Elsevier

Neuroscience

Volume 136, Issue 4, 2005, Pages 1083-1095
Neuroscience

Neuroanatomy
GABAB receptors at glutamatergic synapses in the rat striatum

https://doi.org/10.1016/j.neuroscience.2005.07.013Get rights and content

Abstract

Although multiple effects of GABAB receptor activation on synaptic transmission in the striatum have been described, the precise locations of the receptors mediating these effects have not been determined. To address this issue, we carried out pre-embedding immunogold electron microscopy in the rat using antibodies against the GABAB receptor subunits, GABAB1 and GABAB2. In addition, to investigate the relationship between GABAB receptors and glutamatergic striatal afferents, we used antibodies against the vesicular glutamate transporters, vesicular glutamate transporter 1 and vesicular glutamate transporter 2, as markers for glutamatergic terminals. Immunolabeling for GABAB1 and GABAB2 was widely and similarly distributed in the striatum, with immunogold particles localized at both presynaptic and postsynaptic sites. The most commonly labeled structures were dendritic shafts and spines, as well as terminals forming asymmetric and symmetric synapses. In postsynaptic structures, the majority of labeling associated with the plasma membrane was localized at extrasynaptic sites, although immunogold particles were also found at the postsynaptic specialization of some symmetric, putative GABAergic synapses. Labeling in axon terminals was located within, or at the edge of, the presynaptic active zone, as well as at extrasynaptic sites. Double labeling for GABAB receptor subunits and vesicular glutamate transporters revealed that labeling for both GABAB1 and GABAB2 was localized on glutamatergic axon terminals that expressed either vesicular glutamate transporter 1 or vesicular glutamate transporter 2. The patterns of innervation of striatal neurons by the vesicular glutamate transporter 1- and vesicular glutamate transporter 2-positive terminals suggest that they are selective markers of corticostriatal and thalamostriatal afferents, respectively. These results thus provide evidence that presynaptic GABAB heteroreceptors are in a position to modulate the two major excitatory inputs to striatal spiny projection neurons arising in the cortex and thalamus. In addition, presynaptic GABAB autoreceptors are present on the terminals of spiny projection neurons and/or striatal GABAergic interneurons. Furthermore, the data indicate that GABA may also affect the excitability of striatal neurons via postsynaptic GABAB receptors.

Section snippets

Animals and tissue preparation

The experiments were carried out on 12 male Sprague–Dawley rats (180–300 g; Charles River, Margate, Kent, UK). Environmental conditions for housing of the rats and all procedures that were performed were in accordance with the Animals (Scientific Procedures) Act of 1986 (UK). Every effort was made to use the minimum number of animals and to minimize suffering. They were deeply anesthetized with sodium pentobarbitone (200 mg/kg; i.p.; Sagatal; Rhône Mérieux, Tallaght, Dublin, Ireland) and perfused

Subcellular localization of GABAB1 and GABAB2 in the striatum

Immunolabeling for GABAB1 and GABAB2 was identified by the presence of the electron dense immunogold particles. Labeled structures generally contained multiple particles and it is on this labeling that our qualitative description of the distribution is based. However, structures were observed that possessed only one immunogold particle and, although we cannot be sure that they do not represent background labeling, the profile of structures labeled was similar to those labeled by multiple

Localization of GABAB receptor subunits in the striatum

Studies in knockout mice lacking either GABAB1 or GABAB2 have provided the most compelling evidence to date that GABAB receptors require both subunits in order to function normally in vivo (Prosser et al 2001, Schuler et al 2001, Gassmann et al 2004). In situ hybridization studies, however, have demonstrated that GABAB2 mRNA is at low or undetectable levels in rat striatum, whereas GABAB1 is more strongly expressed in both embryonic and adult striatum (Durkin et al 1999, Clark et al 2000,

Conclusions

The findings of the present study provide an anatomical substrate for the presynaptic effects of GABAB receptor stimulation at glutamatergic cortical and thalamic synapses in the striatum thus underpinning the close interactions between the major inhibitory and excitatory neurotransmitter systems in this nucleus. They also demonstrate that stimulation of synaptic and extrasynaptic GABAB receptors is likely to affect GABAergic transmission at both presynaptic and postsynaptic sites.

Acknowledgments

The work was funded by the Medical Research Council UK and the Research Grants Council of Hong Kong. C.J.L. is in receipt of a Medical Research Council studentship. We thank Caroline Francis, Ben Micklem, and Liz Norman for their expert technical assistance.

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  • Cited by (0)

    1

    Present address: Department of Physiology, Qingdao University, Qingdao 266021, China.

    2

    These authors contributed equally to this manuscript.

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