The impact of postsynaptic calcium on synaptic transmission--its role in long-term potentiation

A developmental switch of AMPA receptor subunits in neocortical pyramidal neurons

AMPA receptors mediate most of the fast excitatory neurotransmission in the brain, and those lacking the glutamate receptor 2 (GluR2) subunit are Ca(2+)-permeable and expressed in cortical structures primarily by inhibitory interneurons. Here we report that synaptic AMPA receptors of excitatory layer 5 pyramidal neurons in the rat neocortex are deficient in GluR2 in early development, approximately before postnatal day 16, as evidenced by their inwardly rectifying current-voltage relationship, blockade of AMPA receptor-mediated EPSCs by external and internal polyamines, permeability to Ca(2+), and GluR2 immunoreactivity. Overall, these results indicate that neocortical pyramidal neurons undergo a developmental switch in the Ca(2+) permeability of their AMPA receptors through an alteration of their subunit composition. This has important implications for plasticity and neurotoxicity.

cAMP/Ca2+ response element-binding protein function is essential for ocular dominance plasticity

The monocular deprivation model of amblyopia is characterized by a reduction in cortical responses to stimulation of the deprived eye. Although the effects of monocular deprivation on the primary visual cortex have been well characterized physiologically and anatomically, the molecular mechanisms underlying ocular dominance plasticity remain unknown. Previous studies have indicated that the transcription factor adenosine cAMP/Ca(2+) response element-binding protein (CREB) is activated during monocular deprivation. However, it remains unknown whether CREB function is required for the loss of cortical responses to the deprived eye. To address this issue, we used the herpes simplex virus (HSV) to express a dominant negative form of CREB (HSV-mCREB) containing a single point mutation that prevents its activation. Quantitative single-unit electrophysiology showed that cortical expression of this mutated form of CREB during monocular deprivation prevented the loss of responses to the deprived eye. This effect was specific and not related to viral toxicity, because overexpression of functional CREB or expression of beta-galactosidase using HSV injections did not prevent the ocular dominance shift during monocular deprivation. Additional evidence for specificity was provided by the finding that blockade of ocular dominance plasticity was reversible; animals treated with HSV-mCREB recovered ocular dominance plasticity when mCREB expression declined. Moreover, this effect did not result from a suppression of sensory responses caused by the viral infection because neurons in infected cortex responded normally to visual stimulation. These findings demonstrate that CREB function is essential for ocular dominance plasticity.

Aging and calcium buffering in adrenergic neurons

The aging process at the cellular, organ and whole organism levels is in many respects a mystery. A common bias among those who study aging is that cellular homeostasis "generally falls apart". The assumption of a general deterioration in cellular homeostasis does not take into account that many individuals age quite well maintaining even robust physiological and mental functions. One facet of aging studies that has come to the forefront is the impact of age on the control of the ion messenger, calcium. Emerging evidence suggests that despite age-related declines in any one component or multiple components of the calcium buffering systems, compensatory mechanisms may be able to maintain overall calcium homeostasis. This brief review focuses specifically on the ability of peripheral neurons to maintain control of the ion messenger calcium with advancing age. In addition, the idea that the impact of age on calcium homeostasis may be more subtle due to complex and integrated mechanisms that control this ion is discussed.

Modification of glutamate binding sites in newborn brain during hypoglycemia

We have shown that acute insulin-induced hypoglycemia leads to specific changes in the cerebral NMDA receptor-associated ion channel in the newborn piglet. The present study tests the hypothesis that exposure to acute hypoglycemia in the newborn will alter the glutamate binding site of both NMDA and kainate receptors. Studies were performed in 3-6 days-old piglets randomized to control (n=6) or hypoglycemic (n=6) groups. Hypoglycemia was maintained for 120 min using insulin infusion. Saturation binding assays were performed in cerebral cell membranes using (3)H-glutamate or (3)H-kainate to determine the characteristics of the glutamate binding sites of the NMDA and kainate receptors, respectively. The concentration of glucose in cerebral cortex was 10-fold less in hypoglycemic piglets than in controls (P<0.05). Brain ATP was not significantly decreased during hypoglycemia, but phosphocreatine decreased from control of 6.6 +/- 1.3 micromoles/g brain to 3.2 +/- 1.9 micromoles/g brain in hypoglycemic piglets. The B(max) for NMDA-displaceable (3)H-glutamate binding was 992 +/- 64 fmol/mg protein in hypoglycemic animals, significantly higher than the control value of 746 +/- 42 fmol/mg protein. However, the dissociation constant for glutamate was unchanged during hypoglycemia. The (3)H-kainate binding studies demonstrated no change in B(max) of high-affinity kainate receptors during hypoglycemia. In contrast, the affinity of the kainate receptor glutamate binding site significantly increased compared to control. Thus, acute hypoglycemia in the newborn piglet had specific effects on the glutamate binding sites of the NMDA and kainate receptors that could be due to alterations in cell membrane lipids or modification of receptor proteins.

Postsynaptic Modulation of AMPA Receptor-Mediated Synaptic Responses and LTP by the Type 3 Ryanodine Receptor

The precise function of ryanodine receptors (RyRs) in synaptic transmission is unknown, but three of their subtypes are expressed in the brain. We examined the roleof RyRs in excitatory synaptic transmission in hippocampal slices, using type 3 RyR (RyR3)-deficient mice. The alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionic acid (AMPA) receptor-mediated basal synaptic responses in the CA1 region of mutant mice were smaller than those of wild-type mice, while there was no difference in N-methyl-d-aspartate receptor-mediated responses, suggesting selective postsynaptic modification of AMPA receptors by RyR3. The expression of synaptic AMPA receptor subunits examined by Western blotting or immunohistochemistry was indistinguishable, suggesting that the smaller AMPA synaptic responses in mutant mice were not due to the reduced number of synaptic AMPA receptors. Although the initial potentiation following tetanic stimulation of afferent fibers was similar, long-term potentiation (LTP) was smaller in mutant mice. There were no differences in presynaptic electrophysiological properties. We thus conclude that RyR3 postsynaptically regulates the properties of AMPA receptors and LTP.

Contribution of postsynaptic Ca2+ to the induction of post-tetanic potentiation in the neural circuit for siphon withdrawal in Aplysia

Recent studies in Aplysia have revealed a novel postsynaptic Ca(2+) component to posttetanic potentiation (PTP) at the siphon sensory to motor neuron (SN-MN) synapse. Here we asked whether the postsynaptic Ca(2+) component of PTP was a special feature of the SN-MN synapse, and if so, whether it reflected a unique property of the SN or the MN. We examined whether postsynaptic injection of BAPTA reduced PTP at SN synapses onto different postsynaptic targets by comparing PTP at SN-MN and SN-interneuron (L29) synapses. We also examined PTP at L29-MN synapses. Postsynaptic BAPTA reduced PTP only at the SN-MN synapse; it did not affect PTP at either the SN-L29 or the L29-MN synapse, indicating that the SN and the MN do not require postsynaptic Ca(2+) for PTP with all other synaptic partners. The postsynaptic Ca(2+) component of PTP is present at other Aplysia SN-MN synapses; tail SN-MN synapses also showed reduced PTP when the MN was injected with BAPTA. Surprisingly, in both tail and siphon SN-MN synapses, there was an inverse relationship between the initial size of the EPSP and the postsynaptic component to PTP; only the initially weak SN-MN synapses showed a BAPTA-sensitive component. Homosynaptic depression of initially strong SN-MN synapses into the size range of initially weak synapses did not confer postsynaptic Ca(2+) sensitivity to PTP. Finally, the postsynaptic Ca(2+) component of PTP could be induced in the presence of APV, indicating that it is not mediated by NMDA receptors. These results suggest a dual model for PTP at the SN-MN synapse, in which a postsynaptic Ca(2+) contribution summates with the conventional presynaptic mechanisms to yield an enhanced form of PTP.

Regulation of spine calcium dynamics by rapid spine motility

Dendritic spines receive most excitatory inputs in the CNS and compartmentalize calcium. Spines also undergo rapid morphological changes, although the function of this motility is still unclear. We have investigated the effect of spine movement on spine calcium dynamics with two-photon photobleaching of enhanced green fluorescent protein and calcium imaging of action potential-elicited transients in spines from layer 2/3 pyramidal neurons in mouse visual cortex slices. The elongation or retraction of the spine neck during spine motility alters the diffusional coupling between spine and dendrite and significantly changes calcium decay kinetics in spines. Our results demonstrate that the spine's ability to compartmentalize calcium is constantly changing.

A simple method using 31P-NMR spectroscopy for the study of protein phosphorylation

Nonradioactive 31P-NMR spectroscopy has previously been used for the study of protein phosphorylations. However, the procedures does not seem to be easy for non-experts of this field, hence, this approach has not been widely used. We introduce here a simple protocol with 31P-NMR spectroscopy to study in vitro phosphorylation in receptor proteins. The effectiveness of this method was verified using synthetic peptides and recombinant proteins of the C-terminus of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor, whose phosphorylations are considered to have important roles in synaptic plasticity. We show that a decrease in the pH of the sample solution after the phosphorylation reaction is critical for the separation of the phosphorylation signals. In the analysis of the C-terminal portion of the GluR2 AMPA receptor, the phosphorylation sites of which had not hitherto been well clarified, we found the presence of at least three protein kinase C (PKC) phosphorylation sites. Furthermore, this method allows prediction of the origins of each of the phosphorylation peaks. Thus, the techniques we described here is useful for examination of protein phosphorylation and permits us to safely conduct repetitive experiments.

Mechanisms of calcium decay kinetics in hippocampal spines: role of spine calcium pumps and calcium diffusion through the spine neck in biochemical compartmentalization

Dendritic spines receive most excitatory inputs in the CNS and compartmentalize calcium. Although the mechanisms of calcium influx into spines have been explored, it is unknown what determines the calcium decay kinetics in spines. With two-photon microscopy we investigate action potential-induced calcium dynamics in spines from rat CA1 pyramidal neurons in slices. The [Ca(2+)](i) in most spines shows two decay kinetics: an initial fast component, during which [Ca(2+)](i) in spines decays to dendritic levels, followed by a slower decay phase in which the spine follows dendritic kinetics. The correlation between [Ca(2+)](i) in spine and dendrite at the breakpoint of the decay kinetics demonstrates diffusional equilibration between spine and dendrite during the slower component. To explain the faster initial decay, we rule out saturation or kinetic effects of endogenous or exogenous buffers and focus instead on (1) active calcium extrusion and (2) buffered diffusion of calcium from spine to dendrite. The presence of an undershoot in most spines indicates that extrusion mechanisms can be intrinsic to the spine. Supporting the two mechanisms, pharmacological blockade of smooth endoplasmic reticulum calcium (SERCA) pumps and the length of the spine neck affect spine decay kinetics. Using a mathematical model, we find that the contribution of calcium pumps and diffusion varies from spine to spine. We conclude that dendritic spines have calcium pumps and that their density and kinetics, together with the morphology of the spine neck, determine the time during which the spine compartmentalizes calcium.

Adrenergic nerves compensate for a decline in calcium buffering during ageing

1. The ubiquitous involvement of intracellular calcium ([Ca2+]i) in multiple neuronal pathways has led investigators to suggest that dysfunction of calcium homeostasis may be the primary mediator of age-related neuronal degeneration. Recently, it was shown that sympathetic neurones from superior cervical ganglion (SCG) of aged rats demonstrate decreased sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA) function and that aged neurones are more dependent upon mitochondria to control K+-evoked [Ca2+]i transients. 2. Therefore, in the present study we investigated age-related changes in ATP-dependent calcium pumps of plasma membrane Ca2+-ATPase (PMCA) and SERCA in acutely dissociated SCG cells from Fischer-344 rats aged 6 and 20 months. To distinguish between PMCA and SERCA pump activity, we applied the Ca2+-ATPase blocker vanadate and measured rates of recovery of K+-evoked [Ca2+]i transients by fura-2 microfluorometry. 3. Young SCG cells showed a biphasic response to vanadate over the vanadate concentration range (0.01-100 microM); however, old SCG cells showed only a single response over the same concentration range. Additionally, old SCG cells showed a greater sensitivity to Ca2+-ATPase blockade by vanadate. 4. The contribution of mitochondrial calcium uptake to regulate [Ca2+]i was also investigated. To measure the impact of mitochondrial calcium uptake, PMCAs and SERCAs were blocked with vanadate (100 microM) and extracellular sodium was replaced with tetraethylammonium (TEA) to block Na+/Ca2+-exchange. Treated SCG cells showed a decline of 50% in rate of recovery of [Ca2+]i in both 6- and 20-month-old cells; however, this effect did not vary with age. 5. These data suggest that there is an age-related decline in function of SERCAs, with an increased reliance on PMCAs to control high K+-evoked [Ca2+]i transients. In addition, there appears to be no age-related change in the capacity of the mitochondria to restore [Ca2+]i transients to basal levels.

Intracellular chloride and calcium transients evoked by gamma-aminobutyric acid and glycine in neurons of the rat inferior colliculus

Microfluorometric recordings showed that the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated transient increases in the intracellular Cl- concentration in neurons of the inferior colliculus (IC) from acutely isolated slices of the rat auditory midbrain. Current recordings in gramicidin-perforated patch mode disclosed that GABA and glycine mainly evoked inward or biphasic currents. These currents were dependent on HCO3- and characterized by a continuous shift of their reversal potential (E(GABA/gly)) in the positive direction. In HCO3- -buffered saline, GABA and glycine could also evoke an increase in the intracellular Ca2+ concentration. Ca2+ transients occurred only with large depolarizations and were blocked by Cd2+, suggesting an activation of voltage-gated Ca2+ channels. However, in the absence of HCO3-, only a small rise, if any, in the intracellular Ca2+ concentration could be evoked by GABA or glycine. We suggest that the activation of GABAA or glycine receptors results in an acute accumulation of Cl- that is enhanced by the depolarization owing to HCO3- efflux, thus shifting E(GABA/gly) to more positive values. A subsequent activation of these receptors would result in a strenghtened depolarization and an enlarged Ca2+ influx that might play a role in the stabilization of inhibitory synapses in the auditory pathway.

Hippocampal synaptic plasticity and cognition

Discoveries made over the past 20 years have greatly improved our understanding of how the brain functions. This article focuses on the relation between memory and cellular mechanisms of neuronal and synaptic plasticity in the hippocampus. Several studies indicate that the hippocampal formation is a crucial element of the neurobiological bases of higher cognitive function. Severe damage to the hippocampal formation is known to produce seemingly permanent anterograde amnesia. A generally accepted hypothesis in neurobiology has been that long-lasting activity-dependent changes in the efficacy of synaptic transmission in the mammalian brain are considered to be of fundamental importance for the development of neural circuitry and for the storage of information. The most compelling and reliable model for such changes has been long-term potentiation (LTP) and long-term depression (LTD) in the hippocampus. Therefore, the possibility of the discovery and development of compounds that, by modulating hippocampal synaptic plasticity, would be useful for the management of dementia and amnesia.

Platelet-activating factor (PAF) acetylhydrolase activity, LIS1 expression, and seizures

Lissencephaly patients are born with severe brain malformations and suffer from recurrent seizures. LIS1, the gene mutated in isolated lissencephaly patients, is a subunit of the heterotrimeric cytosolic enzyme platelet-activating factor acetylhydrolase (PAF-AH), interacts with tubulin, and affects microtubule dynamics. In order to gain molecular insights into the possible involvement of LIS1 in seizures in lissencephaly patients, we induced seizures in rats by injection of kainate. PAF-AH activity was markedly reduced as early as 30 min following initiation of seizures, making this parameter a sensitive indicator of seizure events. PAF-AH activity returned to and surpassed control values 1 week following initiation of seizures. Expression of LIS1 in the dentate gyrus changed significantly in a manner similar to that of PAF-AH enzymatic activity. This is the first correlation found between LIS1 expression and PAF-AH activity. Furthermore, the expression of the alpha2 catalytic subunit, which is the major PAF-AH catalytic subunit in rat adult brain, changed in a dramatic fashion. An additional higher-mobility LIS1 cross-reactive band was detected in samples isolated a week following seizure occurrence. This LIS1 isoform was enriched in the microtubule-associated fraction. We propose that LIS1 expression is an important factor in regulation of PAF-AH activity. We postulate that reductions in LIS1 protein levels found in lissencephaly patients may render them more susceptible to seizures.

Antisense oligonucleotides to C-fos reduce postictal seizure susceptibility following fully kindled seizures in rats

In a previous study we demonstrated that increased FOS expression in the amygdala induced by partial kindling seizures could be attenuated by administering c-Fos specific antisense oligonucleotides. In addition, we found that the administration of c-Fos antisense oligonucleotides at the beginning of the amygdala kindling process facilitated the appearance of stage V kindled seizures. In the present study, we evaluated the effect of the suppression of FOS on the expression and severity of the generalized fully kindled seizures as well as on the susceptibility to additional ictal events during the postictal and interictal periods. We observed that the administration of c-Fos antisense oligonucleotides did not modify the behavioral and electrographic manifestations of generalized stage V kindled seizures. However, c-Fos antisense oligonucleotides significantly reduced the susceptibility to additional ictal events during the postictal refractory period. Hence, the increased FOS protein induced by generalized tonic clonic seizures may participate in postictal mechanisms.

Stress impairs performance in spatial water maze learning tasks

The water maze task has been developed to test spatial learning abilities in rats or mice, and is widely used. Though it has been reported before that numerous cognitive abilities are of importance for learning this task, poor performance is usually interpreted as an impairment of spatial memory formation. Previous investigations that tried to correlate long-term potentiation (LTP) of synaptic transmission with spatial learning abilities in rats reported that injection of drugs or specific gene deletions which blocked the expression of LTP correlated with learning impairments of spatial tasks in a water maze. Recent studies, however, have shown that pretraining enables these animals to learn such spatial tasks even though LTP was still found to be blocked. I investigated to what degree altered fear condition and stress perception could account for the impaired spatial learning when no pretraining is given. In a fear habituation task, unhandled rats preferred a dark over a well lit chamber more than handled animals did, but unhandled rats favoured the lit chamber more in an active avoidance task. They also performed poorly in a spatial water maze task compared with handled rats. Rats pretrained in a radial arm maze performed better in a water maze than non-pretrained rats. No difference between groups was found in a non-spatial water maze task. On the other hand, when pretrained in a water maze, rats performed only marginally better in a radial arm maze compared to non-pretrained animals. Since animals have to be handled to learn a radial arm maze, the difference in this task was not due to stress but most probably due to getting accustomed to the room dimensions prior to learning the spatial task. The results suggest that impaired learning of spatial tasks in the water maze can be due to increased stress and decreased fear conditioning without actually affecting spatial learning abilities. These results question the interpretations of the results of some previously published results of spatial water maze tasks.

Masking of forskolin-induced long-term potentiation by adenosine accumulation in area CA1 of the rat hippocampus

At hippocampal Schaffer collateral-CA1 synapses, activation of beta-adrenergic receptors and adenylyl cyclase increases transmitter release. However, this effect is transient, which is in contrast to that seen at mossy fiber-CA3 synapses, where activation of cyclic-AMP-dependent protein kinase results in long-lasting facilitation of transmitter release, a phenomenon known as a presynaptic form of long-term potentiation. The present study was aimed at investigating whether forskolin, an adenylyl cyclase activator, could produce long-term effects at the Schaffer collateral-CA1 synapses using extracellular recording techniques. As has been reported previously, forskolin persistently increased the amplitude of evoked population spikes without having a long-term effect on the field excitatory postsynaptic potentials. However, under the conditions where adenosine A1 receptors are inhibited, cyclic-AMP metabolism is disrupted or the transport of cyclic-AMP is blocked, forskolin induces long-term potentiation. Forskolin-induced potentiation is associated with a decrease in paired-pulse facilitation and is blocked by the cyclic-AMP-dependent protein kinase inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate. Activation of N-methyl-D-aspartate receptors is not required for forskolin-induced long-term potentiation, because pretreatment of slices with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate did not prevent forskolin-induced potentiation. These results suggest that blockade of adenosine A1 receptors unmasks forskolin-induced long-term potentiation, and activation of cyclic-AMP-dependent protein kinase induces a form of long-term potentiation which is different from that induced by tetanic stimulation.

Calcium-permeable AMPA receptors mediate long-term potentiation in interneurons in the amygdala

Fear conditioning is a paradigm that has been used as a model for emotional learning in animals. The cellular correlate of fear conditioning is thought to be associative N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity within the amygdala. Here we show that glutamatergic synaptic transmission to inhibitory interneurons in the basolateral amygdala is mediated solely by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In contrast to AMPA receptors at inputs to pyramidal neurons, these receptors have an inwardly rectifying current-voltage relationship, indicative of a high permeability to calcium. Tetanic stimulation of inputs to interneurons caused an immediate and sustained increase in the efficacy of these synapses. This potentiation required a rise in postsynaptic calcium, but was independent of NMDA receptor activation. The potentiation of excitatory inputs to interneurons was reflected as an increase in the amplitude of the GABA(A)-mediated inhibitory synaptic current in pyramidal neurons. These results demonstrate that excitatory synapses onto interneurons within a fear conditioning circuit show NMDA-receptor independent long-term potentiation. This plasticity might underlie the increased synchronization of activity between neurons in the basolateral amygdala after fear conditioning.

Metabotropic glutamate receptors are involved in calcium-induced LTP of AMPA and NMDA receptor-mediated responses in the rat hippocampus

Effects of metabotropic glutamate (mGlu) receptors on calcium-induced long-term potentiation (LTP) of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components were investigated in rat hippocampal slices using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs). Calcium-induced LTP comprises a parallel, long-lasting increase of AMPA and NMDA receptor-mediated components. The calcium-induced LTP of the AMPA receptor-mediated component can be significantly attenuated by the use of a selective NMDA antagonist. (R.S)-alpha-methyl-4-carboxyphenylglycine (MCPG), a selective antagonist of mGlu receptors, abolished the long-lasting increase of both AMPA and NMDA receptor-mediated components observed in calcium-induced LTP. In current clamp mode, the application of a high calcium alone or Schaffer fiber stimulation alone (20 Hz) only generated a short-term increase in the firing rate of evoked action potentials. Conversely, a long-term increase in the firing rate was observed if Schaffer fiber stimulation (20 Hz) accompanied the perfusion of high calcium. These results suggest that calcium-induced LTP involves a parallel, long-lasting enhancement in ionotropic AMPA and NMDA receptor-mediated components. More importantly, the mGlu receptor plays a critical role in the establishment of both AMPA and NMDA receptor-mediated components underlying calcium-induced LTP. In addition, the present study also described an experimental condition in which the coapplication of the high calcium pulse and Schaffer fiber stimulation (20 Hz) can synergistically elicit a long-term increase of neuronal excitability.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

PCBs reduce long-term potentiation in the CA1 region of rat hippocampus

Prenatal exposure to polychlorinated biphenyls (PCBs) has been associated with a lower IQ in childhood. We have examined the effects of acute exposure to PCB mixtures and two single congeners on synaptic transmission between Schaffer collaterals and CA1 neurons of the rat hippocampus as well as posttetanic potentiation (PTP), paired pulse facilitation (PPF), and long-term potentiation (LTP). PTP and PPF represent transient increases in transmitter release immediately after stimulation, while LTP is a measure of long-term changes in synaptic plasticity that has been related to learning and memory. LTP, but neither PTP nor PPF, was reduced by Aroclor 1016 in a dose-dependent fashion at concentrations that had little effect on general synaptic transmission. The more highly chlorinated Aroclor 1254 at low concentrations specifically blocked LTP, but at higher concentrations also reduced synaptic transmission. The mono-ortho PCB congener 2,4,4'-trichlorobiphenyl and the coplanar congener 3,3',4,4'-tetrachlorobiphenyl also blocked LTP without effect on PTP or PPF. We conclude that PCBs selectively impair the process of LTP in CA1 neurons of the hippocampus.

Glycine-activated currents are changed by coincident membrane depolarization in developing rat auditory brainstem neurones

1. During early ontogeny, glycine receptors (GlyRs) exert depolarizing responses which may be of developmental relevance. We have used the gramicidin-perforated patch technique to elucidate the mechanism of glycine-activated currents in developing neurones of the rat lateral superior olive (LSO). 2. When the holding potential was set to -60 mV, perforated-patch recordings revealed glycine-induced inward currents in 59 %, outward currents in 5 % and biphasic currents in 34 % of the LSO neurones tested (n = 44). The biphasic currents were characterized by a transient outward phase which was followed by an inward phase. 3. Ion substitution experiments showed that both Cl- and HCO3- contributed to the glycine- induced biphasic current responses. 4. In the biphasic responses, the reversal potential of the glycine-induced current (Egly) depended on the response phase. A strong shift of Egly from a mean of -72 mV during the outward phase of the glycine response to a mean of -51 mV during the inward phase was observed, suggesting a shift of an ion gradient. 5. When the membrane potential was depolarized, 'tail' currents were induced in the presence of glycine. An increased duration or amplitude of the evoked depolarizations resulted in a proportional enlargement of these tail currents, indicating that they were produced by a shift of an ion gradient. Since changes of the HCO3- gradient are negligible, because of the carbonic anhydrase activity, we suggest that these tail currents were caused by a shift of the Cl- gradient. 6. We conclude that Cl- accumulates intracellularly during the activation of GlyRs and, consequently, Egly moves towards more positive values. 7. Coincident depolarizing stimuli enhanced intracellular Cl- accumulation and the shift of Egly, thereby switching hyperpolarizing to depolarizing action. This change could assist in an activity-dependent strengthening and refinement of glycinergic synapses during the maturation of inhibitory connectivity.

A novel brain gene, norbin, induced by treatment of tetraethylammonium in rat hippocampal slice and accompanied with neurite-outgrowth in neuro 2a cells

Tetraethylammonium (TEA) induces long-term potentiation (LTP)-like synaptic enhancement in rat hippocampal slices. To find the genes related to this phenomenon, subtraction screening was performed between the mRNA of TEA-treated slices and that of untreated whole brain. One of the clones induced by the TEA treatment, named as norbin, was expressed only in neural tissues. The predicted protein sequence of norbin consisted of 729 amino acids, and no homologies in the sequence were found with known genes or proteins. Overexpression of norbin in cultured Neuro 2a cells by cDNA transfection induced neurite-outgrowth. Since in the course of neural plasticity the formation of new synapses should occur, the neurite-outgrowth-related protein, norbin, might play an important role in neural plasticity.

Relation between dendritic Ca2+ levels and the polarity of synaptic long-term modifications in rat visual cortex neurons

Long-term changes of synaptic efficacy, in particular when they are use-dependent, are candidate mechanisms for the storage of information in the nervous system. In a variety of brain structures, including the neocortex and hippocampus, synapses are susceptible to long-term potentiation (LTP) and long-term depression (LTD). It has been hypothesized that the polarity of the synaptic gain change depends on the amplitude of the postsynaptic [Ca2+]i rise, the threshold for the induction of LTD being lower than that for the induction of LTP. To test this assumption, we characterized Ca2+ signals in layer II/III pyramidal cells of rat visual cortex slices, using the fluorescent Ca2+ indicator fura-2, during application of stimulation protocols that had been adjusted to reliably induce either LTP or LTD in cells not loaded with fura-2. At dendritic sites activated by the stimulated afferents the intracellular [Ca2+] concentration ([Ca2+]i) reached higher amplitudes and decayed more slowly with stimuli inducing LTP than with those inducing LTD. To directly analyse the functional significance of the observed difference in the Ca2+ signal amplitude, we examined whether a tetanization protocol suitable for the induction of LTP can be converted into a protocol inducing LTD by injecting the postsynaptic cells with Ca2+ chelators that reduce the concentration of effective free Ca2+. In the presence of fura-2 or BAPTA [bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetate], the stimulation protocol that would normally produce LTP induced either LTD or failed to induce synaptic modifications altogether. These results support the hypothesis that the amplitude of the postsynaptic rise in [Ca2+]i is a key factor in the determination of the polarity of synaptic gain change.

Inhibition by ATP of calcium oscillations in rat cultured hippocampal neurones

1 The effect of adenosine 5'-triphosphate (ATP) on glutamatergic synaptic transmission in hippocampus was examined by an indicator of intracellular Ca2+ oscillations. These oscillations were postsynaptic responses by glutamate released from presynaptic sites. ATP completely inhibited the oscillations in a concentration-dependent manner. 2 The ATP-induced inhibition was mediated via P2-purinoceptors since ATP exhibited the inhibitory action even in the presence of P1-purinoceptor antagonists. Also non-hydrolysable ATP analogues and uridine 5'-triphosphate (UTP) inhibited the oscillation. 3 The rank order of agonist potency of ATP analogues for inhibition of the Ca2+ oscillation was as follows: 2-methyl-thio-adenosine 5'-triphosphate > or = ATP > adenosine 5'-O-(3-thiotriphosphate)>UTP> alpha, beta-methylene-adenosine 5'-triphosphate. These inhibitory effects were insensitive to suramin. Judging from this rank order of potency, the inhibitory P2-purinoceptor could be assigned to a subclass of GTP-binding protein coupled-type receptors. 4 The site of action of ATP was thought to be presynaptic since ATP did not affect the postsynaptic Ca2+ responses by glutamate. These results suggest the existence of a presynaptic inhibitory P2-receptor that inhibits glutamate release in the hippocampus.

Retinal activity regulates developmental switches in functional properties and ifenprodil sensitivity of NMDA receptors in the lateral geniculate nucleus

Previous studies have shown that marked changes occur in the kinetic properties of N-methyl-D-aspartate (NMDA) receptors during development of the visual pathways. In the lateral geniculate nucleus (LGN) of the ferret, excitatory postsynaptic currents (EPSCs) induced by activation of NMDA receptors display a very slow decay time during the first postnatal month, then become shorter in duration following eye-opening (around postnatal day 32; P32). In view of the critical role that NMDA receptors play in activity-dependent refinement of visual connections during development, we have examined the mechanisms that underlie these changes and how they are regulated. To examine the role of retinal activity, whole-cell recordings were conducted in the LGN slice preparation obtained from normal ferrets and ferrets treated with continuous intraocular application of tetrodotoxin (TTX) from P25 until the time of recording. Blockade of ganglion cell activity with TTX prevented the changes in decay rate of the postsynaptic current induced by NMDA receptors. Treated animals older than P40 had NMDA-EPSCs markedly longer in duration than normal animals at a similar age, resembling responses present in normal newborn animals. To examine whether changes in subunit composition of the NMDA receptor may contribute to the maturation of its kinetic properties, we used the antagonist ifenprodil, which produces selective inhibition of heteromeric NMDA receptors containing the NR-2B subunit. Ifenprodil induced profound inhibition of NMDA receptor activity in normal young animals and TTX-treated mature animals, but substantially less inhibition in normal mature animals. These findings indicate that retinal activity is required for the developmental switch from a juvenile form of the NMDA receptor to a more mature form, possibly affecting NR2 subunit expression.

Calcium- and calmodulin-dependent phosphorylation of AMPA type glutamate receptor subunits by endogenous protein kinases in the post-synaptic density

We have detected immunoreactivities of AMPA receptor subunits GluR1-4 in post-synaptic density (PSD) fraction and tested whether they can be phosphorylated by endogenous kinases. Incubation of PSD with Ca2+ and calmodulin increased phosphorylation of GluR1 and GluR2/3. The phosphorylation of GluR1 was largely blocked by a Ca2+/calmodulin-dependent protein kinase type II inhibitor. Thus Ca2+/calmodulin-dependent phosphorylation of glutamate receptor may be a mechanism underlying enhanced post-synaptic receptor responsiveness in LTP.

Regional alterations in calcium homeostasis in the primate brain following chronic aluminium exposure

The present study investigates the possible effects of chronic aluminium exposure on the various aspects of calcium homeostasis in the primate central nervous system. Aluminium administration caused a marked decline in the activity of Ca2+ ATPase in the monkey brain. The total calcium content was also significantly raised following aluminium exposure. Concomittant to the increase in the calcium content, the levels of lipid peroxidation were also augmented in the aluminium treated animals, thereby further accentuating the aluminium induced neuronal damage. In addition, aluminium had an inhibitory effect on the depolarization induced 45Ca2+ uptake via the voltage operated channels. The results presented herein, indicate that the toxic effects of aluminium could be mediated through modifications in the intracellular calcium homeostasis with resultant altered neuronal function.

Mitochondrial dysfunction and calcium perturbation induced by traumatic brain injury

Traumatic brain injury (TBI) is associated with primary and secondary injury. A thorough understanding of secondary injury will help to develop effective treatments and improve patient outcome. In this study, the GM model of controlled cortical impact injury (CCII) of Lighthall (1988) was used with modification to induce lateral TBI in rats. Forebrain mitochondria isolated from ipsilateral (IH) and contralateral (CH) hemispheres to impact showed a distinct difference. With glutamate + malate as substrates, mitochondria from the IH showed a significant decrease in State 3 respiratory rates, respiratory control indices (RCI), and P/O ratios. This decrease occurred as early as 1 h and persisted for at least 14 days following TBI. The State 3 respiratory rates, RCI, and P/O ratios could be restored to sham values by the addition of EGTA to the assay mixture. A significant amount of Ca2+ was found to be adsorbed to the mitochondria of both the IH and the CH with higher values seen in the IH. The rate of energy-linked Ca2+ transport in the IH was significantly decreased at 6 and 12 h. These data indicate that CCII-induced TBI perturbs cellular Ca2+ homeostasis and results in excessive Ca2+ adsorption to the mitochondrial membrane, which subsequently inhibits the respiratory chain-linked electron transfer and energy transduction.

Increased levels of plasma cholesteryl ester hydroperoxides in patients with subarachnoid hemorrhage

The pathophysiology of subarachnoid hemorrhage (SAH) may involve free radical production and lipid peroxidation. We examined plasma levels of cholesteryl ester hydroperoxides (CEOOH) and antioxidants in 25 patients with SAH, and 10 neurologic controls with lacunar stroke. Patients with SAH had significantly increased plasma levels of CEOOH, which peaked on day 5 after the ictus. Concentrations of CEOOH were significantly increased, and ascorbic acid concentrations were significantly decreased in patients who developed vasospasm compared with patients without vasospasm. Increased levels of CEOOH were associated with increased mortality and correlated with clinical outcome scales. These results implicate oxidative stress in the pathogenesis of SAH and suggest that measurements of CEOOH in plasma may be useful both prognostically as well as in monitoring therapeutic interventions.

Evaluation of carbon-11 labeled KF15372 and its ethyl and methyl derivatives as a potential CNS adenosine A1 receptor ligand

We prepared [11C]KF15372 ([1-propyl-11C]8-dicyclopropylmethyl-1,3-dipropylxanthine, refs 10, 13) as well as its 11C-ethyl and 11C-methyl derivatives ([11C]EPDX and [11C]MPDX), and examined the potential of the three compounds as PET ligands for CNS adenosine A1 receptors. The three compounds had high affinity for the A1 receptors in vitro in the following order; [11C]EPDX > [11C]KF15372 > [11C]MPDX. In mice, the highest initial brain uptake was found in [11C]MPDX followed by [11C]EPDX and [11C]KF15372, but the level of [11C]MPDX decreased faster than those of the other two compounds. The uptake of each compound was decreased by carrier KF15372, but not by an A2A antagonist, indicating the selective affinity for the A1 receptors. Autoradiography with [11C]MPDX ex vivo demonstrated decreased A1 receptor binding in the superior colliculus of rats deprived of retino-collicular fibers by contralateral eye enucleation. These results show that three compounds have potential as PET ligands for CNS adenosine A1 receptors.

Long-term potentiation: a good model for learning and memory?

1. Long-term potentiation (LTP), long-term depression (LTD), and depotentiation of synaptic activity have been suggested to model synaptic plastic changes that occur during learning. Recent reports however show that neither LTP induced by high frequency stimulation (HFS) in the dentate, CA3, or CA1, nor depotentiation in area CA1 of the hippocampus, are reliable models of the learning abilities of rats or mice. LTD cannot reliably be obtained in the hippocampus in vivo and might be an artefact caused by altered inhibitory transmission. 2. Experiments with gene deletion ('knock out') mice strains show that mice that do not express HFS-induced LTP in the dentate are able to learn spatial tasks. 3. Studies of the effect of NMDA receptor blockers also showed that HFS-induced LTP in the dentate is not a model for processes that occur during learning. Studies using drugs that act on metabotropic glutamate receptors showed that HFS-induced LTP or depotentiation of LTP in area CA1 are not models for learning mechanisms either. 4. Neither in vivo recording of naturally-occurring LTP in the dentate nor synaptic saturation experiments in the hippocampus was able to support the theory that LTP occurs during learning. 5. While in vitro experiments are essential tools to investigate cellular and subcellular mechanisms that underlay synaptic transmission, measurements of LTP, LTD, or DP are not reliable models for learning processes and cannot replace experiments with intact animals that learn spatial tasks.

Implication of ATP receptors in brain functions

The possible implication of P2-purinoceptors in brain functions is reviewed. Involvement of P2-purinoceptors in memory and learning (Section 2) is suggested by ATP release from hippocampal slices [Wieraszko, A., Goldsmith, G. and Seyfried, T. N. (1989) Brain Res. 485, 244-250], induction of fast synaptic currents in cultured hippocampal neurons [Inoue, K., Nakazawa, K., Fujimori, W. and Takanaka, A. (1992a) Neurosci. Lett. 134, 294-299] and long-lasting enhancement of the population spikes [Wieraszko, A. and Seyfried, T. N. (1989) Brain Res. 491, 356-359; Nishimura, S., Mohri, M., Okada, Y. and Mori, M. (1990) Brain Res. 525, 165-169; Fujii, S., Kato, H., Furuse, H., Ito, K., Osada, H., Hamaguchi, T. and Kuroda, Y. (1995) Neurosci, Lett. 187, 130-132], as well as ATP release on glutamate stimulation to evoke an increase in intracellular Ca2+ in hippocampal cells [Inoue, K., Koizum, S. and Nakazawa, K. (1995) NeuroReport 6, 437-440]. Moreover, mRNAs for certain types of P2x-purinoceptors are present in the hippocampus [Collo, G., North, R. A., Kawashima, E., Merlo-Pich, E., Neidhart, S., Surprenant, A. and Buell, G. (1996) J. Neurosci. 16, 2495-2507]. It is likely, therefore, that ATP may be involved in modulation of synaptic efficiency in the hippocampus. The implication of ATP in schizophrenia is suggested by the fact that antipsychotic drugs inhibit ATP-evoked responses in PC12 cells [Koizumi, S., Ikeda, M., Nakazawa, K., Inoue, K., Ito, K. and Inoue, K. (1995b) Biochem. Biophys. Res. Commun. 210, 624-630] without blocking the action of dopamine D2 receptors. Involvement of P2-purinoceptors in Sections 4 ("Pain and cognition") and 5 ("Central regulation of the autonomic system") are also discussed.

Purkinje neurons express the SERCA3 isoform of the organellar type Ca(2+)-transport ATPase

We report the distribution of the sarco(endo)plasmic reticulum Ca2+ ATPase 3 (SERCA3) isoform in the rat brain. Compared to SERCA2 isoform, which is found in all brain regions, SERCA3 is specifically expressed in the Purkinje neurons. This conclusion is based on immunochemical observations using SERCA3- and SERCA2b-specific antibodies, in-situ hybridization using SERCA3-specific oligonucleotide probes and single-cell reverse transcription-polymerase chain reaction (RT-PCR). Immunocytochemistry clearly revealed the expression of SERCA3 in the cell body and in the dentritic processes of the Purkinje neurons. Single-cell ratio RT-PCR showed that Purkinje neurons expressed 3-fold lower levels of SERCA3 mRNA compared to SERCA2 mRNA. SERCA3 expression is very low or absent in the rat cerebrum and brainstem. It is known that the SERCA3 Ca2+ pump has an approximately 5-fold lower affinity for Ca2+ when expressed in COS cells as compared to other SERCA members [15]. If this property is also valid in a neuronal context, the expression of the SERCA3 Ca(2+)-pump isoform could have important functional implications for the regulation of the cytosolic Ca2+ concentration in Purkinje neurons.

Inhibition by NMDA of carbachol-stimulated inositol tetrakisphosphate accumulation in rat brain cortical slices

The present studies examined the effect of NMDA on carbachol-stimulated accumulation of inositol polyphosphates, with emphasis on the accumulation of inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4), at short time periods in rat brain cortical slices. There was a stimulatory effect of NMDA on accumulation of labeled inositol mono-, bis- and trisphosphates but not on labeled inositol tetrakisphosphates. In the presence of carbachol Ins 1,3,4,5-P4 accumulation was preferentially inhibited by NMDA at early time periods (within 30 seconds after NMDA addition). Subsequently, total phosphoinositide breakdown was inhibited by NMDA. NMDA did not stimulate accumulation of total Ins 1,3,4,5-P4 but immediately inhibited carbachol stimulated accumulation of Ins 1,3,4,5-P4. The inhibitory effect of NMDA (1 mM) was not mimicked by increasing K+ in the medium from 10 to 30 mM. However 30 mM K+ reversed the inhibitory effect of 1 mM NMDA on carbachol-stimulated Ins 1,3,4,5-P4. Parallel experiments with veratridine (a sodium channel activator) suggest that the early inhibitory effects of NMDA on Ins 1,3,4,5-P4 accumulation are not due to decreases in ATP availability or elevations in intracellular Na+. These data indicate that NMDA increases inositol mono-, bis- and trisphosphate accumulation while blocking muscarinic cholinergic stimulated accumulation of Ins 1,3,4,5-P4.

Expression of type 1 inositol 1,4,5-trisphosphate receptor during axogenesis and synaptic contact in the central and peripheral nervous system of developing rat

Release of intracellular Ca2+ is triggered by the second messenger inositol 1,4,5-trisphosphate, which binds to the inositol 1,4,5-trisphosphate receptor and gates the opening of an intrinsic calcium channel in the endoplasmic reticulum. In order to understand the importance of this mechanism in development, we have examined the distribution of the type 1 inositol 1,4,5-trisphosphate receptor during development, in some areas of the rat brain and spinal cord and in peripheral neurons, using in situ hybridization and immunohistochemistry. In brain, we find that type 1 inositol 1,4,5-trisphosphate receptor is expressed in neurons from very early in development; low levels of expression are first detected after the neurons have migrated to their final positions, when they start to differentiate and begin axonal growth. Increasing levels of expression are observed later in development, during the time of synaptogenesis and dendritic contact. Glial cells do not express type 1 inositol 1,4,5-trisphosphate receptor, except for a transient period of expression, probably by oligodendrocytes, in developing fibre tracts during the onset of myelination. In contrast with the brain, both grey and white matter of the spinal cord express type 1 inositol 1,4,5-trisphosphate receptor throughout development, and it remains present in the adult spinal cord. We also show, for the first time, that type 1 inositol 1,4,5-trisphosphate receptor is expressed in the peripheral nervous system. Strong labelling was observed in the dorsal root ganglia and during development this expression seems to coincide with the onset of axogenesis. These results suggest that type 1 inositol 1,4,5-trisphosphate may be involved in the regulatory mechanism controlling Ca2+ levels in neurons during the periods of cell differentiation, axogenesis and synaptogenesis.

Ionotropic glutamate receptors. Their possible role in the expression of hippocampal synaptic plasticity

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds--the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and the N-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/ LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/ decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.

Intra- vs extracellular calcium regulation of neurotransmitter-stimulated phosphoinositide breakdown

The dependence on Ca2+ of basal, glutamate- and carbachol-stimulated phosphoinositide (PI) turnover was studied on 8-day old rat brain synaptoneurosomes. For that purpose, intracellular and extracellular Ca2+ concentrations were buffered by bis-(alpha-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, in its tetra(acetoxymethyl)-ester form (BAPTA-AM) and in its free acid form (BAPTA), respectively. The effects of both forms of the calcium chelator intracellular and extracellular Ca2+ buffering on intracellular and extracellular Ca2+ concentration ([Ca2+]i and [Ca2+]e) were determined with fluorimetric assay using fura2, either in its acetoxymethyl ester form (fura2-AM) or in its free acid form. Intracellular chelation of Ca2+ ions with BAPTA-AM induced a dose-dependent reduction of the [Ca2+]i. Basal inositol phosphate (IP) formation was slightly affected by this [Ca2+]i buffering, while glutamate and carbachol stimulations of PI hydrolysis were similarly diminished. Chelation of extracellular Ca2+ ions with BAPTA produced a reduction of both [Ca2+]e and [Ca2+]i. Basal IP accumulation was maximally inhibited by 50%. The carbachol-induced PI hydrolysis was completely inhibited in the presence of 200 microM BAPTA, while a substantial residual glutamate-elicited IP response remained (40% of the control response). It is concluded that [Ca2+]i of synaptoneurosomes is not critical for basal and neurotransmitter-stimulated IP formation, whilst [Ca2+]e is critical. Glutamate may, in part, stimulate PI breakdown in a Ca(2+)-insensitive way.

Ryanodine receptor-mediated intracellular calcium release in rat cerebellar Purkinje neurones

1. Ryanodine receptor-mediated Ca2+ release was investigated in Purkinje neurones of rat cerebellar slices by using whole-cell patch-clamp recordings combined with fluorometric digital imaging of cytoplasmic Ca2+ concentration ([Ca2+]i). 2. Caffeine caused a transient increase in [Ca2+]i in the somata and dendrites of Purkinje neurones. Caffeine-induced Ca2+ transients were not associated with a membrane inward current and persisted in Ca(2+)-free external solutions, indicating that they are caused by Ca2+ released from intracellular stores. The amplitudes of the caffeine-mediated elevations in [Ca2+]i were strongly dependent on the baseline level of [Ca2+]i. 3. Intracellular application of Ruthenium Red through the patch pipette blocked caffeine-induced Ca2+ transients in Purkinje neurones. Ryanodine when applied either intra- or extracellularly caused a use-dependent block of caffeine-induced Ca2+ release. 4. Depolarization-induced Ca2+ transients were strongly prolonged by caffeine. Several lines of evidence suggest that these prolongations reflect Ca(2+)-induced Ca2+ release. 5. Despite the presence of skeletal muscle type ryanodine receptors in Purkinje neurones, depolarizing pulses failed to induce any changes in [Ca2+]i when the influx of Ca2+ through voltage-gated channels was prevented by using Ca(2+)-free solution, or when applying blockers of voltage-gated Ca2+ channels. 6. Dendritic Ca2+ transients produced by stimulation of the excitatory climbing fibre synaptic input were also prolonged by caffeine, indicating that ryanodine receptor-mediated release of Ca2+ may be involved in synaptic signalling in cerebellar Purkinje neurones. 7. Ryanodine receptor-mediated release of Ca2+ in cerebellar Purkinje neurones can be explained by a model in which release of Ca2+ is strongly facilitated by the co-operative action of Ca2+, caffeine and/or ryanodine. Our results suggest that Ca2+ release in these central neurones becomes prominent only during episodes of intensive electrical activity associated with increased Ca2+ entry.

The biochemistry of learning and memory

An overview of some of the biochemical and molecular events involved in the process of learning and memory are presented in a short review. Two invertebrate models of learning are considered: the gill-withdrawal reflex of Aplysia and avoidance learning in Drosophila melanogaster. Particular attention is paid to the biochemical mechanisms underlying both the development of long-term potentiation (LTP) and passive avoidance learning (PAL) in the young chick. The role of several biological molecules in learning and memory are considered, for example, protein kinase C (PKC), Ca(++)-Calmodulin kinase II (CaMKII), GAP-43, and glutamate receptors.

Imaging of calcium variations in living dendritic spines of cultured rat hippocampal neurons

1. Cultured rat hippocampal neurons were loaded with the Ca2+ indicator fura-2 through micropipettes and visualized with an inverted microscope equipped with a high power objective and a cooled CCD camera. The responses of dendritic spines and their parent dendrites to stimuli which evoke a rise of [Ca2+]i were monitored. 2. NMDA caused a rapid and transient rise in [Ca2+]i, which was more evident in the spine than in the parent dendrite. The recovery in both compartments had the same time course, and was dependent on normal [Na+]o. 3. Application of alpha-latrotoxin, which causes release of neurotransmitters from terminals, produced a rise of [Ca2+]i in the dendritic spines, more than in their parent dendrites. Prolonged exposure to the drug eliminated the spine/dendrite disparity. 4. The presence of voltage-gated calcium channels in dendritic spines is indicated by the enhanced calcium rise in spines rather than dendrites of cells depolarized by either intracellular current injection or by raising [K+]o. This rise was attenuated by nifedipine or verapamil, both L-type channel blockers. 5. It is suggested that the dendritic spine constitutes an independent calcium compartment that is closely linked to the parent dendrite.

Long-term potentiation at low temperature is stronger in hippocampal slices from hibernating Turkish hamsters compared to warm-acclimated hamsters and rats

Tetanus-induced long-term potentiation (LTP) of population action potentials at 22 degrees C was investigated in area CA1 of hippocampal slices prepared from hibernating (HH) and warm-acclimated Turkish hamsters (WH) and rats. LTP elicited at this temperature was significantly stronger in HH slices compared to WH and rat slices. Hibernation-related improvement of the ability to develop long-lasting enhancement of synaptic transmission at low temperatures is interpreted as supporting hippocampal function during arousal from hibernation.

Molecular correlates between reactive and developmental plasticity in the rat hippocampus

Area CA3 of the hippocampus is the most epileptogenic structure of the brain. Various studies have shown that kainate-induced experimental epilepsy in rats and human cases of epilepsy are associated with sprouting of the mossy fibers of the dentate granule neurons and selective loss of pyramidal neurons, notably in the CA3-CA4 areas of Ammon's horn. In experimental models of epilepsy, brief seizure activity initiates a cascade of molecular alterations that will contribute to changes in the expression of numerous genes, which can last several weeks. The products of some of these genes will contribute to the permanent state of enhanced synaptic efficiency, to the sprouting and formation of novel excitatory synapses, and possibly to neuronal cell loss. The expression of genes encoding transcription factors and numerous growth factors is rapidly altered following seizure episodes. Based on observations in vivo and in vitro in cultured hippocampal neurons, it is hypothesized that an interplay between transcription and growth factors, because of their pleiotropic effects on the regulation of effector genes, may be instrumental in coupling transient extracellular stimuli to irreversible cellular alterations.