Hippocampal kindling in the rat: intrastructural differences

Higher intrinsic network excitability in ventral compared with the dorsal hippocampus is controlled less effectively by GABAB receptors

BACKGROUND

Elucidating specializations of the intrinsic neuronal network between the dorsal and the ventral hippocampus is a recently emerging area of research that is expected to help us understand the mechanisms underlying large scale functional diversification along the hippocampus. The aim of this study was to characterize spontaneous network activity between the dorsal and the ventral hippocampus induced under conditions of partial or complete blockade of GABAergic inhibition (i.e. disinhibition).

RESULTS

Using field recordings from the CA3 and CA1 fields of hippocampal slices from adult rats I found that ventral compared with dorsal hippocampus slices displayed higher propensity for and higher frequency of occurrence of spontaneous field potentials (spfps) at every level of disinhibition. Also NMDA receptor-depended spfps (spfps(-nmda)) occurred with higher probability more frequently and were larger in the ventral compared with the dorsal hippocampus. Importantly, blockade of GABA(B) receptors produced a stronger effect in enhancing the probability of generation of spfps and spfps(-nmda) in the dorsal compared with the ventral hippocampal slices and increased spfps(-nmda) only in dorsal slices.

CONCLUSION

These results demonstrate a higher intrinsic neuronal excitability of the ventral compared with the dorsal local circuitry with the considerable contribution of NMDA receptors. Furthermore, the GABA(B) receptors control the total and the NMDA receptor-dependent excitation much less effectively in the ventral part of the hippocampus. It is proposed that NMDA and GABA(B) receptors significantly contribute to differentiate local network dynamics between the dorsal and the ventral hippocampus with important implications in the information processing performed along the long hippocampal axis.

Sensory stimulation reduces seizure severity but not afterdischarge duration of partial seizures kindled in the hippocampus at threshold intensities

Epilepsy is a family of neurological disorders that result in seizure activity that is characterized by transient hypersynchronous activation of a large population of neurons. In animal models, focal tetanic electrical stimulation of sufficient duration and intensity, can elicit epileptiform activity, that if repeated results in progressive intensification of seizure activity known as kindling. Kindling serves as a model of partial as well as secondarily generalized temporal lobe epilepsy. We utilized hippocampal kindling to provide a means of evaluating the effect of sensory stimulation on the duration and severity of the induced seizure activity. Sensory stimuli targeted either the olfactory, auditory or somatosensory systems in an attempt to retard or suppress seizure activity. To that end, rats were chronically implanted with electrodes in the CA1 region of dorsal hippocampus and kindled once daily until the seizure behaviour was fully generalized. Kindling stimulation consisted of daily application of 1-s trains of biphasic square wave pulses applied at a frequency of 60Hz, at the afterdischarge (AD) threshold. Sensory stimulation was applied 6-8s after the kindling stimulation every third day. One group of rats received a different sensory stimulus (novel) every third day, while another group was presented with the same sensory stimulus (repeated) every third day. Kindling stimulation applied to the dorsal hippocampus resulted in progression of the AD characteristics and seizure behavior, which typically developed very slowly in the early stages. The application of both the novel and repeated sensory stimulation during partial seizures (stages 1 and 2) resulted in a reduction in the seizure severity but not in the afterdischarge duration. Sensory stimulation delivered during secondarily generalized seizures (stages 4 and 5) failed to affect either parameter.

Activation of NMDA receptors in rat dentate gyrus granule cells by spontaneous and evoked transmitter release

Activation of N-methyl-D-aspartate (NMDA) receptors by synaptically released glutamate in the nervous system is usually studied using evoked events mediated by a complex mixture of AMPA, kainate, and NMDA receptors. Here we have characterized pharmacologically isolated spontaneous NMDA receptor-mediated synaptic events and compared them to stimulus evoked excitatory postsynaptic currents (EPSCs) in the same cell to distinguish between various modes of activation of NMDA receptors. Spontaneous NMDA receptor-mediated EPSCs recorded at 34 degrees C in dentate gyrus granule cells (DGGC) have a frequency of 2.5 +/- 0.3 Hz and an average peak amplitude of 13.2 +/- 0.8 pA, a 10-90% rise time of 5.4 +/- 0.3 ms, and a decay time constant of 42.1 +/- 2.1 ms. The single-channel conductance estimated by nonstationary fluctuation analysis was 60 +/- 5 pS. The amplitudes (46.5 +/- 6.4 pA) and 10-90% rise times (18 +/- 2.3 ms) of EPSCs evoked from the entorhinal cortex/subiculum border are significantly larger than the same parameters for spontaneous events (paired t-test, P < 0.05, n = 17). Perfusion of 50 microM D(-)-2-amino-5-phosphonopentanoic acid blocked all spontaneous activity and caused a significant baseline current shift of 18.8 +/- 3.0 pA, thus identifying a tonic conductance mediated by NMDA receptors. The NR2B antagonist ifenprodil (10 microM) significantly reduced the frequency of spontaneous events but had no effect on their kinetics or on the baseline current or variance. At the same time, the peak current and charge of stimulus-evoked events were significantly diminished by ifenprodil. Thus spontaneous NMDA receptor-mediated events in DGGC are predominantly mediated by NR2A or possibly NR2A/NR2B receptors while the activation of NR2B receptors reduces the excitability of entorhinal afferents either directly or through an effect on the entorhinal cells.

Hippocampal cell proliferation and epileptogenesis after audiogenic kindling are not accompanied by mossy fiber sprouting or Fluoro-Jade staining

Repetitive sound-induced seizures, known as audiogenic kindling (AK), gradually induce the transference of epileptic activity from brainstem to forebrain structures along with behavioral changes. The aim of our work was to correlate the behavioral changes observed during the AK with possible alterations in neuronal proliferation, cell death, hippocampal mossy fiber sprouting and in the EEG pattern of Wistar audiogenic rats, a genetically susceptible strain from our laboratory. Susceptible and non-susceptible animals were submitted to repeated sound stimulations for 14-16 days and hippocampal mitotic activity was studied through the incorporation of bromodeoxyuridine (BrdU). Cell death and mossy fiber sprouting were assessed, respectively, by using Fluoro-Jade and Timm staining, 2 and 32 days after the last kindling stimulation. In addition, we used immunofluorescent double labeling for a glial and a mitotic marker to evaluate newly born cell identity. Some animals had hippocampus and amygdala electrodes for EEG recordings. Our results show that kindled animals with 6-11 generalized limbic seizures (class IV-V) had increased cell proliferation in the dentate gyrus when compared with animals with zero or one to three seizures. BrdU-positive cells labeled on day 2 and on day 32 were both GFAP negative. In the later group, rounded and well-defined BrdU-positive/GFAP-negative nuclei were seen in different portions of the granule cell layer. We did not observe any Fluoro-Jade or differential Timm staining in kindled animals at both killing times. However, EEG recordings showed intense epileptic activity in the hippocampus and amygdala of all animals with limbic seizures.Therefore, our data indicate that AK-induced limbic epileptogenicity is able to increase the hippocampal mitotic rate, even though it does not seem to promote neuronal death or mossy fiber sprouting in the supragranular layer of the dentate gyrus.

Increase in GAP-43 and GFAP immunoreactivity in the rat hippocampus subsequent to perforant path kindling

Kindling is an animal model of epilepsy which is accompanied by morphological and biochemical changes in the brain, including sprouting of fibers and increased transmitter release. Here we have examined the immunocytochemical expression of 1) GAP-43, a growth-associated protein, which is a neuron-specific PKC substrate, particularly expressed in development and regeneration and 2) glial fibrillary acidic protein (GFAP), part of the astrocytic cytoskeleton, after perforant path kindling. Subsequent to kindling, GAP-43 immunoreactivity was increased in CA1 stratum lacunosum-moleculare and the inner and outer molecular layer of the fascia dentata. Other hippocampal subregions showed a lower increase. GFAP immunoreactivity was increased in the entire hippocampus, but especially in stratum lacunosum-moleculare of the CA1 and the hilus of fascia dentata. The difference between the number of GFAP-positive profiles in the hippocampus of control rats and in fully kindled rats was found to be non-significant. We interpret these findings as being related to both plastic neuronal changes and possible neuronal degeneration.

Are differences in dorsal hippocampal kindling related to amygdala-piriform area excitability?

It has been suggested that several structures associated with the amygdala-piriform (A-P) area are important, if not critical, for convulsive generalization of limbic seizures. In experiment 1, when examining the development of convulsive seizures kindled from the dorsal hippocampus (cornus ammonis; DH), a broad range of kindling rates was observed. Independent of electrode location within the hippocampus, kindling rates were correlated positively with both local and, more dramatically, distant excitability (measured by the duration of the primary and secondary hippocampal afterdischarges, respectively) at all stages of epileptogenesis. Because kindling rates were bimodally distributed, we bisected the distribution into 'faster' and 'slower' kindling rats. Here we examined the magnitude of both electrophysiological and behavioral differences between 'faster' and 'slower' rats and their changes over time. The 'faster' rats had longer primary and secondary afterdischarge (AD) durations than 'slower' rats throughout all stages of kindling. With the appearance of generalized convulsions, the 'faster' rats showed longer latencies to clonus onset, with longer clonus and AD durations than 'slower' rats. Also, the generalized convulsions of 'faster' rats appeared during a much enlarged secondary AD period, while 'slower' rats convulsed during primary AD. In both groups, convulsions were invariably associated with increased discharge in A-P associated structures. We interpreted the differences between 'faster' and 'slower' DH rats to reflect genetic differences in excitability in both local and A-P associated structures. If the DH kindling profile of the 'faster' rats differed from 'slower' rats largely because of naturally greater excitability in A-P associated structures, then experimentally increased excitability in those structures (via amygdala kindling) in a random sample of rats should duplicate much of the 'faster' DH kindling profile. In experiment 2, this outcome was observed. In conclusion, we suggest that either natural or induced differences in the excitability of A-P associated structures affect both the genesis and the profile of convulsive generalization of limbic kindled seizures.

Regenerative, all-or-none electrographic seizures in the rat hippocampal slice in Mg-free and physiological medium

All-or-none electrographic seizures (EGSs) were studied in hippocampal slices from young (21- to 38-day-old) rats in medium containing low (0 mM) or physiological (0.9 mM) levels of magnesium, with and without the GABAB agonist baclofen. Extracellular recording and stimulation were performed in stratum pyramidale and stratum radiatum of CA3, respectively. EGS activity was induced by exposure to low-Mg medium or by delivering repetitive stimulus trains in physiological Mg medium. After EGS activity had stabilized, the EGSs were tested for all-or-none behavior by varying the number of pulses in a train. An EGS was considered all-or-none if subthreshold stimulation produced no afterdischarge bursts, and if the EGS duration was largely independent of the number of suprathreshold stimulus pulses. According to this measure, EGSs in Mg-free + baclofen medium were all-or-none. EGSs evoked in physiological Mg medium were also all-or-none, although the threshold was higher, and the EGS duration lower, than in Mg-free medium. This all-or-none characteristic was observed whether the EGSs were induced by prior exposure to Mg-free medium or by repetitive stimulation, and in the presence and absence of baclofen. The all-or-none characteristic suggests that while the triggering mechanism for EGSs is strongly dependent on stimulus intensity, regenerative mechanisms--independent of stimulus intensity--are responsible for the maintenance of EGSs. EGSs are also terminated by mechanisms not dependent on stimulus intensity.

Kindling with stimulation of the dentate gyrus. I. Characterization of electrographic and behavioral events

Once daily for 60 days, hooded rats received unilateral high-frequency stimulation in the hilus of the dentate gyrus (DG), at an intensity sufficient to evoke epileptiform afterdischarge (AD). Although most rats eventually developed generalized stage-5 seizures (Generalized group), some did not progress beyond partial stage-1 or stage-2 seizures (Partial group). Hilar kindling also displayed several other characteristics that distinguished it from typical limbic kindling, including low rate of development, marked instability of the seizures, and little or no growth in duration of AD.

NMDA antagonists differentiate epileptogenesis from seizure expression in an in vitro model

In an electrographic model of seizures in the hippocampal slice, both of the N-methyl-D-aspartate (NMDA) antagonists 2-amino-5-phosphonovaleric acid and 5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK-801) prevented the progressive development of seizures but did not block previously induced seizures. Thus, a process dependent on the NMDA receptor-ionophore complex establishes a long-lasting, seizure-prone state; thereafter the seizures depend on non-NMDA receptor-ionophore mechanisms. This suggests that there is an important distinction between epileptogenesis and seizure expression and between antiepileptogenic and anticonvulsant pharmacological agents.

Hippocampal kindling enhances excitatory amino acid receptor-mediated polyphosphoinositide hydrolysis in the hippocampus and amygdala/pyriform cortex

We recently demonstrated that a long-lasting increase in ibotenate-stimulated polyphosphoinositide (PPI) hydrolysis in the amygdala/pyriform cortex (AM/PC) is associated with seizure susceptibility of amygdala (AM)-kindled rats. The present study examined (1) whether ibotenate-stimulated PPI hydrolysis would be lastingly enhanced in the hippocampus (HIPP) and AM/PC of the HIPP-kindled rats and (2) whether similar changes would be found in the early stage of HIPP kindling. Although ibotenate-stimulated accumulation of [3H]inositol 1-phosphate ([ 3H]IP1) increased significantly in the HIPP 24 h, 5 days, and 15 days after the last seizure of fully developed HIPP-kindled rats, no statistically significant increase was found in the HIPP 30 days after the last seizure. In the AM/PC, 10(-3) M ibotenate-stimulated [3H]IP1 accumulation significantly increased by 91%, 91%, 86% and 90%, 24 h, 5 days, 15 days and 30 days after the last seizure, respectively. There was no significant increase in ibotenate-stimulated [3H]IP1 accumulation 7 days after the last stimulation in the HIPP and AM/PC of rats which had undergone electrical stimulation only 5 times in the HIPP. These results indicate that (1) PPI hydrolysis coupled to excitatory amino acid receptors increases long-lastingly in the AM/PC regardless of the primary kindled site, and (2) these changes do not occur in the early stage of HIPP-kindling.