Biophysical properties of scorpion alpha-toxin-sensitive background sodium channel contributing to the pacemaker activity in insect neurosecretory cells (DUM neurons).
A scorpion alpha-toxin-sensitive background sodium channel was characterized in short-term cultured adult cockroach dorsal unpaired median (DUM) neurons using the cell-attached patch-clamp configuration. Under control conditions, spontaneous sodium currents were recorded at different steady-state holding potentials, including the range of normal resting membrane potential. At -50 mV, the sodium current was observed as unclustered, single openings. For potentials more negative than -70 mV, investigated patches contained large unitary current steps appearing generally in bursts. These background channels were blocked by tetrodotoxin (TTX, 100 nm), and replacing sodium with TMA-Cl led to a complete loss of channel activity. The current-voltage relationship has a slope conductance of 36 pS. At -50 mV, the mean open time constant was 0.22 +/- 0.05 ms (n = 5). The curve of the open probability versus holding potentials was bell-shaped, with its maximum (0.008 +/- 0.004; n = 5) at -50 mV. LqhalphaIT (10-8 m) altered the background channel activity in a time-dependent manner. At -50 mV, the channel activity appeared in bursts. The linear current-voltage relationship of the LqhalphaIT-modified sodium current determined for the first three well-resolved open states gave three conductance levels: 34, 69 and 104 pS, and reversed at the same extrapolated reversal potential (+52 mV). LqhalphaIT increased the open probability but did not affect either the bell-shaped voltage dependence or the open time constant. Mammal toxin AaHII induced very similar effects on background sodium channels but at a concentration 100 x higher than LqhalphaIT. At 10-7 m, LqhalphaIT produced longer silence periods interrupted by bursts of increased channel activity. Whole-cell experiments suggested that background sodium channels can provide the depolarizing drive for DUM neurons essential to maintain beating pacemaker activity, and revealed that 10-7 m LqhalphaIT transformed a beating pacemaker activity into a rhythmic bursting.
Lapied
B
B
Laboratoire de Neurophysiologie, UPRES EA 2647, Université d Angers, France.Bruno.Lapied@univ-angers.fr
Stankiewicz
M
M
Grolleau
F
F
Rochat
H
H
Zlotkin
E
E
Pelhate
M
M
D000818:Animals; D001683:Biological Clocks; D055592:Biophysical Phenomena; D001703:Biophysics; D003058:Cockroaches; D017952:Ganglia, Invertebrate / Q000166:cytology / Q000187:drug effects*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects*; D009490:Neurosecretory Systems / Q000187:drug effects*; D009498:Neurotoxins / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D011336:Probability; D030162:Reptilian Proteins; D012604:Scorpion Venoms / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology
D009498:Neurotoxins; D030162:Reptilian Proteins; D012604:Scorpion Venoms; D015222:Sodium Channels; C053969:scorpion toxin II, Androctonus; D013779:Tetrodotoxin