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Muscimol diffusion after intracerebral microinjections: a reevaluation based on electrophysiological and
Jean-Marc Edeline1, Bernard Hars, Elizabeth Hennevin
1Laboratoire de Neurobiologie de l'Apprentissage, de la Mémoire et de la Communication, UMR CNRS 8620, Université Paris-Sud, 91405 Orsay, France. Jean-Marc.Edeline@ibaic.u-psud.fr
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 microg/microl) either into the nucleus basalis magnocellularis (0.1-0.4 microl) or into the thalamic reticular nucleus (RE, 0.05-0.1 microl). 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 microl injection produced a labeled area of 5.25 mm(2) 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 microl, systematically led to a larger labeled area at the injection site (8-12 mm(2)) 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.
Insights
Intracerebral muscimol injections, used to inactivate brain regions, diffuse further than commonly assumed. Electrophysiological and autoradiographic data show muscimol spread up to 3.25 mm from the injection site in rats.
Area of Science:
- Neuroscience
- Pharmacology
Background:
- Intracerebral muscimol injections are a standard method for reversibly inactivating specific brain structures in research.
- The extent of muscimol diffusion from the injection site has not been thoroughly quantified, potentially impacting experimental interpretations.
Purpose of the Study:
- To electrophysiologically and autoradiographically quantify the diffusion of muscimol following intracerebral microinjections in rats.
- To determine the factors influencing muscimol spread, such as injection volume and survival time.
Main Methods:
- Multiunit recordings were performed in the thalamic reticular nucleus (RE) and auditory thalamus of rats after muscimol injection into the nucleus basalis magnocellularis or RE.
- Autoradiographic studies utilized optical density (OD) and high-resolution radioimaging to measure the labeled area and diffusion distance after varying survival times and injection volumes.
- Electrophysiological recordings assessed neuronal activity changes up to 3 mm from the injection site.
Main Results:
- Neuronal activity decreased up to 3 mm from the injection site, with the timing dependent on distance, suggesting significant muscimol diffusion.
- Autoradiography revealed that a 0.05 microl injection resulted in a labeled area of 5.25 mm² and 1.7 mm rostrocaudal diffusion after 15 minutes.
- Increased survival time (60 min) or volume (0.1 microl) led to larger labeled areas (8-12 mm²) and greater diffusion (2.0-2.5 mm), with radioimaging indicating spread up to 3.25 mm.
Conclusions:
- Muscimol diffusion from intracerebral microinjection sites is considerably larger than often presumed.
- These findings have important implications for interpreting behavioral studies using muscimol to inactivate brain regions, suggesting a broader impact than intended.
- Accurate quantification of diffusion is crucial for precise experimental design and interpretation in neuroscience research.