Illuminating the multifaceted roles of neurotransmission in shaping neuronal circuitry

Abstract

Across the nervous system, neurons form highly stereotypic patterns of synaptic connections that are designed to serve specific functions. Mature wiring patterns are often attained upon the refinement of early, less precise connectivity. Much work has led to the prevailing view that many developing circuits are sculpted by activity-dependent competition among converging afferents, which results in the elimination of unwanted synapses and the maintenance and strengthening of desired connections. Studies of the vertebrate retina, however, have recently revealed that activity can play a role in shaping developing circuits without engaging competition among converging inputs that differ in their activity levels. Such neurotransmission-mediated processes can produce stereotypic wiring patterns by promoting selective synapse formation rather than elimination. We discuss how the influence of transmission may also be limited by circuit design and further highlight the importance of transmission beyond development in maintaining wiring specificity and synaptic organization of neural circuits.

Figure 1. Developmental sequences producing stereotypic neuronal connectivity patterns

A. Postsynaptic cells receive a diversity of afferent inputs (convergence), and often, presynaptic cells contact multiple postsynaptic targets (divergence). Adult patterns of synaptic convergence and divergence may be derived from initially imprecise sets of connections that undergo refinement upon maturation. A, B. Biased connectivity can arise from differences in the number of connected presynaptic cells (A, shown here for contacts with two purple cells compared to contact with a single green cell), from differences in the number of synapses (red puncta) made by each axon type (B), or from differences in synaptic strength (not shown). Additionally, different input types could utilize disparate receptor types to achieve differential connectivity.

Figure 2. Retinal connections undergo activity-dependent alterations without competition

A. Basic organization of the vertebrate retina, illustrated here for the mouse. Cone: cone photoreceptor; Rod: rod photoreceptor; HC: horizontal cell; T2, 6, 7, and 8: Cone bipolar cell types; RBC: rod bipolar cell; A17: A17 amacrine cell; AII: AII amacrine cell; A_ON RGC: A_ON retinal ganglion cell; ON, OFF: ON and OFF sublaminae of the inner plexiform layer. B. (Top) Suppressing glutamatergic transmission from all ON bipolar cells by expression of tetanus toxin light chain (TeNT; gray) reduces the number of excitatory synaptic sites (marked by PSD95) between ON but not OFF bipolar cells contacting a bistratified ON-OFF retinal ganglion cell (Kerschensteiner et al., 2009). (Bottom) Synaptic number and size of T6 ON cone bipolar cells are regulated by transmission on a cell-by-cell basis (Okawa et al., 2014). C. Zebrafish H3 HCs contact more UV cones than blue cones. (Top) Blocking UV cone transmission (TeNT; gray) results in increased connectivity with blue cones, but no change in connectivity with UV cones. (Bottom) Blocking blue cone transmission has no effect on connectivity with either cone type (Yoshimatsu et al., 2014).

Figure 3. Uniform alteration of transmission from all inputs can lead to differential connectivity

A. Maximum intensity projection of a confocal image stack showing synaptic contacts (fluorescently tagged PSD95) between Type 6 (T6) and Type 7 (T7) bipolar cells with an A_ON retinal ganglion cell (RGC) in the mouse retina. Bipolar cells shown here are labeled in the grm6-tdTomato transgenic line (Kerschensteiner et al., 2009). Insets show higher magnification of the bipolar cell contacts. Arrows point to synaptic appositions defined by overlap of PSD95 and tdTomato signal in three-dimension. B. Suppressing transmission from all retinal ON bipolar cells differentially reduces the number of synaptic sites (PSD95) between T6, but not T7, bipolar cells (BC) with the A_ON RGC (Morgan et al., 2011). C. Sensory deprivation alters the number of cone photoreceptors contacted by T7 but not T6 BCs compared at 3 weeks after birth (Dunn et al., 2013).

Figure 4. Potential architectural constraints on activity-dependent competition

A. Schematic illustrating the segregated distributions of distinct afferents on the dendritic arbors of four types of neurons: (1) The dorsal and ventral arbors of chick Nucleus Laminaris (NL) neurons are innervated separately by ipsi- and contralateral projections from Nucleus Magnocellularis (NM) neurons; (2) Climbing fibers (CF) and parallel fibers (PF) contact the proximal and distal parts of the purkinje cell (PC) dendritic arbor; (3) Distinct excitatory afferents (PP = Perforant path, MF = mossy fiber, SC = Schaffer collateral) contact different parts of the dendritic arbor of hippocampal CA3 pyramidal neurons; and (4) An ON-OFF retinal ganglion cell (RGC) receives inputs from ON and OFF bipolar cells on dendrites stratifying in separate sublaminae. B. The axonal terminals of each bipolar cell type (blue) tile such that there is minimal overlap of their territories (magenta polygon). C. Comparison of the axon terminals of mouse Type 6 (T6) bipolar cells in control, TeNT and diphtheria toxin (DTA)-expressing retinas. TeNT: suppressed transmission in the axon; DTA: neighboring bipolar cells ablated. Shown are z-projections of the digital masks of individual axon terminals. Scale bar applies to all. D. Within the axonal territory of inactive (TeNT) T6 bipolar cells, there is no increase in the number of synapses (estimated by PSD95 expression) between the postsynaptic RGC and its other bipolar afferents (presumed T7 here). However, T7 bipolar cells gain many more synapses with the RGC when T6 bipolar cells are ablated (DTA). Note that PSD95 puncta apposed to TeNT-expressing T6 bipolar cells are larger compared to those apposed to non-TeNT bipolar cells. For C and D see Okawa et al., 2014.

Figure 5. Neurotransmission regulates synapse formation and maintenance

A. Monosynaptic inputs from neighboring star pyramidal cells in Layer 4 of the visual cortex increase in amplitude (Amp) in monocularly deprived rats (Maffei et al., 2004) B. Sensory deprivation selectively reduces mGluR6 expression on dendritic tips of cone bipolar cells (CBC) but not rod bipolar cells (RBC) (Dunn et al., 2013). C. Star pyramidal neurons in the visual cortex receive GABAergic input from fast spiking (FS) and regular-spiking nonpyramidal (RSNP) interneurons. These two inhibitory synapses are differentially modulated by visual deprivation. Response amplitude of RSNP synapses increases whereas synapses made by FS neurons show reduced amplitude. D. RBC axon terminals and A17 amacrine cell neurites form reciprocal synapses. RBCs release glutamate (glu) that drives release of GABA from the A17, which inhibits further release of glutamate from the RBC terminal. In GAD1 knockout (KO) mouse retina, GABA release is impaired (gray) resulting in a selective decrease of α1 subunit-containing GABA_A receptors (GABA_ARα1) at RBC terminals. α3 subunit-containing GABA_A receptors (GABA_ARα3) and GABA_C receptors (GABA_CR) are unaffected. AMPA receptor (GluA) expression on A17 processes assessed by glutamate evoked responses remains unperturbed in the GAD1 KO retina (Schubert et al., 2013).

Figure 6. Structural and connectivity defects in transmission mutant models of retinal disease

A. Mice lacking phototransduction channels from both rods and cones (blue: CNGA3/B1) are a model of retinitis pigmentosa. Neurotransmission is also perturbed in mice with mutations in the voltage-gated calcium channel Ca_V1.4 (red) located at photoreceptor axon terminals, amongst which nob2 mice are a model for congenital stationary night blindness. Rod: rod photoreceptor, Cone: cone photoreceptor, HC: horizontal cell, RBC: rod bipolar cell, CBC: cone bipolar cell. B. Rod terminals retract whereas RBC dendrites and HC processes sprout into the outer nuclear layer, in both CNGA3/B1 double knockout (dKO) mice and Ca_V1.4 mutants (shown by the stripes of blue and red). In Ca_V1.4 mutants, retraction of cone terminals is rarely observed. But the cones often form extra branches (shown in red). CBCs in general appear to be less affected in all transmission mutants. C. Defects at the rod photoreceptor synapse found in different transmission mutants. nob4 mice lack metabotropic glutamate receptor expression (mGluR6). See text for references.