Sympathoexcitatory neurons of rostral ventrolateral medulla exhibit pacemaker properties in the presence of a glutamate-receptor antagonist

Abstract

Intracisternal (i.c.) administration of the glutamate-receptor antagonist kynurenate to halothane-anesthetized rats (paralyzed, ventilated) produced an initial hypertension associated with an increase in lumbar sympathetic nerve discharge. Kynurenate (i.c.) blocked or greatly reduced all sympathetic reflexes investigated (somatosympathetic 70% reduction; vagal pressor and depressor responses, 100%; hypothalamic mixed responses, 90%; baroreflex, 100%) and increased the firing rate of retiiculospinal sympathoexcitatory cells of the rostral ventrolateral medulla (PGCL-SE neurons) by 33%. After i.c. kynurenate, these cells exhibited a rhythmic, non-bursting firing pattern which could be reset by spinal cord stimulation only when antidromic spikes were elicited. Cells with similar characteristics were recorded in the nucleus paragigantocellularis lateralis (PGCL) in an in vitro rat bulb preparation perfused through the basilar artery. Their ‘pacemaker-like’ discharge pattern was observed even in the absence of kynurenate and was reset by orthodromic activation. Cells with similar characteristics were also recorded within the PGCL in 500-μm coronal slices in vitro. At 37°C their discharge rate was similar to that of PGCL-SE neurons recorded in vivo after i.c. kynurenate; it was also pacemaker-like and was insensitive to glutamate receptor blockade. It is suggested that the tonic discharge of PGCL-SE neurons is normally due to an intrinsic pacemaker activity which is modulated in vivo by a variety of synaptic inputs.