Single-unit responses of serotonergic neurons to glucose and insulin administration in behaving cats

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.