Dynamic changes of CB1-receptor expression in hippocampi of epileptic mice and humans

Abstract

The endocannabinoid system plays a central role in retrograde synaptic communication, and controls both glutamatergic and gamma-aminobutyric acid (GABA)ergic transmission via type 1 cannabinoid (CB1) receptor. Both in sclerotic human hippocampi and in the chronic phase of pilocarpine-induced epilepsy in mice with sclerosis, CB1-receptor-positive interneuron somata were preserved both in the dentate gyrus and in the CA1 area, and the density of CB1-immunostained fibers increased considerably in the dentate molecular layer. This suggests that, although CB1 receptors are known to be reduced in density on glutamatergic axons, the CB1-receptor-expressing GABAergic axons sprout, or there is an increase of CB1-receptor levels on these fibers. The changes of CB1 immunostaining in association with the GABAergic inhibitory system appear to correlate with the severity of pyramidal cell loss in the CA1 subfield. These results confirm the involvement of the endocannabinoid system associated with GABAergic transmission in human temporal lobe epilepsy (TLE), as well as in the chronic phase of the pilocarpine model in mice. Pharmacotherapy aimed at the modulation of endocannabinoid-mediated retrograde synaptic signaling should take into account the opposite change in CB1-receptor expression observed on glutamatergic versus GABAergic axon terminals.

Figure 1

CB1 immunostaining in control and sclerotic epileptic human dentate gyrus. A) Low power light micrograph showing the distribution of CB1-positive profiles in the control human dentate gyrus. Granule cells are always negative for CB1. B) Sclerotic epileptic patients show remarkable differences from the controls. The density and staining intensity of CB1-positive fibers is considerably increased in the stratum moleculare. C) Density of CB1 receptor immunopositive fibers in control (N=3) and sclerotic epileptic patients (N=10) revealed by confocal laeser scanning microscope. The intensity of CB1 receptor staining is elevated in epileptic samples. The difference between control and epileptic samples was highly significant (p<0,05; Student t test). D) A CB1 receptor-positiv terminal from the stratum moleculare of an epileptic patient labelled by immunogold technique establishes symmetric synapse on a dendrite. All of the terminals stained by this CB1 antibody established symmetric synapses. Scales: A,B:50 μm; D:0,5 μm

Figure 2

CB1 immunostaining in the dentate gyrus of a control (A, B, C) and a “strong” epileptic (D, E, F) mouse. In control samples (A,B), dense staining of CB1-positive fibers can be found in the molecular layer of the DG. In the stratum moleculare of sclerotic animals (D, E) the intensity of CB1 immunostaining is elevated. The most frequent postsynaptic targets of CB1 receptor-positive terminals establishing symmetric synapses (arrows) are cell bodies (C) and dendrites (F) Scales: A,D: 200 μm; B,E: 50 μm; C,F:1 μm.