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The Mauthner Cell Half a Century Later: A Neurobiological Model for Decision-Making?

2005; Cell Press; Volume: 47; Issue: 1 Linguagem: Inglês

10.1016/j.neuron.2005.05.019

1097-4199

Henri Korn, Donald S. Faber,

Neural dynamics and brain function

The Mauthner (M) cell is a critical element in a vital escape "reflex" triggered by abrupt or threatening events. Its properties at the molecular and synaptic levels, their various forms of plasticity, and the design of its networks, are all well adapted for this survival function. They guarantee that this behavior is appropriately unilateral, variable, and unpredictable. The M cell sets the behavioral threshold, and, acting in concert with other elements of the brainstem escape network, determines when, where, and how the escape is executed. The Mauthner (M) cell is a critical element in a vital escape "reflex" triggered by abrupt or threatening events. Its properties at the molecular and synaptic levels, their various forms of plasticity, and the design of its networks, are all well adapted for this survival function. They guarantee that this behavior is appropriately unilateral, variable, and unpredictable. The M cell sets the behavioral threshold, and, acting in concert with other elements of the brainstem escape network, determines when, where, and how the escape is executed. The Mauthner (M) cell is a critical element in a vital escape "reflex" that can be triggered by abrupt or threatening events, and this neuron determines whether or not there will be a response. The modern era of M cell studies began about 50 years ago. The initial tone was set on electrophysiological (Furshpan and Furukawa, 1962Furshpan E.J. Furukawa T. Intracellular and extracellular responses of several regions of the Mauthner cell of the goldfish.J. Neurophysiol. 1962; 25: 732-771PubMed Google Scholar) and behavioral (Wilson, 1959Wilson D.M. Function of giant Mauthner's neurons in the lungfish.Science. 1959; 129: 841-842Crossref PubMed Google Scholar) grounds. Since then, studies of this neuron and its networks have often opened new directions for work in the more popular model systems of today. This privileged role derives from its morphological and electrophysiological identifiability in fish and amphibia, particularly teleosts, and from the fact that most of this research has been carried out with in vivo preparations. The many investigations dealing with this complex system made it possible to gradually reconstruct the wiring diagram of the underlying neuronal networks (Figure 1) and to appreciate their functional properties, including their remarkable plasticity and adaptability to a continuously varying environment. Thus, even though a large number of studies were concerned with basic aspects of synaptic transmission and excitability, they now converge on higher-order issues related to the mechanisms and information processing, or decision-making operations involved in the choice of a behavior and its subsequent execution. Several structural features made the M cell ideal for morphological studies (Cajal, 1908Cajal S.R Sur un noyau spécial du nerf vestibulaire des poissons et des oiseaux.Trab. Lab. Invest. Biol. Univ. Madrid. 1908; 6: 1-20Google Scholar, Bodian, 1937Bodian D. The structure of the vertebrate synapse. A study of the axon endings on Mauthner's cell and neighboring centers in the goldfish.J. Comp. Neurol. 1937; 68: 117-159Crossref Google Scholar; see Zottoli, 1978Zottoli S.J. Comparative morphology of the Mauthner cell in fish and amphibians.in: Faber D.S. Korn H. Neurobiology of the Mauthner Cell. Raven Press, New York1978: 13-45Google Scholar). These include its large size, limited number (two per individual), and a stereotyped gross morphology with two major dendrites, a large crossed axon that descends in the spinal cord, and an initial segment surrounded by a particularly dense neuropil called the axon cap. Authoritative descriptions of synaptic structure were obtained at light and electron microscopic levels, including the defining features of mixed electrical and chemical excitatory synapses and of various types of inhibitory terminals and the soma-dendritic distribution of their endings (Nakajima, 1974Nakajima Y. Fine structure of the synaptic endings on the Mauthner cell of the goldfish.J. Comp. Neurol. 1974; 156: 375-402Crossref Google Scholar). The M cell has also been a privileged model for developmental investigations (see Kimmel and Model, 1978Kimmel C.B. Model P.G. Developmental studies of the Mauthner cell.in: Faber D.S. Korn H. Neurobiology of the Mauthner Cell. Raven Press, New York1978: 183-217Google Scholar). Particular attention was paid to factors and cues influencing cellular determination (Detwiler, 1933Detwiler S.R. Further experiments upon the extirpation of Mauthner's neurones in amphibian embryos (Amblystoma mexicanum).J. Exp. Zool. 1933; 64: 415-431Crossref Google Scholar, Stefanelli, 1951Stefanelli A. The Mauthnerian apparatus in the Ichthyopsida: Its nature and function and correlated problems of neurohistogenesis.Q. Rev. Biol. 1951; 26: 17-34Crossref PubMed Google Scholar), guidance and orientation (Oppenheimer, 1942Oppenheimer J.M. The decussation of ectopic Mauthner fibers in fundulus embryos.J. Comp. Neurol. 1942; 77: 577-587Crossref Google Scholar, Swisher and Hibbard, 1967Swisher J.E. Hibbard E. The course of Mauthner axons in Janus-headed Xenopus embryos.J. Exp. Zool. 1967; 165: 433-440Crossref Google Scholar), and neuronal differentiation (Leghissa, 1941Leghissa S. Sviluppo dell'apparato del Mauthner in larve di Ambystoma mexicanum (Axolotl).Arch. Zool. Ital. 1941; 29: 213-253Google Scholar). For some of these experiments, prospective hindbrain regions were transplanted in the belly (Stefanelli, 1951Stefanelli A. The Mauthnerian apparatus in the Ichthyopsida: Its nature and function and correlated problems of neurohistogenesis.Q. Rev. Biol. 1951; 26: 17-34Crossref PubMed Google Scholar) or midbrain (Model, 1978Model P.G. Regulation of the Mauthner cell following unilateral rotation of the prospective hindbrain in axolotl (Ambystoma mexicanum) neurulae.Brain Res. 1978; 153: 135-143Crossref PubMed Scopus (1) Google Scholar, Swisher and Hibbard, 1967Swisher J.E. Hibbard E. The course of Mauthner axons in Janus-headed Xenopus embryos.J. Exp. Zool. 1967; 165: 433-440Crossref Google Scholar) of several embryonic species, thus serving as some of the earliest examples of grafts in the nervous system. Study of the M cell system has contributed to fundamental descriptions of the primary forms of communication between neurons that are conserved throughout metazoan phylogeny, particularly the basic properties of electrical and chemical interactions. Its accessibility for a wide range of experimental approaches, including simultaneous recordings from the presynaptic and postsynaptic sides of identified connections with intracellular staining, or from different regions of the M cell, stems from one striking feature. Specifically, when the M cell is activated, the extracellular currents associated with its action potential produce a negative field potential that can be as large as 20–40 mV close to the axon hillock. This discovery by Furshpan and Furukawa, 1962Furshpan E.J. Furukawa T. Intracellular and extracellular responses of several regions of the Mauthner cell of the goldfish.J. Neurophysiol. 1962; 25: 732-771PubMed Google Scholar signaled the beginning of the modern era of M cell research. Remarkably, these currents also underlie a class of neuronal interactions that still tend to be overlooked in other preparations, despite their potential functional relevance. Nonsynaptically mediated electrical inhibition was demonstrated beautifully by Furukawa and Furshpan, 1963Furukawa T. Furshpan E.J. Two inhibitory mechanisms in the Mauthner neurons of goldfish.J. Neurophysiol. 1963; 26: 140-176PubMed Google Scholar. They found that activation of the M cell's recurrent collateral network causes an immediate inhibition of this cell. This early inhibition is correlated with an extracellular positive field, called the extrinsic hyperpolarizing potential (EHP), in the central core of the axon cap. It is due to outward currents generated by nearby axons that flow inward across the M cell axon hillock and hyperpolarize it. The EHP and its effect on excitability are monophasic because the presynaptic action potentials fail to propagate actively to the axonal terminals. These presynaptic cells constitute a defined population of inhibitory interneurons with processes that terminate on the soma and proximal dendrites of the M cell (Faber and Korn, 1973Faber D.S. Korn H. A neuronal inhibition mediated electrically.Science. 1973; 179: 577-578Crossref PubMed Google Scholar, Korn and Faber, 1975Korn H. Faber D.S. An electrically mediated inhibition in goldfish medulla.J. Neurophysiol. 1975; 38: 452-471PubMed Google Scholar). Conversely, and also a consequence of the high resistance of the axon cap that channels current intracellularly, when the M cell fires, its field hyperpolarizes these interneurons, producing a passive hyperpolarizing potential (PHP). Discovery of the PHP was essential for a number of subsequent studies, because it allowed reliable identification of axons presynaptic to the M cell. Field effects, also known as ephaptic interactions (Faber and Korn, 1989Faber D.S. Korn H. Electrical field effects: Their relevance in central neural networks.Physiol. Rev. 1989; 69: 821-863PubMed Google Scholar), represent a powerful mechanism for synchronizing neuronal populations, such as cerebellar interneurons (Korn and Axelrad, 1980Korn H. Axelrad H. Electrical inhibition of Purkinje cells in the cerebellum of the rat.Proc. Natl. Acad. Sci. USA. 1980; 77: 6244-6247Crossref PubMed Google Scholar) and hippocampal pyramidal cells in mammals (Dudek et al., 1998Dudek F.E. Yasamura T. Rash J.E. Non-synaptic mechanisms in seizures and epileptogenesis.Cell Biol. Int. 1998; 22: 793-805Crossref PubMed Scopus (76) Google Scholar), including during seizures. Throughout the first half of the twentieth century, a controversy raged over whether synaptic transmission in the vertebrate central nervous system is electrical or chemical, as described in detail by Eccles, 1964Eccles J.C. The Physiology of Synapses. Springer-Verlag, Berlin-Göttingen-Heidelberg1964Crossref Google Scholar. It seemed that the issue was resolved in favor of the latter hypothesis with the advent of motoneuron intracellular recordings in the early 1950s. However, the question of electrical transmission reappeared shortly thereafter. The issue was provoked by electron microscopic evidence in a few systems (Bennett et al., 1963Bennett M.V. Aljure E. Nakajima Y. Pappas G.D. Electrotonic junctions between teleost spinal neurons: electrophysiology and ultrastructure.Science. 1963; 19: 262-264Crossref Google Scholar, Robertson, 1961Robertson J.D. Ultrastructure of excitable membranes and the crayfish median-giant synapse.Ann. N Y Acad. Sci. 1961; 6: 339-389Google Scholar) of what are now called gap junctions. A notable example is the gap junction between a large myelinated club ending of an eighth nerve afferent and the lateral dendrite of the M cell, which was interpreted by Robertson et al., 1963Robertson J.D. Bodenheimer T.S. Stage D.E. The ultrastructure of Mauthner cell synapses and nodes in goldfish brains.J. Cell Biol. 1963; 19: 159-199Crossref PubMed Google Scholar as being suggestive of electrical transmission. Separate intracellular recordings from the pre- and postsynaptic elements demonstrated the electrotonic flow of current in both directions across M cell gap junctions (Furshpan, 1964Furshpan E.J. "Electrical transmission" at an excitatory synapse in a vertebrate brain.Science. 1964; 144: 878-880Crossref PubMed Google Scholar). Furthermore, EM data suggested that these junctions are, in fact, mixed, i.e., they have both electrical and chemical transmission (Nakajima, 1974Nakajima Y. Fine structure of the synaptic endings on the Mauthner cell of the goldfish.J. Comp. Neurol. 1974; 156: 375-402Crossref Google Scholar). Since then, electrical coupling has been found in an increasing number of central structures, with morphologically mixed synapses in some (inferior olive, cortex, lateral vestibular nucleus, retina, and hippocampus; see Pereda et al., 2003Pereda A. O'Brien J. Nagy J.I. Bukauskas F. Davidson K.G. Kamasawa N. Yasumura T. Rash J.E. Connexin 35 mediates electrical transmission at mixed synapses on Mauthner cells.J. Neurosci. 2003; 23: 7489-7503PubMed Google Scholar). However, the combined morphological and electrophysiological accessibility of the M cell, and of its auditory afferents, allowed in-depth study of their properties. For example, such recordings proved the hypothesis of dual transmission at single terminals (Lin and Faber, 1988aLin J.-W. Faber D.S. Synaptic transmission mediated by single club endings on the goldfish Mauthner cell. I. Characteristics of electrotonic and chemical postsynaptic potentials.J. Neurosci. 1988; 8: 1302-1312PubMed Google Scholar) and demonstrated that electrical transmission is amplified by a subthreshold voltage-dependent sodium current in the presynaptic endings, a mechanism that could be important in cases of weak coupling between dendrites (Curti and Pereda, 2004Curti S. Pereda A. Voltage-dependent enhancement of electrical coupling by a subthreshold sodium current.J. Neurosci. 2004; 24: 3999-4010Crossref PubMed Scopus (16) Google Scholar). Recently, connexin35, the fish ortholog of the neuron-specific human and mouse connexin36, was localized to these junctions and to others in the goldfish brain, using a combination of confocal microscopy and freeze-fracture replica immunogold labeling (Pereda et al., 2003Pereda A. O'Brien J. Nagy J.I. Bukauskas F. Davidson K.G. Kamasawa N. Yasumura T. Rash J.E. Connexin 35 mediates electrical transmission at mixed synapses on Mauthner cells.J. Neurosci. 2003; 23: 7489-7503PubMed Google Scholar). A subunit of the NMDA glutamate receptor is in postsynaptic densities quite close to the gap junction plaques, providing a potential substrate for various forms of activity-dependent synaptic plasticity at these contacts (see below). Excitatory inputs diverge to the M cell and its inhibitory interneurons. Graded afferent stimulations revealed one of the operative rules postulated to link inhibition to excitation: the disynaptic inhibition produced by the bilateral feed-forward (or commissural) glycinergic pathway dominates for the weak strengths, and, only when it is saturated, can excitation bring the cell to threshold; (although the inhibition is disynaptic, it occurs without time lag relative to excitation, because all of the elements across the pathway have an electrical component). This parallel inhibitory pathway controls the effectiveness of the M cell's excitatory inputs. The output circuit includes a powerful Renshaw-like (or collateral) feedback loop, and these interneurons can also be activated by sensory afferents. The same basic design, i.e., convergence of feedforward and feedback inhibitory connections onto common targets, pertains to a number of central circuits in vertebrates, including those in the mammalian brain. The M cell is also a prototype for understanding targeting and integration of inputs to specific local postsynaptic domains. Visual and statoacoustic inputs to this neuron are segregated to separate dendrites. Furthermore, different components of the latter pathway (auditory and vestibular otoliths, lateral line) are localized to specific regions of the lateral dendrite (Faber and Korn, 1978Faber D.S. Korn H. Electrophysiology of the Mauthner cell: basic properties, synaptic mechanisms, and associated networks.in: Faber D.S. Korn H. Neurobiology of the Mauthner Cell. Raven Press, New York1978: 47-131Google Scholar). The chemical map of transmitter systems along this dendrite, determined with iontophoresis (Diamond, 1963Diamond J. Variation in the sensitivity to gamma-aminobutyric acid of different regions of the Mauthner neurone.Nature. 1963; 199: 773-775Crossref Scopus (1) Google Scholar, Diamond and Huxley, 1968Diamond J. Huxley The activation and distribution of GABA and L-glutamate receptors on goldfish Mauthner neurones: an analysis of dendritic remote inhibition.J. Physiol. 1968; 194: 669-723PubMed Google Scholar), pharmacology (Wolszon et al., 1997Wolszon L.R. Pereda A.E. Faber D.S. A fast synaptic potential mediated by NMDA and non-NMDA receptors.J. Neurophysiol. 1997; 78: 2693-2706PubMed Google Scholar), and immunocytochemistry (Sur et al., 1994Sur C. Korn H. Triller A. Colocalization of somatostatin with GABA or glutamate in distinct afferent terminals presynaptic to the Mauthner cell.J. Neurosci. 1994; 14: 576-589PubMed Google Scholar), also indicated regional specialization: AMPA, glycine, and GABA receptors are distributed throughout, while NMDA receptors and dopaminergic inputs are restricted to the distal dendritic region (with colocalization of somatostatin and GABA or glutamate in some terminals) (Sur et al., 1994Sur C. Korn H. Triller A. Colocalization of somatostatin with GABA or glutamate in distinct afferent terminals presynaptic to the Mauthner cell.J. Neurosci. 1994; 14: 576-589PubMed Google Scholar). Serotonergic inputs are excluded from the dendrite (Figure 2, see also Korn et al., 1990Korn H. Faber D.S. Triller A. Convergence of morphological physiological, and immunocytochemical techniques for the study of single Mauthner cells.in: Björkland A. Hökfelt T. Wouterlood F.G. Van den Pol A.N. Handbook of Chemical Neuroanatomy, Volume 8, Analysis of Neuronal Microcircuits and Synaptic Interactions. Elsevier, Amsterdam1990: 403-480Google Scholar). Diagram of the cell and of the distribution of its afferent synaptic endings, some of which are localized in discrete regions, particularly around the soma, the initial part of the axon, and the distal lateral dendrite. The cell is subdivided according to regions defined by the dominance of a given transmitter, rather than by common morphological boundaries. For each area, the relative weights of the indicated substances, are proportional to the character size (modified from Korn et al., 1990Korn H. Faber D.S. Triller A. Convergence of morphological physiological, and immunocytochemical techniques for the study of single Mauthner cells.in: Björkland A. Hökfelt T. Wouterlood F.G. Van den Pol A.N. Handbook of Chemical Neuroanatomy, Volume 8, Analysis of Neuronal Microcircuits and Synaptic Interactions. Elsevier, Amsterdam1990: 403-480Google Scholar, used with permission from Elsevier). (Inset) Junctions recognized by electron microscopy, which carry gap junctions (arrows), chemical, or mixed synapses. They are excitatory (LMCE, large myelinated club endings; MCE, small myelinated club endings; LVB, large vesicle boutons) or inhibitory (UCE, unmyelinated club endings; SVB, small vesicle boutons). Terminals of the thin fibers that spiral around the M axon (SF) are excitatory, according to Scott et al. (Scott et al., 1994Scott J.W. Zottoli S.J. Beatty N.P. Korn H. Origin and function of spiral fibers projecting to the goldfish Mauthner cell.J. Comp. Neurol. 1994; 339: 76-90Crossref PubMed Scopus (18) Google Scholar) (modified from Nakajima, 1974Nakajima Y. Fine structure of the synaptic endings on the Mauthner cell of the goldfish.J. Comp. Neurol. 1974; 156: 375-402Crossref Google Scholar, used with permission from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.). The ability to record intradendritically from the M cell and its small membrane time constant were essential for quantifying the strength of the two forms that synaptic inhibition can manifest (Fukami et al., 1965Fukami Y. Furukawa T. Asada Y. Excitability changes of the Mauthner cell during collateral inhibition.J. Gen. Physiol. 1965; 48: 581-600Crossref PubMed Google Scholar) depending upon the Cl− equilibrium potential (Furukawa and Furshpan, 1963Furukawa T. Furshpan E.J. Two inhibitory mechanisms in the Mauthner neurons of goldfish.J. Neurophysiol. 1963; 26: 140-176PubMed Google Scholar, Furukawa et al., 1963Furukawa T. Fukami Y. Asada Y. A third type of inhibition in the Mauthner cell of goldfish.J. Neurophysiol. 1963; 26: 759-774PubMed Google Scholar). That is, it can appear as a shunt of an excitatory input, as initially demonstrated in crayfish muscle (Fatt and Katz, 1953Fatt P. Katz B. The effect of inhibitory nerve impulses on a crustacean muscle fibre.J. Physiol. 1953; 121: 374-389PubMed Google Scholar), and as a frank and prolonged change in membrane potential. These properties also made it possible to establish the reality and specificity of dendritic, or "remote," inhibition (Diamond and Huxley, 1968Diamond J. Huxley The activation and distribution of GABA and L-glutamate receptors on goldfish Mauthner neurones: an analysis of dendritic remote inhibition.J. Physiol. 1968; 194: 669-723PubMed Google Scholar), as first postulated by Frank, 1959Frank K. Basic mechanisms of synaptic transmission in the central nervous system.I.R.E. Trans. Med. Electron. 1959; ME-6: 85-88Crossref Google Scholar and subsequently observed by Llinas and Terzuolo, 1965Llinas R. Terzuolo C.A. Mechanisms of supraspinal actions upon spinal cord activities.Reticular inhibitory mechanisms upon flexor motoneurons.J. Neurophysiol. 1965; 28: 413-422PubMed Google Scholar. Thus, in some cases inhibition can be purely shunting due to an increased conductance, and its effectiveness can be highly localized and restricted to a distance of ∼50 μm along the large primary lateral dendrite. This conclusion was validated with predictions based on an equivalent circuit model of the M cell implemented by Furukawa (Furukawa, 1966Furukawa T. Synaptic interaction at the Mauthner cell of goldfish.Prog. Brain Res. 1966; 21: 44-70Crossref PubMed Scopus (7) Google Scholar) and by Huxley (Diamond and Huxley, 1968Diamond J. Huxley The activation and distribution of GABA and L-glutamate receptors on goldfish Mauthner neurones: an analysis of dendritic remote inhibition.J. Physiol. 1968; 194: 669-723PubMed Google Scholar), with results that matched experimental observations. Furthermore, these issues have resurfaced, as in vitro patch-clamp and imaging techniques allowed them to be addressed in a wide range of mammalian neurons (Andersen, 1990Andersen P. Synaptic integration in hippocampal CA1 pyramids.Prog. Brain Res. 1990; 83: 215-222Crossref PubMed Google Scholar, Fregnac et al., 2003Fregnac Y. Monier C. Chavane F. Baudot P. Graham L. Shunting inhibition, a silent step in visual cortical computation.J. Physiol. (Paris). 2003; 97: 441-451Crossref PubMed Scopus (14) Google Scholar). The physical separation between adjacent synaptic units in the CNS might appear to preclude crosstalk between them. However, the postsynaptic conductance changes evoked by two separate inhibitory inputs with adjacent terminals summate supralinearly when they are co-activated. Kinetic modeling of quantal currents, based on biophysical and morphological parameters and patterned after pioneering models of the neuromuscular junction (Land et al., 1980Land B.R. Salpeter E.E. Salpeter M.M. Acetylcholine receptor site density affects the rising phase of miniature endplate currents.Proc. Natl. Acad. Sci. USA. 1980; 77: 3736-3740Crossref PubMed Google Scholar), indicated that this effect is due to lateral diffusion of transmitter (Faber and Korn, 1988Faber D.S. Korn H. Synergism at central synapses due to lateral diffusion of transmitter.Proc. Natl. Acad. Sci. USA. 1988; 85: 8708-8712Crossref PubMed Google Scholar). This type of facilitation depends on rapid diffusion of glycine from one synapse to the next (0.5–1.0 μm in 300 μsec) and on the requirement that glycine receptors be at least doubly liganded to open. Previously described for the snake neuromuscular junction after cholinesterase inhibition (Hartzell et al., 1975Hartzell H.C. Kuffler S.W. Yoshikami D. Post-synaptic potentiation: interaction between quanta of acetylcholine at the skeletal neuromuscular synapse.J. Physiol. 1975; 251: 427-463Crossref PubMed Scopus (151) Google Scholar), but unrecognized in the CNS, this phenomenon, later referred to as "spillover" (Kullmann et al., 1999Kullmann D.M. Min M.Y. Asztely F. Rusakov D.A. Extracellular glutamate diffusion determines the occupancy of glutamate receptors at CA1 synapses in the hippocampus.Philos. Trans. R. Soc. Lond. B Biol. Sci. 1999; 28: 395-402Crossref Google Scholar), is important in various forms of synaptic plasticity. Analysis of the kinetics of M cell responses to iontophoretic application of glycine and GABA suggested the presence of distinct receptors for the two amino acids, with their effects being primarily diffusion-limited (Diamond and Roper, 1973Diamond J. Roper S. Analysis of Mauthner cell responses to iontophoretically delivered pulses of GABA, glycine and L-glutamate.J. Physiol. 1973; 232: 113-128PubMed Google Scholar). When fluctuation analysis, first developed for isolated preparations (Anderson and Stevens, 1973Anderson C.R. Stevens C.F. Voltage clamp analysis of acetylcholine produced end-plate current fluctuations at frog neuromuscular junction.J. Physiol. 1973; 235: 655-691PubMed Google Scholar), was used in the M cell, the mean open time of both glycine- and GABA-activated channels corresponded to the decay time constant of unitary IPSPs, suggesting that diffusion is quite rapid and channel kinetics are rate-limiting in physiological conditions (Faber and Korn, 1980Faber D.S. Korn H. Single-shot channel activation accounts for duration of inhibitory postsynaptic potentials in a central neuron.Science. 1980; 208: 612-615Crossref PubMed Google Scholar). Interestingly, Werman in Mazliah (described in Faber and Korn, 1978Faber D.S. Korn H. Electrophysiology of the Mauthner cell: basic properties, synaptic mechanisms, and associated networks.in: Faber D.S. Korn H. Neurobiology of the Mauthner Cell. Raven Press, New York1978: 47-131Google Scholar) had obtained reliable steady-state dose-response curves for GABA and glycine receptor interactions at the M cell's lateral dendrite, as required theoretically (Werman, 1969Werman R. An electrophysiological approach to drug-receptor mechanisms.Comp. Biochem. Physiol. 1969; 30: 997-1017Crossref PubMed Google Scholar). Furthermore, their results indicated that GABA can allosterically modify the glycine receptor, thereby increasing the affinity of glycine to its binding site (see Faber and Korn, 1978Faber D.S. Korn H. Electrophysiology of the Mauthner cell: basic properties, synaptic mechanisms, and associated networks.in: Faber D.S. Korn H. Neurobiology of the Mauthner Cell. Raven Press, New York1978: 47-131Google Scholar). The still unresolved problem of whether the M cell glycine receptor could be activated by GABA acquired functional relevance when it emerged that the two inhibitory transmitters are colocalized in the same presynaptic terminals in the M cell system and in mammals (Triller et al., 1987Triller A. Cluzeaud F. Korn H. Gamma-aminobutyric-containing terminals can be apposed to glycine receptors at central synapses.J. Cell Biol. 1987; 104: 947-956Crossref PubMed Google Scholar, Ottersen et al., 1988Ottersen O.P. Davanger S. Somogyi P. Colocalization of glycine-like and GABA-like immunoreactivities in Golgi cell terminals in the rat cerebellum: a postembedding light and electron microscopic study.Brain Res. 1988; 450: 342-353Crossref PubMed Google Scholar, Todd and Sullivan, 1990Todd A.J. Sullivan A.C. Light microscopic study of the coexistence of GABA and glycine-like immunoreactivities in the spinal cord of the rat.J. Comp. Neurol. 1990; 296: 496-505Crossref PubMed Scopus (271) Google Scholar). Furthermore, GABAergic terminals can be apposed to postsynaptic GlyRs (Triller et al., 1987Triller A. Cluzeaud F. Korn H. Gamma-aminobutyric-containing terminals can be apposed to glycine receptors at central synapses.J. Cell Biol. 1987; 104: 947-956Crossref PubMed Google Scholar), and, in rat spinal cord both transmitters can be coreleased from individual vesicles (Jonas et al., 1998Jonas P. Bischofberger J. Sandkühler J. Co-release of two fast neurotransmitters at a central synapse.Science. 1998; 281: 419-424Crossref PubMed Scopus (530) Google Scholar). A novel subunit of the glycine receptor, αZ1, with a high degree of homology with mammalian α subunits, was cloned from zebrafish brain (David-Watine et al., 1999David-Watine B. Goblet C. de Saint Jan D. Fucile S. Devignot V. Bregestovski P. Korn H. Cloning, expression and electrophysiological characterization of glycine receptor alpha subunit from zebrafish.Neuroscience. 1999; 90: 303-317Crossref PubMed Scopus (30) Google Scholar). It is present in the M cell (Imboden et al., 2001Imboden M. Devignot V. Korn H. Goblet C. Regional distribution of glycine receptor messenger RNA in the central nervous system of zebrafish.Neuroscience. 2001; 103: 811-830Crossref PubMed Scopus (18) Google Scholar), and the homomeric receptor that it forms can be activated by both transmitters, albeit with different kinetics and EC50s (Fucile et al., 1999Fucile S. De Saint Jan D. David-Watine B. Korn H. Bregestovski P. Comparison of glycine and GABA actions on the zebrafish homomeric glycine receptor.J. Physiol. 1999; 517: 369-383Crossref PubMed Scopus (38) Google Scholar), as is also the case for human α1 and α2 homomeric receptors (De Saint Jan et al., 2001De Saint Jan D. David-Watine B. Korn H. Bregestovski P. Activation of human α1 and α2 homomeric glycine receptors by taurine and GABA.J. Neurophysiol. 2001; 535: 741-755Google Scholar). Quantal Release. The discovery that the presynaptic interneurons that can be identified by the presence of a PHP (see above) mediate glycinergic inhibition of the M cell (Korn and Faber, 1976Korn H. Faber D.S. Vertebrate central nervous system: same neurons mediate both electrical and chemical inhibitions.Science. 1976; 194: 1166-1169Crossref PubMed Google Scholar)