Retinal waves are governed by collective network properties

The information content of spontaneous retinal waves

Spontaneous neural activity that is present in the mammalian retina before the onset of vision is required for the refinement of retinotopy in the lateral geniculate nucleus and superior colliculus. This paper explores the information content of this retinal activity, with the goal of determining constraints on the nature of the developmental mechanisms that use it. Through information-theoretic analysis of multielectrode and calcium-imaging experiments, we show that the spontaneous retinal activity present early in development provides information about the relative positions of retinal ganglion cells and can, in principle, be used at retinogeniculate and retinocollicular synapses to refine retinotopy. Remarkably, we find that most retinotopic information provided by retinal waves exists on relatively coarse time scales, suggesting that developmental mechanisms must be sensitive to timing differences from 100 msec up to 2 sec to make optimal use of it. In fact, a simple Hebbian-type learning rule with a correlation window on the order of seconds is able to extract the bulk of the available information. These findings are consistent with bursts of action potentials (rather than single spikes) being the unit of information used during development and suggest new experimental approaches for studying developmental plasticity of the retinogeniculate and retinocollicular synapses. More generally, these results demonstrate how the properties of neuronal systems can be inferred from the statistics of their input.

Initiation and propagation of calcium-dependent action potentials in a coupled network of olfactory interneurons

Coherent oscillatory electrical activity and apical-basal wave propagation have been described previously in the procerebral (PC) lobe, an olfactory center of the terrestrial slug Limax maximus. In this study, we investigate the physiological basis of oscillatory activity and wave propagation in the PC lobe. Calcium green dextran was locally deposited in the PC lobe; this led to cellular uptake and transport of dye by bursting and nonbursting neurons of the PC lobe. The change of intracellular calcium concentration was measured at several different positions in neurites of individual bursting neurons in the PC lobe with a two-photon laser-scanning microscope. Fluorescence measurements were also made from neurons intracellularly injected with calcium green-1. Two different morphological classes of bursting neurons were found, varicose (VB) and smooth (SB). Our results from concurrent optical and intracellular recordings suggest that Ca2+ is the major carrier for the inward current during action potentials of bursting neurons. Intracellular recordings from bursting neurons with nystatin perforated-patch electrodes made while simultaneously recording the local field potential (LFP) with extracellular electrodes indicate that the burster spikes are precisely phase-locked to the periodic LFP events. By referencing successive calcium measurements to the common LFP signal, we could therefore accurately determine the relative timing of calcium transients at different points along a neurite. Measuring the relation of temporal to spatial differences allowed us to estimate the velocity of action potential propagation, which was 4.3 +/- 0.2 (SE) mm/s in VBs, and 1.3 +/- 0.2 mm/s in SB.

Modeling of spontaneous activity in developing spinal cord using activity-dependent depression in an excitatory network

Spontaneous episodic activity is a general feature of developing neural networks. In the chick spinal cord, the activity comprises episodes of rhythmic discharge (duration 5-90 sec; cycle rate 0.1-2 Hz) that recur every 2-30 min. The activity does not depend on specialized connectivity or intrinsic bursting neurons and is generated by a network of functionally excitatory connections. Here, we develop an idealized, qualitative model of a homogeneous, excitatory recurrent network that could account for the multiple time-scale spontaneous activity in the embryonic chick spinal cord. We show that cycling can arise from the interplay between excitatory connectivity and fast synaptic depression. The slow episodic behavior is attributable to a slow activity-dependent network depression that is modeled either as a modulation of cellular excitability or as synaptic depression. Although the two descriptions share many features, the model with a slow synaptic depression accounts better for the experimental observations during blockade of excitatory synapses.

Neurotransmitters and gap junctions in developing neural circuits

A growing body of evidence suggests that highly correlated, spontaneous neural activity plays an important role in shaping connections in the developing nervous system prior to the maturation of sensory afferents. In this article we discuss the mechanisms involved in the generation and the regulation of spontaneous activity patterns in the developing retina and the developing neocortex. Spontaneous activity in the developing retina propagates across the ganglion cell layer as waves of action potentials and drives rhythmic increases in intracellular calcium in retinal neurons. Retinal waves are mediated by a combination of chemical synaptic transmission and gap junctions, and the circuitry responsible for generating retinal waves changes with age and between species. In the developing cortex, spontaneous calcium elevations propagate across clusters of cortical neurons called domains. Cortical domains are generated by a regenerative mechanism involving second messenger diffusion through gap junctions and subsequent calcium release from internal stores. The neocortical gap junction system is regulated by glutamate-triggered second messenger systems as well as neuromodulatory transmitters, suggesting extensive interactions between synaptic transmission and information flow through gap junctions. The interaction between gap junctions and chemical synaptic transmission observed in these developing networks represent a powerful mechanism by which activity across large groups of neurons can be correlated.

Developmental changes in the neurotransmitter regulation of correlated spontaneous retinal activity

Synchronized spontaneous rhythmic activity is a feature common to many parts of the developing nervous system. In the early visual system, before vision, developing circuits in the retina generate synchronized patterns of bursting activity that contain information useful for patterning connections between retinal ganglion cells and their central targets. However, how developing retinal circuits generate and regulate these spontaneous activity patterns is still incompletely understood. Here we show that in developing retinal circuits, the nature of excitatory neurotransmission driving correlated bursting activity in ganglion cells is not fixed but undergoes a developmental shift from cholinergic to glutamatergic transmission. In addition, we show that this shift occurs as presynaptic glutamatergic bipolar cells form functional connections onto the ganglion cells, implicating the role of bipolar cells in providing endogenous drive to bursting activity later in development. This transition coincides with the period when subsets of ganglion cells (On and Off cells) develop distinct activity patterns that are thought to underlie the refinement of their connectivity with their central targets. Here, our results suggest that the differences in activity patterns of On and Off ganglion cells may be conferred by differential synaptic drive from On and Off bipolar cells, respectively. Taken together, our results suggest that the regulation of patterned spontaneous activity by neurotransmitters undergoes systematic change as new cellular elements are added to developing circuits and also that these new elements can help specify distinct activity patterns appropriate for shaping connectivity patterns at later ages.

Dynamics of retinal waves are controlled by cyclic AMP

Waves of spontaneous activity sweep across the developing mammalian retina and influence the pattern of central connections made by ganglion cell axons. These waves are driven by synaptic input from amacrine cells. We show that cholinergic synaptic transmission during waves is not blocked by TTX, indicating that release from starburst amacrine cells is independent of sodium action potentials. The spatiotemporal properties of the waves are regulated by endogenous release of adenosine, which sets intracellular cAMP levels through activation of A2 receptors present on developing amacrine and ganglion cells. Increasing cAMP levels increase the size, speed, and frequency of the waves. Conversely, inhibiting adenylate cyclase or PKA prevents wave activity. Together, these results imply a novel mechanism in which levels of cAMP within an immature retinal circuit regulate the precise spatial and temporal patterns of spontaneous neural activity.