Homeostatic significance of interleukin-1β in the cingulate cortex

Keeping the internal environment constant in a continuously changing external environment, i.e. the maintenance of homeostasis is crucial for the living organisms and it is served by several mechanisms. Interleukin-1 (IL-1) is one of the most important cytokines, peptide molecules that are responsible for the communication among the immune, neural and other cells of the organism in immune and inflammatory processes. Of the two isoforms of this primary cytokine, the ‘beta’ form appears to be biologically a lot more important. The IL-1β is known to have numerous functional effects both in the immune and nervous systems, and its role has already been shown in homeostatic regulatory processes as well [1]. The target area of our present series of experiments was the anterior cingulate cortex which is known to be essential in motivational, emotional and cognitive control mechanisms. As part of the limbic system, it is supposed to be also involved in the organization of homeostatic processes. In microelectrophysiological experiments of our laboratory, the functional presence of IL-1β responsive neural cells has been elucidated in this cortical area. Based on all the above, the goal of the highlighted study [2] was to reveal whether cytokine mechanisms of the cingulate cortex are important in the central regulation of homeostasis. To do so, changes in food and water intake, body temperature, and other metabolic functions, i.e. homeostatic effects of direct bilateral microinjection of IL-1β into the cingulate cortex of adult male Wistar rats have been investigated. To clarify the role of cyclooxygenase (COX) – mediated mechanisms in the mode of the postulated action of IL-1β, the effect of intracerebral pretreatment by the COX-inhibitor paracetamol was tested in our study.

Guide cannulas for the IL-1β or vehicle microinjections were implanted on the surface of the dura above the cingulate cortex in a stereotaxic operation under ketamine anesthesia. One week later, IL-1β; paracetamol or sterile phosphate-buffered saline (PBS), in the sham control animals, were administered into the cingulate cortex as bilateral microinjections through the previously implanted guide cannulas (Figure 1).

Figure 1.

Guide cannulas for the IL-1β or vehicle microinjections were implanted in a stereotaxic operation under ketamine anesthesia. After one week of recovery, IL-1β, paracetamol or sterile PBS were administered into the cingulate cortex by means of a microinfusion pump through the previously implanted guide cannulas. Body temperature of the animals was measured by means of a digital thermometer just before and two hours after the microinjections. Measurement of relevant metabolites was performed 20 minutes after the microinjections by means of a cold chemistry photometer. Interleukin-1β caused significant elevation of body temperature (*p < 0.001) and significant decrease in the plasma levels of HDL and total cholesterol (*HDL: p < 0.001; total cholesterol: p < 0.005). Food and water intakes and blood glucose levels of the animals did not change remarkably after the intracerebral microinjections.

Food and water consumption of the rats were monitored for one day after the microinjections: short- (2 h), medium- (12 h) and long-term (24 h) measurements were performed. Animals were divided into four groups in these experiments: half of both the cytokine-treated and the control animals received paracetamol pretreatment 25 minutes before the administration of IL-1β or PBS. These experiments were carried out after 24 hours of food deprivation.

Body temperature of the same four groups of animals was measured rectally, just before and two hours after the intracerebral microinjections, by means of a digital thermometer.

In order to examine the blood glucose levels of cytokine treated and control rats, a standard glucose tolerance test was performed 12 hours following the food deprivation. Measurements were carried out right before and 20 minutes after the cerebral microinjections of IL-1β or PBS, as well as 9, 18, 30, 60 and 120 minutes after the sugar load. Blood samples were taken from the tail vein of the rats and blood glucose levels were determined electrochemically by means of a semi-automatic glucometer.

Relevant plasma metabolites (total cholesterol, HDL, LDH, triglycerides, uric acid) of the animals were also determined following 12 hours of food deprivation. Blood samples were obtained after decapitation of the rats 20 minutes following the IL-1β or PBS microinjections. A cold chemistry photometer was employed for these examinations.

All experiments were performed under protocols with approved ethical permissions.

Although the food and water intake decreasing effect of the cytokine is one of the most widely known consequences of the IL-1β microinjection in other brain structures [3], the cingulate cortical administration of IL-1β had no significant effect on these parameters. Instead, this primary cytokine applied into the cingulate cortex was shown to result in the elevation of body temperature. Despite the well known pyrogenic effect of intracerebrally administered IL-1β [4], it was the first time to demonstrate its thermoregulatory role in this cortical area. The body temperature of the cytokine treated animals increased significantly compared to the control and the paracetamol pretreated groups. That is, the involvement of cyclooxygenase mediated processes in the IL-1β induced fever was also substantiated. Nevertheless, the paracetamol prevented the body temperature rising effect only partially, which necessarily means the involvement of other mechanisms as well. The details and other potential elements of this presumably complex mechanism are yet to be fully explored.

The blood glucose levels of the interleukin-1β treated rats were higher throughout the whole glucose tolerance test compared to values of the vehicle treated animals, but the difference did not reach the level of significance. Interpretations about the effect of cytokines on glucose metabolism are contradictory in the literature, presumably it is determined by its local available amount and the spatial and temporal pattern of its production [5]. In contrast to causing no major alteration in the blood glucose levels, a significant decrease was observed in the plasma concentrations of HDL and total cholesterol after the cytokine microinjection into the cingulate cortex. Plasma levels of triglycerides were marginally higher in the IL-1β treated group, but in the concentrations of the other measured metabolites there was no remarkable alteration compared to the control animals. Cytokines are thought to be involved in the mediation of the alterations of plasma levels of triglycerides and lipoproteins in infectious and inflammatory diseases, but these results are, again, contradictory. In a previous study of our laboratory, for instance, IL-1β injected directly into the nucleus accumbens resulted in significant elevation of not only the triglycerides, but the plasma cholesterol as well [1]. Results make it clear that IL-1β mediated central control mechanisms affecting lipid metabolism do necessarily exist. The actual changes of plasma concentrations of lipid metabolism associated metabolites, however, depend on several interrelated regulatory factors.

After all, based on the findings of the highlighted study, it is reasonable to suppose that the IL-1β mediated processes in the cingulate cortex have important role in the central regulation of homeostasis. The close control of body temperature, and the actual concentration of relevant plasma metabolites are of great significance in the maintenance of the homeostatic balance of the organism. Elucidating the above mentioned processes lead to the better understanding of the complex interactions among the regulatory systems of the healthy organism, and also those found in such feeding-metabolic disorders, like obesity and diabetes mellitus that are known to impose a rapidly increasing heavy burden on the modern societies.

Funding Statement

University of Pecs, Medical School, Research Fund PTE AOK-KA 2013/34039/1