Abstract
Morphine- and ethanol-induced stimulation of neuronal firing of ventral tegmental area (VTA) dopaminergic neurons and of dopamine (DA) transmission in the shell of the nucleus accumbens (AcbSh) represents a crucial electrophysiological and neurochemical response underlying the ability of these compounds to elicit motivated behaviors and trigger a cascade of plasticity-related biochemical events. Previous studies indicate that the standardized methanolic extract of Withania somnifera roots (WSE) prevents morphine- and ethanol-elicited conditioned place preference and oral ethanol self-administration. Aim of the present research was to investigate whether WSE may also interfere with the ability of morphine and ethanol to stimulate VTA dopaminergic neurons and thus AcbSh DA transmission as assessed in male Sprague-Dawley rats by means of patch-clamp recordings in mesencephalic slices and in vivo brain microdialysis, respectively. Morphine and ethanol significantly stimulated spontaneous firing rate of VTA neurons and DA transmission in the AcbSh. WSE, at concentrations (200–400 μg/ml) that significantly reduce spontaneous neuronal firing of VTA DA neurons via a GABAA- but not GABAB-mediated mechanism, suppressed the stimulatory actions of both morphine and ethanol. Moreover, in vivo administration of WSE at a dose (75 mg/kg) that fails to affect basal DA transmission, significantly prevented both morphine- and ethanol-elicited increases of DA in the AcbSh. Overall, these results highlight the ability of WSE to interfere with morphine- and ethanol-mediated central effects and suggest a mechanistic interpretation of the efficacy of this extract to prevent the motivational properties of these compounds.
Highlights
Withania somnifera (WS) Dunal, known as Ashwagandha or Indian Ginseng, is a plant widely used in traditional Ayurvedic medicine in India since antiquity (Kulkarni and Dhir, 2008)
Effect of Withania somnifera roots (WSE) on Spontaneous and Morphine- and Ethanol-Induced Increase in Firing Rate of ventral tegmental area (VTA) DA Neurons Identification of single VTA DA neurons was confirmed by the presence of HCN-mediated Ih currents, evoked by hyperpolarizing the cell membrane from −65 to 115 mV, and of a regular spontaneous firing rate (4.84 ± 0.4 Hz, n = 40) (Figure 2A, control) (Ungless and Grace, 2012) In addition, the stimulatory effect induced by the GABAB receptor antagonist SCH 50911 (10 μM) (Figure 2G) on firing rate further proved the dopaminergic phenotype of the recorded VTA neurons (Margolis et al, 2012)
In order to evaluate quantitatively the effects of WSE, alone or in combination with morphine or ethanol, on the spontaneous firing activity of DA neurons in rat VTA slices, spike discharge was recorded in the cellattached configuration
Summary
Withania somnifera (WS) Dunal (family Solanaceae), known as Ashwagandha or Indian Ginseng, is a plant widely used in traditional Ayurvedic medicine in India since antiquity (Kulkarni and Dhir, 2008). The preclinical search of new therapeutic approaches, including the application and development of strategies based on the use of substances obtained from herbal remedies or from whole herbal extracts (Carai et al, 2000; Lu et al, 2009; Liu et al, 2011; Zhang et al, 2014), appears nowadays highly desirable In this regard, the standardized methanolic extract of WS roots (WSE) has recently been shown to affect the ability of morphine (Ruiu et al, 2013) and ethanol (Spina et al, 2015) to elicit acquisition and expression of conditioned place preference (CPP) in mice as well as the acquisition and maintenance of oral ethanol self-administration in rats (Peana et al, 2014). One of the most likely candidate mechanism by which WSE impairs acquisition and expression of morphine- and ethanol-elicited CPP (Ruiu et al, 2013; Spina et al, 2015) and acquisition and maintenance of ethanol oral selfadministration (Peana et al, 2014) is represented by the ability of WSE to affect morphine- and ethanol-elicited increase of the spontaneous firing rate of VTA DA neurons as well as of phasic DA transmission in the AcbSh
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