Regular Article
Muscimol Diffusion after Intracerebral Microinjections: A Reevaluation Based on Electrophysiological and Autoradiographic Quantifications

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Abstract

Intracerebral muscimol injection is widely used to inactivate discrete brain structures during behavioral tasks. However, little effort has been made to quantify the extent of muscimol diffusion. The authors report here electrophysiological and autoradiographic results obtained after muscimol injection (1 μg/μl) either into the nucleus basalis magnocellularis (0.1–0.4 μl) or into the thalamic reticular nucleus (RE, 0.05–0.1 μl). In 52 rats, multiunit recordings were collected either in the RE or in the auditory thalamus during the 2 h following muscimol injection. Decreases in neuronal activity were observed up to 3 mm from the injection site; their time of occurrence was a function of the distance between the injection and recording sites. Because these decreases cannot be explained by physiological effects, they likely reflected muscimol diffusion up to the recording sites. Autoradiographic studies involved 25 rats and different experimental conditions. Optical density (OD) measures indicated that after a survival time of 15 min, a 0.05-μl injection produced a labeled area of 5.25 mm2 at the injection site and a rostrocaudal labeling of 1.7 mm. Increasing the survival time to 60 min, or increasing the injected volume to 0.1 μl, systematically led to a larger labeled area at the injection site (8–12 mm2) and to a larger rostrocaudal diffusion (2.0–2.5 mm). Direct quantifications of radioactivity by a high-resolution radioimager validated the OD measures and even indicated a larger muscimol diffusion (up to 3.25 mm). Thus, these data point out that muscimol diffusion after intracerebral microinjection is larger than usually supposed. The relationships between these results and those obtained in behavioral studies are discussed.

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    We gratefully acknowledge Françoise Tardieux (Biospace Mesures), who kindly quantified the brain sections with the β-Imager 2000. We thank I. Delamanche for assistance in histology and autoradiography, P. Lanièce for giving us the first opportunity to use a radioimager, and R. Mastrippolito for calculation of the theoretical muscimol diffusion. Special thanks go to Jessica Dubois for detailed explanations on the laws of Fickian diffusion. N.C. was supported by a doctoral fellowship from the MENESR.

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    Address correspondence and reprint requests to Jean-Marc Edeline, NAMC, UMR 8620, Université Paris-Sud, Bât 446, 91405 Orsay cedex, France. Fax: 33-1-69-15-77-26. E-mail: [email protected].

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