Long-term behavioral and biochemical effects of an ultra-low dose of Δ9-tetrahydrocannabinol (THC): neuroprotection and ERK signaling.

We have previously reported that a single injection of an ultra-low dose of delta-9-tetrahydrocannabinol (THC; the psychoactive ingredient of marijuana) protected the brain from pentylenentetrazole (PTZ)-induced cognitive deficits when applied 1-7 days before or 1-3 days after the insult. In the present study we expanded the protective profile of THC by showing that it protected mice from cognitive deficits that were induced by a variety of other neuronal insults, including pentobarbital-induced deep anesthesia, repeated treatment with 3,4 methylenedioxymethamphetamine (MDMA; "ecstasy") and exposure to carbon monoxide. The protective effect of THC lasted for at least 7 weeks. The same ultra-low dose of THC (0.002 mg/kg, a dose that is 3-4 orders of magnitude lower than the doses that produce the known acute effects of the drug in mice) induced long-lasting (7 weeks) modifications of extracellular signal-regulated kinase (ERK) activity in the hippocampus, frontal cortex and cerebellum of the mice. The alterations in ERK activity paralleled changes in its activating enzyme MEK and its inactivating enzyme MKP-1. Furthermore, a single treatment with the low dose of THC elevated the level of pCREB (phosphorylated cAMP response element-binding protein) in the hippocampus and the level of BDNF (brain-derived neurotrophic factor) in the frontal cortex. These long-lasting effects indicate that a single treatment with an ultra-low dose of THC can modify brain plasticity and induce long-term behavioral and developmental effects in the brain.

The dual neuroprotective-neurotoxic profile of cannabinoid drugs.

Extensive in vitro and in vivo studies have shown that cannabinoid drugs have neuroprotective properties and suggested that the endocannabinoid system may be involved in endogenous neuroprotective mechanisms. On the other hand, neurotoxic effects of cannabinoids in vitro and in vivo were also described. Several possible explanations for these dual, opposite effects of cannabinoids on cellular fate were suggested, and it is conceivable that various factors may determine the final outcome of the cannabinoid effect in vivo. In the current review, we focus on one of the possible reasons for the dual neuroprotective/neurotoxic effects of cannabinoids in vivo, namely, the opposite effects of low versus high doses of cannabinoids. While many studies reported neuroprotective effects of the conventional doses of cannabinoids in various experimental models for acute brain injuries, we have shown that a single administration of an extremely low dose of Δ(9) -tetrahydrocannabinol (THC) (3-4 orders of magnitude lower than the conventional doses) to mice induced long-lasting mild cognitive deficits that affected various aspects of memory and learning. These findings led to the idea that this low dose of THC, which induces minor damage to the brain, may activate preconditioning and/or postconditioning mechanisms and thus will protect the brain from more severe insults. Indeed, our recent findings support this assumption and show that a pre- or a postconditioning treatment with extremely low doses of THC, several days before or after brain injury, provides effective long-term cognitive neuroprotection. The future therapeutical potential of these findings is discussed.

Pre- and post-conditioning treatment with an ultra-low dose of Δ9-tetrahydrocannabinol (THC) protects against pentylenetetrazole (PTZ)-induced cognitive damage.

Preconditioning, a phenomenon where a minor noxious stimulus protects from a subsequent more severe insult, and post-conditioning, where the protective intervention is applied following the insult, offer new insight into the neuronal mechanism(s) of neuroprotection and may provide new strategies for the prevention and treatment of brain damage. We have previously reported that a single administration of an extremely low dose of Δ(9)-tetrahydrocannabinol (THC; the psychoactive ingredient of marijuana) to mice induced minor long-lasting cognitive deficits. In the present study we examined the possibility that such a low dose of THC will protect the mice from more severe cognitive deficits induced by the epileptogenic drug pentylenetetrazole (PTZ). THC (0.002 mg/kg, a dose that is 3-4 orders of magnitude lower than the doses that induce the conventional effects of THC) was administered 1-7 days before, or 1-3 days after the injection of PTZ (60 mg/kg). The consequences of this treatment were studied 3-7 weeks later by various behavioral tests that evaluated different aspects of memory and learning. We found that a single administration of THC either before or after PTZ abolished the PTZ-induced long-lasting cognitive deficits. Biochemical studies indicated a concomitant reduction in phosphorylated-ERK (extracellular signal-regulated kinase) in the cerebella of mice 7 weeks following the injection of THC. Our results suggest that a pre- or post-conditioning treatment with extremely low doses of THC, several days before or after brain injury, may provide safe and effective long-term neuroprotection.