A neuromodulatory role of interleukin-1beta in the hippocampus

Abstract

It is widely accepted that interleukin-1beta (IL-1beta), a cytokine produced not only by immune cells but also by glial cells and certain neurons influences brain functions during infectious and inflammatory processes. It is still unclear, however, whether IL-1 production is triggered under nonpathological conditions during activation of a discrete neuronal population and whether this production has functional implications. Here, we show in vivo and in vitro that IL-1beta gene expression is substantially increased during long-term potentiation of synaptic transmission, a process considered to underlie certain forms of learning and memory. The increase in gene expression was long lasting, specific to potentiation, and could be prevented by blockade of potentiation with the N-methyl-D-aspartate (NMDA) receptor antagonist, (+/-)-2-amino-5-phosphonopentanoic acid (AP-5). Furthermore, blockade of IL-1 receptors by the specific interleukin-1 receptor antagonist (IL-1ra) resulted in a reversible impairment of long-term potentiation maintenance without affecting its induction. These results show for the first time that the production of biologically significant amounts of IL-1beta in the brain can be induced by a sustained increase in the activity of a discrete population of neurons and suggest a physiological involvement of this cytokine in synaptic plasticity.

Figure 1

IL-1β gene expression during LTP in hippocampal slices. (A Left) High frequency stimulation induced a robust LTP in comparison to controls. (Right) Analog traces of a representative recording obtained during baseline recording (Lower trace) and 10 min after tetanization (Upper trace). Arrow indicates time of tetanization. (B) IL-1β mRNA expression at different times after induction of LTP. Group 1 (control): slices without tetanic stimulation. The total time of incubation was comparable for all slices. Results of RT-PCR are expressed as mean ± SEM of the ratio IL-1β cDNA and the competitive fragment (cf). Group 1, n = 4; group 2, n = 8; group 3, n = 6; and group 4, n = 4. lL-1β gene expression was significantly increased in groups 3 and 4 as compared with the control group (P < 0.05; ANOVA followed by Fisher test for multiple comparisons). (C) Representative ethidium bromide-stained agarose gel showing RT-PCR amplified transcripts. Lane numbers correspond to the groups shown in B. M, 100-bp ladder molecular weight marker; no RT, RT-PCR without addition of reverse transcriptase; −, RT-PCR without addition of cDNA.

Figure 2

IL-1β gene expression during LTP in vivo. (A) IL-1β mRNA expression 8 hr after tetanization. Thirty minutes before tetanization, animals received either physiological saline (groups 1, 3, and 4) or AP-5 (group 2) administered i.c.v. Black bars indicate measurements performed in ipsilateral (i) and white bars in contralateral (c) hippocampi. Group 1: hippocampi of animals recorded under baseline conditions without tetanic stimulation. Group 2: hippocampi of animals in which the expression of LTP after tetanization was blocked by AP-5. Group 3: hippocampi of animals showing a potentiation that returned to baseline within 3 hr. Group 4: hippocampi of animals with a robust LTP lasting for 8 hr. Results of RT-PCR are expressed as mean ± SEM of the ratio IL-1β cDNA and the competitive fragment (cf). n = 4 per group. Group 4_i differs significantly from all other groups (P < 0.05; ANOVA followed by Fisher test for multiple comparisons). (B) Representative ethidium bromide-stained agarose gel showing the amplified transcripts of a RT-PCR obtained from ipsi- and contralateral hippocampi that were subjected to different experimental conditions (lane numbers correspond to the groups shown in A). The line graphs display representative analog traces recorded during baseline (Top), immediately after tetanization (Middle) and 8 hr after tetanus (Bottom). M, 100-bp ladder molecular weight marker; no RT, RT-PCR without addition of reverse transcriptase; −, RT-PCR without addition of cDNA.

Figure 3

Blockade of IL-1 receptors inhibits LTP maintenance in hippocampal slices. Ninety to 100 min after inducing LTP in hippocampal slices, IL-1ra was perfused for 40 min (n = 5). The same procedure was followed in control slices omitting the antagonist in the perfusion fluid (n = 6). Results (mean ± SEM) are expressed as percentage of the pretetanic PS-amplitude of each preparation. Each point in the curves shows the average of two consecutive recordings. Application of IL-1ra resulted in a complete elimination of potentiation 1–2 hr after starting IL-1ra perfusion. (P < 0.05, ANOVA, followed by the Fisher test for multiple comparisons). Representative analog traces corresponding to the points indicated by the arrows are shown in the upper part.

Figure 4

Blockade of IL-1 receptors causes a reversible inhibition of LTP maintenance in vivo. Either physiological saline (control) or IL-1ra was administered i.c.v. Data are plotted as average change from baseline response (mean ± SEM). The times of tetanization and of drug application are indicated by an arrow. (Top) Application of IL-1ra 30 min before tetanization did not affect subsequent LTP (n = 6). (Middle) When IL-1ra was applied immediately after tetanus no statistically significant effects could be observed (n = 7). (Bottom) IL-ra infused 90 min post-tetanus caused a marked decrease of potentiation (n = 8, ∗, P < 0.05, ANOVA, followed by the Fisher test for multiple comparisons), which lasted for ≈40 min.