Activators and inactivators of calcium channels: effects in the central nervous system

Nicardipine and MK-801 attenuate platelet-activating factor increases following cerebral ischemia-reperfusion in gerbils

The effects of pretreatment with nicardipine (dihydropyridine Ca2+ channel antagonist), Bay K8644 (dihydropyridine Ca2+ channel agonist), and MK-801 (N-methyl-D-aspartate-receptor antagonist) on changes of platelet-activating factor (PAF) concentrations in transient ischemic brain are reported. The tissue concentration of PAF increases significantly in hippocampus, cortex and thalamus by 210%, 169% and 168% of controls without ischemia-reperfusion, respectively after 1 h of reperfusion. Nicardipine (5 mg/kg) reduces the accumulation of PAF, the remaining increases in hippocampus, cortex and thalamus being 151%, 138% and 145% of the controls, respectively. In contrast, Bay K8644 (2.5 mg/kg) enhances the accumulation of PAF, its concentrations in hippocampus, cortex and thalamus being 376%, 233% and 204% of the controls, respectively. The Bay K8644 enhancement in hippocampus is completely inhibited by pretreatment of nicardipine (5 mg/kg). MK-801 (10 mg/kg) reduces the accumulation of PAF, the remaining increases in hippocampus, cortex and thalamus being 152%, 147% and 144% of the controls, respectively. Moreover, brain tissue from animals subjected to the combined pretreatment with nicardipine (5 mg/kg) and MK-301 (10 mg/kg) indicates there is greater inhibition of ischemia-induced PAF increases than with either drug alone. These results indicate that PAF production in the ischemic brain may be regulated by Ca2+ influx through voltage-sensitive Ca2+ channels which are antagonized and agonized by nicardipine and Bay K8644, respectively and receptor-operated Ca2+ channels which are antagonized by MK-801. Because it is known that increases of intracellular Ca2+ in the brain accompany ischemia and early periods of reperfusion and that PAF exhibits neurotoxicity, the present findings support the role of PAF as a mediator in ischemia-induced brain damage at early stages of reperfusion.

Block of P-type Ca2+ channels by the NMDA receptor antagonist eliprodil in acutely dissociated rat Purkinje cells

The effect of eliprodil on P-type Ca2+ channels was investigated in acutely dissociated rat Purkinje neurons, by using the whole-cell patch-clamp technique. Eliprodil inhibited in a reversible manner the omega-agatoxin-IVA-sensitive Ba2+ current elicited by step depolarizations from a -80 mV holding voltage (IC50 = 1.9 microM). The Ba2+ current showed steady-state inactivation (V1/2 = -61 mV) which was shifted toward more positive values when the intracellular Ca2+ buffering was increased. In these conditions, the potency of eliprodil was decreased (IC50 = 8.2 microM), suggesting a modulation by intracellular Ca2+ of the eliprodil blockade. The potency of eliprodil was not modified at more depolarized holding potentials and was not dependent on the frequency at which the step-depolarizations were applied (0-0.2 Hz) indicating a lack of voltage and use dependence of the eliprodil blockade. When eliprodil was applied in the patch-pipette at a concentration which causes full block when applied externally, the Ba2+ current amplitude was not affected and external application of eliprodil was still efficacious, indicating an extracellular location of the binding site. Analysis of the time course of recovery from Ca2+ channel blockade obtained by concomitant application of eliprodil with Cd2+, omega-agatoxin-IVA or fluspirilene, indicated that these later compounds did not interact with eliprodil, suggesting that eliprodil acts at a different site. These results demonstrate that eliprodil blocks P-type Ca2+ channels in cerebellar Purkinje neurons and suggest that this property may contribute to its neuroprotective activity.

Reduction by lifarizine of the neuronal damage induced by cerebral ischaemia in rodents

1. The objective of this study was to evaluate the broad neurocytoprotective potential of the novel sodium-calcium ion channel modulator, lifarizine (RS-87476), in two rodent 72 h survival models of forebrain ischaemia. 2. Under fluothane anaesthesia, rats were subjected to 10 min four vessel occlusion and gerbils to either (i) 5 or (ii) 10 min bilateral carotid artery occlusion. 3. Rats were dosed parenterally solely post-ischaemia (reperfusion) in a series of five studies covering a range of intra-arterial/intraperitoneal (i.a./i.p.) combination doses from 2/10, 5/20, 20/100, 50/200 and 100/500 micrograms kg-1, where the initial loading dose was injected i.a. at 5 min. An i.p. dose was given at 15 min and repeated twice daily. In a sixth study, treatment at 50/200 micrograms kg-1 was deferred for 1 h. 4. Gerbils were treated (i) 15 min pre-ischaemia with either (a) 250, (b) 500 micrograms kg-1 i.p., or (c) 5 mg kg-1 by gavage (p.o.) for 3 days then at 1 h pre-ischaemia. Animals treated as (ii) received 500 micrograms kg-1 i.p. 15 min pre-ischaemia. The above doses were repeated twice daily for 3 days post-ischaemia for the respective groups. 5. In rats, the protective effect of lifarizine was regionally and cumulatively assessed in six brain regions (anterior and posterior neocortex, hippocampal CA1 subfield, thalamus, striatum, cerebellar Purkinje cells-brain stem) at each dose level. Cumulative (total) means +/-s.e.mean neurohistopathological scores(0-4) of 1.16+/-0.09 (n=5), 1.02+/-0.10 (n=5), 0.93+/-0.06 (n=6), 0.79+/-0.09 (n=9) and 0.45+/-0.16(n = 7), respectively, were obtained for the above treatment groups compared to the control (2.01 +/- 0.17,n = 16) group (P<0.0035). The score for the 1 h deferred treatment group was also significant at 0.77 +/- 0.10, n =5 (P< 0.0035). The normal group without ischaemia showed a score of 0.52 +/- 0.09 (n = 6).6. In gerbils, (i) percentage delayed neuronal death (DND) of hippocampal pyramidal cells in the CA1subfield was prevented at 250 (a) and 500 microg kg-' i.p. (b) (27.2+/- 14.6, n=6 and 26.9+/- 10.4%, n= 10 respectively, P<0.02) compared to controls (78.3+/-8.5%, n= 12) and by 5 mg kg-1 p.o. (c) (2.9+/-0.8%,n =l1, P <0.002). Mean +/- s.e.mean total brain scores (0-4) for each of 4 different features denoting cerebral 'oedema' were lower for normal brains (1.60 +/-0.34, n =6) and reduced in animals dosed at 250(a) (3.00+/-0.79, n=6) and 500 microg kg-1 i.p. (b) (3.75 0.36, n= 10) compared to controls (6.58+/-1.00,n = 12) (P< 0.02 -0.03). There was a linear relationship (r = 0.97) between the 'oedema' scores and percentage CA1 DND. Percentage CA1 DND in response to 10 min ischaemia (ii) was reduced(53.0+/-21.0%, n=6, P<0.05) compared to controls (100.0+/-0.0%, n=7).7 The significant neuroprotection shown by lifarizine in rodents substantiates findings in other species.These observations, together with its effect on ion channels and efficacy at extremely low doses offers novelty and suggests a broad spectrum of activity in ischaemia.

Neuroprotective properties of lifarizine compared with those of other agents in a mouse model of focal cerebral ischaemia

1. Changes in the peripheral type benzodiazepine binding site density following middle cerebral artery occlusion in the mouse, have been used as a marker of neuronal damage. These sites can be identified using the selective ligand [3H]-PK 11195 located on non neuronal cells, macrophages and astroglia, within the CNS. Glial cell proliferation and macrophage invasion is an unvoidable sequelae to cerebral ischaemic injury, secondary to neuronal loss. Following occlusion of the left middle cerebral artery (left MCA) a reproducible lesion was found in the parietal cortex within 7 days which gave rise to a significant increase in [3H]-PK 11195 binding. 2. Treatment of animals with the sodium channel blocker, lifarizine, significantly reduced the ischaemia-induced increase in [3H]-PK 11195 binding when given either 30 min pre-ischaemia and three times daily for 7 days at 0.5 mg kg-1, i.p. (P < 0.01) or delayed until 15 min post-ischaemia and three times daily for 7 days at 0.5 mg kg-1, i.p. (P < 0.001). Lifarizine was an effective neuroprotective agent in this model of focal ischaemia in the mouse. 3. Lifarizine also showed a dose-related protection against the ischaemia-induced increase in [3H]-PK 11195 binding with significant protection at doses of 0.1 mg kg-1, i.p. (P < 0.05), 0.25 mg kg-1, i.p. (P < 0.01) or 0.5 mg kg-1, i.p. (P < 0.01) 15 min post-ischaemia and b.i.d. for 7 days. No significant change is seen in the Kd for [3H]-PK 11195. The first dose could be delayed for up to 4 h after cerebralartery cauterization and protection was maintained.4. Phenytoin (28 mg kg-1, i.v. 15 min and 24 h post-ischaemia) was also neuroprotective in this model(P<0.01). This agent is thought to interact with voltage-dependent sodium channels to effect its anticonvulsantactions and this mechanism may also underlie its neuroprotective actions in focal cerebralischaemia.5. Agents with other mechanisms of action were also shown to have significant neuroprotection in this model. The non-competitive NMDA antagonist, MK 801, showed significant neuroprotection in the model when given at 0.5 mg kg-1, i.p. 30 min pre-ischaemia with t.i.d. dosing for 7 days (P< 0.001). The dihydropyridine calcium antagonist, nimodipine was not protective when given using the same dosing protocol as MK 801, 0.5 mg kg-1 30 min pre-occlusion and three times daily for 7 days but showed significant protection when given at 0.05 mg kg-1 15 min post-ischaemia and three times daily for 7days. The lipid peroxidation inhibitor, tirilazad (single dose 1 mg kg-1, i.v.) showed significant neuroprotection when given 5 min post-ischaemia but not when the first dose was delayed for 4 h.

Differential contribution of L- and N-type calcium channels on rat hippocampal acetylcholine release

Bay K 8644, nimodipine and omega-conotoxin GVIA (omega-CgTx) were used to study the different contribution of voltage-sensitive calcium channels (VSCC) to [3H]acetylcholine ([[3H]ACh) release in rat hippocampal synaptosomes. In our experimental conditions, the percentage of calcium-dependent ACh release was approximately 80%. Nimodipine (0.01-10 microM) and Bay 8644 (0.01-10 microM) were not able to modify the [3H]ACh release under stimulating conditions (15 mM K+). Nevertheless, when K+ concentration was reduced to 8 mM, a significant increase in [3H]ACh release was observed at 1 and 10 microM of Bay K 8644. Nimodipine (0.01-10 microM) failed to reverse the effect of Bay K 8644 on [3H]ACh release. Finally, omega-CgTx (0.001-1 microM) caused a concentration-dependent reduction of [3H]ACh release in K+ (15 mM)-stimulating conditions. These results suggest that the N-type VSCC probably play a predominant role in regulating the [3H]ACh release in synaptosomes from rat hippocampus.

Effects of calcium channel blockers on the kinetics of voltage-dependent changes in synaptosomal calcium concentrations

Synaptosomal preparations from rat cerebral cortex have been used in stopped-flow fluorescence studies to measure rapid changes in intrasynaptosomal calcium concentrations upon depolarization. Synaptosomes were loaded with the fluorescent calcium chelating dye, Fura-2, by incubation with the membrane permeant acetoxymethyl ester derivative. Depolarization by elevated external K+ concentration resulted in a rapid increase in cytoplasmic Ca2+ as measured by a quench in Fura-2 fluorescence when excited at 390 nm. The fluorescence change could be reasonably fit by a single exponential process with an apparent rate of 10-15 s-1 and the magnitude of the response was voltage-dependent, increasing with increasing external K+ over the range of 5-30 mM. The observed quench was blocked by micromolar concentrations of the inorganic calcium channel blockers, Cd2+, Co2+ and La3+. Nimodipine, a dihydropyridine which blocks L-type calcium channels, inhibited only 10-15% of the flux response while nitrendipine had no consistent effect. omega-Conotoxin GVIA, a blocker of N-type channels in many species, had only a small inhibitory effect at high (1-10 microM) concentrations. The response was, however, inhibited by pre-incubation of the synaptosomes with venom of the funnel web spider. Agelenopsis aperta (0.1-300 micrograms/ml). Inhibition was observed with both a purified polyamine fraction (FTX) from the venom (IC50 = 4 nl/ml) and a purified peptide toxin, omega-AgaIVA (IC50 = 30 nM). These results indicate that voltage-dependent Ca2+ uptake by mammalian nerve terminals is mediated primarily by channels that are insensitive to dihydropyridines and omega-conotoxin GVIA but are sensitive to components of funnel web spider venom.

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca2+]i in rat cortex synaptosomes were measured using the Ca2+ indicator Fura-2. Ca2+ influx was induced by veratridine in a concentration-dependent manner (1-10 microM). The resulting increase in [Ca2+]i was inhibited by tetrodotoxin (TTX). K+ (18 mM) increased the [Ca2+]i which was not influenced by TTX. K(+)-channel blockers such as 4-aminopyridine, alpha- and delta-dendrotoxin pre se were ineffective. The veratridine-induced Ca2+ influx in synaptosomes was reduced by L-type Ca(2+)-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. omega-Conotoxin, and N-type Ca(2+)-channel blocker, did not inhibit the veratridine-stimulated [Ca2+]i increase. Bay K 8644, and L-channel agonist, stimulated an increase of [Ca2+]i in synaptosomes which was not sensitive to TTX. R-N6-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A1-receptors and alpha 2-adrenoceptors, respectively, did not influence the veratridine-stimulated [Ca2+]i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca2+]i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca2+ influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+ mechanisms of presynaptic neuromodulation.

The effect of lifarizine (RS-87476), a novel sodium and calcium channel modulator, on ischaemic dopamine depletion in the corpus striatum of the gerbil

1. Unilateral ligation of the right common carotid artery in the anaesthetized gerbil for 3 h caused a 62.7% decrease in ipsilateral dopamine in the corpus striatum from 1.40 (+/- 0.13, n = 27) micrograms g-1 in the non-ischaemic hemisphere to 0.47 (+/- 0.07, n = 27) micrograms g-1 in the ischaemic hemisphere (all results are expressed as mean +/- s.e. mean). In sham-operated animals there were no differences in the dopamine levels (1.31 +/- 0.14 micrograms g-1, n = 11, left; 1.27 +/- 0.13 micrograms g-1, n = 11 in the right hemisphere). Animals with intact communicating arteries in the circulus arteriosus were excluded. 2. Lifarizine (RS-87476; 250, 500, but not 50, micrograms kg-1, i.p.) protected against this dopamine depletion showing only a 9.2% decrease at 250 micrograms kg-1, i.p. (P < 0.01) and no decrease at 500 micrograms kg-1, i.p. (P < 0.01). 3. Nicardipine (250 micrograms kg-1, p.o.) was effective when administered chronically once daily for 10 days (26.6% decrease, P < 0.05) but not when administered acutely at 50 micrograms kg-1, i.p.

The effect of flunarizine on central nervous system oxygen toxicity in rats

The toxicity of hyperbaric oxygen in the central nervous system is expressed by generalized tonic-clonic seizures. In the search for drugs effective against these seizures, we tested flunarizine, a calcium antagonist known to have antiepileptic properties and only minimal cardiovascular side effects. 49 rats with chronic cortical electrodes were injected i.p. with six different doses of flunarizine (10-300 mg/kg) or vehicle, before exposure to 0.5 MPa oxygen. Two doses of flunarizine and vehicle were given to rats exposed to oxygen with 5% CO2 at an absolute pressure of 0.5 MPa. EEG and spectral analysis of background EEG activity were monitored. The duration of the latent period before the appearance of electrical discharges in the EEG was used as an index of oxygen toxicity. There was no statistical difference between the duration of the latent periods for the seven groups treated by flunarizine or by vehicle on exposure to 0.5 MPa pure oxygen (P = 0.9 in ANOVA), but on exposure to oxygen with CO2, there was significant prolongation of the latent periods in comparison with vehicle (P < 0.001). Our results suggest that on exposure to hyperbaric oxygen, the antiepileptic effect of flunarizine might be masked, probably by its cerebral antivasoconstrictive effect.

Thrombolysis in acute ischemic stroke: does it work?

BACKGROUND

This article is presented to provoke further discussion regarding the use of thrombolytic drugs to treat acute ischemic stroke.

SUMMARY OF REVIEW

Overview analysis of the six randomized trials of thrombolysis in acute ischemic stroke available in the world literature shows a 20% increase in the odds of death and a 30% reduction in the odds of death or deterioration (both with wide confidence intervals, neither result significant) after thrombolytic treatment for acute ischemic stroke. Exclusion of the two trials conducted without the benefit of computed tomographic scanning shows a 37% reduction in the odds of death (95% confidence interval, 74% reduction to 40% excess) and a significant reduction of 56% in the odds of death or deterioration after thrombolytic treatment (95% confidence interval, 20-76% reduction; 2p = 0.007). Analysis of all published studies (randomized and nonrandomized) shows that there does not appear to be an excess risk of hemorrhagic transformation of the cerebral infarct or of severe edema formation.

CONCLUSIONS

We believe the present evidence is sufficiently encouraging to warrant proper testing of thrombolysis in sufficiently large and well-designed randomized clinical trials to influence clinical practice.

Pathophysiology and treatment of focal cerebral ischemia. Part II: Mechanisms of damage and treatment

The mechanisms that give rise to ischemic brain damage have not been definitively determined, but considerable evidence exists that three major factors are involved: increases in the intercellular cytosolic calcium concentration (Ca++i), acidosis, and production of free radicals. A nonphysiological rise in Ca++i due to a disturbed pump/leak relationship for calcium is believed to cause cell damage by overactivation of lipases and proteases and possibly also of endonucleases, and by alterations of protein phosphorylation, which secondarily affects protein synthesis and genome expression. The severity of this disturbance depends on the density of ischemia. In complete or near-complete ischemia of the cardiac arrest type, pump activity has ceased and the calcium leak is enhanced by the massive release of excitatory amino acids. As a result, multiple calcium channels are opened. This is probably the scenario in the focus of an ischemic lesion due to middle cerebral artery occlusion. Such ischemic tissues can be salvaged only by recirculation, and any brain damage incurred is delayed, suggesting that the calcium transient gives rise to sustained changes in membrane function and metabolism. If the ischemia is less dense, as in the penumbral zone of a focal ischemic lesion, pump failure may be moderate and the leak may be only slightly or intermittently enhanced. These differences in the pump/leak relationship for calcium explain why calcium and glutamate antagonists may lack effect on the cardiac arrest type of ischemia, while decreasing infarct size in focal ischemia. The adverse effects of acidosis may be exerted by several mechanisms. When the ischemia is sustained, acidosis may promote edema formation by inducing Na+ and Cl- accumulation via coupled Na+/H+ and Cl-/HCO3- exchange; however, it may also prevent recovery of mitochondrial metabolism and resumption of H+ extrusion. If the ischemia is transient, pronounced intraischemic acidosis triggers delayed damage characterized by gross edema and seizures. Possibly, this is a result of free-radical formation. If the ischemia is moderate, as in the penumbral zone of a focal ischemic lesion, the effect of acidosis is controversial. In fact, enhanced glucolysis may then be beneficial. Although free radicals have long been assumed to be mediators of ischemic cell death, it is only recently that more substantial evidence of their participation has been produced. It now seems likely that one major target of free radicals is the microvasculature, and that free radicals and other mediators of inflammatory reactions (such as platelet-activating factor) aggravate the ischemic lesion by causing microvascular dysfunction and blood-brain barrier disruption.(ABSTRACT TRUNCATED AT 400 WORDS)

Drugs acting on calcium channels: potential treatment for ischaemic stroke

Calcium subserves a ubiquitous role in the organisation of cell function. Ca2+ channels which control influx may be modified in disease states. Animal models of cerebral ischaemia do present some problems when investigating potential therapies involving Ca2+ channels. However, it is important not to be too rigid in searching for models which exactly mimic the human disease state, when even the best experimental approaches fall short of such an ideal. There are differences between different classes of calcium entry blocking drugs with regard to their activity on Ca2+ channels and transmembrane Ca2+ movement. Some calcium antagonists may also affect ion channels other than Ca2+, and this potential is exemplified by the novel ion channel modulator RS-87476, which affords experimental neurocytoprotection. Limitation of intracellular Na+ influx during ischaemia-induced depolarization may be useful.

An examination of the anticonvulsant properties of voltage-sensitive calcium channel inhibitors in amygdala kindled seizures

Voltage-sensitive calcium (VSC) channels may contribute to epileptogenesis. A systematic examination of the anticonvulsant efficacy of different classes of VSC channel inhibitors, however, is lacking in chronic seizure models. The present study evaluated representatives from three different classes of VSC channel inhibitors for their protection against amygdala kindled seizures. Adult male rats (n = 12) were kindled to stage 5 seizures (GS), and a threshold intensity required to evoke a GS was determined. The Ca(++)-channel inhibitors (verapamil 0, 10, 20, 40 mg/kg; nimodipine 0, 5, 25, 50 mg/kg; nitrendipine 0, 25, 50, 100 mg/kg and flunarizine 0, 20, 40, 80 mg/kg) were administered 60-90 min prior to amygdala stimulation at the established threshold. None of the drugs altered threshold for inducing a seizure. Verapamil, a phenylalkylamine, and the dihydropyridines nimodipine and nitrendipine were without effect on kindled seizures. The diphenylalkylamine, flunarizine, was found to be the most efficacious, reducing AD duration and duration of clonic seizure activity by more than 60% in most animals. Flunarizine also decreased the severity of behavioral seizures, with 40% of the animals displaying Stage 1-2 seizures only. It is concluded that some VSC Ca(++)-channel inhibitors do possess anticonvulsant potential. Thus influx of extracellular calcium through VSC channels may contribute to the expression of kindled seizures.

Effects of calcium channel agonist and antagonists on calcium-dependent events in CA1 hippocampal neurons

The effects of a variety of calcium channel modulators on different calcium-dependent events in CA1 pyramidal hippocampal neurons were analysed using intracellular recordings in an in vitro slice preparation. The following substances were tested: the dihydropyridine calcium agonist BAY K 8644, the dihydropyridine calcium antagonist nimodipine, the phenylalkylamine verapamil and the snail toxin omega-conotoxin GVIA (omega-CgTx). BAY K 8644 increased the repolarization time of the after hyperpolarization (AHP) following a spike burst. This effect was antagonized by nimodipine. BAY K 8644 also prolonged the calcium spike and, in some cases, increased the size of the synaptic events resulting from activation of the Schaffer collateral/commissural system. Nimodipine decreased the size of the AHP in some neurons but had no consistent effect on synaptic events. Verapamil at low concentrations (1-10 microM) had no significant effects on the calcium-dependent events in the hippocampus. Increasing the concentration (up to 100 microM) led to a progressive suppression of the AHP and of the slow inhibitory postsynaptic potential (IPSP), probably via an action on potassium conductances. In addition, the baclofen-induced hyperpolarization was blocked by verapamil. Interestingly, at this higher concentration, verapamil could suppress the AHP without depressing the calcium spike. omega-CgTx selectively blocked the synaptic events (especially the IPSPs) but had no effect on non-synaptic events. This last compound exhibits a high degree of selectivity, acting on N-type calcium channels which are involved in neurotransmitter release. Our results provide evidence that different classes of agents which act on calcium channels can be used to discriminate between different calcium-dependent responses in CA1 hippocampal neurons.

Calbindin-D-28k-like immunoreactive structures in the olfactory bulb and anterior olfactory nucleus of the human adult: distribution and cell typology--partial complementarity with parvalbumin

Calbindin-D-28k and parvalbumin are calcium-binding proteins. The laminar distribution and morphological features of calbindin-D-28k-like immunoreactive structures were studied in 60-microns-thick sections of the human olfactory bulb. Except for the olfactory nerve layer, immunoreactive neurons were present in all layers of the olfactory bulb. They reached highest densities in the external plexiform layer and internal granule cell layer. Considerable numbers of calbindin-like nerve cells were also found in the olfactory tract and in distal portions of the anterior olfactory nucleus. When comparing the distribution of calbindin-positive structures to that of parvalbumin-positive ones a partially complementary distribution pattern was found. Calbindin-like immunoreactive portions of the anterior olfactory nucleus and olfactory tract were mirrored by immunonegative areas in adjacent sections stained for parvalbumin. Using the combined pigment-Nissl procedure we observed the presence of lipofuscin deposits in nearly 80% of all the calbindin-immunoreactive neurons analysed. Moreover, analysis of their lipofuscin deposits rendered the further differentiation of morphologically similar neuronal subpopulations possible. In contrast, all parvalbumin-like immunoreactive neurons remained free of lipofuscin granules.

Omega-conotoxin GVIA blocks synaptic transmission in the CA1 field of the hippocampus

The effects of omega-conotoxin GVIA (omega-CgTx), a peptide isolated from the venom of a marine mollusc, were studied in rat hippocampal neurons. Intracellular recordings from the CA1 area were made for the purpose in in vitro slice preparations. Omega-CgTx (0.1-1 microM) rapidly and irreversibly blocked the EPSP and the IPSPs elicited by electrical stimulation of Schaffer collaterals/commissural fibers. Omega-CgTx also blocked the slow cholinergic EPSP induced by electrical stimulation of cholinergic afferents. The postsynaptic effects of baclofen or carbachol remained unchanged in the presence of omega-CgTx and other postsynaptic calcium-dependent events such as afterhyperpolarization were not affected by omega-CgTx. These results suggest a presynaptic action of omega-CgTx through the blockade of neurotransmitter release. Omega-CgTx might act in the hippocampus by blocking presynaptic N-type voltage-sensitive calcium channels.