The data presented here demonstrate a novel activity of CBG, as an appetite stimulant at 120–240 mg/kg. At these doses, CBG increased food intake, predominantly via stimulation of appetitive phase feeding behaviours. Furthermore, CBG acutely administered at doses ≤120 mg/kg did not elicit any detrimental neuromotor effects on locomotor activity, balance, fine motor control or muscular strength, and at 240 mg/kg appears to have some stimulant activity.

The widely utilised and validated feeding paradigm employed in this study is designed to sensitively detect even relatively small hyperphagic actions of cannabinoid agents (Williams et al. 1998). The paradigm provides detailed analysis of meal microstructure, allowing quantification of discrete behaviours during the appetitive and consummatory phases of feeding. The incorporation of an infrared locomotor activity monitoring system provides additional measures of ambulatory activity and rearing behaviour, allowing differentiation of locomotion-dependent and locomotion-independent effects of drugs on feeding behaviour. The use of this paradigm in the present study allows the direct comparison of the effects of CBG to previously published results using the same paradigm for the eCB anandamide (Williams and Kirkham 2002a); the purified pCBs ∆9-THC, CBD, cannabiniol (CBN) and cannabidiolic acid (Williams et al. 1998; Williams and Kirkham 2002b; Williams and Kirkham 2002a; Farrimond et al. 2012b; Brierley et al. 2016); and low- and non-∆9-THC cannabis extracts and analogues (Farrimond et al. 2010a; Farrimond et al. 2010b; Farrimond et al. 2012a). It should be noted that, unlike these previous studies of ∆9-THC, the doses of CBG used in the present study are considerably higher than concentrations typically found in whole C. sativa preparations. As such, the hyperphagic activity of CBG reported here is unlikely to meaningfully contribute to the appetite-stimulating effects of cannabis consumption in humans.

In this study, administration of CBG at 120–240 mg/kg dose dependently increased total food intake over the 2-h test period. This is in contrast to previous studies of various ∆9-THC formulations, which elicit a robust increase in intake during hour 1 followed by a compensatory decrease during hour 2 (Farrimond et al. 2010a; Farrimond et al. 2012a). The pCB CBN elicits a similar biphasic effect on food intake during this 2-h paradigm, with hyperphagia blocked by the CB 1 R antagonist SR141716, indicative of a ∆9-THC-like mechanism of action (Farrimond et al. 2012b). In our study, CBG also increased appetitive phase feeding behaviour, with the onset of feeding advanced by approximately 30 min, from 83 to 54 min. However, this is somewhat in contrast to previous studies of ∆9-THC formulations and CBN, in which feeding was initiated within 10–20 min, despite similar long latencies in vehicle groups (Farrimond et al. 2010a; Farrimond et al. 2012a; Farrimond et al. 2012b). Hence, it appears that whilst CBG may stimulate the appetitive component of feeding behaviour, it does so to a lesser degree than ∆9-THC and CBN.

Whilst the CBG-induced increase in feeding frequency and decrease in latency are consistent with stimulation of the appetitive component of feeding, the modest effects on intra-meal factors provide little evidence for stimulation of the consummatory component. Given that a significant effect of CBG was only evident on the cumulative size of meals 1 and 2, it is apparent that increased consumption is predominantly driven by the dose-dependent increase in feeding frequency, rather than significant increase in individual meal sizes. Similarly, the lack of significantly increased durations of individual meals does not support a stimulatory effect of CBG on the consummatory component of feeding behaviour. Differences are thus again evident between consummatory meal microstructure parameters following administration of CBG, and those of ∆9-THC formulations, which are typified by robust increases in both the size and duration of the first meal consumed (Farrimond et al. 2010a). Considered overall, the alterations in food intake and meal pattern microstructure induced by CBG demonstrate a dose-dependent hyperphagic effect, predominantly mediated by stimulation of the appetitive component of feeding behaviour.

Such differences in patterns of feeding behaviour stimulation between CBG and pCBs acting directly as CB 1 R agonists are consistent with the limited in vitro pharmacodynamic data on CBG, which have shown that whilst it has some affinity for this receptor, it does not appear to activate it (Cascio et al. 2010; Pertwee et al. 2010). Given that CBG has been shown to be one of the most effective pCBs at inhibiting AEA reuptake (De Petrocellis et al. 2011), it is instead possible that it elicits CB 1 R-mediated hyperphagia in an indirect manner, via upregulation of orexigenic endocannabinoid tone (Kirkham et al. 2002; Reyes-Cabello et al. 2012). The TRPV1 agonist activity of CBG could conceivably contribute to such a mechanism, given the recent observation that TRPV1 agonists can themselves inhibit AEA reuptake (Hofmann et al. 2014). Alternatively, CBG-induced hyperphagia may be mediated by its activity (to date only observed in vitro) as a highly potent agonist of α2-adrenoceptors (Cascio et al. 2010). Consistent with this, stimulation of α2-adrenoceptors in the hypothalamic paraventricular nucleus has been shown to have hyperphagic effects in satiated rats (Wellman et al. 1993; Taksande et al. 2011), whilst administration of the α2-adrenoceptor agonist clonidine into the median raphe nucleus had orexigenic effects in free feeding (Mansur et al. 2010) but not fasted or food-restricted rats (Ribas et al. 2012). Whilst the above studies suggest that central α2-adrenoceptor activation may be involved in the hyperphagic activity of CBG, it should be noted that recent cardiovascular safety assays in dog did not reveal any effects on cardiovascular parameters (T. Hill, personal communication), indicating that α2-adrenoceptor agonism may not be the predominant action for CBG. Given that cannabinoids acting as CB 1 R agonists have demonstrated limited clinical utility as appetite stimulants, the possibility that CBG induces hyperphagia via indirect and/or CB 1 R-independent mechanisms warrants urgent further investigation, as this pCB may represent a valuable novel therapeutic option for such applications.

A further interesting observation from the feeding experiment is the stimulation of ambulatory activity over the 2-h test duration. These data support the predicted lack of sedative effect for the 240 mg/kg dose based on results up to 120 mg/kg in the neuromotor test battery. However, they are not wholly consistent, given that a non-significant increase in activity during the feeding experiment was observed at 120 mg/kg, which was not observed in the open field test. This is not inherently contradictory however, as it is plausible that differences in test environment, and the considerably longer test duration and drug exposure time (180 vs 65 min from drug administration), allow the detection of effects too subtle to be observed in the open field.

The coincident increases in total food intake and ambulatory activity suggest the following two possible alternative interpretations of these data: that increased locomotor activity is an artefact of increased food seeking; or that increased food intake is secondary to increased activity or general arousal. For the first interpretation to be valid, any compound which increases food intake by a similar magnitude in this system would have to also increase locomotor activity levels. However, validation studies of the feeding and activity cages, using 0.5–2-mg/kg ∆9-THC-containing formulations, resulted in the expected stimulation of feeding behaviours but did not increase locomotor activity (unpublished observations). Given these data, and video observations showing that the majority of animals’ activities in the cages were exploratory rather than food seeking, it is apparent that the activity data do indeed represent generalised locomotor stimulation. For this locomotor stimulation to be the primary driver of increased food intake, via a general arousal mechanism, patterns of activity and food intake would have to closely mirror one another, both in terms of temporal profile and dose response. Upon close inspection of hourly intake and activity levels, it can be observed that whilst intake levels in hour 2 are very similar to hour 1 (and indeed 10 % higher in the 240-mg/kg group), activity levels in hour 2 are approximately half that in hour 1(data not shown). Further evidence of the disconnect between activity and intake can been seen in the dose response, with the highest intakes during hour 1 in the 120 mg/kg group, in contrast to the highest activity levels being in the 240 mg/kg group. These data thus argue against the interpretation that the hyperphagic activity of CBG is driven by generalised arousal, but rather that this compound directly stimulates motivation to feed, with coincident feeding-independent locomotor activation apparent at the highest dose. Whilst beyond the scope of the present study, this apparent stimulant effect of higher CBG doses warrants further investigation in models which can assess locomotor activation over extended time periods, without any confounding effects of feeding stimulation.