An investigation into the effects of Sceletium tortuosum in a methamphetamine addiction model in Sprague Dawley rats

Abstract

Background: Methamphetamine (MA), globally known as “crystal meth”, is a highly addictive psychostimulant, which can be easily synthesized by using over the counter medicine and common household items. It induces intense euphoria, insomnia, anxiety, aggressive tendencies, hyperactivity and psychosis. Millions of people world-wide are addicted to this substance, in fact, after cannabis and opioids, MA is the third most abused illicit substance in the world. Despite this, there is currently no available treatment for MA addiction and abuse. MA elicits its effects by a variety of pathways, including the monoaminergic-, inflammatory and cyclic adenosine monophosphate (cAMP)-phosphodiesterase 4 (PDE4)-cAMP response element binding protein (CREB) pathways. In South Africa MA is locally known as “Tik”, and its usage amongst the youth, especially in the Western Cape has become a major cause of concern. These factors raise the need for a safe, cost-effective treatment for MA addiction. The majority of South Africans use herbal medicine to treat psychiatric diseases and recent clinical studies began to focus on the clinical utility of antioxidants of plant origin in treating these disorders. There is however a lack of pre-clinical evidence on the safety and efficacy of such plant extracts. Sceletium tortuosum (ST), is an indigenous plant, which gained attention for being a PDE4- and serotonin reuptake inhibitor, which helps to alleviate symptoms of psychiatric diseases such as anxiety and depression. Moreover, recent studies have shown that PDE4 inhibitors might have the potential to attenuate some of the effects caused by MA addiction. Given ST’s inherent pharmacological properties, this study set out to evaluate the effects of a standardized extract of ST (Zembrin®= Z®) and its potential in modifying the bio-behavioural changes induced by MA exposure, during the development of MA addiction in a rat model. Aims: The aims of the current study were to evaluate whether sub-acute MA exposure causes behavioural- (e.g., reward, psychosis and hyperactivity) and neurochemical (regional brain monoamine levels) alterations in Sprague dawley (SD) rats and whether concurrent administration of Z® could prevent these bio-behavioural changes. Further, this study also investigated the bio- behavioural effects of Z® alone in healthy SD rats. Methodology: Eighty-four healthy male SD rats were divided into 7 groups (n = 12/group). Exposure and treatment continued for 8 days. To confirm the MA-exposure regimen one group received saline exposure intraperitoneally and saline administration through oral gavage and a second group received MA and saline exposure (intraperitoneally) on every alternative day and received concurrent oral saline administration. Thereafter four groups received alternative day MA and saline exposure (intraperitoneally) and concurrent administration of different doses of Z® (5mg/kg, 10mg/kg, 25mg/kg and 50mg/kg) via oral gavage. The last group received intraperitoneal saline exposure and concurrent Z® 25mg/kg oral administration. All rats were subjected to the open field test (OFT), a

pre-pulse inhibition (PPI) test and a conditioned place preference (CPP) test to determine locomotor, sensorimotor gating and addictive-like behaviours respectively, after which striatal and hippocampal brain tissue were harvested for dopamine (DA), serotonin (5-HT), noradrenaline (NA) and their respective metabolite analysis. Results: MA alone induced significant addictive- (increased time spend in drug-paired compartment in the CPP) and psychotic (decreased %PPI) -like behaviour, with no significant effect on locomotor activity (OFT). Apart from significantly increasing hippocampal DA levels, MA did not induce any changes in any other striatal or hippocampal monoamine levels. Different doses of Z® did not affect MA-induced PPI deficits, although MA plus Z® 25mg/kg significantly decreased locomotor activity in the OFT. Concurrent MA and Z® 50mg/kg administration tended to increase place preference for MA in the CPP test. Concurrent saline and Z® 25mg/kg administration significantly decreased the average %PPI. Neurochemically, concurrent MA and Z® 25mg/kg administration increased striatal 5-HT- and 3,4-Dihydroxyphenylacetic acid (DOPAC) levels as well as hippocampal 5-HT levels. MA together with Z® 50mg/kg caused a significant increase in striatal 5-HT- and DOPAC levels and tended to increase striatal DA levels. Concurrent MA and Z® 5mg/kg tended to increase striatal DA and to decrease hippocampal NA. Saline with concurrent Z® 25mg/kg administration caused a significantly decrease in hippocampal NA- and 5-hydroxyindoleacetic acid (5-HIAA) levels. Conclusions: Concurrent MA and Z® 25mg/kg decreased locomotor activity, while concurrent MA and Z® 25mg/kg, as well as MA and Z® 50mg/kg administration significantly increased striatal- and hippocampal monoamine levels, without affecting PPI. Administration of Z® 50mg/kg during MA exposure tended to increase CPP. Concurrent saline and Z® 25mg/kg administration however significantly induced PPI deficits and significantly decreased hippocampal monoamine levels. Tentatively, Z® acts as a stimulant during MA exposure, hinting at its potential to act as a substitute for MA by preventing MA associated withdrawal symptoms, while the abusers receive other therapeutical interventions. This is similar to bupropion which is also a non-addictive stimulant drug (which increases DA levels by binding to the dopamine transporter (DAT) and inhibiting DA reuptake), that is used to treat MA withdrawal symptoms. However, further studies investigating Z® post MA exposure (viz. during the withdrawal stage), as well as the mechanisms of action of Z® are encouraged.