Influence 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. Preliminary data suggests that cotinine, the primary metabolite of nicotine, enhances the effects of nicotine on breathing, consistent with its presumed action as a modulator of nAChRs. Here, to build on this preliminary data, 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 control rat pups and rat pups that were exposed to nicotine in utero and after birth (developmental nicotine exposure, DNE). Respiratory motor nerve bursting was recorded from the 4th cervical ventral root in brainstem spinal‐cord preparations studied in two experimental groups. In the first group, preparations were perfused with traditional aCSF that did not contain cotinine (Control). In a second group, 400 nM cotinine was added to the aCSF during dissection and throughout the experiment (Cotinine). The frequency and amplitude of bursts were recorded using the following protocol: a 30‐minute baseline recording where amplitude and frequency of bursts were stable, 20‐minutes of 500 nM nicotine application, and a 30‐minute washout. To assess the effects of cotinine alone, additional control experiments were performed where preparations were perfused with traditional aCSF during the baseline period, followed by application of 400 nM cotinine aCSF for 60 minutes. Results from these experiments indicate that: (1)400 nM cotinine alone does not alter burst amplitude or frequency in either control (n=6, 2‐way ANOVA: Amplitude; P=0.4487, Frequency; P=0.1020) or DNE preparations (n=6, Amplitude; P=0.1338, Frequency; P=0.1034) (2) 500 nM nicotine decreased burst amplitude and increased burst frequency in both control (n=8, for Amplitude and Frequency; P<0.0001) and DNE preparations (n=8, for Amplitude and Frequency; P<0.0001); and (3)cotinine had no effect on the nicotine‐ mediated increase in frequency in either treatment group (n=8 for Control; P=0.5105, n=8 for DNE; P=0.0762), but enhanced the nicotine‐ mediated decrease in amplitude in DNE preparations (n=8, P=0.0227) but not in control preparations (n=8, P=0.2716). 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.