COMPARISON OF NEURONS IN RAT MEDULLA-OBLONGATA WITH FOS IMMUNOREACTIVITY EVOKED BY SEIZURES, CHEMORECEPTOR, OR BARORECEPTOR STIMULATION

Neurons in rat medulla oblongata with Fos immunoreactivity as a marker of synaptic excitation evoked by pentylenetetrazole-induced seizures were compared with cell populations activated by the stimulation of ch emoreceptor and baroreceptor afferent pathways. Chemoreceptors were st imulated by placing rats in a hypoxic gas mixture (7% oxygen) for 2 h. Baroreceptors were activated by phenylephrine-induced hypertension. S eizures and hypoxia induced Fos immunoreactivity in neurons with simil ar anatomical distributions in the nucleus tractus solitarius, dorsal motor nucleus of the vagus, and ventrolateral medulla. Hypertension wa s associated with Fos immunoreactivity in an overlapping anatomical di stribution compared to seizures and hypoxia, but in a more restricted pattern. A similar proportion of catecholaminergic cells of medulla ob longata (cells immunoreactive for catecholamine synthetic enzymes, tyr osine hydroxylase or phenylethanolamine-N-methyltransferase) had Fos i mmunostaining after seizures and hypoxia (P > 0.05), while significant ly fewer were activated by hypertension (P < 0.05). The majority of ty rosine hydroxylase-immunoreactive cells in caudal ventrolateral medull a were activated by both seizures and hypoxia (mean per cents, 79 and 67%, respectively). Since cell populations activated by seizures and h ypoxia are indistinguishable, and a majority of tyrosine hydroxylase-r eactive cells in caudal ventrolateral medulla are independently activa ted by each stimulus, it may be inferred that some impulses originatin g from seizures and chemoreceptor afferent pathways converge to a comm on set of neurons. These observations identify neurons in rat medulla oblongata which may mediate the impact of seizures on central processi ng of chemoreceptor afferent activity. Copyright (C) 1996 IBRO. Publis hed by Elsevier Science Ltd.