Calcium channels in cellular membranes

The olivocerebellar projection mediates ibogaine-induced degeneration of Purkinje cells: a model of indirect, trans-synaptic excitotoxicity

Ibogaine, an indole alkaloid that causes hallucinations, tremor, and ataxia, produces cerebellar neurotoxicity in rats, manifested by degeneration of Purkinje cells aligned in narrow parasagittal bands that are coextensive with activated glial cells. Harmaline, a closely related alkaloid that excites inferior olivary neurons, causes the same pattern of Purkinje cell degeneration, providing a clue to the mechanism of toxicity. We have proposed that ibogaine, like harmaline, excites neurons in the inferior olive, leading to sustained release of glutamate at climbing fiber synapses on Purkinje cells. The objective of this study was to test the hypothesis that increased climbing fiber activity induced by ibogaine mediates excitotoxic Purkinje cell degeneration. The inferior olive was pharmacologically ablated in rats by a neurotoxic drug regimen using 3-acetylpyridine, and cerebellar damage attributed to subsequent administration of ibogaine was analyzed using immunocytochemical markers for neurons and glial cells. The results show that ibogaine administered after inferior olive ablation produced little or no Purkinje cell degeneration or glial activation. That a lesion of the inferior olive almost completely prevents the neurotoxicity demonstrates that ibogaine is not directly toxic to Purkinje cells, but that the toxicity is indirect and dependent on integrity of the olivocerebellar projection. We postulate that ibogaine-induced activation of inferior olivary neurons leads to release of glutamate simultaneously at hundreds of climbing fiber terminals distributed widely over the surface of each Purkinje cell. The unique circuitry of the olivocerebellar projection provides this system with maximum synaptic security, a feature that confers on Purkinje cells a high degree of vulnerability to excitotoxic injury.

Pharmacologically distinct presynaptic calcium channels in cerebellar excitatory and inhibitory synapses

We have used whole-cell patch clamp recordings and pharmacological blockers of Ca channels to compare the pharmacology of Ca channels that mediate synaptic transmission at the three types of synapses innervating Purkinje cells in rat cerebellar slices. Both parallel fiber and climbing fiber excitatory synapses were sensitive to the P-type Ca channel blocker, omega-AgaIVA and the P/Q/N-type channel blocker, omega-conotoxin MVIIC. Transmission at inhibitory interneuronal synapses was not suppressed by these toxins, or by the N-type (omega-conotoxins GVIA and MVIIA) or L-type (nimodipine) channel blockers. Inhibitory transmission could be inhibited by Ni2+ and amiloride, but only at concentrations (IC50 approximately 300 microM) that affect other types of Ca channels. These results indicate that excitatory and inhibitory presynaptic terminals of the cerebellar cortex possess different types of voltage-gated Ca channels. The excitatory terminals contain P-type, Q-type and N-type Ca channels, with P-type channels playing the most prominent role. The inhibitory terminals possess quite different type(s) of Ca channel. The heterogeneous distribution of Ca channel types should impart unique properties to transmitter release from the excitatory and inhibitory terminals.

Pharmacological characterisation of voltage-sensitive calcium channels and neurotransmitter release from mouse cerebellar granule cells in culture

Using subtype-specific Ca-channel blockers, we have characterised the voltage-sensitive Ca2+ currents as well as neurotransmitter release from cultured mouse cerebellar granule cells. The whole cell version of the patch clamp technique was adapted to monitor the isolated Ca-channel currents. The currents were activated at potentials more positive than -40 mV and were composed of at least four pharmacological distinct components being sensitive to nifedipine (35%), omega-conotoxin GVIA (10%), and omega-agatoxin IVA (42%) corresponding to L-, N-, and P-channel-mediated currents. The insensitive fraction (13%) possibly represented R channels. High potassium-evoked release of 3H-D-aspartate was used as a model of synaptic release. These studies were performed at relatively mild stimulation conditions (30 mM K+, 0.4 mM Ca2+), and 85% of the evoked release was Ca2+ dependent as well as tetrodotoxin and Cd2+ sensitive. Nifedipine and omega-agatoxin IVA dose dependently (IC50 values of 10 nM and 0.7 nM, respectively) blocked most of the release, whereas omega-conotoxin MVIIA (IC50 = 5 nM) caused partial blockage. The results indicate that several subtypes of voltage-sensitive Ca channels are present in mouse cerebellar granule cells. Furthermore, the data suggest that L, N, and P channels act in concert in the neurotransmitter release process.

Age-dependent modification of aspecific cellular effects of the benzodiazepine flunitrazepam

The experiments presented here demonstrate an aspecific effect of the benzodiazepine derivative flunitrazepam (FNZ). It differs in sites and mechanisms of action, both from benzodiazepine (BZ) specific effects on gamma-aminobutyric acid (GABA) blocking transmission via central BZ receptors and from BZ effects mediated by peripheral BZ receptors. The aspecific effect of FNZ can suitably be examined on isolated and identified neurons of the mollusc Lymnaea stagnalis (pond snail). The physiological sites of action are outside the synaptic zone, on the neuron somatic membrane and affect 'intrinsic' properties of membrane, including calcium, calcium-activated potassium and chloride channels. The aspecific FNZ effect exerts an influence on the metabolism of the cell by decreasing the permeability of the calcium channel, diminishing the excitability of the neuron membrane, and hyperpolarizing the cell, thus potentiating the specific effect of FNZ. The senile alterations of the neuron function intensify the aspecific effects of FNZ to such a degree that it must be taken seriously in consideration in anesthesia of elderly patients.