Ian Spence

Postmortem serotoninergic correlates of cognitive decline in Alzheimer's disease

Serotonin1A receptor density and serotonin concentration were measured in the postmortem neocortex of 17 AD patients who had been prospectively assessed every four months with the Mini-Mental State Examination (MMSE) for a mean of 2.6 years till death. In the frontal cortex, serotonin levels correlated negatively with the annual rate of MMSE decline, while serotonin1A receptor density was positively correlated with the rate of MMSE decline. Our study suggests that reduced serotonin levels and increased serotonin1A receptor density are markers for accelerated cognitive decline in AD, and provides support for the use of serotonin1A antagonists in the treatment of AD.

Chronic cerebral hypoperfusion and impaired neuronal function in rats.

Background and Purpose Studies in acute cerebral ischemia have shown that reductions in cerebral blood flow of up to 50% do not lead to infarction or alterations in neuronal electric activity. Little is known about the effects of chronic reductions in cerebral blood flow. The purpose of this study was to evaluate neuronal electrophysiological function in brain that had been subjected to a chronic reduction of cerebral blood flow of less than 50%. Based on existing knowledge of thresholds of cerebral ischemia, neuronal electrophysiological function should be unaffected by hypoperfusion of this magnitude. Methods An arteriovenous fistula model in the rat was used to induce chronic cerebral hypoperfusion with reductions of cerebral blood flow of 25% to 50% as measured previously by 14C-labeled autoradiography. Using in vitro electrophysiological brain slice techniques, long-term potentiation in hippocampal CA1 neurons was examined extracellularly after 6 months of chronic noninfarctional cerebral hypoperfusion. Brains were also examined histologically at this time for evidence of cerebral infarction. Results There was no evidence of cerebral infarction. Longterm potentiation was produced in 9 of 12 control animals and only 2 of 8 hypoperfused animals. This difference was significant (P<.05) and demonstrated that long-term potentiation was impaired in animals with chronic hypoperfusion. Conclusions Noninfarctional reductions in cerebral blood flow of up to 50% do impair neuronal function in chronic cerebral ischemia, a result quite distinct from that seen in acute ischemia. The threshold for neuronal dysfunction in chronic cerebral hypoperfusion is lower than that found in acute cerebral ischemia, suggesting that duration as well as severity of ischemic insult determines cellular viability. Chronic hypoperfusion may lead to a noninfarctional state with impaired neuronal function, a category of chronic cerebral ischemia not previously identified.

Chronic cerebral hypoperfusion: pathological and behavioral consequences.

OBJECTIVE Although the effects of acute ischemic insults to the brain are well known, the effects related to chronic ischemia are poorly delineated. The pathological and behavioral changes induced by a chronic noninfarctional reduction in cerebral blood flow of 25 to 50% maintained for 6 months were assessed. METHODS In each of 18 male Sprague-Dawley rats, an arteriovenous fistula was created in the neck via an anastomosis between the right external jugular vein and the right common carotid artery to induce cerebral hypoperfusion. Nineteen age-matched animals comprised a control group. Six months after surgery, the animals were examined using light and electron microscopic techniques, as well as via a battery of behavioral tests (motor, open field, and T-maze). RESULTS Examination of the hippocampus by using light microscopy revealed disorganization of the CA1 sector with an increased number of astrocytes. Transmission electron microscopy of the CA1 region demonstrated neurons with increased lipofuscin pigment and central nucleoli and astrocytes with more numerous cytosolic mitochondria. Motor performance testing revealed no gross motor deficits, although open-field assessment demonstrated increased exploratory behavior in rats with fistulas. Finally, T-maze testing results suggested that errors in working memory were more common in rats undergoing chronic cerebral hypoperfusion (P < 0.05). CONCLUSIONS These findings suggest that chronic reductions in cerebral blood flow of a magnitude previously thought to be harmless to neurons (i.e., reduced by 25-50%) do alter neuronal structure and affect whole animal behavior. Such a scenario may be responsible for a symptomatology secondary to arteriovenous steal and severe carotid stenoses. The mechanisms are still unknown.

Actions of robustoxin, a neurotoxic polypeptide from the venom of the male funnel-web spider (Atrax robustus), in anaesthetized monkeys

Robustoxin, a polypeptide consisting of a chain of 42 amino acid residues in a known sequence, has been isolated by cation exchange chromatography from the crude venom of the male funnel-web spider (Atrax robustus). Physiological activity or toxicity in the venom fractions was detected by production of fasciculation in mouse phrenic nerve-hemidiaphragm preparations and by lethality in new-born mice. In the present experiments in Macaca fascicularis monkeys anaesthetized with pentobarbitone, robustoxin (5-30 micrograms/kg infused i.v. over 5 min) produced immediate disturbances in respiration (including dyspnoea and apnoea), blood pressure and heart rate followed by severe hypotension (mean systemic blood pressure less than 50 mmHg) or death due to respiratory and circulatory failure within 196 min. Robustoxin also produced lachrymation, salivation, generalized skeletal muscle fasciculation and a parallel increase in body temperature, and increased firing in skeletal motor and autonomic nerves. These effects closely resembled those produced by i.v. infusions over 5 min of 50 micrograms/kg of crude venom from male A. robustus spiders. Crude venom from female A. robustus spiders (500 micrograms/kg i.v. over 5 min) produced some of the effects elicited by robustoxin and crude venom from male spiders, but to a much less marked extent. It was concluded that robustoxin is responsible for the neurotoxic and lethal effects of human envenomation by male A. robustus spiders.

Studies on the subunit structure of textilotoxin, a potent neurotoxin from the venom of the Australian common brown snake (Pseudonaja textilis)

Textilotoxin is a presynaptic neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis. It has the highest lethality and is structurally the most complex of any known snake venom neurotoxin. It was resolved into its five non-covalently linked subunits in a single step by reverse-phase HPLC. Two of the subunits were identical. The N-terminal amino-acid sequence and amino-acid composition of each subunit were determined. Subunit A was the only one found to possess phospholipase A activity. Separation of textilotoxin into its subunits was reversible and reformed textilotoxin had the same Mr and lethality in mice as the native toxin. Experiments with various unnatural combinations of subunits have led to interesting variations in lethality and Mr of the resulting complexes.

A serotoninergic basis for hyperphagic eating changes in Alzheimer's disease

Hyperphagia and associated eating changes occur frequently in Alzheimers disease (AD) and lead to considerable morbidity. However, the neurochemical basis for these neuropsychiatric behaviours is at present unclear. In this study, we measured serotonin transporters, 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors using radioligand binding assays in the postmortem temporal cortex of a cohort of controls and AD patients longitudinally assessed for hyperphagia. We found significant decreases in 5-HT(4) receptor densities in the hyperphagic, but not normophagic, AD group. Our data suggest that 5-HT(4) receptor deficits may be a specific neurochemical correlate of hyperphagia, and point to the potential pharmacotherapeutic utility of 5-HT(4) agonists for these behaviours in AD.

Involvement of adenosine in synaptic depression induced by a brief period of hypoxia in isolated spinal cord of neonatal rat.

The monosynaptic reflex (MSR), recorded extracellularly from the ventral root isolated, superfused spinal cords of neonatal rats (6-10 days post-partum), was rapidly depressed to 35-45% of control values by either cessation of superfusion (4 min stop-flow period) or by superfusion with anoxic medium (95% N2-5% CO2; 4 min). The depression was reversible, 85-115% recovery occurring after 15 min of restoration of flow or normoxic (95% O2-5% CO2) superfusion. 2-Chloroadenosine, a metabolically stable adenosine analogue, also reversibly inhibited the MSR, an effect which was antagonised by 10(-6) M 8-cyclopentyltheophylline (8-CPT). The depression of the MSR, caused by 4 min of hypoxia (either stop-flow or anoxic superfusion), was prevented by 10(-6) M 8-CPT. These results provide strong evidence for a critical involvement of adenosine in mediating early synaptic depression evoked by a brief period of hypoxia.

The complete amino acid sequence of a post-synaptic neurotoxin isolated from the venom of the Australian death adder snake Acanthophis antarcticus.

1. A lethal neurotoxin (acanthophin d) was isolated from the venom of the Australian death adder snake Acanthophis antarcticus. 2. Acanthophin d consisted of a single polypeptide chain of 74 amino acid residues cross-linked by five disulphide bridges. 3. The results of neurophysiological experiments on murine phrenic nerve hemi-diaphragm preparations were consistent with irreversible post-synaptic blockage of neuromuscular transmission by acanthophin d.

[3H]GR113808 binding to serotonin 5-HT(4) receptors in the postmortem neocortex of Alzheimer disease: a clinicopathological study.

Summary. Abnormalities in neural transmission of serotonin (5-HT) may play a role in both cognitive and neuropsychiatric features of Alzheimer disease (AD). We measured 5-HT4 receptors in the postmortem frontal and temporal cortex of 34 AD subjects and 15 controls by radioligand binding with [3H]GR113808. Receptor binding data was then correlated with prospectively assessed cognitive (Mini-Mental State Examination, MMSE) and behavioral (Present Behavioural Examination, PBE) data. [3H]GR113808 binding affinity (KD) and density (Bmax) in AD were unchanged compared to controls in both cortical regions, and did not correlate with MMSE or PBE data. The binding parameters were also not related to disease duration, senile plaque and neurofibrillary tangle counts, and neuroleptic medication. We conclude that unlike other 5-HT receptors, 5-HT4 receptor binding affinity and density do not seem to be affected in the frontal and temporal cortex in AD and may not have a direct role in the clinical features of the disease.

Chronic Cerebral Hypoperfusion Inhibits Calcium-Induced Long-term Potentiation in Rats

BACKGROUND AND PURPOSE Long-term potentiation (LTP) in the rat hippocampus induced by tetanic stimulation is impaired by chronic cerebral hypoperfusion. The effects of chronic cerebral hypoperfusion on other forms of LTP are unknown. Such data could help delineate the pathways of cellular alteration caused by chronic cerebral hypoperfusion. The in vitro phenomenon of calcium-induced LTP was thus examined in rat hippocampal CA1 cells that had undergone chronic hypoperfusion with a reduction in cerebral blood flow of between 25% and 50% maintained for 26 weeks. METHODS Ten Sprague-Dawley rats had a cervical arteriovenous fistula surgically constructed, and an additional 10 animals were used as age-matched controls. Hippocampal slices were prepared after 26 weeks of hypoperfusion, and in vitro extracellular field potential recordings were taken from the Schäffer collateral CA1 region. Properties of LTP induced through transient exposure to a hypercalcemic solution were analyzed. RESULTS LTP was impaired in animals with an arteriovenous fistula (P < .05). Control animals demonstrated potentiation lasting for the entire 2 hours of recording, whereas fistula animals showed only transient potentiation (< 60 minutes) before returning to baseline values. CONCLUSIONS Calcium-induced LTP is impaired by chronic cerebral hypoperfusion. This form of LTP is different from that induced by tetanic stimulation. It is the most sensitive test available for in vitro detection of the changes induced in neuronal function by chronic noninfarctional reductions in cerebral blood flow of 25% to 50% and may indicate that the most basic cellular parameters involving calcium homeostasis and metabolism are being altered. The precise mechanisms remain to be elucidated, and several postulates are discussed.