Scorpion toxins specific for Na+ and K+ channels, have been studied extensively but relatively little has been done on Ca2+ channel toxins. Toxins in the venom of only two South African scorpions P. transvaalicus and P. granulatus have been found to interact with Ca2+ channels. Kurtoxin isolated from the venom of P. transvaalicus inhibits the T and L-type neuronal Ca2+ channels, whereas KLI and KLII (Kurtoxin-like peptide I and II), isolated from P. granulatus, inhibits T-type Ca2+ channel activity in mouse male germ cells. In this study the effects of fractions isolated from the venom of P. granularus on Cca2+ channels in rat ventricular myocytes were investigated by means of the whole-cell patch clamp technique. Fractions of P. granulatus crude venom were isolated with Sephadex G50 columns (fraction I-IV). Fraction III (PgIII) showed a voltage dependent increase of the inward Ca2+ current and influenced the channel kinetics by shifting the voltage dependence of activation towards more hyperpolarizing membrane potentials and decreased the rate of inactivation and deactivation. The time of the current to reach peak was also delayed. PgIII was further separated by HPLC in an attempt to identify the subfraction/s responsible for the agonistic effect. Subfraction I had an agonistic effect similar to PgIII, whereas subfraction II and III, decreased the Ca2+ current. The observed agonistic effect has not been described in the literature. The identification of new peptide structures with unique functions are important in the field of toxin research. Peptides that target Ca2+ channels can be valuable tools to characterize Ca2+ channels. Ca2+ channels in the heart are implicated in a number of pathological disorders like angina, ischemia, some arrhythmias and hypertension. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2005.

Scorpion toxins specific for Na+ and K+ channels, have been studied extensively but relatively little has been done on Ca2+ channel toxins. Toxins in the venom of only two South African scorpions P. transvaalicus and P. granulatus have been found to interact with Ca2+ channels. Kurtoxin isolated from the venom of P. transvaalicus inhibits the T and L-type neuronal Ca2+ channels, whereas KLI and KLII (Kurtoxin-like peptide I and II), isolated from P. granulatus, inhibits T-type Ca2+ channel activity in mouse male germ cells. In this study the effects of fractions isolated from the venom of P. granularus on Cca2+ channels in rat ventricular myocytes were investigated by means of the whole-cell patch clamp technique. Fractions of P. granulatus crude venom were isolated with Sephadex G50 columns (fraction I-IV). Fraction III (PgIII) showed a voltage dependent increase of the inward Ca2+ current and influenced the channel kinetics by shifting the voltage dependence of activation towards more hyperpolarizing membrane potentials and decreased the rate of inactivation and deactivation. The time of the current to reach peak was also delayed. PgIII was further separated by HPLC in an attempt to identify the subfraction/s responsible for the agonistic effect. Subfraction I had an agonistic effect similar to PgIII, whereas subfraction II and III, decreased the Ca2+ current. The observed agonistic effect has not been described in the literature. The identification of new peptide structures with unique functions are important in the field of toxin research. Peptides that target Ca2+ channels can be valuable tools to characterize Ca2+ channels. Ca2+ channels in the heart are implicated in a number of pathological disorders like angina, ischemia, some arrhythmias and hypertension. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2005.

We study the activity-dependent refinement of sensory systems by using the mouse retinogeniculate system as a model. Spontaneous retinal waves lead to robust excitatory post-synaptic activity in developing relay cells in the dorsal lateral geniculate nucleus (dLGN) of the thalamus and are reportedly needed to help guide the segregation of retinal inputs into eye-specific domains as well as for the pruning of extraneous retinal inputs onto single dLGN relay cells. The composition of retinally evoked post-synaptic activity activated by these retinal waves in dLGN is largely unknown, but based on our in vitro recordings, such activity seems well suited to activate large, long-lasting, high-amplitude depolarizations mediated by L-type Ca2+ channel activation, plateau potentials. Plateau activity prevails early in life, at the peak of retinogeniculate refinement, however, little is known about the factors that contribute to the activation of these events, or the potential role of plateau potentials in mediating activity-dependent remodeling. In this thesis, we examined the factors and stimulus conditions that lead to the activation of plateau activity. We found that many aspects of developing retinogeniculatecircuitry (e.g., the high degree of retinal convergence, the temporal summation of excitatory post-synaptic potentials, and the lack of inhibitory connections) seem to favor their activation at early postnatal ages. We then tested whether such activity is necessary for the refinement of retinal projections, as well as their functional connections onto dLGN cells. To address this, we took a loss-of-function approach and made use of a transgenic mouse that lacks the β3 subunit of the L-type Ca2+ channel. These mutants have far fewer membrane-bound L-type Ca2+ channels and greatly attenuated L-type activity. In β3 nulls, L-type plateau potentials are rarely observed in the dLGN, even at young ages or when repetitive pulses of electrical stimulation are applied to the optic tract. Although these mice have normal stage II and III spontaneous retinal waves, the retinogeniculate projections of β3 null mice fail to segregate properly. In addition, the degree of retinal pruning is impaired. These results suggest that post-synaptic L-type Ca2+ channel activity is necessary to implement the activity-dependent refinement of the retinogeniculate pathway.

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