AJP - Regulatory, Integrative and Comparative Physiology, Vol 257, Issue 6 1345-R1353, Copyright © 1989 by American Physiological Society
Single-unit responses of serotonergic neurons to glucose and insulin administration in behaving cats
C. A. Fornal, W. J. Litto, D. A. Morilak and B. L. Jacobs
Department of Psychology, Princeton University, New Jersey 08544.
Extracellular single-unit activity of serotonergic neurons in the dorsal
raphe nucleus (DRN) was recorded in response to glucose loading and insulin
administration in conscious, freely moving cats. Serotonergic neurons were
identified based on their discharge characteristics, activity across states
of behavioral arousal, response to systemic administration of serotonin
autoreceptor agonists, and histological localization to the DRN. The
spontaneous activity of serotonergic neurons varied in association with
behavioral state, reaching their highest level during arousal and their
lowest level during rapid-eye-movement sleep, when cells typically stopped
firing. The activity of serotonergic DRN neurons was not significantly
altered by a glucose load (500 mg/kg iv) that produced an approximately
3.5-fold increase in blood glucose levels. Furthermore, serotonergic DRN
neuronal activity was not significantly altered after insulin
administration (2-4 IU/kg iv), which lowered blood glucose approximately
50% below control levels, or after the rapid reversal of hypoglycemia by
subsequent glucose administration. These results indicate that the activity
of serotonergic DRN neurons is unrelated to alterations in blood glucose
and is not sensitive to elevations of endogenous circulating insulin levels
or to exogenous insulin administration. Furthermore, changes in the
activity of serotonergic DRN neurons does not appear to be a component of
glucoregulatory mechanisms invoked by either hyper- or hypoglycemia.
Overall, these results do not support a role for serotonergic DRN neurons
in glucoregulation in the cat.
