Breathing and upper airway CO2 in reptiles: role of the nasal and vomeronasal systems

Abstract

The ventilatory response of the garter snake, Thamnophis sirtalis, to 2% CO2 delivered to the upper airways (UA) was measured before and after the olfactory or vomeronasal nerves were transected. The UA (nasal cavities and mouth) were isolated from the gas source inspired into the lungs by inserting an endotracheal T tube into the glottis. CO2 was administered to the UA via a head chamber. The primary ventilatory response to UA CO2 was a significant decrease in ventilatory frequency (f) and minute ventilation. The decrease in f was caused by a significant increase in the pause duration. Tidal volume, expiratory duration, and inspiratory duration were not altered with UA CO2. The f response to UA CO2 was abolished with olfactory nerve transection, whereas vomeronasal nerve transection significantly increased the magnitude of the f depression. These results indicate that CO2-sensitive receptors are located in the nasal epithelium and that the olfactory nerves must be intact for the UA CO2 f response to be observed. In addition, the vomeronasal system appears to modulate the ventilatory response to UA CO2.

The neuronal correlates of intranasal trigeminal function—an ALE meta-analysis of human functional brain imaging data

“…Evidence supporting the first hypothesis, the existence of olfactory receptors responsive to CO 2 , comes from a range of animal models, including bullfrogs (Coates and Ballam, 1990; Sakakibara, 1978), tegu lizards (Coates and Ballam, 1987), reptiles (Coates and Ballam, 1989), and mice (Hu et al, 2007). Furthermore it is possible that olfactory brain areas are activated during the perception of trigeminal CO 2 stimuli via trigeminal ganglion cells with sensory endings in the nasal epithelium that also send branches into the olfactory bulb and the spinal trigeminal complex which were discovered by Schaefer and colleagues (2002).…”

Respiratory and antinociceptive effects of dexmedetomidine and doxapram in ball pythons (Python regius)

“…Conditioning was conducted by placing a snake into the chamber and alternating between 20 minutes of airflow and 10 minutes of no airflow (ie, air inflow and outflow ports closed), with data recorded during the 10-minute period of no airflow. Alternating periods of airflow and no airflow prevented accumulation of CO 2 in the chamber, which can stimulate breathing, 54 increase Vt, increase breath frequency, and increase Ve 35,[55][56][57][58] and allowed for nearly continuous recording for > 8 hours. 18 When the inflow and outflow ports were closed, the pressure transducer detected pressure changes attributable to breathing movements.…”

Topical Inhibition of Nasal Carbonic Anhydrase Affects the CO2 Detection Threshold in Rats

“…Electrophysiological studies of olfactory receptors in salamanders (Getchell and Shephard 1978), frogs (Coates and Ballam 1990), and rats (Coates 2001) show that single olfactory receptor activity or electroolfactograms (EOGs), which measure summated receptor responses, can be recorded in response to CO 2 concentrations ranging from 0.5% to 15%. In addition, studies measuring ventilation in bullfrogs (Sakakibara 1978;Kinkead and Milsom 1996), lizards (Coates and Ballam 1987), and snakes (Coates and Ballam 1989) show that CO 2 delivered to the isolated upper airways causes a depression in ventilation. Transection of the olfactory nerves of bullfrogs (Sakakibara 1978) and garter snakes (Coates and Ballam 1989) eliminates the ventilatory response to upper airway CO 2 , whereas transection of the trigeminal nerves of bullfrogs (Sakakibara 1978) or the vomeronasal nerves of garter snakes (Coates and Ballam 1989) does not affect the response.…”

“…In addition, studies measuring ventilation in bullfrogs (Sakakibara 1978;Kinkead and Milsom 1996), lizards (Coates and Ballam 1987), and snakes (Coates and Ballam 1989) show that CO 2 delivered to the isolated upper airways causes a depression in ventilation. Transection of the olfactory nerves of bullfrogs (Sakakibara 1978) and garter snakes (Coates and Ballam 1989) eliminates the ventilatory response to upper airway CO 2 , whereas transection of the trigeminal nerves of bullfrogs (Sakakibara 1978) or the vomeronasal nerves of garter snakes (Coates and Ballam 1989) does not affect the response. The results from the studies cited above provide further support for the presence of CO 2sensitive olfactory receptors in the amphibians, reptiles, and mammals.…”