THE EFFECT OF COTININE ON BREATHING-RELATED MOTOR OUTPUT

Abstract

Cotinine, the primary metabolite of nicotine, is used as a marker of nicotine exposure due to its long plasma half-life. Previous research shows that perinatal nicotine exposure alters the regions of the brain that control breathing and results in impaired protective breathing reflexes in exposed human and animal neonates. While it has long been known that nicotine has direct effects on brain function and development, it has only recently been established that cotinine may act on the brain as well. Some studies suggest that cotinine may act as an allosteric modulator of nicotinic acetylcholine receptors (nAChRs). Here, we test the hypothesis that cotinine enhances the effects of nicotine on the frequency and amplitude of respiratory motor bursting in the brainstem spinal cord preparation from 1–5-day old rat pups. There were two primary experimental groups: rat pups that were exposed to nicotine in utero and after birth (developmental nicotine exposure, DNE) and control pups unexposed to nicotine (Control). Respiratory motor nerve bursting was recorded from the 4th cervical ventral root in brainstem spinal-cord preparations under three treatment conditions. First, preparations were studied in standard artificial cerebral spinal fluid (aCSF) and bursts were recorded over a 30-minute stable baseline, a 20-minute treatment period of 500 nM nicotine, and a 30-minute washout (Nicotine). Next, the same experimental protocol was carried out in preparations that were studied in aCSF containing 400 nM cotinine (Nicotine + Cotinine). Finally, experiments to assess the effects of cotinine alone were conducted, where baseline bursting was recorded in aCSF, followed by 60 minutes of cotinine application at 400 nM (Cotinine) or 20 minutes at 0.05 mM. Results from these experiments indicate that: (1) 400 nM or 0.05 mM cotinine alone does not alter burst amplitude or frequency in either Control or DNE preparations; and (2) 400 nM cotinine had no effect on the nicotine-mediated increase in frequency in either treatment group, but enhanced the nicotine-mediated decrease in amplitude in DNE preparations and not in Control preparations. We conclude that cotinine may modulate the function of specific nAChR subtypes that mediate the inhibitory effects of nicotine on respiratory motor nerve burst amplitude, and that these receptors may be functionally upregulated by DNE.