Developmental Nicotine Exposure Alters Synaptic Input to Hypoglossal Motoneurons and Is Associated with Altered Function of Upper Airway Muscles

Elapsed time between the onset of nasal occlusion (dashed line) to first discernible GG muscle EMG burst, defined as onset latency. A, B Recordings from representative control (A) and (B) DNE pups, as indicated. The length of the solid line under the EMG tracing represents the latency, which is prolonged in the DNE animal following the onset of nasal occlusion. C, Onset latency in nine control and DNE pups. The horizontal lines represent the mean value. An unpaired t test revealed a significant difference between the groups (p = 0.0018).

Failure to recover from nasal occlusion. A, An example tracing of one of the 12 animals that exhibited continuous breathing difficulties following nasal occlusion, as explained in Results. B, The number of animals in each treatment group that either recovered from nasal occlusion or failed to recover. Note that of the 12 animals that never recovered, nine were from the DNE group, which is a significant difference by χ² analysis (p = 0.0290).

Example traces of AMPA receptor-mediated sEPSCs recorded from XIIMNs. A, B, Representative traces of pharmacologically isolated sEPSCs from a control animal (A) and a DNE animal (B), both studied on P4. Each trace shows the entire 3-min recording period at baseline (upper trace in each panel) and during acute nicotine application (bottom trace in each panel). Inset panels are an expanded view, showing 10 s of the recording at the end of each trace. Note that the size and frequency of events at baseline is similar in the control and DNE animals. In contrast, with acute nicotine challenge (bottom trace in each panel) the DNE cell shows a decrease in sEPSC frequency, whereas there is a modest increase in the control cell.

Individual and average IEI of sEPSCs and mEPSCs at baseline and during acute nicotine challenge. A, Individual and average IEI of AMPA receptor-mediated sEPSCs under baseline conditions and following acute nicotine challenge. Note that in control cells (circles, left of the vertical line) there was a slight although non-significant decline in IEI (i.e., an increase in frequency) on both P1–P2 (filled circles) and P3–P5 (open circles). The IEI in response to an acute nicotine challenge in cells from DNE animals was age-dependent (squares, right of the vertical line). Note that on P1–P2, nicotine challenge decreased the IEI, although as in controls this trend was not significant. Surprisingly, on P3–P5 acute nicotine challenge significantly increased the IEI (p = 0.0021). Moreover, the mean value for IEI in P3–P5 DNE cells during acute nicotine challenge is significantly different from corresponding data in P1–P2 DNE cells (p = 0.0001), and in P3–P5 cells from control animals. B, Individual and average IEI of AMPA receptor-mediated mEPSCs under baseline conditions and following acute nicotine challenge. Note that in the P1–P2 group, both control and DNE cells show a trend toward a decrease in IEI with acute nicotine challenge; however, this was not significant. Analysis of average IEI from control and DNE neurons at P3–P5 shows no differences at baseline and no change in frequency with acute nicotine challenge.

DNE alters the modulation of glutamatergic sEPSCs in response to an acute nicotine challenge but only in cells from pups aged P3–P5. Cumulative probability distributions of glutamatergic sEPSC IEIs in control and DNE cells, at baseline and during acute nicotine challenge. At P1–P2 (A, B), acute nicotine challenge with 0.5 μM nicotine (black dashed lines) caused a left shift, toward shorter IEIs, of glutamatergic sEPSCs in both control and DNE cells [control, p = 0.049 (A) DNE, p < 0.0001 (B)]. In cells from P3–P5 animals (C, D), acute nicotine challenge caused a left shift, toward shorter IEIs in control cells (p < 0.0001; C), but the distribution shifted to the right, toward longer IEIs in the DNE cells (p = 0.012; D). Arrows indicate the direction of the shift with acute nicotine challenge and indicates significant differences with K–S test (see Materials and Methods). Dotted gray lines indicate the 95% confidence intervals of each curve.

Influence of DNE on AMPA receptor-mediated mEPSCs recorded from XIIMNs. A, B, Representative traces of pharmacologically isolated mEPSCs from a control animal (A) and a DNE animal (B) studied on P4. Each trace shows the entire 3 min of recording at baseline (top trace) and during acute nicotine application (bottom trace). Inset panels show an expanded view of 10 s at the end of each trace, as in Figure 4. Note that the size and frequency of events is similar in the control and DNE cell at baseline. However, during acute nicotine challenge, mEPSC frequency increased in control cells but not in DNE cells.

Cumulative probability distributions of glutamatergic mEPSC IEIs in control and DNE cells, at baseline and during acute nicotine challenge. In cells from animals aged P1–P2 (A, B), acute nicotine challenge with 0.5 μM nicotine (black dashed lines) caused a left shift, toward shorter IEIs, of glutamatergic mEPSCs in both control (p < 0.0001; A) and DNE cells (p < 0.0001; B). In cells from animals aged P3–P5 (C, D), acute nicotine challenge caused a significant left shift of glutamatergic mEPSCs in control cells (p = 0.004; C), but there was no change in the distribution of IEIs in DNE cells (p = 0.066; D). Arrows indicate the direction of the shift with acute nicotine challenge and indicates significant differences with K–S test (see Materials and Methods). Dotted gray lines indicate the 95% confidence intervals of each curve.

Activation of postsynaptic AMPA receptors in control cells and DNE cells. A, Representative trace of the AMPA receptor-mediated inward current in XIIMNs from a control animal at P1. B, Individual values for the peak inward current in response to bath application of AMPA. Mean values within each treatment group are indicated by the horizontal lines. There were no differences in the magnitude of the postsynaptic inward current either within or between treatment groups.