Stéphane H. R. Oliet

Activity‐dependent long‐term synaptic changes were investigated at glutamatergic synapses in the supraoptic nucleus (SON) of the rat hypothalamus. In acute hypothalamic slices, high frequency stimulation (HFS) of afferent fibres caused long‐term potentiation (LTP) of the amplitude of AMPA receptor‐mediated excitatory postsynaptic currents (EPSCs) recorded with the whole‐cell patch‐clamp technique. LTP was also obtained in response to membrane depolarization paired with mild afferent stimulation. On the other hand, stimulating the inputs at 5 Hz for 3 min at resting membrane potential caused long‐term depression (LTD) of excitatory transmission in the SON. These forms of synaptic plasticity required the activation of NMDA receptors since they were abolished in the presence of d‐AP5 or ifenprodil, two selective blockers of these receptors. Analysis of paired‐pulse facilitation and trial‐to‐trial variability indicated that LTP and LTD were not associated with changes in the probability of transmitter release, thereby suggesting that the locus of expression of these phenomena was postsynaptic. Using sharp microelectrode recordings in a hypothalamic explant preparation, we found that HFS also generates LTP at functionally defined glutamatergic synapses formed between the organum vasculosum lamina terminalis and SON neurons. Taken together, our findings indicate that glutamatergic synapses in the SON exhibit activity‐dependent long‐term synaptic changes similar to those prevailing in other brain areas. Such forms of plasticity could play an important role in the context of physiological responses, like dehydration or lactation, where the activity of presynaptic glutamatergic neurons is strongly increased.

Astrocytes clear synaptically released glutamate from the extracellular space through high‐affinity transporters present on their plasma membrane. By controlling the extracellular level of the main excitatory transmitter in the central nervous system, astrocytes thus contribute prominently to the regulation of overall cellular excitability and synaptic information processing. We recently investigated the influence of the glial environment on glutamatergic and GABAergic neurotransmission in the supraoptic nucleus of the rat hypothalamus under physiological conditions such as lactation that significantly reduce astrocytic coverage of its neurons. By performing electrophysiological analyses on this unique model of dynamic neuronal‐glial interactions, we have been able to show that the fine astrocytic processes normally enwrapping synapses serve two important functions. First, they govern the level of activation of presynaptic metabotropic glutamate receptors on glutamatergic terminals, thereby regulating synaptic efficacy at excitatory synapses. Second, they act as a physical and functional barrier to diffusion in the extracellular space, limiting spillover of glutamate and other neuroactive substances and therefore contributing to the regulation of heterosynaptic transmission and intercellular communication.

The adult hypothalamic‐neurohypophysial system undergoes activity‐dependent morphological plasticity that modifies the astrocytic enwrapping of its magnocellular neurones. For a long time, the functional consequences of such changes have remained hypothetical. Modifications in the glial environment of neurones are expected to have important physiological repercussions in view of the various functions played by astrocytes in the central nervous system. In particular, glial cells are essential for uptake of neurotransmitters, including glutamate, and for physically and functionally restricting diffusion of neuroactive substances within the extracellular space. Recent studies performed in the supraoptic nucleus of lactating and chronically dehydrated animals, in conditions where astrocytic coverage of neurones is reduced, have revealed a significant impairment of glutamate clearance. The resulting accumulation of the excitatory amino acid in the extracellular space around glutamatergic inputs causes an enhanced activation of presynaptic metabotropic glutamate receptors that inhibit transmitter release. In the supraoptic nucleus of lactating rats, neuroglial remodelling is accompanied by modification of the geometry, size and diffusion properties of the extracellular space. The latter observations suggest that, in the activated supraoptic nucleus, the range of action and the concentration of released neuroactive substances may be significantly enhanced. Overall, our observations indicate that the glial environment of supraoptic neurones influences synaptic glutamatergic transmission, as well as extrasynaptic forms of communication.

Interactions between separate synaptic inputs converging on the same target appear to contribute to the fine-tuning of information processing in the central nervous system. Intersynaptic crosstalk is made possible by transmitter spillover from the synaptic cleft and its diffusion over a distance to neighboring synapses. This is the case for glutamate, which inhibits γ-aminobutyric acid (GABA)ergic transmission in several brain regions through the activation of presynaptic receptors. Such heterosynaptic modulation depends on factors that influence diffusion in the extracellular space (ECS). Because glial cells represent a physical barrier to diffusion and, in addition, are essential for glutamate uptake, we investigated the physiological contribution of the astrocytic environment of neurons to glutamate-mediated intersynaptic communication in the brain. Here we show that the reduced astrocytic coverage of magnocellular neurons occurring in the supraoptic nucleus of lactating rats facilitates diffusion in the ECS, as revealed by tortuosity and volume fraction measurements. Under these conditions, glutamate spillover, monitored through metabotropic glutamate receptor-mediated depression of GABAergic transmission, is greatly enhanced. Conversely, impeding diffusion with dextran largely prevents crosstalk between glutamatergic and GABAergic afferent inputs. Astrocytes, therefore, by hindering diffusion in the ECS, regulate intersynaptic communication between neighboring synapses and, probably, overall volume transmission in the brain.

The presence of group III metabotropic glutamate receptors on GABAergic terminals in the supraoptic nucleus suggests that the level of glutamate in the extracellular space may regulate synaptic strength at inhibitory synapses. To test this hypothesis we examined the consequences of increasing ambient glutamate on GABA‐mediated synaptic activity in supraoptic neurons. The concentration of the excitatory amino acid in the extracellular space was increased pharmacologically by blocking glutamate transporters. Inhibition of the astrocyte‐specific GLT‐1 glutamate transporter led to a reversible decrease in evoked inhibitory postsynaptic current amplitude. This modulation had a presynaptic origin as revealed by analysis of paired‐pulse ratio and miniature inhibitory currents. Furthermore, blocking group III metabotropic glutamate receptors with the specific antagonist MAP4 prevented the depression of GABAergic transmission induced by glutamate transporter blockade. Thus, presynaptic metabotropic glutamate receptors located on inhibitory terminals in the supraoptic nucleus appear to sense changes in ambient glutamate and modify GABA release accordingly. However, it seems that such changes need to reach a certain magnitude because the discrete deficit in glutamate clearance which occurs in the supraoptic nucleus of lactating rats is not sufficient to modulate GABA‐mediated transmission. These results suggest that ambient glutamate contributes to the modulation of synaptic efficacy not only at glutamatergic synapses but also at inhibitory GABAergic synapses.

The supraoptic and paraventricular nuclei of the hypothalamus undergo reversible anatomical changes under conditions of intense neurohypophysial hormone secretion, such as lactation, parturition and chronic dehydration. This morphological remodelling includes a reduction in astrocytic coverage of neurones resulting in an increase in the number and extent of directly juxtaposed somatic and dendritic surfaces. There is a growing body of evidence indicating that such anatomical plasticity is of functional significance. Astrocytic‐dependent clearance of electrolytes and neurotransmitters from the extracellular space appears to be altered under conditions where glial coverage of magnocellular neurones is reduced. Glutamate, for example, has been found to accumulate in the extracellular space in the supraoptic nucleus of lactating animals and cause a modulation of synaptic efficacy. On the other hand, the range of action of substances released from astrocytes and acting on adjacent magnocellular neurones is expected to be limited during such anatomical remodelling. It thus appears that the structural plasticity of the magnocellular nuclei does affect neuroglial interactions, inducing significant changes in signal transmission and processing.

Analysis of excitatory synaptic transmission in the rat hypothalamic supraoptic nucleus revealed that glutamate clearance and, as a consequence, glutamate concentration and diffusion in the extracellular space, is associated with the degree of astrocytic coverage of its neurons. Reduction in glutamate clearance, whether induced pharmacologically or associated with a relative decrease of glial coverage in the vicinity of synapses, affected transmitter release through modulation of presynaptic metabotropic glutamate receptors. Astrocytic wrapping of neurons, therefore, contributes to the regulation of synaptic efficacy in the central nervous system.