Capsaicin and regulation of respiration: interaction with central substance P mechanisms.

The neuropharmacological effects of capsaicin (CAPS) (8-methyl-N-vanillyl-6-nonenamide) have been closely linked to the peptide neurotransmitter substance P (SP). In order to elucidate SP mechanisms in peripheral and central control of breathing we have studied the respiratory effects of CAPS and SP administration to neonatal and adult rats using a whole body plethysmographic method. CAPS (3 and 30 micrograms) induced an immediate apnea after intravenous injection. This effect could be reduced by vagotomy but not further changed by combined vagotomy and glossopharyngectomy. The apnoic periods were followed by periods of tachypnea. Intracerebroventricular (i.c.v.) administration of CAPS resulted in an increased tidal volume (VT) and a decreased respiratory frequency (f), i.e. a respiratory response similar to that seen after i.c.v. SP. No apnoic episodes were seen after i.c.v. injection. The respiratory pattern after acute i.c.v. CAPS administration was not significantly changed by neonatal CAPS pretreatment. However, while saline pretreated control animals responded to an i.c.v. injection of SP with an increase in VT and inspiratory drive (VT/TI), animals pretreated with CAPS responded with a shortening of inspiratory and expiratory time in combination with an increase in VT. Similar changes have been observed in vagotomized animals after SP administration. It is concluded that CAPS elicits apnea via mechanisms located outside the CNS, which cannot be fully deafferented by combined vagotomy and glossopharyngectomy. Furthermore, CAPS i.c.v. induces a stimulation of respiration by a central mechanism of action, possibly due to a release of SP. Neonatal pretreatment with CAPS modifies the respiratory response to i.c.v. SP. This effect might be due to an impairment in tonical afferent SP mechanisms to the central respiratory regulating system and possibly also to an impairment of central SP mechanisms involved in respiration.