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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Locus coeruleus is a central chemoreceptive site in toads

¹Department of Morphology, Stomatology and Physiology, Dental School of Ribeirao Preto, University of São Paulo; ²Department of Animal Morphology and Physiology, São Paulo State University-FCAV at Jaboticabal; and ³Department of Physiology, Faculty of Medicine of Ribeirao Preto, University of São Paulo, Brazil

Submitted 2 February 2006 ; accepted in final form 26 April 2006

The locus coeruleus (LC) has been suggested as a CO₂ chemoreceptor site in mammals. This nucleus is a mesencephalic structure of the amphibian brain and is probably homologous to the LC in mammals. There are no data available for the role of LC in the central chemoreception of amphibians. Thus the present study was designed to investigate whether LC of toads (Bufo schneideri) is a CO₂/H⁺ chemoreceptor site. Fos immunoreactivity was used to verify whether the nucleus is activated by hypercarbia (5% CO₂ in air). In addition, we assessed the role of noradrenergic LC neurons on respiratory and cardiovascular responses to hypercarbia by using 6-hydroxydopamine lesion. To further explore the role of LC in central chemosensitivity, we examined the effects of microinjection of solutions with different pH values (7.2, 7.4, 7.6, 7.8, and 8.0) into the nucleus. Our main findings were that 1) a marked increase in c-fos-positive cells in the LC was induced after 3 h of breathing a hypercarbic gas mixture; 2) chemical lesions in the LC attenuated the increase of the ventilatory response to hypercarbia but did not affect ventilation under resting conditions; and 3) microinjection with acid solutions (pH = 7.2, 7.4, and 7.6) into the LC elicited an increased ventilation, indicating that the LC of toads participates in the central chemoreception.

ventilation; midbrain; brain stem; amphibian; isthmus; bufo; hypercarbia

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