Changes in functional glutamate receptors on a postsynaptic neuron accompany formation and maturation of an identified synapse

Abstract

N-Methyl-D-aspartate (NMDA)-type glutamate receptors play important roles at developing synapses and in activity-dependent synaptic plasticity. Recent studies in Aplysia suggest that NMDA-like receptors may contribute to some forms of plasticity of sensorimotor synapses accompanying associative learning. We examined at various times after plating neurons in culture the contribution of NMDA- and alpha-amino-3 hydroxy-5 methyl-4 isoxazole proprionic acid (AMPA)-like glutamate receptors to responses evoked in motor cell L7 either by action potentials in sensory neurons (SNs) or by focal applications of glutamate. We found that (D,L)-2-amino-5-phosphopentoic acid-sensitive receptors contributed significantly to postsynaptic responses in 1-day cultures but contributed little in the same cultures on day 4. By contrast, postsynaptic responses on day 4 increased significantly in amplitude by the addition of functional 6-cyano-7 nitroquinoxaline-2,3-dione- or 1-(4-aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine hydrochloride-sensitive receptors. Receptors with NMDA-like properties are detected on day 1 only at sites on L7 apposed to SN varicosities, and are not detected on L7 cultured alone. The results indicate that changes in expression and distribution of functional receptors on L7 accompany the formation and maturation of SN synapses. Signals from the SN appear to trigger expression and clustering of functional NMDA-like receptors at sites contacted by presynaptic structures capable of transmitter release. With time, functional AMPA-like receptors are added to these sites enhancing synaptic efficacy. The results are consistent with the idea that the expression and sequential clustering of NMDA- and AMPA-type receptors may be essential for the formation and maturation of central synapses.

Localization of glutamate and glutamate transporters in the sensory neurons ofAplysia

“…Identification of the neurotransmitter used by the sensorimotor synapse in the pleural‐pedal ganglia should prove an important step to uncovering further cellular, molecular, and biochemical processes involved in plasticity at this synapse. Roles of regulation of glutamate uptake, synthesis, and release mechanisms can begin to be investigated, as well as regulation of the postsynaptic receptors (Trudeau and Castellucci, 1995; Conrad et al, 1999). We have initiated experiments to study glutamate uptake in sensory neurons and found that it is regulated during long‐term sensitization (Eskin et al, 1999).…”

Molecular Mechanisms of Memory Storage inAplysia

“…The simplicity of the neuronal circuit underlying these behavioral modifications-including direct monosynaptic connections between identified mechanoreceptor sensory neurons and their follower cells has allowed the analysis of the short-and long-term memory for sensitization to be reduced to the cellular and molecular level. This monosynaptic sensory-to-motor neuron connection, which is thought to be glutamatergic (Dale and Kandel, 1993;Trudeau and Castellucci, 1993;Conrad et al, 1999), can be reconstituted in dissociated cell culture. A number of studies in our laboratory and elsewhere have demonstrated that this simplified in vitro model system reproduces what is observed during behavioral training if the tail shocks are replaced with brief applications of serotonin (5-HT), a modulatory transmitter normally released by sensitizing stimuli in the intact animal (Glanzman et al, 1989;Mackey et al, 1989;Marinesco and Carew, 2002).…”

Presynaptic and Postsynaptic Mechanisms of a Novel Form of Homosynaptic Potentiation atAplysiaSensory-Motor Neuron Synapses

“…Both CamKII and MAP kinase contribute to LTP in hippocampus through insertion of AMPA-type glutamate receptors into the postsynaptic membrane (Malinow and Malenka, 2002). Aplysia sensorymotor neuron PSPs are thought to have both AMPA and NMDA components (Glanzman, 1994;Conrad et al, 1999;Antonov et al, 2003), and insertion of AMPA-type receptors may also contribute to facilitation by serotonin in Aplysia (Chitwood et al, 2001). To begin to test whether that mechanism contributes to homosynaptic potentiation as well, we examined potentiation in the presence of a low concentration (5 M) of the AMPA receptor antagonist DNQX, which should decrease the importance of any selective changes in the AMPA component.…”