Salsolinol Facilitates Glutamatergic Transmission to Dopamine Neurons in the Posterior Ventral Tegmental Area of Rats Guiqin Xie, Jiang-Hong Ye* Department of Anesthesiology, Pharmacology and Physiology, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, New Jersey, United States of America Abstract Although in vivo evidence indicates that salsolinol, the condensation product of acetaldehyde and dopamine, has properties that may contribute to alcohol abuse, the underlying mechanisms have not been fully elucidated. We have reported previously that salsolinol stimulates dopamine neurons in the posterior ventral tegmental area (p-VTA) partly by reducing inhibitory GABAergic transmission, and that ethanol increases glutamatergic transmission to VTA-dopamine neurons via the activation of dopamine D1 receptors (D1Rs). In this study, we tested the hypothesis that salsolinol stimulates dopamine neurons involving activation of D1Rs. By using whole-cell recordings on p-VTA-dopamine neurons in acute brain slices of rats, we found that salsolinol-induced increase in spike frequency of dopamine neurons was substantially attenuated by DL-2-amino-5- phosphono-valeric acid and 6, 7-dinitroquinoxaline-2, 3-dione, the antagonists of glutamatergic N-Methyl-D-aspartic acid and a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Moreover, salsolinol increased the amplitude of evoked excitatory postsynaptic currents (EPSCs) and the frequency but not the amplitude of spontaneous EPSCs. Additionally, SKF83566, a D1R antagonist attenuated the salsolinol-induced facilitation of EPSCs and of spontaneous firing of dopamine neurons. Our data reveal that salsolinol enhances glutamatergic transmission onto dopamine neurons via activation of D1Rs at the glutamatergic afferents in dopamine neurons, which contributes to salsolinol’s stimulating effect on p-VTA dopamine neurons. This appears to be a novel mechanism which contributes toward rewarding properties of salsolinol. Citation: Xie G, Ye J-H (2012) Salsolinol Facilitates Glutamatergic Transmission to Dopamine Neurons in the Posterior Ventral Tegmental Area of Rats. PLoS ONE 7(5): e36716. doi:10.1371/journal.pone.0036716 Editor: Kuei-Sen Hsu, National Cheng Kung University, Taiwan Received February 8, 2012; Accepted April 12, 2012; Published May 10, 2012 Copyright: 2012 Xie, Ye. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work is made possible by the support from National Institutes of Health – National Institute on Alcohol Abuse and Alcoholism [grants AA016964 and AA016618] to JHY. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: ye@umdnj.edu Introduction The racemic mixture of salsolinol ((R) + (s)-salsolinol) is formed by nonenzymatic Pictet-Spengler condensation of dopamine with acetaldehyde, the major metabolite of ethanol in the brains of mammals [1,2]. Salsolinol has been proposed to play a role in the etiology of alcoholism [3], and to the rewarding properties of ethanol [4]. Early animal studies revealed that salsolinol promotes alcohol drinking [5,6]. More recent studies showed that rats self-administer salsolinol into posterior VTA (p-VTA), a key region in the brain reward system [7,8], and that microinjection of salsolinol into the p-VTA of rats induces conditioned place preference [9]. Furthermore, such behaviors seem to depend on activation of dopaminergic (DA) neurons [8] and associated with enhanced dopamine levels in the ipsilateral nucleus accumbens shell [9]. With patch clamp techniques, we recently showed that salsolinol (0.01–1 mM) dose-dependently stimulates DA neurons in the p-VTA in acute midbrain slices of rats. Salsolinol decreased GABAergic synaptic transmission onto p-VTA DA neurons, and gabazine, an antagonist of GABAA receptors substantially attenuated salsolinol-induced increase in firing rate of DA neurons. This phenomenon suggests that salsolinol stimulates DA neurons through a mechanism of disinhibition [10]. Interestingly, gabazine (10 mM) failed to completely abolish salsolinol-induced increase in DA neuron firing, indicating that other mechanisms may also be involved. In addition to potent inhibitory GABAergic afferents [11], VTA DA neurons receive glutamatergic (Gluergic) inputs from diverse brain nuclei [12,13] and from the Gluergic neurons in the VTA [14]. These diverse sources of Gluergic afferents may enable DA neurons to respond to a wide range of environmental stimuli. Such excitatory Gluergic synaptic input is a key component in the regulation of DA cell excitability [15,16] and is known to play an important role in the actions of many drugs of abuse [17,18] including ethanol [19]. Previous in vivo evidence indicates that activation of dopamine D1 receptors (D1Rs) can increase the release of glutamate in the VTA [20]. In recent in vitro studies, we have demonstrated that acute ethanol facilitates Gluergic transmission to VTA DA neurons via the activation of D1Rs at Gluergic afferents [21,22]. This study was set to test the hypothesis that salsolinol stimulates DA neurons involving activation of D1Rs at Gluergic afferents. Methods Experimental procedures All experiments were performed in accordance with the guidelines of the National Institutes of Health Guide for the Care PLoS ONE | www.plosone.org 1 May 2012 | Volume 7 | Issue 5 | e36716