However, all the above studies focused on the role of CX3CL1/CX3CR1 signaling in pathological conditions, ignoring the relevance of CX3CL1/CX3CR1 signaling under physiological conditions. No approach to date has been taken to decipher the significance of defects in CX3CL1/CX3CR1 signaling in physiological condition. In the present study we used CX3CR1−/−, CX3CR1+/−, and wild-type mice to investigate the physiological role of CX3CR1 receptor in cognition and synaptic plasticity. Our results demonstrate for the first time that mice lacking the CX3CR1 receptor show contextual fear conditioning and Morris water maze deficits. CX3CR1 deficiency also affects motor learning. Importantly, mice lacking the receptor have a significant impairment in long-term potentiation (LTP). Infusion with IL-1β receptor antagonist significantly reversed the deficit in cognitive function and impairment in LTP. Our results reveal that under physiological conditions, disruption in CX3CL1 signaling will lead to impairment in cognitive function and synaptic plasticity via increased action of IL-1β.

Cholinergic gating of response to auditory stimuli in rat hippocampus

Rapid decrement of response to repeated stimuli is a characteristic of hippocampal neurons. To assess the possible role in this process of cholinergic afferents from the medial septal nucleus, a series of cholinergic antagonists were administered intraventricularly to chloral hydrate-anesthetized rats. Auditory stimuli were delivered in pairs to the rats, and the evoked response was recorded from an electrode in the CA3 layer of the hippocampus. The most prominent component of the auditory evoked potential recorded in this region (N40) showed over 60% decrement in the amplitude of the response to the second stimulus when the two stimuli were delivered 0.5 s apart. Only neuromuscular-type nicotinic antagonists, alpha-bungarotoxin and (+)-tubocurarine, disrupted this decrement of response to repeated auditory stimuli. The muscarinic antagonist, scopolamine, and the ganglionic-type nicotinic antagonists, kappa-bungarotoxin and mecamylamine, were without effect. The results suggest that a subset of nicotinic receptors mediate the gating of response to auditory stimuli in the hippocampus.

Schizophrenia and nicotinic receptors

&NA; Patients with schizophrenia often cannot respond to important features of their environment and filter out irrelevant stimuli. This dysfunction could be related to an underlying defect in inhibition‐i.e., the brains ability to alter its sensitivity to repeated stimuli. One of the neuronal mechanisms responsible for such inhibitory gating involves the activation of cholinergic nicotinic receptors in the hippocampus. These receptors are diminished in many specimens of hippocampal brain tissue obtained postmortem from schizophrenic patients. In living schizophrenic patients, stimulation of cholinergic receptors by nicotine transiently restores inhibitory gating of evoked responses to sensory stimuli. Many people with schizophrenia are heavy smokers, but the properties of the nicotinic receptor favor only short‐term activation, which may explain why cigarette smoking is only a transient symptomatic remedy. This paper reviews the clinical phenomenology of inhibitory gating deficits in people with schizophrenia, the neurobiology of such gating mechanisms, and the evidence that some individuals with the disorder may have a heritable deficit in the nicotinic cholinergic receptors involved in this neurobiological function. Inhibitory gating deficits are only partly normalized by neuroleptic drugs and are thus a target for new therapeutic strategies for schizophrenia.

Brain-derived neurotrophic factor is required for the establishment of the proper number of dopaminergic neurons in the substantia nigra pars compacta.

Brain-derived neurotrophic factor (BDNF) has been implicated in regulating neuronal survival, differentiation, and synaptic plasticity. Reduced expression of BDNF within the substantia nigra accompanies the deterioration of dopaminergic neurons in Parkinsons disease (PD) patients. Analysis of the effects of long-term BDNF absence from the CNS has been difficult because of the early postnatal lethality of BDNF-/- mice. Mice with a floxed BDNF allele were bred with Wnt1-Cre mice to generate Wnt-BDNFKO mice that lack BDNF from the midbrain-hindbrain (MHB). These mice are viable but exhibit hindlimb clutching and poor rotarod performance. Tyrosine hydroxylase (TH)-positive neuron numbers in the substantia nigra pars compacta (SNC) were estimated using stereological methods, revealing a persistent ∼23% reduction of these cells at postnatal day 21 (P21) in Wnt-BDNFKO mice compared with controls. The diminishment of TH-expressing neurons was present at birth and continued through P120. This deficit appears selective for the dopaminergic population, because at P21, total neuron number within the SNC, defined as neuronal nuclei protein-positive cells, was not significantly reduced. Interestingly, and similar to observations in PD patients, SNC neuron subpopulations are not equally affected. Calbindin- and calretinin-expressing SNC populations show no significant difference between Wnt-BDNFKO mice and controls. Thus, BDNF depletion from the MHB selectively leads to reduced TH expression in a subpopulation of neurons, but it remains unclear whether these cells are lost.

Cord blood rescues stroke-induced changes in splenocyte phenotype and function.

The neuroprotective mechanism of human umbilical cord blood cells (HUCBC) in the rat middle cerebral artery occlusion (MCAO) stroke model remains uncertain. Given the inflammatory sequelae that occur following stroke, we investigated whether HUCBC protection could be derived from the modulation of this immuno-inflammatory event, suggested by the attraction of the HUCBC to the spleen. We found that, following MCAO, rat spleen size was reduced concomitantly with their CD8+ T-cell counts. Interestingly, MCAO-induced spleen size reduction correlated with the extent of ischemic damage, however, HUCBC treatment rescued the spleen weight, splenic CD8+ T-cell counts, as well as the amount of brain injury. Additionally, splenocyte proliferation assays demonstrated that HUCBC treatment opposed MCAO-associated T-cell proliferation by increasing the production of IL-10 while decreasing IFN-gamma. Taken together, these results suggest a novel immunomodulatory mechanism by which HUCBC mediate protection in the rat MCAO model of stroke.

Fractalkine and CX3CR1 regulate hippocampal neurogenesis in adult and aged rats

Microglia have neuroprotective capacities, yet chronic activation can promote neurotoxic inflammation. Neuronal fractalkine (FKN), acting on CX(3)CR1, has been shown to suppress excessive microglia activation. We found that disruption in FKN/CX(3)CR1 signaling in young adult rodents decreased survival and proliferation of neural progenitor cells through IL-1β. Aged rats were found to have decreased levels of hippocampal FKN protein; moreover, interruption of CX(3)CR1 function in these animals did not affect neurogenesis. The age-related loss of FKN could be restored by exogenous FKN reversing the age-related decrease in hippocampal neurogenesis. There were no measureable changes in young animals by the addition of exogenous FKN. The results suggest that FKN/CX(3)CR1 signaling has a regulatory role in modulating hippocampal neurogenesis via mechanisms that involve indirect modification of the niche environment. As elevated neuroinflammation is associated with many age-related neurodegenerative diseases, enhancing FKN/CX(3)CR1 interactions could provide an alternative therapeutic approach to slow age-related neurodegeneration.