Non-cholinergic excitation in neurons after a chronic glutamate receptor blockade

Recovery of network-driven glutamatergic activity in rat hippocampal neurons during chronic glutamate receptor blockade

Previous studies indicated that a long-term decrease in the activity of ionotropic glutamate receptors induces cholinergic activity in rat and mouse hypothalamic neuronal cultures. Here we studied whether a prolonged inactivation of ionotropic glutamate receptors also induces cholinergic activity in hippocampal neurons. Receptor activity was chronically suppressed in rat hippocampal primary neuronal cultures with two proportionally increasing sets of concentrations of NMDA plus non-NMDA receptor antagonists: 100 microM/10 microM AP5/CNQX (1X cultures) and 200 microM/20 microM AP5/CNQX (2X cultures). Using calcium imaging we demonstrate that cholinergic activity does not develop in these cultures. Instead, network-driven glutamate-dependent activity, that normally is detected in hyper-excitable conditions, reappears in each culture group in the presence of these antagonists and can be reversibly suppressed by higher concentrations of AP5/CNQX. This activity is mediated by non-NMDA receptors and is modulated by NMDA receptors. Further, non-NMDA receptors, the general level of glutamate receptor activity and CaMK-dependent signaling are critical for development of this network-driven glutamatergic activity in the presence of receptor antagonists. Using electrophysiology, western blotting and calcium imaging we show that some neuronal parameters are either reduced or not affected by chronic glutamate receptor blockade. However, other parameters (including neuronal excitability, mEPSC frequency, and expression of GluR1, NR1 and betaCaMKII) become up-regulated and, in some cases, proportionally between the non-treated, 1X and 2X cultures. Our data suggest recovery of the network-driven glutamatergic activity after chronic glutamate receptor blockade. This recovery may represent a form of neuronal plasticity that compensates for the prolonged suppression of the activity of glutamate receptors.

Use of calcium imaging for analysis of neuronal gap junction coupling

We recently used Western blots for connexin 36 and neuronal dye coupling with neurobiotin to measure developmental decrease in neuronal gap junction coupling in cell cultures. To ask whether Ca2+ imaging also can be used to measure changes in the amount of neuronal gap junction coupling, we defined a Ca2+ coupling coefficient as the percentage of neurons with bicuculline-induced increases in intracellular Ca2+ that are suppressed by blocking gap junctions. We demonstrate in rat and mouse hypothalamic neuronal cultures that the Ca2+ coupling coefficient decreases during culture development, this decrease is prevented by manipulations that also prevent developmental decrease in neuronal gap junction coupling, and the coefficient is low in cultures lacking connexin 36. The results indicate that Ca2+ imaging is a useful tool to quantify the amount of neuronal gap junction coupling in cultures.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

Regulation of cholinergic phenotype in developing neurons

Specification of neurotransmitter phenotype is critical for neural circuit development and is influenced by intrinsic and extrinsic factors. Recent findings in rat hypothalamus in vitro suggest the role of neurotransmitter glutamate in the regulation of cholinergic phenotype. Here we extended our previous studies on the mechanisms of glutamate-dependent regulation of cholinergic phenotypic properties in hypothalamic neurons. Using immunocytochemistry, electrophysiology, and calcium imaging, we demonstrate that hypothalamic expression of choline acetyltransferase (the cholinergic marker) and responsiveness of neurons to acetylcholine (ACh) receptor agonists increase during chronic administration of an N-methyl-D-aspartate receptor (NMDAR) blocker, MK-801, in developing rats in vivo and genetic and pharmacological inactivation of NMDARs in mouse and rat developing neuronal cultures. In hypothalamic cultures, an inactivation of NMDA receptors also induces ACh-dependent synaptic activity, as do inactivations of PKA, ERK/MAPK, CREB, and NF-kappaB, which are known to be regulated by NMDA receptors. Interestingly, the increase in cholinergic properties in developing neurons that is induced by NMDAR blockade is prevented by the blockade of ACh receptors, suggesting that function of ACh receptor is required for the cholinergic up-regulation. Using dual recording of monosynaptic excitatory postsynaptic currents, we further demonstrate that chronic inactivation of ionotropic glutamate receptors induces the cholinergic phenotype in a subset of glutamatergic neurons. The phenotypic switch is partial as ACh and glutamate are coreleased. The results suggest that developing neurons may not only coexpress multiple transmitter phenotypes, but can also change the phenotypes following changes in signaling in neuronal circuits.