Stimulatory Effect of 5-Hydroxytryptamine (5-HT) on Rat Capsaicin-Sensitive Lung Vagal Sensory Neurons via Activation of 5-HT3 Receptors

FIGURE 1. Experimental records illustrating the effect of perineural capsaicin treatment (PCT) and vagotomy of both cervical vagi on cardiorespiratory responses to injections of 5-HT and capsaicin in an anesthetized rat (body weight: 320 g). Left panels: responses to right-atrial injections of 5-HT (8 μg/kg) before (as control) (A), 15 min after PCT (B), and 20 min after vagotomy (C). Right panels: responses to right-atrial injections of capsaicin (0.75 μg/kg) before (as control) (A), 15 min after PCT (B) and 20 min after vagotomy (C). Injection (0.1 ml) was first slowly injected into the catheter (dead space, 0.2 ml) and then flushed (at arrow) into the circulation by a bolus with saline (0.3 ml). VT, tidal volume; ABP, arterial blood pressure.

FIGURE 2. Apneic responses to intravenous injections of 5-HT in anesthetized, spontaneously breathing rats. (A) Effect of increasing doses of 5-HT on apneic responses in a group of 6 rats. Symbols ∗ and # depict significantly different (P < 0.05) from the responses to 4 and 8 μg/kg of 5-HT, respectively. (B) Apneic (upper panel) and augmented breath (lower panel) responses to 5-HT (8 μg/kg) before and after pretreatment of tropisetron (15 μg/kg) or its vehicle (saline) in two groups of rats (6 for each group). Data are mean ± SEM. One-way and two-way ANOVA tests followed by a post hoc Fisher’s least significant difference test for (A,B), respectively. ∗, significantly different from the corresponding responses before tropisetron (P < 0.05); #, significant difference when corresponding data between vehicle and tropisetron were compared (P < 0.05).

FIGURE 3. (A,C,D) Effect of perineural capsaicin treatment (PCT) and vagotomy on apneic responses to right-atrial injections of 5-HT, capsaicin (0.8 μg/kg) and lung inflation, respectively. (B) Effect of PCT and vagotomy on augmented breath responses to right-atrial injections of 5-HT. Data are mean ± SEM. One-way ANOVA test followed by a post hoc Fisher’s least significant difference test. ∗, significantly different from the control responses (P < 0.05).

FIGURE 4. Effect of increasing doses of 5-HT on activity of capsaicin-sensitive lung vagal (CSLV) afferents in anesthetized, open-chest and ventilated rats. (A–C) Experimental records illustrating responses of a CSLV afferent to right-atrial injections (arrows) of 4, 8, and 16 μg/kg 5-HT in an anesthetized and open-chest rat (body weight: 405 g), respectively. ΔFA, difference between peak FA (5-s average) occurring within 20 s after the injection and baseline FA (10-s average). AP, action potential; Pt, tracheal pressure; ABP, arterial blood pressure. (D) Effect of increasing doses of 5-HT on afferent responses of 10 CSLV fibers in 8 rats. Data are mean ± SEM. One-way ANOVA test followed by a post hoc Fisher’s least significant difference test. ∗ and #, significantly different (P < 0.05) from the responses to 4 and 8 μg/kg of 5-HT, respectively.

FIGURE 5. Experimental records illustrating the responses of a capsaicin-sensitive lung vagal afferent arising from the right upper lobe to 5-HT (16 μg/kg), capsaicin (0.8 μg/kg), lactic acid (18 mg/kg), or phenylbiguanide (PBG; 6 μg/kg) before and after pretreatment with tropisetron in an anesthetized, open-chest and ventilated rat (body weight: 410 g). (A) Responses before tropisetron pretreatment (15 μg/kg). (B) Responses to 5-HT, capsaicin, lactic acid and PBG at 15 min after tropisetron pretreatment, respectively. Stimulant injections were marked by the arrows. AP, action potential; Pt, tracheal pressure; ABP, arterial blood pressure.

FIGURE 6. Effect of pretreatment with tropisetron (15 μg/kg) on capsaicin sensitive lung vagal afferent responses to various stimulants in anesthetized, open-chest rats. The afferent responses to right-atrial injections of 5-HT (16 μg/kg, n = 15; A), capsaicin (0.8 μg/kg, n = 15; B), lactic acid (9 or 18 mg/kg, n = 11; C) and phenylbiguanide (PBG, 4–8 μg/kg, n = 12; D). ΔFA, difference between peak FA (2-s average for capsaicin and lactic acid; 5-s average for 5-HT and PBG) occurring within 20 s after the injection and baseline FA (10-s average). Data are mean ± SEM. Paired t-test ∗, significantly different (P < 0.05) from the corresponding responses before tropisetron.

FIGURE 7. 5-HT-evoked calcium transients in isolated rat capsaicin-sensitive lung vagal (CSLV) neurons. (A) An experimental record illustrating that 5-HT concentration-dependently evoked calcium transients in a jugular ganglion neuron (diameter: 28 μm). 5-HT and capsaicin (Cap; 1 μM) was applied for 30 s each, and potassium chloride (KCl; 60 mM, 15 s) was applied to test cell viability at the end of experiments. Time between 2 consecutive 5-HT challenges was 15 min. [Ca2+]i, intracellular concentration of Ca2+. (B) Group data of 5-HT evoked increase of [Ca2+]i (Δ[Ca2+]i) obtained from 21 CSLV neurons in 12 rats. Data are mean ± SEM. One-way ANOVA test followed by a post hoc Fisher’s least significant difference test. Symbols ∗ and # depict significantly different (P < 0.05) from the responses to 0.3 and 1 μM of 5-HT, respectively.

FIGURE 8. Effect of tropisetron on 5-HT-induced calcium transients in isolated rat capsaicin-sensitive lung vagal (CSLV) neurons. (A) An experimental record illustrating that pretreatment with tropisetron (Trop; 10 nM, 5 min) almost completely prevented the Ca2+ transient evoked by 5-HT (3 μM, 30 s) in a nodose ganglion neuron (diameter: 24 μm). (B) Group data of 5-HT evoked increase of [Ca2+]i (Δ[Ca2+]i) before and after tropisetron pretreatment obtained from 8 CSLV neurons in 5 rats. Time between 2 consecutive 5-HT challenges was 15 min. Data are mean ± SEM. One-way ANOVA test followed by a post hoc Fisher’s least significant difference test. ∗ and #, significantly different (P < 0.05) from the responses before and immediately after tropisetron, respectively.