Adolescent nicotine challenge promotes the future vulnerability to opioid addiction: Involvement of lateral paragigantocellularis neurons.

Tobacco smoking is recognized as a life-threatening risk factor worldwide. Initiation of smoking primarily occurs during adolescence which is a critical developmental phase characterized by specific neurobehavioral alterations. The effect of adolescent nicotine exposure on vulnerability to opioid addiction has not been previously addressed. Furthermore, lateral paragigantocellularis (LPGi) is a key modulator of opiate effects. In this study we investigated the effect of adolescent nicotine treatment on development of morphine tolerance and dependence as well as LPGi neuronal responses to morphine during adulthood. Male Wistar rats received subcutaneous injections of either nicotine or saline during adolescence and then development of morphine tolerance and dependence was assessed during adulthood by tail-flick and withdrawal tests, respectively. In vivo single-unit recording was performed to examine the LPGi neuronal activities. Results indicated that adolescent nicotine exposure significantly facilitates the development of tolerance to analgesic effect of morphine and increases the expression of morphine withdrawal signs in adulthood. Also, it was observed that following adolescent nicotine treatment, the extent of morphine-induced excitation is attenuated in LPGi neurons of adult rats. Moreover, the onset of morphine-induced inhibition was increased in these animals. Neither the baseline, nor the regularity of firing was affected in our observations. It could be concluded that nicotine challenge during adolescence may enhance the future vulnerability to opioid addiction through induction of persistent neuroadaptations in LPGi neurons.