Jean-Marc Goaillard

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Somatodendritic ion channel expression in substantia nigra pars compacta dopaminergic neurons across postnatal development.

Dopaminergic neurons of the substantia nigra pars compacta (SNc) are involved in the control of movement, sleep, reward, learning, and nervous system disorders and disease. To date, a thorough characterization of the ion channel phenotype of this important neuronal population is lacking. Using immunohistochemistry, we analyzed the somatodendritic expression of voltage‐gated ion channel subunits that are involved in pacemaking activity in SNc dopaminergic neurons in 6‐, 21‐, and 40‐day‐old rats. Our results demonstrate that the same complement of somatodendritic ion channels is present in SNc dopaminergic neurons from P6 to P40. The major developmental changes were an increase in the dendritic range of the immunolabeling for the HCN, T‐type calcium, Kv4.3, delayed rectifier, and SK channels. Our study sheds light on the ion channel subunits that contribute to the somatodendritic delayed rectifier (Kv1.3, Kv2.1, Kv3.2, Kv3.3), A‐type (Kv4.3) and calcium‐activated SK (SK1, SK2, SK3) potassium currents, IH (mainly HCN2, HCN4), and the L‐ (Cav1.2, Cav1.3) and T‐type (mainly Cav3.1, Cav3.3) calcium currents in SNc dopaminergic neurons. Finally, no robust differences in voltage‐gated ion channel immunolabeling were observed across the population of SNc dopaminergic neurons for each age examined, suggesting that differing levels of individual ion channels are unlikely to distinguish between specific subpopulations of SNc dopaminergic neurons. This is significant in light of previous studies suggesting that age‐ or region‐associated variations in the expression profile of voltage‐gated ion channels in SNc dopaminergic neurons may underlie their vulnerability to dysfunction and disease.

Physiological epidermal growth factor concentrations activate high affinity receptors to elicit calcium oscillations.

Signaling mediated by the epidermal growth factor (EGF) is crucial in tissue development, homeostasis and tumorigenesis. EGF is mitogenic at picomolar concentrations and is known to bind its receptor on high affinity binding sites depending of the oligomerization state of the receptor (monomer or dimer). In spite of these observations, the cellular response induced by EGF has been mainly characterized for nanomolar concentrations of the growth factor, and a clear definition of the cellular response to circulating (picomolar) concentrations is still lacking. We investigated Ca2+ signaling, an early event in EGF responses, in response to picomolar doses in COS-7 cells where the monomer/dimer equilibrium is unaltered by the synthesis of exogenous EGFR. Using the fluo5F Ca2+ indicator, we found that picomolar concentrations of EGF induced in 50% of the cells a robust oscillatory Ca2+ signal quantitatively similar to the Ca2+ signal induced by nanomolar concentrations. However, responses to nanomolar and picomolar concentrations differed in their underlying mechanisms as the picomolar EGF response involved essentially plasma membrane Ca2+ channels that are not activated by internal Ca2+ store depletion, while the nanomolar EGF response involved internal Ca2+ release. Moreover, while the picomolar EGF response was modulated by charybdotoxin-sensitive K+ channels, the nanomolar response was insensitive to the blockade of these ion channels.

The pros and cons of degeneracy

Drugs could treat neuropathic pain more effectively if they simultaneously targeted two or more types of ion channel.

Neural Networks: More about Flexibility Than Synaptic Strength

The leech heartbeat neural network is famous for its constancy in both architecture and functional output across animals. A recent study, however, has found that the synaptic strengths underlying this constancy are quite variable across animals.

Immature brains don't need GABA to get 'hyper'-excited

It is now well known that nervous system development at prenatal and postnatal stages involves activity-dependent remodelling of neural networks. Activity dependence of neural network development is a widespread phenomenon that has been observed in many different brain areas, including the thalamic nuclei, cortical sensory areas and hippocampus (Moody & Bosma 2005). During the first two postnatal weeks, activity underlies network refinement through hebbian stabilization or removal of synapses. This activity usually takes the form of spontaneous synchronized neuron firing that favours calcium entry, which is necessary for functional and morphological plasticity. One major consequence of this phenomenon is that immature networks are, by nature, more excitable than mature networks. The pathophysiological correlate of this is that immature brains are more susceptible to developing epileptic activity (Sperber et al. 1999). However, whether this hyperexcitability is mainly the consequence of particular excitability properties of individual neurons or of networks and whether it involves mainly excitatory or inhibitory neurons and/or synapses remain to be determined. In a recent issue of The Journal of Physiology, Shao & Dudek (2009) present a very thorough study of the mechanisms underlying epileptiform activity in the CA3 area of the hippocampus at immature developmental stages. Using CA3 minislice preparations (Miles & Wong, 1983), they analysed the population bursts induced by extracellular stimulation after blockade of fast inhibitory GABAergic transmission (GABAA receptor blockade) in immature (postnatal days 9–14) and mature preparations (postnatal days 90–100). As previously described, the authors found that the duration of these bursts is almost two orders of magnitudes longer in the immature compared to the mature hippocampus (14 s vs. 180 ms). Moreover, the burst latency is significantly shorter in immature animals compared to mature animals. Using pharmacological tools, the authors exclude the involvement of GABAB, NMDA and metabotropic glutamate receptors and show that AMPA-mediated glutamatergic transmission is necessary and sufficient to sustain prolonged population bursts. The authors then discovered that the excitatory network has significantly different properties in the immature animal: (i) increased frequency and amplitude of recurrent excitatory synaptic events paradoxically coupled to a lower probability of release of glutamate, (ii) increased intrinsic excitability of excitatory neurons due to an increased membrane resistance and decreased medium AHP (afterhyperpolarization ocurring after a few action potentials). The interplay between these different network properties explains why the immature CA3 is able to generate long population bursts while the mature CA3 is unable to sustain prolonged recurrent activity. In the immature CA3, when an excitatory neuron is stimulated, it induces large synaptic responses in the postsynaptic neuron, which easily fires action potentials due to its high membrane resistance. This spiking also occurs at a shorter latency than in older animals, presumably due to the shorter axons and higher prevalence of axo-axonic coupling in immature preparations (MacVicar & Dudek, 1980; Gomez-Di Cesare et al. 1997). The postsynaptic neuron is then able to quickly recurrently activate its presynaptic partner, which is able to sustain high frequencies of firing due to its almost non-existent AHP. When this presynaptic neuron releases glutamate for the second time onto the postsynaptic neuron, summation of synaptic events occurs due to synaptic facilitation (a classical consequence of the low probability of release), and therefore the likelihood that the postsynaptic neuron will fire an action potential is even higher than after the first release of glutamate. Thus, these properties create a positive feedback loop that will engage the network in an epileptiform regime of activity. In the mature CA3 the high probability of release and the longer latency of synaptic events are unfavourable to summation of synaptic events, and the lower membrane resistance and larger AHP prevent repetitive firing, such that the recurrent wave of activity will die out after a small number of cycles. This work is particularly interesting because it underlines the importance of AMPA receptors and intrinsic excitability in the genesis of epileptiform activity in the immature hippocampal network. Over the past twenty years, an increasing interest has been taken in the role of the depolarizing actions of GABA in epileptiform activity in the immature hippocampus (until postnatal week 2) (Ben-Ari et al. 2007). The experimental paradigm Shao and Dudek used rules out the involvement of depolarizing actions of GABA in the initiation of prolonged population bursts, as all experiments were performed in the presence of GABAA-receptor blockers. Indirectly, this work undermines the role of depolarizing GABA in the genesis of some forms of epileptiform activity in the hippocampus. This timely study echoes the recent results obtained by Rheims et al. (2009) who demonstrated that depolarizing actions of GABA in immature networks may have been overestimated in most studies due to differences in brain metabolism between immature and mature animals, which were usually not taken into account in experimental paradigms. These authors showed that accounting for these metabolic differences significantly diminishes the depolarizing effect of GABA in young networks. Taken together, these two studies support the idea that at least some forms of epileptiform activity observed in the young hippocampus might rely on increased synaptic and intrinsic excitability of excitatory CA3 neurons, rather than on particular properties of GABA neurotransmission.

Neurotransmitter identity and electrophysiological phenotype are genetically coupled in midbrain dopaminergic neurons

Most neuronal types have a well-identified electrical phenotype. It is now admitted that a same phenotype can be produced using multiple biophysical solutions defined by ion channel expression levels. This argues that systems-level approaches are necessary to understand electrical phenotype genesis and stability. Midbrain dopaminergic (DA) neurons, although quite heterogeneous, exhibit a characteristic electrical phenotype. However, the quantitative genetic principles underlying this conserved phenotype remain unknown. Here we investigated the quantitative relationships between ion channels’ gene expression levels in midbrain DA neurons using single-cell microfluidic qPCR. Using multivariate mutual information analysis to decipher high-dimensional statistical dependences, we unravel co-varying gene modules that link neurotransmitter identity and electrical phenotype. We also identify new segregating gene modules underlying the diversity of this neuronal population. We propose that the newly identified genetic coupling between neurotransmitter identity and ion channels may play a homeostatic role in maintaining the electrophysiological phenotype of midbrain DA neurons.

Information topology of gene expression profile in dopaminergic neurons

Extracting high-degree interactions and dependences between variables (pairs, triplets, … k-tuples) is a challenge posed by all omics approaches1, 2. Here we used multivariate mutual information (Ik) analysis3 on single-cell retro-transcription quantitative PCR (sc-RTqPCR) data obtained from midbrain neurons to estimate the k-dimensional topology of their gene expression profiles. 41 mRNAs were quantified and statistical dependences in gene expression levels could be fully described for 21 genes: Ik analysis revealed a complex combinatorial structure including modules of pairs, triplets (up to 6-tuples) sharing strong positive, negative or zero Ik, corresponding to co-varying, clustering and independent sets of genes, respectively. Therefore, Ik analysis simultaneously identified heterogeneity (negative Ik) of the cell population under study and regulatory principles conserved across the population (homogeneity, positive Ik). Moreover, maximum information paths enabled to determine the size and stability of such transcriptional modules. Ik analysis represents a new topological and statistical method of data analysis.

Somato-dendritic processing of postsynaptic potentials II. Role of sub-threshold depolarizing voltage-gated currents

The combination of the large variation in synaptic distance and the cable-filtering properties of dendrites should, in theory, cause the amplitude and temporal characteristics of functionally similar inputs to be highly variable at the final integration site. Although theoretical analyses have predicted such a clear location-dependent variability of synaptic input, there is now considerable evidence indicating that the shape of EPSPs may be relatively independent of synapse location (e.g. in pyramidal neurons of the CA1 region of the hippocampus). The ability to simultaneously record synaptic activity from several locations on the same neuron (distal or proximal dendrites, soma) and the advent of imaging techniques with high spatial and temporal resolution now give the opportunity to understand the real dendritic processing of synaptic information.

Chapter 18 – Synaptic plasticity

Synaptic responses undergo short- and long-term modifications. This chapter examines the mechanisms underlying plasticity in adult synapses. Developmental forms of plasticity are not covered here. The first form of long-term changes of synaptic efficacy is called hebbian plasticity and comprises long-term potentiation (LTP) and long-term depression (LTD). Moreover, there are several forms of LTP and LTD classified by their mechanisms and their mode of induction. We have chosen three examples, the NMDA receptor-dependent LTP, the metabotropic glutamate receptors (mGluR)-dependent LTD in the cerebellum and the spike-timing-dependent plasticity (STDP). The second, more recently discovered, form of long-term changes of synaptic efficacy is called homeostatic plasticity and occurs in response to prolonged changes in activity. Homeostatic plasticity can affect both excitatory and inhibitory synapses, but we have chosen the example of synaptic scaling at glutamatergic synapses to illustrate this type of plasticity.