This study examines the episodic breathing patterns of three disparate
groups of vertebrates. In an in vitro bullfrog brainstem-spinal cord
preparation, episodic breathing was replaced by uniformly spaced breat
hs following transection caudal to the optic chiasma. The same effect
was produced in hibernating squirrels by inhalation of mild anesthesia
. Preliminary data suggest that a similar conversion is also produced
in hibernating squirrels by vagotomy, in conjunction with blockade of
central NMDA-type glutamate receptors. In all cases, even though overa
ll breathing frequency increased, due to elimination of periods of apn
ea, instantaneous breathing frequency slowed. Seals breathe episodical
ly in sleep and when these animals awaken after the start of a breathi
ng episode, breathing also immediately slows. The data presented here
are consistent with the suggestion that in all vertebrates, higher cen
tres can modulate the central rhythm generator for breathing, in both
a positive and a negative fashion. During episodic breathing, in the s
pecies studied here, these modulating influences alternate in a fashio
n that produces periods of apnea alternating with periods of relativel
y high frequency ventilation. (C) 1997 Elsevier Science B.V.