Elsevier

Neuroscience

Volume 146, Issue 1, 25 April 2007, Pages 286-297
Neuroscience

Neuroanatomy
Distribution of serotonin 5-HT2C receptors in the ventral tegmental area

https://doi.org/10.1016/j.neuroscience.2006.12.071Get rights and content

Abstract

Serotonin 2C receptors (5-HT2CR) appear to exert tonic inhibitory influence over dopamine (DA) neurotransmission in the ventral tegmental area (VTA), the origin of the mesolimbic DA system, thought to be important in psychiatric disorders including addiction and schizophrenia. Current literature suggests that the inhibitory influence of 5-HT2CR on DA neurotransmission occurs via indirect activation of GABA inhibitory neurons, rather than via a direct action of 5-HT2CR on DA neurons. The present experiments were performed to establish the distribution of 5-HT2CR protein on DA and GABA neurons in the VTA of male rats via double-label immunofluorescence techniques. The 5-HT2CR protein was found to be co-localized with the GABA synthetic enzyme glutamic acid decarboxylase (GAD), confirming the presence of the 5-HT2CR on GABA neurons within the VTA. The 5-HT2CR immunoreactivity was also present in cells that contained immunoreactivity for tyrosine hydroxylase (TH), the DA synthetic enzyme, validating the localization of 5-HT2CR to DA neurons in the VTA. While the degree of 5-HT2CR+GAD co-localization was similar across the rostro-caudal levels of VTA subnuclei, 5-HT2CR+TH co-localization was highest in the middle relative to rostral and caudal levels of the VTA, particularly in the paranigral, parabrachial, and interfascicular subnuclei. The present results suggest that the inhibitory influence of the 5-HT2CR over DA neurotransmission in the VTA is a multifaceted and complex interplay of 5-HT2CR control of the output of both GABA and DA neurons within this region.

Section snippets

Tissue preparation

Naïve male Sprague–Dawley rats (N=6; 175–199 g; Harlan Sprague–Dawley, Inc., Houston, TX, USA) were used. Rats were deeply anesthetized with pentobarbital (100 mg/kg, i.p., Sigma-Aldrich, St. Louis, MO, USA) and perfused transcardially with phosphate-buffered saline (PBS) followed by 3% paraformaldehyde in PBS. Brains were removed, blocked at mid-pons, and post-fixed for 2 h at room temperature (RT). Brains were then cryoprotected in 30% sucrose for 48 h at 4 °C, rapidly frozen on crushed dry

5-HT2CR-IR in the VTA

Intense 5-HT2CR-IR was observed throughout the rostral (Fig. 1B), middle (Fig. 1E), and caudal levels (Fig. 1H) of all subnuclei in the VTA. The 5-HT2CR-IR in each subnucleus was most prominently expressed in cell bodies (Fig. 1C, F, I, arrows), although labeling of neuronal processes was also detected (Fig. 1C, F, I, arrowheads) particularly within the PBP, RLi, and CLi, albeit less frequently. Furthermore, tiny clusters of 5-HT2CR-IR much smaller in diameter than the cell bodies were also

Discussion

The present study is the first to quantify the occurrence of 5-HT2CR-IR co-localized to subsets of GAD- and TH-IR cells throughout the VTA subnuclei. The data reveal that the 5-HT2CR is located on subpopulations of both GABA and DA neurons in the VTA. While the percentage of 5-HT2CR co-localization with GAD-IR was relatively similar across the levels of the various subnuclei, the proportion of TH-IR cells co-labeled with 5-HT2CR-IR differed significantly across a number of subnuclei, with the

Conclusion

In summary, the present study is the first to demonstrate localization of the 5-HT2CR protein on subpopulations of GABA and DA neurons in the VTA. Although the distribution of the 5-HT2CR on these two neuronal subtypes appears to vary slightly among the rostro-caudal levels of the various subnuclei, the incidence of co-localization of the 5-HT2CR with DA neurons appears to predominate in several subnuclei, particularly in the middle-VTA. While the functional implications of these differences in

Acknowledgments

We would like to thank Erik J. Shank, Sonja J. Stutz, and Dr. Shijing Liu for assisting with image analysis. In addition, we appreciate the assistance of Dr. Thomas Albrecht and Mr. Eugene Knutson at the UTMB Infectious Disease and Toxicology Optical Imaging Core in conducting the confocal microscopy imaging. This research was supported by the National Institute on Drug Abuse DA 00260, DA 07287, DA 13595, DA 15259 and DA 20087. This manuscript was presented by M.J.B. in partial fulfillment of

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