Synaptic Reorganization of the Perisomatic Inhibitory Network in Hippocampi of Temporal Lobe Epileptic Patients

Abstract

GABAergic inhibition and particularly perisomatic inhibition play a crucial role in controlling the firing properties of large principal cell populations. Furthermore, GABAergic network is a key element in the therapy attempting to reduce epileptic activity. Here, we present a review showing the synaptic changes of perisomatic inhibitory neuronal subtypes in the hippocampus of temporal lobe epileptic patients, including parvalbumin- (PV-) containing and cannabinoid Type 1 (CB1) receptor-expressing (and mainly cholecystokinin-positive) perisomatic inhibitory cells, known to control hippocampal synchronies. We have examined the synaptic input of principal cells in the dentate gyrus and Cornu Ammonis region in human control and epileptic hippocampi. Perisomatic inhibitory terminals establishing symmetric synapses were found to be sprouted in the dentate gyrus. Preservation of perisomatic input was found in the Cornu Ammonis 1 and Cornu Ammonis 2 regions, as long as pyramidal cells are present. Higher density of CB1-immunostained terminals was found in the epileptic hippocampus of sclerotic patients, especially in the dentate gyrus. We concluded that both types of (PV- and GABAergic CB1-containing) perisomatic inhibitory cells are mainly preserved or showed sprouting in epileptic samples. The enhanced perisomatic inhibitory signaling may increase principal cell synchronization and contribute to generation of epileptic seizures and interictal spikes.

Figure 1

Light micrographs show the distribution of PV-containing interneurons in the human control (a) and epileptic (b–d) CA1 region. (a) Horizontal PV-positive cells (double arrowheads) are present in the stratum oriens (s. o.). Multipolar PV-positive cells (arrows) located in the stratum pyramidale (s. p.) send their dendrites to all layers (arrowheads). (b) In the nonsclerotic epileptic CA1 region (Type 1, mild) the number of PV-positive elements (somata and dendrites) has decreased, mainly visible in the stratum oriens. (c) In the nonsclerotic CA1 region with patchy cell loss (Type 2) the decrease in the number of PV-positive elements is even more pronounced. In several cases, surviving pyramidal cells (left side) accumulate the chromogene diaminobenzidine (giving aspecific staining), possibly due to cellular degeneration processes. (d) In the sclerotic CA1 region (Type 3) only a few PV-stained cells and dendrites are present. Scale bar: 50 μm.

Figure 2

Camera lucida drawings show PV-positive interneuron with its axonal cloud (a) and the inhomogeneous axonal staining (b) in the human sclerotic (Type 3) dentate gyrus. Insert on (a) shows the complex chandelier formations in the dentate gyrus of the sclerotic hippocampus. (b) Dense PV-positive axonal patches are alternated with lack of stained boutons in the granule cell layer of epileptic patients. Schematic cell bodies indicate the location of PV-positive interneuron somata in the stratum moleculare. s. m.: stratum moleculare, s. g.: stratum granulosum, and h: hilus. Perisomatic inhibitory input included PV-positive (c) and PV-negative (d) symmetrical (presumably inhibitory) synapses both in the human control and in epileptic dentate gyrus (arrows). In the sclerotic hippocampus, mossy fibers were also found to form asymmetrical (presumably excitatory) synapses on granule cell somata ((e), arrowhead). Electron micrographs of PV-negative (f) and PV-positive (g) inhibitory synapses terminating on AISs, in the control and epileptic dentate gyrus, respectively. Note the larger bouton in the epileptic tissue, giving a perforated synapse. The somatic and axonal inhibitory synaptic coverage were found to be increased in all epileptic samples. PV+: parvalbumin-positive, PV−: parvalbumin-negative, MF: mossy fiber, and AIS: axon initial segment. Scale bars: (a) 20 μm, insert: 15 μm, (b) 100 μm, and (c–g) 1 μm.

Figure 3

High magnification light micrographs (a–d) show the somata (upper panels) and the axonal cloud (lower panels) of PV-positive cells in the human control (a) and epileptic (b–d) CA1 region. The number of PV-positive elements decreased with the degree of cell loss in the human epileptic CA1 region. PV-stained axons (lower panels) formed a dense network in the stratum pyramidale of the CA1 region as long as their postsynaptic targets, that is, pyramidal cells, are present (b, c). Note the basket-like formation on (a)–(c) (arrowheads) and chandelier-like formation on (b) (double arrowheads). In the sclerotic CA1 region lacking principal cells hardly any PV-positive axons can be seen (d). Scale bar: (a–d) 20 μm. Electron micrographs show PV-stained axonal boutons contacting pyramidal cell somata (e, f) in the control (e) and epileptic (f) CA2 region. Axon initial segments (AISs, (g) and (h)) were the other main targets of PV-positive axons in the human control (g) and epileptic (h) CA1 region. Scale bars: (a)–(d) 20 μm; (e)–(h) 1 μm.

Figure 4

Light micrographs show the distribution of CB1-immunoreactive elements in the human control (a, c) and epileptic (b–d) dentate gyrus. (a) Dense CB1-immunopositive meshwork was present around the dentate granule cells and in the stratum moleculare (s. m.). The hilus (H) contained less CB1-positive terminals. Arrow points to a CB1-positive interneuron. (b) In epileptic patients with hippocampal sclerosis the density of CB1-positive fibers has been increased in the stratum moleculare and granulosum. Note the dispersion of granule cells. Arrows point to CB1-positive interneurons. (c) CB1-immunostained terminals were present in the hilus, in homogenous distribution. Arrow points to a CB1-positive interneuron. (d) In the hilus of the epileptic hippocampus more CB1-positive terminals were present; they often formed dense network around surviving mossy cells or interneurons (arrowheads).

Figure 5

Light micrographs show the distribution of CB1-immunopositive fibers in the human control (a) and epileptic (b-c) CA3 regions. (a) Homogeneous CB1-immunopositive meshwork was present around the pyramidal cells. (b) In epileptic patients without hippocampal sclerosis the density of CB1-positive fibers has been increased moderately. (c) Further increase in density has been observed in the CA3 region of the sclerotic hippocampi, if pyramidal cells were preserved. Arrows point to pyramidal cell bodies surrounded by basket-like formations of CB1-positive fibers. Scale bar: 20 μm.

Figure 6

Light micrographs show the distribution of CB1-immunopositive fibers in the human control (a) and epileptic (b-c) CA1 regions. (a) CB1-immunopositive axonal network was present in the stratum pyramidale. (b) In epileptic patients without hippocampal sclerosis the density of CB1-positive fibers has been slightly increased. (c) The density of CB1-positive axons has been decreased in the sclerotic CA1 region. Arrows point to pyramidal cell bodies surrounded by basket-like formations of CB1-positive fibers. Arrowheads show axons. Scale bar: 25 μm.