Maturation of synaptic partners: functional phenotype and synaptic organization tuned in synchrony

Maturation of principal neurons of the medial nucleus of the trapezoid body (MNTB) was assessed in the context of the developmental organization and activity of their presynaptic afferents, which grow rapidly to form calyces of Held and to establish mono-innervation between postnatal days (P)2 and 4. MNTB neurons and their inputs were studied from embryonic day (E)17, when the nucleus was first discernable, until P14 after the onset of hearing. Using a novel slice preparation containing portions of the cochlea, cochlear nucleus and MNTB, we determined that synaptic inputs form onto MNTB neurons at E17 and stimulation of the cochlear nucleus can evoke action potentials (APs) and Ca(2+) signals. We analysed converging inputs onto individual MNTB neurons and found that competition among inputs was resolved quickly, as a single large input, typically larger than 4 nA, emerged from P3-P4. During calyx growth but before hearing onset, MNTB cells acquired their mature, phasic firing property and quantitative real-time PCR confirmed a coincident increase in low threshold K(+) channel mRNA. These events occurred in concert with an increase in somatic surface area and a 7-fold increase in the current threshold (30 to >200 pA) required to evoke action potentials, as input resistance (R(in)) settled from embryonic values greater than 1 GΩ to approximately 200 MΩ. We postulate that the postsynaptic transition from hyperexcitability to decreased excitability during calyx growth could provide a mechanism to establish the mature 1:1 innervation by selecting the winning calyceal input based on synaptic strength. By comparing biophysical maturation of the postsynaptic cell to alterations in presynaptic organization, we propose that maturation of synaptic partners is coordinated by synaptic activity in a process that is likely to generalize to other neural systems.

Serial sectioning and electron microscopy of large tissue volumes for 3D analysis and reconstruction: a case study of the calyx of Held

Serial section electron microscopy is typically applied to investigation of small tissue volumes encompassing subcellular structures. However, in neurobiology, the need to relate subcellular structure to organization of neural circuits can require investigation of large tissue volumes at ultrastructural resolution. Analysis of ultrastructure and three-dimensional reconstruction of even one to a few cells is time consuming, and still does not generate the necessary numbers of observations to form well-grounded insights into biological principles. We describe an assemblage of existing computer-based methods and strategies for graphical analysis of large photographic montages to accomplish the study of multiple neurons through large tissue volumes. Sample preparation, data collection and subsequent analyses can be completed within 3-4 months. These methods generate extremely large data sets that can be mined in future studies of nervous system organization.

Synaptogenesis of the calyx of Held: rapid onset of function and one-to-one morphological innervation

Synaptogenesis during early development is thought to follow a canonical program whereby synapses increase rapidly in number and individual axons multiply-innervate nearby targets. Typically, a subset of inputs then out-competes all others through experience-driven processes to establish stable, long-lasting contacts. We investigated the formation of the calyx of Held, probably the largest nerve terminal in the mammalian CNS. Many basic functional and morphological features of calyx growth have not been studied previously, including whether mono-innervation, a hallmark of this system in adult animals, is established early in development. Evoked postsynaptic currents, recorded from neonatal mice between postnatal day 1 (P1) and P4, increased dramatically from -0.14 +/- 0.04 nA at P1 to -6.71 +/- 0.65 nA at P4 with sharp jumps between P2 and P4. These are the first functional assays of these nascent synapses for ages less than P3. AMPA and NMDA receptor-mediated currents were prominent across this age range. Electron microscopy (EM) revealed a concomitant increase, beginning at P2, in the prevalence of postsynaptic densities (16-fold) and adhering contacts (73-fold) by P4. Therefore, both functional and structural data showed that young calyces could form within 2 d, well before the onset of hearing around P8. Convergence of developing calyces onto postsynaptic targets, indicative of competitive processes that precede mono-innervation, was rare (4 of 29) at P4 as assessed using minimal stimulation electrophysiology protocols. Serial EM sectioning through 19 P4 cells further established the paucity (2 of 19) of convergence. These data indicate that calyces of Held follow a noncanonical program to establish targeted innervation that occurs over a rapid time course and precedes auditory experience.

Activation of mGluR5 modulates GABA(A) receptor function in retinal amacrine cells

Amacrine cells in the vertebrate retina receive glutamatergic input from bipolar cells and make synapses onto bipolar cells, ganglion cells, and other amacrine cells. Recent studies indicate that amacrine cells express metabotropic glutamate receptors (mGluRs) and that signaling within the inner plexiform layer (IPL) of the retina might be modulated by mGluR activity. This study tests the hypothesis that activation of mGluR5 modulates GABA(A) receptor function in retinal amacrine cells. Whole cell voltage-clamp recordings were combined with pharmacology to establish the identity of the ionotropic GABA receptors expressed in primary cultures of chick amacrine cells and to determine how mGluR5 activity affected the behavior of those receptors. Application of GABA (20 microM) produced currents that reversed at -58.2 +/- 0.9 mV, near the predicted Cl(-) reversal potential of -59 mV. The GABA(A) receptor antagonist, bicuculline (50 microM), completely blocked the GABA-gated currents. cis-4-Aminocrotonic acid (CACA; 100 microM), a GABA(C) receptor agonist, produced small currents that were not blocked by the GABA(C) antagonist, (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA; 20 microM), but were completely blocked by bicuculline. These results indicate that cultured amacrine cells express GABA(A) receptors exclusively. Activating mGluR5 with (RS)-2-chloro-5-hydroxyphenylglycine (CHPG; 300 microM) enhanced GABA-gated currents by 10.0 +/- 1.5%. Buffering internal Ca(2+) with BAPTA (10 mM) blocked the CHPG-dependent enhancement. Activation of PKC with the cell-permeable PKC activators (-)-7-octylindolactam V, phorbol 12-myristate 13 acetate (PMA), or 1-oleoyl-2-acetyl-sn-glycerol (OAG) also enhanced GABA-gated currents in a dose-dependent manner. Preactivation of PKC occluded the mGluR5-dependent enhancement, and inhibition of Ca-dependent PKC isotypes with Gö6976 (35 nM) suppressed the effects of mGluR5 activation, suggesting that mGluR5 and PKC are part of the same pathway. To determine if mGluR5-dependent enhancement occurred at synaptic GABA(A) receptors, postsynaptic currents were recorded in the presence of CHPG. On average, the mean amplitudes of the quantal events were increased by about 18% when mGluR5 was activated. These results indicate that activation of mGluR5 enhances GABA-gated current in cultured amacrine cells in a manner that is both Ca(2+)- and PKC-dependent. These results support the possibility that glutamate released from bipolar cells can modulate the function of GABAergic amacrine cells and alter signaling in the inner plexiform layer.