ABSTRACT:Communication between neurons rests on their capacity to change their firing pattern to encode different messages. For several vital functions, such as respiration and mastication, neurons need to generate a rhythmic firing pattern. Here we show in the rat trigeminal sensori-motor circuit for mastication that this ability depends on regulation of the extracellular Ca 2+ concentration ([Ca 2+ ] e ) by astrocytes. In this circuit, astrocytes respond to sensory stimuli that induce neuronal rhythmic activity, and their blockade with a Ca 2+ chelator prevents neurons from generating a rhythmic bursting pattern.This ability is restored by adding S100β, an astrocytic Ca 2+ -binding protein, to the extracellular space, while application of an anti-S100β antibody prevents generation of rhythmic activity. These results indicate that astrocytes regulate a fundamental neuronal property: the capacity to change firing pattern. These findings may have broad implications for many other neural networks whose functions depend on the generation of rhythmic activity.

No abstract

There is increasing evidence that a subpopulation of neurons in the dorsal principal sensory trigeminal nucleus are not simple sensory relays to the thalamus but may form the core of the central pattern generating circuits responsible for mastication. In this paper, we used whole cell patch recordings in brain stem slices of young rats to show that these neurons have intrinsic bursting abilities that persist in absence of extracellular Ca(2+). Application of different K(+) channel blockers affected duration and firing rate of bursts, but left bursting ability intact. Bursting was voltage dependent and was abolished by low concentrations of Na(+) channel blockers. The proportion of bursting neurons increased dramatically in the second postnatal week, in parallel with profound changes in several electrophysiological properties. This is the period in which masticatory movements appear and mature. Bursting was associated with the development of an afterdepolarization that depend on maturation of a persistent sodium conductance (I(NaP)). An interesting finding was that the occurrence of bursting and the magnitude of I(NaP) were both modulated by the extracellular concentration of Ca(2+). Lowering extracellular [Ca(2+)] increased both I(NaP) and probability of bursting. We suggest that these mechanisms underlie burst generation in mastication and that similar processes may be found in other motor pattern generators.

Stimuli that induce rhythmic firing in trigeminal neurons also increase astrocytic coupling and reveal networks that define the boundaries of this particular population. Rhythmic firing depends on astrocytic coupling which in turn depends on S100β. In many nervous functions that rely on the ability of neuronal networks to generate a rhythmic pattern of activity, coordination of firing is an essential feature. Astrocytes play an important role in some of these networks, but the contribution of astrocytic coupling remains poorly defined. Here we investigate the modulation and organization of astrocytic networks in the dorsal part of the trigeminal main sensory nucleus (NVsnpr), which forms part of the network generating chewing movements. Using whole-cell recordings and the dye coupling approach by filling a single astrocyte with biocytin to reveal astrocytic networks, we showed that coupling is limited under resting conditions, but increases importantly under conditions that induce rhythmic firing in NVsnpr neurons. These are: repetitive electrical stimulation of the sensory inputs to the nucleus, local application of NMDA and decrease of extracellular Ca . We have previously shown that rhythmic firing induced in NVsnpr neurons by these stimuli depends on astrocytes and their Ca -binding protein S100β. Here we show that extracellular blockade of S100β also prevents the increase in astrocytic coupling induced by local application of NMDA. Most of the networks were small and remained confined to the functionally distinct area of dorsal NVsnpr. Disrupting coupling by perfusion with the nonspecific gap junction blocker, carbenoxolone or with GAP26, a selective inhibitor of connexin 43, mostly expressed in astrocytes, abolished NMDA-induced rhythmic firing in NVsnpr neurons. These results suggest that astrocytic coupling is regulated by sensory inputs, necessary for neuronal bursting, and organized in a region specific manner.

In this paper, we investigated the influence of synapses on the cell bodies of trigeminal muscle spindle afferents that lie in the trigeminal mesencephalic nucleus (NVmes), using intracellular recordings in brain stem slices of young rats. Three types of synaptic responses could be evoked by electrical stimulation of the adjacent supratrigeminal, motor, and main sensory nuclei and the intertrigeminal area: monophasic depolarizing postsynaptic potentials (PSPs), biphasic PSPs, and all or none action potentials without underlying excitatory PSPs (EPSPs). Many PSPs and spikes were abolished by bath-application of 6,7-dinitroquinoxaline (DNQX) alone or combined with D,L-2-amino-5-phosphonovaleric acid (APV), suggesting that they are mediated by non-N-methyl-D-aspartate (NMDA) and NMDA glutamatergic receptors, while some action potentials were sensitive to bicuculline, indicating involvement of GABAA receptors. A number of cells showed spontaneous membrane potential oscillations, and stimulation of synaptic inputs increased the amplitude of the oscillations for several cycles, which often triggered repetitive firing. Furthermore, the oscillatory rhythm was reset by the stimulation. Our results show that synaptic inputs to muscle primary afferent neurons in NVmes from neighboring areas are mainly excitatory and that they cause firing. In addition, the inputs synchronize intrinsic oscillations, which may lead to sustained, synchronous firing in a subpopulation of afferents. This may be of importance during rapid biting and during the mastication of very hard or tough foods.

The main axons of mammalian sensory neurons are usually viewed as passive transmitters of sensory information. However, the spindle afferents of jaw-closing muscles behave as if action potential traffic along their central axons is phasically regulated during rhythmic jaw movements. In this paper, we used brainstem slices containing the cell bodies, stem axons, and central axons of these sensory afferents to show that GABA applied to the descending central (caudal) axon often abolished antidromic action potentials that were elicited by electrical stimulation of the tract containing the caudal axons of the recorded cells. This effect of GABA was most often not associated with a change in membrane potential of the soma and was still present in a calcium-free medium. It was mimicked by local applications of muscimol on the axons and was blocked by bath applications of picrotoxin, suggesting activation of GABA(A) receptors located on the descending axon. Antidromic action potentials could also be blocked by electrical stimulation of local interneurons, and this effect was prevented by bath application of picrotoxin, suggesting that it results from the activation of GABA(A) receptors after the release of endogenous GABA. We suggest that blockage is caused mainly by shunting within the caudal axon and that motor command circuits use this mechanism to disconnect the rostral and caudal compartments of the central axon, which allows the two parts of the neuron to perform different functions during movement.

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

Pairing a cutaneous electrical stimulus of the hind-paw with stimulation of the basal forebrain produces long-term cholinergic enhancement of the responsiveness to a tactile stimulus. A short period of pairing (20 trials) increased the area of the two main components of the evoked potential by 37.1 +/- 13.5% (+/- SEM) and 37.9 +/- 6.8%, respectively. The effects lasted for the duration of the experiment (> 2 h). The enhancement could be blocked by either MK-801, an NMDA receptor antagonist or by L-NAME, a nitric-oxide-synthase inhibitor when they were given prior to pairing. Control experiments with skin stimulation alone and basal forebrain stimulation alone had only small long-term effects (approximately 10%) on the size of the evoked potential. Thus, long-term cholinergic enhancement, attributable to disinhibition and increased release of acetylcholine in the cortex during neuronal excitation by other sources, and so named because it is blocked by atropine, may be a form of long-term potentiation. The existence of such a mechanism for the control of cortical neuronal plasticity identifies the basal forebrain as a powerful modulator of long-lasting changes in cortical neuronal excitability.