Before vision, retinal ganglion cells produce spontaneous waves of action potentials. A crucial question is whether this spontaneous activity is transmitted to lateral geniculate nucleus (LGN) neurons. Using a novel in vitro preparation, we report that LGN neurons receive periodic barrages of postsynaptic currents from the retina that drive them to fire bursts of action potentials. Groups of LGN neurons are highly correlated in their firing. Experiments in wild-type and NMDAR1 knockout mice show that NMDA receptor activation is not necessary for firing. The transmission of the highly correlated retinal activity to the LGN supports the hypothesis that retinal waves drive retinogeniculate synaptic remodeling. Because LGN neurons are driven to fire action potentials, this spontaneous activity could also act more centrally to influence synaptic modification within the developing visual cortex.

SUMMARY1. The properties of chemoreceptor and baroreceptor neurones in the petrosal ganglion of the cat were examined in vitro with intracellular micro-electrodes.2. Chemoreceptor neurones with myelinated axons (average conduction velocity, 11 m/s) showed action potentials with a hump on the falling phase, followed by a prolonged after-hyperpolarization (average duration, 260 ms).3. The duration of the hump present in the action potential of chemoreceptor neurones was positively correlated with the duration of the after-hyperpolarization.4. In response to prolonged depolarization, chemoreceptor neurones showed only one or a few action potentials at the beginning of the depolarization.5. Two types of baroreceptors neurones with myelinated axons were found: fast (F) baroreceptors (average conduction velocity, 33 m/s) and slow (S) baroreceptors (average conduction velocity, 10 m/s).6. F baroreceptors had action potentials without a hump followed by a short after-hyperpolarization (average duration, 43 ms), while S baroreceptors had spikes similar to those found in chemoreceptors except for a shorter hyperpolarization (average duration, 145 ms).7. Both types of baroreceptor neurones fired repetitively throughout prolonged depolarization.8. It is concluded that, in the petrosal ganglion, primary sensory neurones orginating a given type of sensory terminal share a particular set of electrophysiological properties.

Repetitive stimulation of synaptic connections in the cerebral cortex often induces short-term synaptic depression (STD), a property directly related to the probability of transmitter release and critical for the computational properties of the network. In order to explore how spontaneous activity in the network affects this property, we first studied STD in cortical slices that were either silent or that displayed spontaneous rhythmic slow oscillations resembling those recorded during slow wave sleep in vivo. STD was considerably reduced by the occurrence of spontaneous rhythmic activity in the cortical network. Once the rhythmic activity started, depression decreased over time in parallel with the duration and intensity of the ongoing activity until a plateau was reached. Thalamocortical and intracortical synaptic potentials studied in vivo also showed stronger depression in a silent than in an active cortical network, and the depression values in the active cortical network in vivo were indistinguishable from those found in active slices in vitro. We suggest that this phenomenon is due to the different steady states of the synapses in active and in silent networks.

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