We have shown previously that the inactivation of the zinc finger gene Krox-20 affects hindbrain segmentation, resulting in the elimination of rhombomeres 3 and 5. We demonstrate here that Krox-20 homozygous mutant mice exhibit abnormally slow respiratory and jaw opening rhythms, indicating that a modification of hindbrain segmentation influences the function of neuronal networks after birth. Central neuronal networks that control respiratory frequency are made predominantly depressant by the elimination of a previously undescribed rhythm-promoting system. Recordings of rhythmic activity from the isolated hindbrain following progressive tissue transections indicate that the reorganization takes place in the caudal pontine reticular formation. The newborn (PO) Krox-20-/- mice, in which apneas are ten times longer than in wild-type animals, may be a valuable model for the study of life-threatening apneas during early infancy.

Voltage-sensitive K+ channels were studied in rat cerebellar Purkinje neurons in culture using the single-channel recording technique. Recordings in the cell-attached and outside-out configuration revealed multiple voltage-sensitive K+ channel types in patches from both the somatic and the dendritic regions. K+ channel types were present in all patches studied. The same channel types were observed in somatic and dendritic recordings. Channel types were identified by reversal potential, single-channel conductance, voltage sensitivity, and patterns of activity. In cell-attached patches recorded under physiological conditions, 3 channel types were identified. Mean single-channel conductances were 92, 57, and 12 pS. All 3 channel types were activated by membrane depolarization. Similar channel types were identified in inside-out and outside-out patches recorded under physiological conditions. Two additional channel types were identified in the outside-out patches, with mean single-channel conductances of 41 and 26 pS. In cell-attached recordings under symmetrical K+ conditions, 6 channel types were identified. Mean single-channel conductances were 222, 134, 39, 25, 14, and 15 pS. Channel types with mean conductances of 222, 134, and 39 pS required membrane depolarization for activation. A comparison of channel properties indicated that these channel types correlated with the 3 channel types observed in cell-attached patches under physiological conditions. The 3 smaller-conductance channel types (25, 14, and 15 pS) were active at potentials around rest or at hyperpolarized membrane potentials. Two K+ channel types (39 and 25 pS) were commonly associated with the late phase of extracellularly recorded spontaneous spike events, suggesting a functional role in the repolarizing phase of somatic and dendritic action potentials. These results demonstrate that voltage-sensitive K+ channels are a prominent component of both the somatic and the dendritic membrane of the cerebellar Purkinje neuron and support the view that multiple voltage-sensitive K+ channel types contribute to the membrane functions of both cellular regions in this CNS neuronal type.

SUMMARY1. Respiratory neurones of mammals are rhythmically active because their membrane potential fluctuates periodically over a voltage range of -70 to -55 mV. These respiratory drive potentials lead to periodic discharges of bursts of action potentials lasting for 1-2 s. The neuronal processes stabilizing this rhythmic activity involve excitatory and inhibitory synaptic processes that interact with specific membrane properties of the postsynaptic neurones. In the present experiments, performed on dorsal and ventral groups of respiratory neurones under in vivo and in vitro conditions, we verified the modulating feature of such intrinsic neuronal properties.2. Intrinsic neuronal properties involve Ca2" mechanisms that lead to intracellular Ca2' accumulation, and consequently to activation of Ca2+-dependent K+ currents.3. Blockade of intracellular Ca2+ accumulation significantly changed the amplitude and pattern of respiratory drive potentials, and blocked initial hyperpolarizing shifts of the membrane potential following each period of synaptic activation.4. The data demonstrate that postsynaptic activities and action potential discharges activatelow andhighvoltage-activatedCa2+ currentsleadingto intracellular Ca2+ accumulation and to activation of Ca2+-dependent K+ currents that significantly modulate the voltage response of medullary respiratory neurones to on-going synaptic activation. These intrinsic membrane properties also seem to be involved in the processes controlling termination of rhythmic burst discharges.

Neurones within the ventral and ventrolateral nuclei of the solitary tract were analyzed under single-electrode current- and voltage-clamp conditions in rat brainstem slices. We present direct and indirect evidence for the existence of five different sorts of membrane currents: a tetrodotoxin-sensitive sodium current, a tetrodotoxin-resistant calcium current, a calcium-dependent potassium current, a non-inactivating potassium current which is inhibited by muscarine, an inactivating potassium current, which is inhibited by 4-aminopyridine. These membrane properties do not produce spontaneous bursting in these neurones. Assuming that neurones with such properties belong to the respiratory network, we discuss how conductances of this type may be involved in mechanisms regulating central respiratory activity.