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The squid Giant Synapse

Anatomy

This chemical synaptic junction was first recognized by J.Z.Young in 1939. The giant synapse (Fig 1) lies in the stellate ganglion on each side of the midline at the posterior wall of the squid’s muscular mantel. Activation of this synapse triggers the synchronous contraction of the mantle musculature that is responsible for the forceful ejection of a jet stream of water from the mantle. This water propulsion allows the squid to move rapidly through the water and even to jump through the surface of the water (breaking the air water barrier) to escape predators. The information to the mantle is transmitted via a chain consisting of three giant neuronal elements organized in sequence. The first is located in the ventral magnocellular lobe, central to the eyes. It serves as a central integrating manifold that receives all sensory information and consists of two symmetrical neurons (I). They, in turn, contact secondary neurons (one in each side) in the dorsal magnocellular lobe and (II) and in turn contact the tertiary giant axons in the stellate ganglion (III, one in each side of the mantle). These latter are the giant axons that the work of Alan Hodgkin and Andrew Huxley made famous. Each secondary axon branches at the stellate ganglion and contacts all the tertiary axons. Information concerning relevant sensory input is thus relay from the sense organs in the cephalic ganglion (the squid’s brain) to the contractile muscular mantle activated directly by the tertiary giant axons.

Fig 1 Upper left, side view of a squid. Upper right, the area inside the black square is enlarged in the diagram below, showing the giant neuronal system with the first (red) second (green) and third giant neuronal elements (brown). The arrows indicate the direction of transmission flow from the head ganglion towards the mantle. The light blue funnel is the site for water flow following rapid water expulsion when the mantel contracts. (modified from Lllinas 1999).

Electrophysiology

Many essential elements of how all chemical synapses function were first discovered by studying the squid giant synapse. Early electrophysiological studies demonstrated the chemical nature of transmission at this synanpse by making simultaneous intracellular recording from the presynaptic and postsynaptic terminals in vitro (Bullock and Hagiwara (1957), Hagiwara and Tasaki (1958), Takeuchi and Takeuchi (1962). Classical experiments later on demonstrated that, in the absence of action potentials, transmission could occur (Bloedel et al (1966), Katz and Miledi (1967), Kusano et al (1967). The calcium hypothesis for synaptic transmission was directly demonstrated in this synapse by showing that at the equilibrium potential for calcium (+60 mV, calcium does not enter the presynaptic terminal) no transmitter is released. Thus, calcium entry and not the change in the transmembrane electric field is responsible for transmitter release Katz and Miledi (1967). A direct measure of the transmembrane current was first determined in the mid nineteen seventies (Llinas et al 1976 a&b) by direct presynaptic voltage clamp studies that determined the relationship between the time course and amplitude of the calcium current and the amount of transmitter release directly determined (Llinas et al 1981, Augustine et al 1985) Fig 2. This preparation continues to be the most useful for the study of the molecular and cell biological basis for transmitter release. Other important new mammalian preparations are not available for such studies such as the Calyx of Held.

Fig 2. Upper image: Enlarged picture of the squid stellate ganglion showing the giant synapse. Intracellular dye injection was used to stain the presynaptic axon green and the postsynaptic axon red. The presynpatic fiber has seven branches, each for one giant tertiary axon. Only the last postsynaptic axon on the right is colored. Middle image: A) Simultaneous intracellular recording from the presynaptic fiber (pre) and the postsynaptic axon (post). The synaptic action potential releases a transmitter substance (glutamate) that acts on the postsynaptic receptors and activates the postsynaptic action potential. B & C) Synaptic transmission can be evoked with either a square voltage pulse (B) or an artificial action potential wave form (C) these are delivered to a command amplifier as shown in D. D. Diagram of a command amplifier (CO) and current injection amplifier (I) with a feedback control via presynaptic voltage (Pre V). The response to these stimuli is recorded as a current (Im) and displayed in green in E and F. E) Synaptic transmission and calcium current (ICa, green) evoked by a square voltage pulse (Pre). F) Calcium current (green) and postsynaptic potential (Post) evoked by an artificial action potential (Pre). Note that in F the calcium curren starts during the down swing of the presynaptic action potential. (modified from Llinas 1999) Lower Image: Left, Voltage clamp records illustrating the relation between transmembrane voltage in mV (square wave at the bottom of each record) Calcium current amplitude in nA (middle record) and postsynaptic potential in mV. Time mark one ms. The voltage steps are generates from a holding potential of -170mV. (Llinas et al 1981). Right image: Relation between voltage and current for the “on” (red plot) and “tail” (white plot) calcium current. Voltage in mV current in nA. (modified from Llinas 1999)

References

• Young J.Z. (1939) “Fused neurons and synaptic contacts in the giant nerve fibers of cephalopods” Phil. Trans. Roy. Soc 229:465-503
• Bullock TH and Hagiwara S. (1963) “Intracellular recording from the giant synapse of the squid” J. Gen. Physiol. 40:565-577
• Hagiwara S. and Tasaki I. (1958) “A study on the mechanism of impulse transmission across the giant synapse of the squid “ J. Physiol.. 143:114-137
• Takeuchi A, and Takeuchi N. (1962) “Electrical changes in the pre and postsynaptic axons of the giant synapse of Loligo” J. Gen Physiol. 45:1181-1193
• Bloedel JR., Gage, PWQ., Llinas R. and Quastel DMJ. (1966)“Transmitter release at the squid giant synapse in the presence of tetrodotoxin” Nature, 212:49-50
• Katz B and Miledi R.(1967) “A study of synaptic transmission in the absence of nerve impulses” J.Physiol 192:407-436
• Kusano K. Livengood, DR and Werman R. (1967) “ Correlation of transmitter release with membrane properties of the presynaptic fiber of the squid giant synapse” J. Gen. Physiol 50:2579-2601
• Llinas R, Steinberg IZ. and Walton K. (1976) “Presynaptic calcium currents and their relation to synaptic transmission. Voltage clamp study in the squid giant synapse and theoretical model for the calcium gate”PNAS 73:2918-2922
• Llinas R, Steinberg IZ. and Walton K.(1981) (a) “Presynaptic calcium currents in the squid giant synapse. Biophys J. 33: 289-321
• Llinas R, Steinberg IZ. and Walton K.(1981) (b) “Relationship between presynaptic calcium current and postsynaptic potential in squid giant synapse” Biophys J. 33: 323-352.
• Augustine GJ, Charlton MP, Smith SJ (1985) “ Calcium entry into voltage clamped presynaptic terminals of squid” J.Physiol 367:163-181
• Llinas R. (1999) “The squid Giant Synapse” Oxford U. Press

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