No abstract

Valproic acid (VPA) is widely prescribed to treat epilepsy. Maternal VPA use is, however, clinically restricted because of the severe risk that VPA may cause neurodevelopmental disorders in offspring, such as autism spectrum disorder. Understanding the negative action of VPA may help to prevent VPA-induced neurodevelopmental disorders. Astrocytes play a vital role in neurodevelopment and synapse function; however, the impact of VPA on astrocyte involvement in neurodevelopment and synapse function has not been examined. In this study, we examined whether exposure of cultured astrocytes to VPA alters neuronal morphology and synapse function of co-cultured neurons. We show that synaptic transmission by inhibitory neurons was small because VPA-exposed astrocytes reduced the number of inhibitory synapses. However, synaptic transmission by excitatory neurons and the number of excitatory synapses were normal with VPA-exposed astrocytes. VPA-exposed astrocytes did not affect the morphology of inhibitory neurons. These data indicate that VPA-exposed astrocytes impair synaptogenesis specifically of inhibitory neurons. Our results indicate that maternal use of VPA would affect not only neurons but also astrocytes and would result in perturbed astrocyte-mediated neurodevelopment.

27The astrocyte, a major glial cell type, is involved in formation and maturation of 28 synapses, and thus contributes to sustainable synaptic transmission between neurons. 29 Given that the animals in the higher phylogenetic tree have brains with higher density of 30 glial cells with respect to neurons, there is a possibility that the relative astrocytic 31 density directly influences synaptic transmission. However, the notion has not been 32 tested thoroughly. Here we addressed it, by using a primary culture preparation where 33 single hippocampal neurons are surrounded by a variable but countable number of 34 cortical astrocytes in dot-patterned microislands, and recording synaptic transmission by 35 patch-clamp electrophysiology. Neurons with a higher astrocytic density showed a 36 higher amplitude of evoked excitatory postsynaptic current (EPSC) than that of neurons 37 with a lower astrocytic density. The size of readily releasable pool of synaptic vesicles 38 per neuron was significantly higher. The frequency of spontaneous synaptic 39 transmission (miniature EPSC) was higher, but the amplitude was unchanged. The 40 number of morphologically identified glutamatergic synapses was unchanged, but the 41 number of functional ones was increased, indicating a lower ratio of presynaptically 42 silent synapses. Taken together, the higher astrocytic density enhanced excitatory 43 synaptic transmission by increasing the number of functional synapses through 44 presynaptic un-silencing. 45 46 Keywords Abbreviations 50 ANLS: astrocyte-neuron lactate shuttle 51 APV: (2R)-amino-5-phosphonovaleric acid 52 CNQX: 6-cyano-7-nitroquinoxaline-2,3-dione 53 DAPI: 4',6-diamidino-2-phenylindole 54 EPSC: excitatory postsynaptic current 55 FM1-43: N-(3-triethylammoniumpropyl)-4-(4-(dibutyl amino) styryl) pyridinium 56 dibromide 57 HDG: high-density group 58 LDG: low-density group 59 LED: light-emitting diode 60 MAP2: microtubule-associated protein 2 61 mEPSC: miniature excitatory postsynaptic current 62 PBS: phosphate-buffered saline 63 Pvr: vesicular release probability 64 RRP: readily releasable pool 65 TTX: tetrodotoxin 66 VGLUT1: vesicular glutamate transporter 1 67 Vh: holding potential 68 69While neurons are indispensable for information processing through neural circuits, it 71 has been reported that glial cells play essential roles in brain physiology and 72 development (Barres, 2008). Dynamic information processing in the brain is not only 73 due to the interaction between neurons, but also to the interaction between neurons and 74 astrocytes, a major type of glial cells. In particular, the structure and function of 75 synapses, the basic information-processing units of neurons, are closely regulated by 76 astrocytes. For example, astrocytes are involved in the regulation of synaptic strength 77 (Haydon, 2001) and synapse formation (Nedergaard et al., 2003). Neurons co-cultured 78 with astrocytes have higher synaptic efficacy compared with neurons in the absence of 79 astrocytes, based on direct contact with and humoral ...

Information processing in the brain is performed not only by neurons but also by glial cells. Specifically, a major glial cell type, the astrocyte, is in charge of forming and maturing synapses to establish sustainable synaptic transmission in the brain. Generally, higher animals have larger brains. There is a possibility that the number of astrocytes determines the intricate brain function in which a higher animal's brain has a higher density of astrocytes. This study took advantage of a primary co-culture system using a single autaptic hippocampal neuron with dot-patterned cortical astrocytes. This preparation enables the proper and systematic counting of astrocytes surrounding a single neuron. As a result, a hippocampal neuron with a higher density of astrocytes showed more excellent excitatory synaptic transmission than that of neurons with a lower density of astrocytes. This result was accompanied by a significant increase in the pool of readily releasable synaptic vesicles. The number of morphologically identified glutamatergic synapses was comparable, but the percentage of functional ones was increased, indicating a lower ratio of presynaptically silent synapses. Taken together, the higher astrocytic density enhanced excitatory synaptic transmission by increasing the fraction of functional synapses through presynaptic un-silencing.