Introduction and commentary to: M.I. Mityushov (1954) "Conditioned reflex secretion of insulin"

Adaptive short-term associative conditioning in the pancreatic β-cell

This study associates cholinergic stimulation of the pancreatic β-cell electrical activity with a short-term memory phenomenon. Glucose pulses applied to a basal glucose concentration induce depolarizing waves which are used to estimate the evolution of the β-cell glucose sensitivity. Exposure to carbamoylcholine (carbachol) increases the size of the glucose-induced depolarizing waves. This change appears after carbachol withdrawal and implies a temporal potentiation of sensitivity (TPS) lasting up to one hour. TPS induction requires the simultaneous action of carbachol and glucose. The substitution of glucose with the secretagogues glyceraldehyde or 2-ketoisocaproate mimics glucose-induced TPS, while palmitate does not. TPS is not produced if the membrane is kept hyperpolarized by diazoxide. Glucose can be replaced by tolbutamide, suggesting a role of depolarization and a subsequent increase in intracellular calcium concentration. A role for kinases is suggested because staurosporine prevents TPS induction. Cycloheximide does not impair TPS induction, indicating that de novo protein synthesis is not required. The fact that the two inputs acting simultaneously produce an effect that lasts up to one hour without requiring de novo protein synthesis suggests that TPS constitutes a case of short-term associative conditioning in non-neural tissue. The convergence of basal glucose levels and muscarinic activation happens physiologically during the cephalic phase of digestion, in order to later absorb incoming fuels. Our data reveals that the role of the cephalic phase may be extended, increasing nutrient sensitivity during meals while remaining low between them.

Interactions between the central nervous system and pancreatic islet secretions: a historical perspective

The endocrine pancreas is richly innervated with sympathetic and parasympathetic projections from the brain. In the mid-20th century, it was established that α-adrenergic activation inhibits, whereas cholinergic stimulation promotes, insulin secretion; this demonstrated the importance of the sympathetic and parasympathetic systems in pancreatic endocrine function. It was later established that insulin injected peripherally could act within the brain, leading to the discovery of insulin and insulin receptors within the brain and the receptor-mediated transport of insulin into the central nervous system from endothelial cells. The insulin receptor within the central nervous system is widely distributed, reflecting insulin's diverse range of actions, including acting as an adiposity signal to reduce food intake and increase energy expenditure, regulation of systemic glucose responses, altering sympathetic activity, and involvement in cognitive function. As observed with central insulin administration, the pancreatic hormones glucagon, somatostatin, pancreatic polypeptide, and amylin can each also reduce food intake. Pancreatic and also gut hormones are released cephalically, in what is an important mechanism to prepare the body for a meal and prevent excessive postprandial hyperglycemia.

Classically conditioned responses following repeated insulin and glucose administration in humans

This paper describes the neural basis and the role of Pavlovian conditioning in the modification of blood glucose and related endocrine parameters after repeated insulin and glucose administration. Pavlovian conditioning requires that conditioned stimulus (CS) and unconditioned stimulus (US) are both detected in the central nervous system (CNS), where the CS-US association takes place. We will therefore elucidate the detectability of insulin and glucose in the CNS. Since current data focus almost exclusively on animals, we conducted a placebo-controlled insulin conditioning experiment in humans (Experiment 1). Compared with the control group with CS-placebo pairings throughout, the experimental group with previous CS-insulin pairings in the acquisition phase showed a conditioned decrease in blood glucose and a trend for a conditioned baseline insulin increase, and an increase in cortisol levels relative to baseline and cumulative number of neuroglycopenic symptoms in the CS-placebo test session. The conditionability of glucose administration also had to be examined; experiments using an arbitrary CS and glucose are extremely rare, even in animals. Glucose is the natural stimulus for endogenous insulin secretion, so studies on cephalic-phase insulin release (CPIR) will be reviewed in this paper. We implemented a placebo-controlled three-group design (Experiment 2): Subjects received either CS-insulin, CS-glucose or CS-placebo pairings during the acquisition. Together, our results demonstrate the conditionability mainly of insulin, but also of glucose effects in healthy humans. The clinical relevance and future research perspectives are outlined with an emphasis on insulin in the brain and its role in learning and memory.