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Cannabinoids and cannabinoid receptors play an important role in development and differentiation of the nervous system, but the mechanisms behind that role have not been fully elucidated. We have examined the effects of synthetic and endogenous cannabinoids and related polyunsaturated fatty acids upon mouse embryonal carcinoma P19 stem cell viability - before, during and after retinoic acid (RA)-induced neural differentiation. Experiments were also performed to investigate whether the cannabinoids affect the differentiation of P19-derived neurons by measuring the development and growth of neurites and intracellular acetylcholinesterase activity.

Both synthetic and endogenous cannabinoids as well as related fatty acids produced a concentration-dependent decrease in undifferentiated P19 cell viability, but induction of the neural pathway reduced the sensitivity to the cytotoxic effects, and in differentiated neurons anandamide and related fatty acids showed no cytotoxicity. However, synthetic cannabinoids such as HU 210, HU 211 and WIN 55,212-2 produced cytotoxicity in both undifferentiated and differentiated cells, but there was a right-shifted concentration-effect curve in RA-induced cells and differentiated neurons compared with the undifferentiated cells.

HU 210 produced a time- and concentration-dependent decrease in cell number, percentage of cells expressing neurites, number of neurites per cell and neurite length. Statistically significant inhibition was seen at a concentration of 1 µM to 3 µM, and this was confirmed by the measurement of intracellular acetylcholinesterase activity, an enzyme that is dramatically increased during the differentiation process, where HU 210 significantly decreased the activity after six and nine days of exposure. However, these effects of HU 210 could only be observed in the same concentration range as those affecting neuronal viability. Anandamide, on the other hand, had modest effect on measured markers of neuronal differentiation but decreased the fraction of neurite expressing cells and neurite length after nine days of exposure at a concentration ≥ 10 µM. No effect on the acetylcholinesterase activity was observed.

It is concluded that cannabinoids and related fatty acids have cytotoxic effects in undifferentiated P19 embryonal carcinoma cells, but induction of the neuronal pathway reduces the sensitivity to the cytotoxic effects. The synthetic cannabinoids are more potent than the endogenous cannabinoids and fatty acids in causing cytotoxicity in differentiated neurons, but the CB-induced decrease in neurite formation and acetylcholinesterase activity in RA-induced P19-derived neurons occurs only at concentrations that cause measurable neuronal cell death.