Pressor responses from electrical or glutamate stimulations of the dorsal or ventrolateral medulla

In rats, rabbits, and cats anesthetized with α-chloralose and urethan, responses of the pressor areas of the dorsal portion (DM) and ventrolateral portion (VLM) of medulla and pons were compared. Electrical stimulation (monopolar square-wave pulses) on monosodium glutamate solution (Glu 100-200 nl, 1 M) was delivered through an electrode-needle tubing connected to a Hamilton syringe for semimicroinjection. In all three of these species, pressor responses were elicited from both DM and VLM by either Glu or electrical stimulation. The most active parts of DM were found in the dorsomedial reticular formation of the rostral medulla to midmedulla. In the pons and caudal medulla, the Glu-induced response was mild, although the electrically induced response was marked. Application of kainic acid (KA) to either DM or VLM produced an initial pressor response but was followed by a reduction of the pressure rise on subsequent electrical stimulation. Glu, unlike electrical stimulation, excites neural perikarya, not fibers of passage. KA initially excites the neural perikarya before causing damage that spares axons. These results thus suggest that both DM and VLM contain neural perikarya that mediate pressor effects.