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 chemoreceptor and baroreceptor afferent pathways. Chemoreceptors were stimulated by placing rats in a hypoxic gas mixture (7% oxygen) for 2 h. Baroreceptors were activated by phenylephrine-induced hypertension. Seizures and hypoxia induced Fos immunoreactivity in neurons with similar anatomical distributions in the nucleus tractus solitarius, dorsal motor nucleus of the vagus, and ventrolateral medulla. Hypertension was associated with Fos immunoreactivity in an overlapping anatomical distribution compared to seizures and hypoxia, but in a more restricted pattern. A similar proportion of catecholaminergic cells of medulla oblongata (cells immunoreactive for catecholamine synthetic enzymes, tyrosine hydroxylase or phenylethanolamine-N-methyltransferase) had Fos immunostaining after seizures and hypoxia (P > 0.05), while significantly fewer were activated by hypertension (P < 0.05). The majority of tyrosine hydroxylase-immunoreactive cells in caudal ventrolateral medulla were activated by both seizures and hypoxia (mean per cents, 79 and 67%, respectively). Since cell populations activated by seizures and hypoxia are indistinguishable, and a majority of tyrosine hydroxylase-reactive cells in caudal ventrolateral medulla are independently activated by each stimulus, it may be inferred that some impulses originating from seizures and chemoreceptor afferent pathways converge to a common set of neurons. These observations identify neurons in rat medulla oblongata which may mediate the impact of seizures on central processing of chemoreceptor afferent activity.