In summary, it can be seen that opioids exert a variety of stimulatory effects on signal transduction. These include stimulation of the cyclic AMP formation, phosphinoside
hydrolysis, and elevation of intracellular calcium, by mobilization from intracellular stores and by stimulating influx. At a cellular level these changes may underlie an opioid stimulation of neuronal activity. In addition, the stimulatory effects of opioids may play a part in the development of tolerance to opioid drugs.