Metabotropic glutamate receptor mGluR5 subcellular distribution and developmental expression in hypothalamus

Metabotropic glutamate receptor activation modulates kainate and serotonin calcium response in astrocytes

Although metabotropic glutamate receptor (mGluR) modulation has been studied extensively in neurons, it has not been investigated in astrocytes. We studied modulation of glutamate-evoked calcium rises in primary astrocyte cultures using fura-2 ratiometric digital calcium imaging. Calcium plays a key role as a second messenger system in astrocytes, both in regulation of many subcellular processes and in long distance intercellular signaling. Suprachiasmatic nucleus (SCN) and cortical astrocytes showed striking differences in sensitivity to glutamate and to mGluR agonists, even after several weeks in culture. Kainate-evoked intracellular calcium rises were inhibited by concurrent application of the type I and II mGluR agonists quisqualate (10 micro;M), trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (100-500 micro;M), and (2S-1'S-2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) (10 micro;M). Inhibition mediated by L-CCG-I had long-lasting effects (>45 min) in approximately 30% of the SCN astrocytes tested. The inhibition could be mimicked by the L-type calcium channel blocker nimodipine (1 micro;M) as well as by protein kinase C (PKC) activators phorbol 12,13-dibutyrate (10 micro;M) and phorbol 12-myristate 13-acetate (500 nM), and blocked by the PKC inactivator (+/-)-1-(5-isoquinolinesulfonyl)-2-methylpiperazine (200 micro;M), suggesting a mechanism involving PKC modulation of L-type calcium channels. In contrast, mGluRs modulated serotonin (5HT)-evoked calcium rises through a different mechanism. The type III mGluR agonist L-2-amino-4-phosphonobutyrate consistently inhibited 5HT-evoked calcium rises, whereas in a smaller number of cells quisqualate and L-CCG-I showed both inhibitory and additive effects. Unlike the mGluR-kainate interaction, which required a pretreatment with an mGluR agonist and was insensitive to pertussis toxin (PTx), the mGluR modulation of 5HT actions was rapid and was blocked by PTx. These data suggest that glutamate, acting at several metabotropic receptors expressed by astrocytes, could modulate glial activity evoked by neurotransmitters and thereby influence the ongoing modulation of neurons by astrocytes.

Local synaptic release of glutamate from neurons in the rat hypothalamic arcuate nucleus

1. The hypothalamic arcuate nucleus (ARC) contains neuroendocrine neurons that regulate endocrine secretions by releasing substances which control anterior pituitary hormonal release into the portal blood stream. Many neuroactive substances have been identified in the ARC, but the existence of excitatory neurons in the ARC and the identity of an excitatory transmitter have not been investigated physiologically. 2. In the present experiments using whole-cell current- and voltage-clamp recording of neurons from cultures and slices of the ARC, we demonstrate for the first time that some of the neurons in the ARC secrete glutamate as their transmitter. 3. Using microdrop stimulation of presynaptic neurons in ARC slices, we found that local axons from these glutamatergic neurons make local synaptic contact with other neurons in the ARC and that all evoked excitatory postsynaptic potentials could be blocked by the selective ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and D,L-2-amino-5-phosphonovalerate (AP5; 100 microM). To determine the identity of ARC neurons postsynaptic to local glutamatergic neurons, we used antidromic stimulation to reveal that many of these cells were neuroendocrine neurons by virtue of their maintaining axon terminals in the median eminence. 4. In ARC cultures, postsynaptic potentials, both excitatory and inhibitory, were virtually eliminated by the glutamate receptor antagonists AP5 and CNQX, underlining the functional importance of glutamate within this part of the neuroendocrine brain. 5. GABA was secreted by a subset of ARC neurons from local axons. The GABAA receptor antagonist bicuculline released glutamatergic neurons from chronic inhibition mediated by synaptically released GABA, resulting in further depolarization and an increase in the amplitude and frequency of glutamate-mediated excitatory postsynaptic potentials.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Putative excitatory amino acid projections to the suprachiasmatic nucleus in the rat

Retrograde axonal transport of the select neuronal tracer [3H]D-aspartate was used to demonstrate possible sources of excitatory input to the suprachiasmatic nucleus (SCN) in the albino rat. Following injection of [3H]D-aspartate into the SCN, neurons were retrogradely labeled in the infralimbic cortex, the lateral septal nucleus, the paraventricular thalamic nucleus, the medial preoptic area, the ventromedial, dorsomedial and posterior hypothalamic nuclei, the zona incerta, the intergeniculate leaflet and the ventral subiculum. Retinal ganglion cells, which project to the SCN and use glutamate as a neurotransmitter, were not labeled in our [3H]D-aspartate experiments, demonstrating a limitation of this method (i.e., false negatives). Our results show that the [3H]D-aspartate neuronal tracer labels a subset of areas known to project to the SCN, indicating these areas as likely sources of excitatory input to the SCN.

Enhanced early developmental expression of the metabotropic glutamate receptor mGluR5 in rat brain: protein, mRNA splice variants, and regional distribution

Glutamate stimulates phosphatidyl inositol hydrolysis and mobilizes intracellular calcium through the mediation of metabotropic glutamate receptors (mGluRs), in particular the "Group I" receptors mGluRs, mGluR1, and mGluR5. This activity is markedly enhanced in developing brain relative to the adult. To determine whether this may be due to an increased amount of mGluR5 present in the developing brain, we examined mGluR5 expression using western blotting to measure mGluR5 protein reverse transcription polymerase chain reaction (RT-PCR) to measure mGluR5 mRNA, and immunocytochemistry to assess the regional distribution of mGluR5 morphologically. Western blotting revealed that in all brain regions examined there is more mGluR5 protein present in developing brain than in the adult. In most regions, the developmental decrease was over two-fold. Total mGluR5 mRNA also decreased with development in most regions, but to a much lesser extent than the protein, suggesting that there is considerable post-transcriptional regulation of the expression of this receptor. RT-PCR analysis also demonstrated that in most regions the mGluR5a splice variant is most abundant in the young animals but mGluR5b predominates in the adult. Light microscopic immunocytochemistry indicated that expression is widespread in developing brain, and that the developmental decrease in receptor concentration is due to both an increased growth of receptor-poor tissue regions and decreased expression within receptor-rich regions.