Infrared Laser Light Elicits Cardiorespiratory Responses when Applied to the Nucleus Tractus Solitarius in the In Situ Rat Brainstem Preparation

Abstract

The application of infrared laser (IR) light has been shown to alter central and peripheral neural activity. We hypothesized that cardiorespiratory activity would be an ideal system to test the effects of IR on spontaneous activity that is generated by a complex neural circuit. In the present study, we examined the responses elicited by IR stimuli delivered unilaterally to the intermediate subnucleus of the nucleus of the solitary tract (NTSINT) in the in situ brainstem preparation. The data show that IR light evoked changes in cardiorespiratory motor activity in a stimulus intensity‐dependent manner.These studies used the in situ arterially perfused brainstem preparation of male Sprague‐Dawley rat pups (P15–P25). We recorded outputs from the phrenic nerve, the cervical vagus nerve, and thoracic sympathetic nerve and monitored heart rate and perfusion pressure. A 200 mm optic fiber was positioned on the dorsal surface of the brainstem above the NTSINT. The IR light (1450nm) stimulus was 200 Hz pulse frequency, 200 ms pulses duration for 30 s. In order to gain an insight into the potential neurochemistry associated with the IR light‐induced responses, we also examined the effects elicited by the microinjection of the excitatory neurotransmitter, glutamate (20 to 30 nL of a 10 mM solution) into the NTSINT.Low radiant exposure levels of IR light stimuli (4.78 J/cm2 per pulse) decreased phrenic nerve activity (consistent with a decrease in breathing frequency), decreased bursting of vagal activity, inhibition of thoracic sympathetic nerve, a decrease in heart rate and a decrease in perfusion pressure. Upon cessation of IR light application was often followed by a post‐stimulus response. This delayed effect consisted of a further decrease in breathing rate, thoracic sympathetic bursting and perfusion pressure. Increasing the radiant exposure to 5.57 J/cm2 per pulse evoked a strong baroreflex‐like response but only during IR light stimulation. Further, increases in radiant exposure levels evoked the immediate effect, other behaviors like swallowing; and a post‐stimulus response (10 to 30 seconds) such increases in breathing frequency. However, the cardiorespiratory patterning had changed and returning to a 4.78 J/cm2 per pulse IR light stimulus did not evoked comparable response in cardiorespiratory patterning as before. The microinjections of glutamate into the NTSINT elicited cardiorespiratory responses that were similar to those elicited by the lower intensity IR light stimuli.These data indicate that the even though IR light stimuli are focally applied they are strong enough to elicit network effects on respiratory and sympathetic activities. The mechanisms by which IR light elicits these profound changes in cardiorespiratory circuit function remain to be elucidated.Support or Funding InformationGlaxoSmithKline (GSK) FundingCWRU Internal Number: RES510692