Retinotectal Synapses Research Articles

1304 Long-term potentiation is blocked by prior activity in the goldfish retino-tectal synapses

1304 Long-term potentiation is blocked by prior activity in the goldfish retino-tectal synapses

In vivo development of neuronal structure and function.

The formation of the nervous system requires a coordinated development of neuronal morphology and synaptic physiology. We have used the frog retinotectal system to examine the development of brain structure and function. The retinotectal system of frogs is well known for its developmental and experience-dependent synaptic plasticity. As the animal grows, retinotectal axon arbors migrate considerable distances within the contralateral optic tectum in order to maintain a high degree of refinement in the retinotopic map. The capacity of the arbors to move within the tectal neuropil indicates that the retinotectal synapses and axon arbor branches are dynamic. The maintenance of retinotopy despite the axonal mobility indicates that there must be a mechanism which controls sites of synapse formation and stabilization. Considerable evidence supports the hypothesis that N-methyl-d-aspartate (NMDA) receptor activity is required to maintain the retinotopic map (Cline 1991), but it is still not clear how activity influences the neuronal

Target-independent diversification and target-specific projection of chemically defined retinal ganglion cell subsets.

In diverse vertebrate species, defined subsets of retinal ganglion cells (RGCs, the neurons that project from retina to brain) are distinguishable on the basis of their dendritic morphology, physiological properties, neurotransmitter content and synaptic targets. Little is known about when this diversity arises, whether diversification requires target-derived signals, and how subtype-specific projection patterns are established. Here, we have used markers for two chemically defined RGC subsets in chick retina to address these issues. Antibodies to substance P (SP) and the nicotine acetylcholine receptor (AChR) beta 2 subunit label two small ( < 10%), mutually exclusive groups of RGCs in mature retina. SP and AChRs accumulate in distinct RGCs before retinotectal synapses have formed. Moreover, both populations of RGCs form in retinae that develop following tectal ablation or transplantation to the coelomic cavity. Thus, RGC subsets acquire distinct neurotransmitter phenotypes in the absence of extraretinal cues. In the mature optic tectum, SP- and AChR-positive RGC axonal arbors are confined to distinct retinorecipient (synaptic) laminae. In the developing tectum, SP- and AChR-positive axons are initially intermingled in a superficial fiber layer, but then enter and arborize in appropriate laminae soon after those laminae form. Importantly, SP-positive axons, which synapse in a superficial lamina, never extend into the deeper, AChR-positive lamina. Tectal interneurons rich in SP receptors are concentrated in the lamina to which SP-positive RGC axons project, and a set of cholinergic (choline acetyltransferase-positive) tectal projection neurons elaborate dendrites in the lamina to which AChR-positive RGC axons project. These populations of tectal neurons, which are likely targets of the RGC subsets, form in tecta that develop following enucleation. Thus, RGCs and their targets can diversify in each others absence. Accordingly, we propose that the lamina-selective connectivity we observe reflects the presence of complementary cues on RGC subsets and their laminar targets.

Concanavalin A reduces habituation in the tectum of the frog

In the tectum of Rana pipiens, responses to repeated flashes of light to the ipsilateral eye display considerable habituation. We have employed the plant lectin concanavalin A (Con A), which can diminish desensitization of glutamate receptors in vitro, in order to examine whether desensitization of glutamate receptors contributes to this habituation. The ipsilateral eye's projection reaches each tectal lobe indirectly, being relayed from the opposite tectal lobe via the tecto-isthmo-tectal projection. One of the sites along this pathway at which habituation may take place is the retinotectal synapse, where glutamate is a putative transmitter. Pretreatment of one lobe of the tectum with Con A significantly diminished the habituation of responses recorded in the other tectal lobe to light offset, with less of an effect on responses to light onset. These data suggest that OFF habituation may reflect glutamate receptor desensitization at the retinotectal synapse. In contrast, recordings from retinotectal terminals indicate that a primary site of habituation of ON responses is within the retina.

The contributions of NMDA, non-NMDA, and GABA receptors to postsynaptic responses in neurons of the optic tectum

Activation of the NMDA subtype of glutamate receptor has been implicated in activity-dependent development and plasticity in several systems, including the retinotectal system of amphibians. To gain a better understanding of the response properties of tectal neurons, with particular emphasis on the role of both non-NMDA and NMDA glutamate receptors, we have developed an in vitro slice preparation of the diencephalon and midbrain of frog (Rana pipiens) tadpoles. In these slices, we electrically stimulated the optic tract and recorded both mono- and polysynaptic responses in single tectal neurons using whole-cell voltage clamp or current clamp. By including biocytin in the recording electrode, we were also able to determine the location and morphology of many of these neurons. Using these techniques, we found that the current-voltage (I-V) relations for both mono- and polysynaptic responses of tectal neurons showed voltage dependence only in the presence of extracellular Mg2+. This dependence reflects the hyperpolarization-dependent block of the NMDA channel by Mg2+. Bath application of 6-cyano-7-nitroquinoxaline-2,3-dione, a non-NMDA glutamate receptor antagonist, reduced both mono- and polysynaptic responses of tectal neurons. Bath application of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (DL-APV) strongly reduced polysynaptic responses. When neurons were depolarized by the voltage clamp, relieving the Mg(2+)-dependent block of the NMDA channel, DL-APV application also reduced monosynaptic responses. Application of the GABAA receptor antagonist (-)bicuculline methiodide significantly increased the polysynaptic responses of tectal neurons, reflecting block of inhibition. We further confirmed the presence of these three types of receptors by examining postsynaptic currents evoked by iontophoretic application of the three agonists, NMDA, (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and GABA. These results confirm that the dominant excitatory transmitter in the tectum appears to be glutamate. Furthermore, the retinotectal synapses (i.e., monosynaptic currents) express functional NMDA receptors that are voltage dependent and are not responsible for the bulk of normal excitatory transmission. Polysynaptic responses, however, are mediated by both non-NMDA and NMDA receptors, and inhibition plays a significant role in sculpting these polysynaptic responses.

Nicotinic depolarization of optic nerve terminals augments synaptic transmission

The role of acetylcholine (ACh) as the neurotransmitter at the vertebrate neuromuscular junction has served as a model in the study of synaptic physiology throughout the nervous system, but its function in the brain has remained obscure. Nicotinic ACh receptors are found on afferent nerve terminals in several regions of vertebrate brain 6,18,19,23, and nicotinic agonists can cause transmitter release (as measured biochemically) 14,20. Yet there is no direct evidence that activation of these receptors modulates synaptic function. Here I report that nicotinic agonists directly depolarize optic nerve terminals in goldfish tectum recorded via sucrose gap and simultaneously enhance synaptic transmission recorded via tectal field potentials. I also show that a recurrent cholinergic circuit in tectum, acting on these terminals, initiates antidromic impulses that cause repeated transmission at the retinotectal synapses.

Paired pulse facilitation of summated intracellular synaptic potentials in single retinotectal fibers in the frog

Facilitation of the second of two consecutive test EEG quanta (the summated monosynaptic potentials of the synapses of one axon arborizing in layer F of the frog tectum) was investigated in the normal and under conditions of raised extracellular Ca2+ and Mg2+ concentration. Intensification of paired-pulse facilitation (×1.4–2.4) was observed at the shortest interstimulus intervals (of 2.5–5 msec). The distribution of maximum levels for facilitation of EEG quanta was bimodal at levels 1.95 and 1.65, on the basis of which two groups were identified, one potentiating EEG quanta more than the other. The time course of paired-pulse facilitation of both groups of EEG quanta can be broken down into two exponential components with time constants of 5–6, 140–150 and 6–8, 60–70 msec respectively. Bimodal distribution of maximum paired-pulse levels in the normal, together with findings from experiments involving raised Ca2+ and Mg2+ concentrations would indicate that facilitation of frog retinotectal synapses is dependent on the quantal release of transmitter; it may thus be postulated that this release reaches near-saturation point in the normal. It is suggested that two types of axonal terminal arborizations whose synapses differ in the quantal content of transmitter release are present in layer F of the frog tectum. These axonal arbors could well originate from different class 3 and 5 retinal detectors.

Investigation of retino-tectal synapses in frogs by recording extracellular epsps produced by synapses of a single axon

Investigation of retino-tectal synapses in frogs by recording extracellular epsps produced by synapses of a single axon

Morphogenesis and physiogenesis of the retino-tectal connection in the chicken. II. The retino-tectal synapses.

The preceding paper (Eager 1976) was concerned with the development of retinal ganglion cells and the maturation of photoreceptors. Here we discuss the formation of retino-tectal synapses as studied in terms of both morphology and physiology. The early response characteristics of the first and second postsynaptic tectal cells were also studied. The first retino-tectal synapses, sparse in number, were found with the electron microscope on the eleventh day of incubation. Strict criteria for their identification were employed. At the same stage postsynaptic potentials could be recorded for the first time. These potentials could be recognized as postsynaptic by the application of various techniques: combined ortho- and antidromic stimulation, variation of stimulus intensities, surface mapping, field potential profiles, and application of double shocks of varying intervals. From day twelve onward single unit recordings were also possible and confirmed these results. We were able to establish that morphological criteria for recognition of optic nerve synapses are also criteria for when they start to function. Tectal cells first respond with long-lasting repetitive discharges. Only a few days later the response burst has become shorter and is followed by post-excitatory inhibition. When retino-tectal synapses start to function the excitation is not confined to the superficial tectal cells, but reaches deeper neurons which are possibly tectal output cells. The results of this and the preceding paper (Eager 1976) indicate that the development of the retino-tectal connection in the chicken differs from that in the frog in several fundamental respects.

Studies on the development of the chick optic tectum. IV. An autoradiographic study of the development of retino-tectal connections

The normal pattern of innervation of the optic tectum has been studied autoradiographically in a series of chick embryos which were injected intraocularly with [ 3H]proline at intervals between the 6th and 21st days of incubation. From the distribution of the radioactively labeled proteins transported in the rapid phase of axonal flow, it is evident that retinal fibers first enter the tectum late on the 6th day of incubation and then spread across its surface from its rostrolateral aspect to its caudo-dorso-medial pole during the ensuing 6 days. At the 9th day, when the fibers have grown across the surface of the rostral half of the tectum, there is no indication that the terminal portions of the axons have left the stratum opticum to enter the outer layers of the stratum griseum et fibrosum superficiale. The first suggestion of such an invasion of the straum griseum et fibrosum superficiale is found at day 10, when labeled fibers can be seen in this stratum, over a restricted, oval, area near the center of the tectum. Over the course of the next 2 days the region of the stratum griseum et fibrosum superficiale occupied by retinal fibers expands, more-or-less concentrically, until by day 14 some labeled fibers are seen in the outer part of the stratum, throughout the tectum. Concurrent with the ingrowth of optic nerve fibers the final cytoarchitectonic differentiation of the outer layers of the tectum occurs so that by day 18 the autoradiographs show a pattern of labeling of layers a–f of the stratum griseum et fibrosum superficiale similar to that seen in mature, post-hatched chicks. Since the region of the tectum which is first innervated by retinal fibers corresponds to the projection field of the region around the upper end of the choroid fissure where the first ganglion cells are generated, it would appear that the axons of the first-formed ganglion cells grow over the rostral surface of the tectum before establishing the first retino-tectal synapses near the central portion of the tectum. Subsequently, as the ganglion cell population grows concentrically from around the area centralis, there is a parallel expansion of the region in which retino-tectal synapses are being formed in the tectum.