Pavlov's studies on the digestive glands, which years later earned him the Nobel Prize, highlighted, for the first time, the so-called "cephalic or psychic secretions." These secretions occur anticipatorily, with the mere sight, taste, or smell of food we are about to consume, before the nutrients themselves reach the digestive tract. These secretions are essential during feeding behavior, as evidenced by numerous clinical cases, such as those of Tom or Eddie, reported by Terry Powley (Department of Psychology, Purdue University). In both cases, the patients could not ingest food orally, so it had to be administered directly into the stomach via a tube. However, this procedure produced adverse effects such as gastrointestinal discomfort, vomiting, etc. Tom and Eddie quickly discovered that the mere smell or taste of nutrients just before they were administered intragastrically eliminated the aversive consequences, resulting in normal weight gain.
It is believed that these psychic secretions depend on the connections, possibly orexinergic, established by the lateral hypothalamus with groups of neurons in the brainstem. However, the precise functional location of these brainstem centers is still not well known. The present study published by Juan Manuel Jiménez Ramos, a member of CIMCYC and professor in the Department of Psychobiology, aimed precisely at this objective. Specifically, it sought to functionally identify the inferior salivatory nucleus (ISN), which directly controls the parotid salivary glands. These glands are especially important in feeding behavior due to the high amylase content in their secretion. To specifically stimulate the somas of the ISN and not the efferent or afferent fibers to the nucleus, an NMDA receptor agonist, present only in the somas, was administered in the rostrodorsal region of the bulbar reticular formation in rats, where the ISN was suspected to be anatomically located (see figure). The results were spectacular, as during the hour following the NMDA microinjection, the animals secreted more than 1000 milligrams of parotid saliva, with only basal secretion observed in the rest of the glands. Moreover, this potent secretory effect completely disappeared when the parotids were blocked with cholinergic antagonists but not with adrenergic antagonists. These results demonstrate, for the first time, that these rostrodorsal neurons control parotid secretion through a cholinergic/parasympathetic pathway, thus functionally constituting the ISN. In the future, this nucleus should be related to the feeding behavior of animals.
Full reference:
Ramos, J. M. J. (2024). Parotid hypersalivation after inferior salivatory nucleus glutamate/NMDA receptor excitation in the rat. Physiology & Behavior, 280, 114564. https://doi.org/10.1016/j.physbeh.2024.114564