The current study investigated the effect of CBD or CBDA treatment on the expression of behavioural sensitisation to METH, which is thought to model some aspects of METH-induced addiction and psychosis. The major findings of this study were that, in rats which underwent METH sensitisation, CBD at 40 and 80 mg/kg decreased this sensitised response. Treatment with the 40 mg/kg dose of CBD also significantly reduced the locomotor effect of METH administration in non-sensitised animals. There were no effects of treatment with CBDA on any treatment group.

The behavioural sensitisation to METH observed here is in accordance with the literature (Pierce and Kalivas 1997; Ago et al. 2012) and our previous studies (Wearne et al. 2015; Wearne et al. 2017), where repeated exposure to METH produces an enhanced locomotor response after an extended withdrawal period compared to an acute METH challenge (Robinson and Becker 1986; Vanderschuren and Kalivas 2000). This hyperlocomotion resulting from the METH sensitisation paradigm is thought to be a model of some aspects of the emergence of psychotic symptoms and METH addiction in humans that occur with repeated METH use. The ability of antipsychotics to reverse such sensitisation gives some indication of predictive validity (Jones et al. 2011; Akiyama et al. 2011). Our results also show that CBD itself did not decrease locomotion in the rats treated only with saline (SS), suggesting that its inhibitory effect on hyperlocomotion is not due to motor impairment and is consistent with numerous prior studies highlighting showing CBD does not inhibit locomotor activity in rodents (ElBatsh et al. 2012, Todd and Arnold 2016). These results agree with previous preclinical studies demonstrating that CBD reduces hyperlocomotion induced by amphetamine and ketamine (Moreira and Guimarães 2005), thus suggesting that CBD has antipsychotic-like effects without the detrimental motor side effects.

Systemic injections of either 40 or 80 mg/kg CBD prior to the METH challenge in sensitised rats significantly decreased the hyperlocomotor effects to a similar extent. This suggests limited dose-dependency of the effect of CBD in METH sensitised animals, somewhat in contrast to our prior study of METH self-administration and relapse, where only the highest dose of CBD (80 mg/kg i.p.) was effective in reducing addiction-related behaviours (Hay et al. 2018). Further, in the acute METH challenged animals in the present study (group SM), only the 40 mg/kg and not the 80 mg/kg dose of CBD effectively reduced locomotor hyperactivity. This was clearly illustrated in the time course data, where only 40 mg/kg significantly reduced the acute locomotor effect of METH at 10–20 min post challenge. The reason for this is not readily apparent. The behavioural effects of cannabinoids do sometimes follow a bell-shaped curve (for review see Blessing et al. 2015) and this may reflect the recruitment of different receptor and enzymatic targets with ascending doses of CBD.

The diverse actions of CBD on multiple receptor types (Seeman 2016; Morales et al. 2017) may also explain the differential effects of CBD in sensitised and non-sensitised animals. For example, chronic METH administration alters 5-HT_1A receptors, vanilloid receptor 1 (TRPV1), peroxisome proliferator-activated receptor γ (PPARγ) and dopamine D2 receptors (Ago et al. 2006; Maeda et al. 2007; Tian et al. 2010; Granado et al. 2011), providing possible avenues for CBD to have enhanced effects in sensitised animals compared to controls (Seeman 2016; Morales et al. 2017). The ability for CBD to reduce acute stimulant effects appears inconsistent, with some studies showing effective reductions after acute doses (30 and 60 mg/kg; Moreira and Guimarães 2005), and others reporting that only repeated doses of CBD are able to reduce the acute hyperactive effects of amphetamine (Long et al. 2010). On the other hand, some reported no significant effect of CBD administration on acute psychostimulant-induced behaviours (Valvassori et al. 2011). Further studies are required to determine the optimum treatment regimen and mechanism of action of CBD, following either acute or chronic treatment with METH.

The mechanisms through which CBD exerts its antipsychotic effects are still under investigation as are the brain regions involved. The dopamine-rich region of the nucleus accumbens is relevant to antipsychotic effectiveness on the positive symptoms of schizophrenia, while effects in the dorsal striatum are related to motoric side effects (Seeman 2002; Strange 2001). Several clinical and preclinical studies have revealed that CBD can strongly modulate the mesolimbic dopamine system (Bhattacharyya et al. 2010; Valvassori et al. 2011; Renard et al. 2016a). Consistent with this, direct infusion of CBD into the shell region of the nucleus accumbens reduced behavioural sensitisation to amphetamine and amphetamine-induced sensorimotor gating deficits in rats (Renard et al. 2016a). In humans, CBD normalised abnormal activity in brain structures linked to triggering psychosis in patients at high risk of psychosis (Davies and Bhattacharyya 2019; Allen et al. 2016; Bhattacharyya et al. 2018). Notably, Seeman (2016) showed CBD is a partial agonist at dopamine D2 receptors, behaving in a similar manner to the antipsychotic drug aripiprazole. Thus, this pharmacological property of CBD might subserve its ability to reduce METH-induced behavioural sensitisation (Seeman 2016).

In contrast to CBD, CBDA treatment did not reduce the hyperlocomotion caused by METH sensitisation in the present study. Our prediction that CBDA might inhibit the development of sensitisation to METH was partly based on the strong 5-HT_1A receptor agonist effects of this cannabinoid (Bolognini et al. 2013; Rock and Parker 2013, 2015), given that 5-HT_1A receptor agonists are known to inhibit the expression of METH sensitisation (Ago et al. 2006). Surprisingly, however, this hypothesis was not supported by the current findings where CBDA failed to modulate METH-induced behavioural sensitisation. This lack of effect of CBDA could be due to other differences in the pharmacological targets of CBD and CBDA. Moreover, the dose range employed here was much lower for CBDA than CBD, although the doses chosen were based on CBDA’s potent anti-emetic and anxiolytic effects in rats and mice (Pertwee et al. 2018; Rock and Parker 2013, 2015, Rock et al. 2015; Assareh et al. 2020). Future studies might examine higher doses, given that CBDA was anticonvulsant at ≥10 mg/kg in a mouse model of childhood epilepsy (equivalent to a 5 mg/kg dose in rats) (Anderson et al. 2019a). In our experiments, CBDA did not affect baseline activity levels at any dose tested. This is consistent with a previously published report showing that CBDA does not suppress spontaneous locomotor activity at doses ≤ 1 mg/kg (Rock et al. 2014).

Comparisons of activity across the 60-min time course also revealed differences in activity of CBD and CBDA treatment on METH-pretreated rats. While CBD 40 and 80 mg/kg were able to significantly decrease locomotor activity, especially at the 50–60 min part of the test when compared to VEH, all doses of CBDA failed to significantly change behaviour throughout the entire session. It is known that the levels of CBD in the rat brain have a T_max of 120 min following intraperitoneal administration according to Deiana et al. (2012). In line with this, our results show that 90 min after the injection there was still a strong effect of both CBD doses in suppressing locomotor activity in the latter part of the 1-h session when compared to the 0–30-min bin.

This work further underlines the promising therapeutic potential of CBD on METH-induced addiction and psychosis. One limitation of the current data is the use of a within-subject design for challenge testing where rats were exposed to all examined doses of CBD. While the effects of CBD were controlled for by counterbalancing doses and allowing at least 48 h between tests, the repeated exposure to METH in the acute METH group may have initiated some sensitisation. However, when examining the data, it is clear that the acute METH effects were significantly lower than the sensitised group, providing confidence in our data. Further studies should explore the effect of chronic administration of CBD on METH sensitisation, given that a recent study showed an effect of chronic CBD administration to prevent the development of cocaine sensitisation in a conditioned place preference (CPP) paradigm (Chesworth and Karl, 2020). However, others report an effect of chronic CBD administration to reduce CPP, but not behavioural sensitisation to cocaine (Luján et al. 2018). Furthermore, considering that sex differences exist between males and females for the expression of cocaine sensitisation (Hu and Becker 2003), it is relevant for future studies to test the effect of CBD or CBDA treatment on METH sensitised responses in female rats. It is also important to acknowledge that the sensitisation protocol presents some advantages and disadvantages for modelling addiction and psychosis that should be considered. On the one hand, the protocol uses simple sub-chronic drug delivery to produce enduring effects on reward circuity, however the face validity is impacted by experimenter delivered drug of limited amount, and does not model the social constructs surrounding human drug use (Kuhn et al 2019). Despite these considerations, the model provides important basic knowledge to further medications discovery.

In summary, the present study showed a sensitised locomotor response to METH in male rats pretreated and that CBD, but not CBDA treatment, was able to attenuate hyperlocomotion characteristic of METH sensitisation, at pharmacologically effective doses. Future studies should explore the neuropharmacological mechanisms and associated brain circuitry involved in these effects of CBD. The data further reinforce the view that CBD might serve as a novel pharmacotherapy for METH-induced addiction and psychosis.