A group of cortical interneurons expressing μ-opioid receptor-like immunoreactivity: a double immunofluorescence study in the rat cerebral cortex

doi:10.1016/S0306-4522(00)00124-X

Neuroscience

Volume 98, Issue 2, June 2000, Pages 221-231

https://doi.org/10.1016/S0306-4522(00)00124-X Get rights and content

Abstract

μ-Opioid receptor-expressing neurons in the rat cerebral neocortex were characterized by an immunolabeling method with an antibody to a carboxyl terminal portion of the receptor. They were small, bipolar, vertically elongated, non-pyramidal neurons, and scattered mainly in layers II–IV. We examined chemical characteristics of μ-opioid receptor-expressing neocortical neurons by the double immunofluorescence method. Almost all neuronal cell bodies expressing μ-opioid receptor-like immunoreactivity showed immunoreactivity for GABA, suggesting that they were cortical inhibitory interneurons. μ-Opioid receptor-immunoreactive neurons were further studied by the double staining method with markers for the subgroups of cortical GABAergic neurons. Immunoreactivities for vasoactive intestinal polypeptide, corticotropin releasing factor, choline acetyltransferase, calretinin and cholecystokinin were found in 92, 79, 67, 35 and 35% of μ-opioid receptor-immunoreactive cortical neurons, respectively. In contrast, less than 10% of μ-opioid receptor-immunoreactive neurons showed immunoreactivity for parvalbumin, calbindin, somatostatin, neuropeptide Y or nitric oxide synthase. Moreover, μ-opioid receptor-immunoreactive neurons very frequently exhibited preproenkephalin immunoreactivity, but not preprodynorphin immunoreactivity.

The present results indicate that μ-opioid receptor-expressing neurons belong to a distinct subgroup of neocortical GABAergic neurons, because vasoactive intestinal polypeptide, corticotropin releasing factor, choline acetyltransferase, calretinin and cholecystokinin have often been reported to coexist with one another in single neocortical neurons. Methionine–enkephalin, which is a major product of the preproenkephalin gene, is known to be one of the most potent endogenous ligands for μ-opioid receptor. Thus, the expression of μ-opioid receptor in preproenkephalin-producing neurons suggested that μ-opioid receptor serves as an autoreceptor for the subpopulation of GABAergic interneurons at a single-neuron or population level.

Section snippets

Experimental procedures

The procedures of the present experiments were approved by the Animal Care and Use Committee at the Graduate School of Medicine in Kyoto University, Japan.

Results

μ-Opioid receptor-immunoreactive neurons in the cerebral cortex were small, bipolar, and vertically elongated in shape (Fig. 1a,b) except those found in deep layer VI, most of which were multipolar (Fig. 1c). They were distributed throughout layers II–VI, more frequently in the superficial layers than in the deep layers, of the neocortical regions (Fig. 2). The number of MOR-immunoreactive neurons per two frontal 20-μm-thick sections of the frontoparietal neocortical regions was 303±59

Discussion

The present study showed that MOR-expressing neurons were mostly small, vertically elongated, bipolar cells and scattered more frequently in the superficial layers than in the deep layers throughout the cerebral neocortex, and that almost all MOR-expressing neocortical neurons belonged to GABAergic neurons. Furthermore, MOR-expressing neurons were chemically characterized by production of VIP, CRF, ChAT, calretinin and cholecystokinin (Table 1).

Since the present immunocytochemical study was

Acknowledgements

We are grateful for the photographic help of Mr A. Uesugi. This work was supported by Grants-in-Aids from the Ministry of Education, Science, Sports and Culture of Japan (09480211, 11145221, 11170299, 11878151, 12053245), and by Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists 4601).

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^§: Present address: Tokyo Metropolitan Institute for Neuroscience, Fuchu, Tokyo 183-8526, Japan

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A group of cortical interneurons expressing μ-opioid receptor-like immunoreactivity: a double immunofluorescence study in the rat cerebral cortex

Abstract

Section snippets

Experimental procedures

Results

Discussion

Acknowledgements

Fedn Eur. biochem. Socs Lett.

Brain Res.

Brain Res.

Fedn Eur. biochem. Socs Lett.

Brain Res.

Trends pharmac. Sci.

Brain Res.

Trends pharmac. Sci.

Neuroscience

Neurosci. Lett.

Neuroscience

Trends Neurosci.

Brain Res.

Brain Res.

Neuroscience

Brain Res.

Brain Res.

Trends Neurosci.

J. Chem. Neuroanat.

Brain Res.

Brain Res.

Neuroscience

Neurosci. Res.

Molec. Brain Res.

Brain Res.

Neurosci. Lett.

Peptides

Neuron

Neurosci. Lett.

Neuroscience

Fedn Eur. biochem. Socs Lett.

Fedn Eur. biochem. Socs Lett.

Expression of the prodynorphin gene in the developing and adult cerebral cortex of the rat: an in situ hybridization study

J. comp. Neurol.

Distribution and targeting of a μ-opioid receptor (MOR1) in brain and spinal cord

J. Neurosci.

Characterization and distribution of a cloned rat μ-opioid receptor

J. Neurochem.

Parvalbumin in most γ-aminobutyric acid-containing neurons of the rat cerebral cortex

Science

Molecular cloning and functional expression of a μ-opioid receptor from rat brain

Molec. Pharmac.

Expression of mu opioid receptor mRNA in rat brain: an in situ hybridization study at the single cell level

J. comp. Neurol.