Sequential Development of Electrical and Chemical Synaptic Connections Generates a Specific Behavioral Circuit in the Leech

Abstract

Neuronal circuits form during embryonic life, even before synapses are completely mature. Developmental changes can be quantitative (e.g., connections become stronger and more reliable) or qualitative (e.g., synapses form, are lost, or switch from electrical to chemical or from excitatory to inhibitory). To explore how these synaptic events contribute to behavioral circuits, we have studied the formation of a circuit that produces local bending (LB) behavior in leech embryos. This circuit is composed of three layers of neurons: mechanosensory neurons, interneurons, and motor neurons. The only inhibition in this circuit is in the motor neuron layer; it allows the animal to contract on one side while relaxing the opposite side. LB develops in two stages: initially touching the body wall causes circumferential indentation (CI), an embryonic behavior in which contraction takes place around the whole perimeter of the segment touched; one or 2 d later, the same touch elicits adult-like LB. Application of bicuculline methiodide in embryos capable of LB switched the behavior back into CI, indicating that the development of GABAergic connections turns CI into LB. Using voltage-sensitive dyes and electrophysiological recordings, we found that electrical synapses were present early and produced CI. Inhibition appeared later, shaping the circuit that was already connected by electrical synapses and producing the adult behavior, LB.

From synapses to behavior: Development of a sensory‐motor circuit in the leech

“…Our experiments revealed that the circuit is formed first by electrical synapses at the stage when touching an embryo generates CI (Eisenhart, 2000;Marin-Burgin et al, 2005. Chemical synapses, including inhibitory connections, appear later in development, modifying this circuit that initially includes only electrical connections.…”

“…There are, however, a few animals in which specific neuronal circuits are well understood at the level of each participating identified neuron and its connections (Marder and Calabrese, 1996;Kristan et al, 2005;Marder et al, 2005;Katz, 2007), and some laboratories are beginning to look closely at how those circuits are established during development (Casasnovas and Meyrand, 1995;Fenelon et al, 1999;Le Feuvre et al, 1999;Kristan et al, 2000;Marin-Burgin et al, 2005.…”

“…Initially, CI appears and disappears spontaneously, but *1 day later it can be evoked by moderate touch to the skin (Reynolds et al, 1998;Marin-Burgin et al, 2006). Over the course of two more days of development, CI is gradually replaced by LB (Reynolds et al, 1998;Marin-Burgin et al, 2005), which persists into adulthood. LB is evoked by the same mechanical stimulation as CI, but unlike CI it is directed [ Fig.…”

Transient electrical coupling regulates formation of neuronal networks

“…It is becoming clear that interplay between electrical and chemical modes of neural communication have important impacts on neural network formation. The formation of specific neural circuits in leech embryos involves the development of inhibitory, GABAergic connections that shape pre-existing electrical synaptic connections (Marin-Burgin et al, 2005;Marin-Burgin et al, 2006). Interestingly, most electrical synapses in the juvenile neocortex on mammals couple inhibitory networks of GABA-ergic interneurons (Galarreta and Hestrin, 1999;Gibson et al, 1999;Hestrin and Galarreta, 2005).…”

In vivo imaging of growth cone and filopodial dynamics: Evidence for contact‐mediated retraction of filopodia leading to the tiling of sibling processes

“…Axonal pathfinding and growth cone-cell/substrate interactions have been studied extensively in the embryonic leech because its large, identified cells allow for intracellular recordings and injections with dye, even in early embryos (Kuwada, 1985;Wang and Macagno, 1997;Baker at al., 2003;Marin-Burgin et al, 2005). One uniquely accessible peripheral cell in the embryo is the comb cell (CC).…”