Modulation of Ca channels in peripheral neurons

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.

Effects of menthol and cold on histamine-induced itch and skin reactions in man

The effects of cooling and topical application of menthol on histamine-induced itch, wheal and flare reactions of the left lower arm were investigated in a threefold cross-over design with 15 healthy male volunteers. Lowering skin temperature by cooling from 32.8 +/- 0.3 degrees C to 29.7 +/- 0.5 degrees C reduced itch intensity from 260 +/- 47 units to 55 +/- 12 units (visual analogue scale) and flare diameters from 39.0 +/- 2.0 mm to 30.2 +/- 1.8 mm; wheal reactions were not affected. A similar reduction in itch was found under menthol (42 +/- 14 units) although skin temperature was not decreased. These findings suggest a central inhibitory effect of cold sensitive A-delta fibre activation on itch.

Effects of three fragments of parathyroid hormone on calcium channel currents in neonatal rat ventricular cells

The effects of different fragments of bovine parathyroid hormone, bPTH-(1-34), bPTH-(1-84) and bPTH-(3-34), on two types of calcium channel currents in neonatal rat ventricular cells were compared in the present study. bPTH-(1-34) increased the amplitude of L channel currents, but not of the T channel currents. This effect of bPTH-(1-34) was sustained after a complete washout of the peptide from the bath. The intact PTH molecule, bPTH-(1-84), also increased L channel currents but not affecting T channel currents. While bPTH-(3-34) did not affect the amplitudes of either L or T channel currents by itself, pretreatment of cells with bPTH-(3-34) abolished the effects of both bPTH-(1-34) and bPTH-(1-84) on L channel currents. Moreover, the kinetics of L channel currents in the presence of bPTH-(1-34) or bPTH-(3-34) were different. bPTH-(1-34) increased the time constant of activation, but not of inactivation, of L channel currents from 1.8 to 2.5 ms (P < 0.05). In contrast, bPTH-(3-34) decreased the time constant of inactivation, but not of activation, of L channel currents from 159 to 117 ms (P < 0.05). These results indicate that different fragments of PTH exert different effects on the amplitudes or kinetics of cardiac calcium channel currents.

omega-Conotoxin reduces facilitation of transmitter release at the frog neuromuscular junction

We have examined the effects of the peptide toxin omega-conotoxin GVIA (omega-CgTx), a known calcium channel blocker, on stimulation-induced changes in end-plate potential (EPP) amplitude at the frog neuromuscular junction. We found that the addition of this toxin in submicromolar concentrations reduced both the control EPP amplitude and the increase in EPP amplitude that normally occurs during repetitive stimulation under low quantal conditions. These effects of omega-CgTx developed slowly following its addition to the bathing solution, were concentration-dependent and were essentially irreversible. The effects of omega-CgTx appeared to result from reductions in the facilitation and augmentation components of stimulation-induced increases in release. While the effects of omega-CgTx on EPP amplitude could be reversed by increasing the extracellular concentration of Ca2+, we were unable to reverse the effects of the toxin on stimulation-induced increases in EPP amplitude. Thus it appears that omega-CgTx has a dual effect on neuromuscular transmission, perhaps by acting at two different presynaptic sites.

Calciseptine, a peptide isolated from black mamba venom, is a specific blocker of the L-type calcium channel

The venom of the black mamba contains a 60-amino acid peptide called calciseptine. The peptide has been fully sequenced. It is a smooth muscle relaxant and an inhibitor of cardiac contractions. Its physiological action resembles that of drugs, such as the 1,4-dihydropyridines, which are important in the treatment of cardiovascular diseases. Calciseptine, like the 1,4-dihydropyridines, selectively blocks L-type Ca2+ channels and is totally inactive on other voltage-dependent Ca2+ channels such as N-type and T-type channels. To our knowledge, it is the only natural polypeptide that has been shown to be a specific inhibitor of L-type Ca2+ channels.

Na+ currents through low-voltage-activated Ca2+ channels of chick sensory neurones: block by external Ca2+ and Mg2+

1. Whole-cell currents through low-voltage-activated (LVA) Ca2+ channels carried by monovalent cations were studied in chick dorsal root ganglion (DRG) cells. 2. With 120 mM [Na+] on both sides of the membrane and [Ca2+]o less than or equal to 100 microM, the currents reversed at 0 mV. Their half-times of activation and inactivation were strictly voltage-dependent and decreased to near-constant values of 0.6-0.85 and 40 ms, respectively, at positive membrane potentials. The longer activation times were observed with [Ca2+]o greater than or equal to 50 microM. 3. The selectivity of the Ca2+ channel for monovalent ions with reference to internal Na+ was evaluated from the reversal potential. The Li+ and Na+ permeabilities were similar. The permeability ratios of K+ and Rb+ were 0.45, and 0.33 for Cs+. 4. Micromolar increases in [Ca2+]o produced small voltage shifts of half-times of activation (less than or equal to +3 mV at 10 microM and +10 mV at 500 microM), but strongly depressed the Na+ current. The Ca2(+)-induced block of Na+ current satisfied a 1:1 stoichiometry with an apparent KD of 1.8 microM at -20 mV. The block was, however, relieved with more positive and negative potentials, with KDs of 55 and 8.5 microM at +90 and -110 mV, respectively. 5. Relaxation time constants of block and unblock of Na+ currents through the LVA Ca2+ channel were measured on step changes to and from membrane potentials at which pronounced Ca2(+)-induced block occurred. 6. At -20 mV, the time constants of block decreased with micromolar increase in [Ca2+]o in line with a blocking rate coefficient of 1.9 x 10(8) M-1 s-1, but settled to values of 0.18 ms at [Ca2+]o beyond 50 microM. The Na+ currents were unblocked with time constant (tau u) of around 0.25 ms at strongly positive and negative membrane potentials at 22 degrees C. 7. Tau u failed to show any obvious dependence on [Ca2+]o up to the millimolar range. This finding contradicts suggestions that removal of the block occurs in a [Ca2+]o-dependent manner as a result of an increased probability of Ca2+ ion mobilization by repulsive forces with increased Ca2+ occupation of the channel. 8. The time course of unblock of Na+ currents was strongly temperature-dependent showing a Q10 of 2.5 for tau u. 9. The voltage dependence of the Na+ current block by Ca2+ ions is best accounted for by a single, centrally located Ca2+ binding site.(ABSTRACT TRUNCATED AT 400 WORDS)