Morten S. Jensen

Histochemically-reactive zinc in amyloid plaques, angiopathy, and degenerating neurons of Alzheimer's diseased brains

Excess brain zinc has been implicated in Alzheimers neuropathology. Here we evaluated that hypothesis by searching the brains of Alzheimers patients for abnormal zinc deposits. Using histochemical methods, we found vivid Zn2+ staining in the amyloid deposits of dense-core (senile) plaques, in the amyloid angiopathy surrounding diseased blood vessels, and in the somata and dendrites of neurons showing the characteristic neurofibrillary tangles (NFT) of Alzheimers. In contrast, brains from age-matched, non-demented subjects showed only occasional staining for Zn2+ in scattered neurons and possible plaques. A role of abnormal zinc metabolism in Alzheimers neuropathology is suggested.

Ionic basis of spike after‐depolarization and burst generation in adult rat hippocampal CA1 pyramidal cells.

1. Intracellular recordings in adult rat hippocampal slices were used to identify the ionic conductances underlying active spike after‐depolarization (ADP) and intrinsic burst firing in the somata of CA1 pyramidal cells (PCs). To test the ‘Ca2+ hypothesis’, Ca2+ currents were suppressed by replacing the Ca2+ in the saline with either Mn2+ or Mg2+. Alternatively, the inorganic Ca2+ channel blockers Cd2+ (0.5 mM) or Ni2+ (2 mM) were added to the saline. To test the ‘Na+ hypothesis’, Na+ currents were blocked with tetrodotoxin (TTX; 0.5 microM). 2. The suppression of Ca2+ currents blocked the fast after‐hyperpolarization (AHP) generated by the fast Ca(2+)‐gated K+ current Ic, while enhancing the amplitude and duration of active spike ADPS. 3. Evoked and spontaneous burst firing was preserved undiminished following Ca2+ current suppression, while the propensity to fire bursts increased in many cases. The postburst medium AHP (generated primarily by the muscarine‐sensitive voltage‐gated K+ current, IM) was not affected by this treatment, which blocked the slow AHP (generated by the slow Ca(2+)‐gated K+ current, IAHP). 4. TTX strongly suppressed active ADPs and intrinsic bursts before substantially reducing the threshold, rate of rise and amplitude of solitary spikes. 5. In Ca(2+)‐free saline, caesium‐filled PCs generated large, plateau ADPs following an initial burst of fast spikes. Application of TTX suppressed these ADPs before solitary fast spikes appeared to be reduced. 6. Injection of brief, just subthreshold depolarizing current pulses into bursters evoked slow depolarizing potentials lasting up to 50 ms. These persisted after suppression of Ca2+ currents and were entirely blocked by TTX. 7. We conclude that active spike ADPs and intrinsic bursts in the somata of adult CA1 PCs are generated by a low voltage‐gated, persistent Na+ current. Burst termination is mediated by voltage‐gated K+ currents activated during the burst (most likely IM), rather than by the Ca(2+)‐gated K+ currents Ic and IAHP. The latter currents downregulate the innate tendency of CA1 PCs to burst (Ic) and limit the rate of spontaneous burst firing (IAHP).

The slow inhibitory postsynaptic potential in rat hippocampal CA1 neurones is blocked by intracellular injection of QX-314.

Intracellular recordings were made from CA1 pyramidal neurones in the rat hippocampus slice preparation. The recording electrodes contained potassium acetate (4 M) with or without the quaternary lidocaine derivative, QX-314 (50 mM). Both fast (f) and slow (s) inhibitory postsynaptic potentials (IPSP) were evoked by low-frequency orthodromic stimulation. The s-IPSP was rapidly reduced by QX-314 injection. It decreased along a similar time course to the dV/dt of the action potential (AP). The f-IPSP and excitatory postsynaptic potential were not significantly reduced in size at a time when the s-IPSP was virtually abolished by QX-314. It is concluded that conductance through the K+ channels which are coupled to GABAB receptors is readily blocked by QX-314, while the Cl- channels which are coupled to GABAA receptors and the cation channels coupled to the glutamate receptors are relatively resistant to the local anaesthetic.

Effects of new non‐N‐methyl‐D‐aspartate antagonists on synaptic transmission in the in vitro rat hippocampus.

1. The effects of new, potent non‐N‐methyl‐D‐aspartate (NMDA) receptor antagonists, 6,7‐dinitroquinoxaline‐2,3‐dione (DNQX) and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), have been examined using intra‐ and extracellular recordings in the hippocampal slice preparation. In terms of potency and selectivity, the action of the two blockers was similar and CNQX was used in most experiments. 2. CNQX reduced the responses to ionophoretic applications of the non‐NMDA agonists kainate (KAI) and quisqualate (QUIS) with IC50 values of 1.2 and 4.8 microM, respectively. In Mg2+‐free solutions responses to NMDA were generally not affected by concentrations of CNQX up to 25 microM. 3. The action of CNQX was only slowly and poorly reversible on washing. Responses to QUIS and KAI were also reversibly reduced by ionophoretic application of CNQX. 4. CNQX blocked the evoked EPSP in CA1 and CA3 neurones with an IC50 of around 2 microM, which is similar to the IC50 for responses to KAI. CNQX was without effect on the passive membrane properties, the afferent volley and paired pulse potentiation. 5. In the presence of CNQX (greater than 5 microM) a small EPSP remained which was largest in CA1 neurones. It was blocked by low concentrations of the NMDA receptor antagonist (+/‐)‐2‐amino‐5‐phosphonovaleric acid (APV), was markedly enhanced on removing Mg2+ ions from the bathing medium and, in voltage‐clamp experiments, showed a potential dependence which is characteristic of the NMDA ionophore. 6. The latency of the APV‐sensitive EPSP in CA1 was the same as the CNQX‐sensitive EPSP, indicating that NMDA receptors participate in monosynaptic excitation. 7. Feedback and feed‐forward inhibition in both area CA1 and CA3 were sensitive to CNQX. There seemed to be two components of the inhibition, both of which appear to be GABAergic since they could be blocked by picrotoxin (PTX), but only one of which was blocked by CNQX. The CNQX‐resistant IPSP was not affected by APV. 8. In conclusion, quinoxalinediones have been used to demonstrate that non‐NMDA receptors mediate the majority of the EPSP. Additionally, a component of the EPSP in CA1 is mediated by NMDA receptors and is manifested at resting membrane potentials and in the presence of Mg2+.

SPIKE AFTER-DEPOLARIZATION AND BURST GENERATION IN ADULT RAT HIPPOCAMPAL CA1 PYRAMIDAL CELLS

1. Intracellular recordings in adult rat hippocampal slices were used to investigate the properties and origins of intrinsically generated bursts in the somata of CA1 pyramidal cells (PCs). The CA1 PCs were classified as either non‐bursters or bursters according to the firing patterns evoked by intrasomatically applied long ( > or = 100 ms) depolarizing current pulses. Non‐bursters generated stimulus‐graded trains of independent action potentials, whereas bursters generated clusters of three or more closely spaced spikes riding on a distinct depolarizing envelope. 2. In all PCs fast spike repolarization was incomplete and ended at a potential approximately 10 mV more positive than resting potential. Solitary spikes were followed by a distinct after‐depolarizing potential (ADP) lasting 20‐40 ms. The ADP in most non‐bursters declined monotonically to baseline (‘passive’ ADP), whereas in most bursters it remained steady or even re‐depolarized before declining to baseline (‘active’ ADP). 3. Active, but not passive, ADPs were associated with an apparent increase in input conductance. They were maximal in amplitude when the spike was evoked from resting potential and were reduced by mild depolarization or hyperpolarization (+/‐ 2 mV). 4. Evoked and spontaneous burst firing was sensitive to small changes in membrane potential. In most cases maximal bursts were generated at resting potential and were curtailed by small depolarizations or hyperpolarizations (+/‐ 5 mV). 5. Bursts comprising clusters of spikelets (‘d‐spikes’) were observed in 12% of the bursters. Some of the d‐spikes attained threshold for triggering full somatic spikes. Gradually hyperpolarizing these neurones blocked somatic spikes before blocking d‐spikes, suggesting that the latter are generated at more remote sites. 6. The data suggest that active ADPs and intrinsic bursts in the somata of adult CA1 PCs are generated by a slow, voltage‐gated inward current. Bursts arise in neurones in which this current is sufficiently large to generate suprathreshold ADPs, and thereby initiate a regenerative process of spike recruitment and slow depolarization.

Cognitive deficits in the rat chronic mild stress model for depression: Relation to anhedonic-like responses

The chronic mild stress (CMS) protocol is widely used to evoke depressive-like behaviours in laboratory rats. The aim of the present study was to examine the effects of chronic stress on cognitive performance. About 70% of rats exposed to 7 weeks of chronic mild stress showed a gradual reduction in consumption of a sucrose solution, indicating an anhedonic-like state. The remaining rats did not reduce their sucrose intake, but appeared resilient to the stress-induced effects on sucrose intake. Cognitive profiling of the CMS rats revealed that chronic stress had a negative effect on performance in the spontaneous alternation test, possibly reflecting a deficit in working memory. This effect was independent of whether the stressed rats were anhedonic-like or stress-resilient as measured by their sucrose intake. CMS did not influence performance in passive avoidance and auditory cued fear conditioning, however, in rats displaying an anhedonic-like profile, CMS increased freezing behaviour in contextual fear conditioning.

Episodic memory deficits are not related to altered glutamatergic synaptic transmission and plasticity in the CA1 hippocampus of the APPswe/PS1ΔE9-deleted transgenic mice model of β-amyloidosis

Alzheimers disease (AD) is characterized by progressive memory impairment and the formation of amyloid plaques in the brain. Dysfunctional excitatory synaptic transmission and synaptic plasticity are generally accepted as primary events in the development of AD, and beta-amyloid is intimately involved. Here we describe age related differences in learning, memory, synaptic transmission and long-term potentiation (LTP) in wild type and APPswe/PS1DeltaE9 mice, which produce increasing amounts of Abeta1-42 with age. The mice have both age related and age-independent deficits in radial arm water maze performance. Blind studies of hippocampal slices from transgenic and wild type mice demonstrate that transgenic mice have impaired transient LTP and that the degree of impairment is not related to age from 3 to 12 months. The deficiencies in transient LTP may be related to the behavioral deficits that did not progress with age. The accumulation of beta-amyloid and the episodic memory deficits, both of which increased with age, were not accompanied by an alteration in synaptic transmission or sustained LTP in the in vitro hippocampal slices.

Transient and sustained types of long-term potentiation in the CA1 area of the rat hippocampus

Synaptic potentiation induced by high frequency stimulation was investigated by recording field excitatory postsynaptic potentials (f‐EPSPs) in rat hippocampal slices. Potentiation consisted of a transient period of decaying f‐EPSPs (short‐term potentiation, STP) that led to a plateau of continuously potentiated f‐EPSPs (long‐term potentiation, LTP). Here we show that a previously unknown type of transient, use‐dependent, long‐lasting potentiation (t‐LTP) can account for STP. t‐LTP could be stored for more than 6 h and its decay was caused by synaptic activation. Both the expression and the decay of t‐LTP were input specific. t‐LTP was induced differently from conventional LTP in that the amplitude of t‐LTP was dependent upon the stimulation frequency, whereas the magnitude of LTP was dependent on the number of stimuli in the induction train. The decay of t‐LTP could not be prevented by the blockage of glutamate receptors, but was prevented by the blockage of stimulus‐evoked neurotransmitter release, suggesting that t‐LTP is expressed presynaptically. Paired‐pulse stimulation experiments showed that the decay of t‐LTP was mediated by a decrease in the probability of neurotransmitter release. The decline of t‐LTP could be prolonged by the activation of NMDA receptors. Hence, both single and paired‐pulse stimuli prolonged the decline of the t‐LTP. This decline could be prevented by high frequency burst stimulation (200 Hz). We conclude that t‐LTP allows dynamic modulation of synaptic transmission by providing not only spatial association but also temporal convergence between synaptic inputs. Therefore, t‐LTP might be a substrate for the encoding of synaptic memory.

The capillary dysfunction hypothesis of Alzheimer's disease

It is widely accepted that hypoperfusion and changes in capillary morphology are involved in the etiopathogenesis of Alzheimers disease (AD). This is difficult to reconcile with the hyperperfusion observed in young high-risk subjects. Differences in the way cerebral blood flow (CBF) is coupled with the local metabolic needs during different phases of the disease can explain this apparent paradox. This review describes this coupling in terms of a model of cerebral oxygen availability that takes into consideration the heterogeneity of capillary blood flow patterns. The model predicts that moderate increases in heterogeneity requires elevated CBF in order to maintain adequate oxygenation. However, with progressive increases in heterogeneity, the resulting low tissue oxygen tension will require a suppression of CBF in order to maintain tissue metabolism. The observed biphasic nature of CBF responses in preclinical AD and AD is therefore consistent with progressive disturbances of capillary flow patterns. Salient features of the model are discussed in the context of AD pathology along with potential sources of increased capillary flow heterogeneity.

GABAB receptors play a major role in paired-pulse facilitation in area CA1 of the rat hippocampus.

Extracellular recordings of field potentials in area CA1 of the rat hippocampal slice have been used to investigate paired-pulse facilitation. Field potentials were evoked by maximal stimulation of the Schaffer collateral/commissural fibres. The height of the population spike (PS) in stratum pyramidale (str. pyr.) and the area under the field excitatory postsynaptic potential (EPSP) following the PS in the stratum radiatum (str. rad.) were quantified. These values were used to describe the time course of paired-pulse facilitation. Facilitation of the PS was maximal 50 ms after the conditioning pulse and was present over a period of about 500 ms. However, facilitation of the late area (LA) of the field EPSP was maximal afer 125 ms and had an overall duration of 1-2 s. The N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), had no effect on paired-pulse facilitation of either the LA or the PS. The gamma-aminobutyric acid-B (GABAB) agonist baclofen increased facilitation of the PS. This was mainly due to a reduction of the unconditioned response. Facilitation of the LA was reduced by both baclofen and the GABAB antagonist, 2-OH-saclofen. Baclofen increased the LA of the unconditioned response, while this was unaffected by 2-OH-saclofen. The LA of facilitated responses was decreased by 2-OH-saclofen while the effect of baclofen on these responses was more complex. Baclofen reduced the LA of maximally facilitated responses, while the LA of slightly facilitated responses was increased. The results show that different mechanisms are involved in the facilitation of the LA and the PS. Furthermore, activation of GABAB receptors makes a large contribution to paired-pulse facilitation of the field EPSP. It is also suggested that recording of extracellular fields in str. rad. in response to paired-pulse stimulation provides a simple electrophysiological model for testing the effect of agents which act at the GABAB receptor.