Under in vivo conditions, periodic burst discharges of medullary respirator
y neurons of mature cat typically start with a rebound depolarization when
inhibition through antagonistic neurons stops. This rebound can be blocked
by ionophoretically applied extracellular Cd2+. A similar Cd2+-sensitive re
bound depolarization is triggered by hyperpolarizing current pulses even in
the presence of extracellular tetrodotoxin (TTX) and tetraethylammonium (T
EA). In current-clamp mode, the current/voltage (I/V) curves rectify outwar
dly at positive voltages, and this rectification is blocked by Cd2+. Intrac
ellular injection of the L-type Ca2+-channel blocker methoxy-verapamil chan
ges the spontaneous activity patterns of neurons. In those neurons that typ
ically show augmenting patterns, the membrane depolarization is slowed down
, while in those neurons that have a declining pattern, voltage changes bec
ome augmenting. Voltage-clamp measurements reveal a transient, low-voltage-
activated T-type Ca2+ current. The current is deinactivated at -100 mV and
almost completely inactivated at -60 mV. Depolarizing voltage commands star
ting from more positive holding potentials evoke sustained Ca2+ currents th
at reach a maximum at 0 mV. The sustained L-type Ca2+ currents are complete
ly blocked by extracellular Cd2+. We conclude that low- and high-voltage-ac
tivated Ca2+ currents are expressed in all types of respiratory neurons and
play an essential role in rhythm generation and pattern formation in adult
cats in vivo.