The ionic mechanisms underlying the termination of action-potential (AP) bursts and postburst afterhyperpolarization (AHP) in intrinsically bursting (IB) neocortical neurons were investigated by performing intracellular recordings in thin slices of rat sensorimotor cortex. The blockade of Ca2+-activated K+currents enhanced postburst depolarizing afterpotentials, but had inconsistent and minor effects on the amplitude and duration of AHPs. On the contrary, experimental conditions resulting in reduction of voltage-dependent Na+ entry into the cells caused a significant decrease of AHP amplitude. Slice perfusion with a modified artificial cerebrospinal fluid in which LiCl (40 mM) partially replaced NaCl had negligible effects on the properties of individual APs, whereas it consistently increased burst length and led to an approximately 30% reduction in the amplitude of AHPs following individual bursts or short trains of stimulus-induced APs. Experiments performed by partially replacing Na+ ions with choline revealed a comparable reduction in AHP amplitude associated with an inhibition of bursting activity. Moreover, in voltage-clamp experiments carried out in both in situ and acutely isolated neurons, partial substitution of extracellular NaCl with LiCl significantly and reversibly reduced the amplitude of K+ currents evoked by depolarizing stimuli above-threshold for Na+-current activation. The above effect of Na+-to-Li+substitution was not seen when voltage-gated Na+ currents were blocked with TTX, indicating the presence of a specific K+-current component activated by voltage-dependent Na+ (but not Li+) influx. The above findings suggest that a Na+-activated K+ current recruited by the Na+ entry secondary to burst discharge significantly contributes to AHP generation and the maintenance of rhythmic burst recurrence during sustained depolarizations in neocortical IB neurons.

This combined longitudinal analysis of the disease progression, which suggested an impairment of neurotransmission, neuronal integrity and a reversible activation of brain inflammatory processes, might represent a more quantitative approach to compare the differential effects of treatments in slowing down or reversing HD in rodent models with potential applications to human patients.

No abstract

Summary:Purpose: The murine homeobox-containing Otx gene is required for correct nervous system and sense organ development. Otxl-'-mice obtained by replacing Otx with the lac Z gene show developmental abnormalities of the cerebellum, mesencephalon, and cerebral cortex associated with spontaneous epileptic seizures (1). The epileptogenic mechanisms accounting for these seizures were investigated by means of electrophysiological recordings made from neocortical slices.Methods: The 400-km slices were prepared from the somatosensory cortex of Otxl-'-and Otxl"" mice, and the current clamp intracellular recordings were obtained from layer V pyramidal neurons by means of pipettes containing K+ acetate 1.5 molL and biocytin 2% (pH 7.3).Results: Synaptic responses could be evoked in the neocortical pyramidal neurons by electrically stimulating the underlying white matter. y-Aminobutyric acid A/B-mediated inhibitory postsynaptic potentials were more pronounced in the Otxl-'-than in the control pyramidal neurons from the earliest postnatal period; multisynaptic excitatory postsynaptic potentials were significantly more expressed in the Otxl-'-mice also at the end of the first postnatal month, when they were only rarely encountered in controls.Conclusion: Excessive excitatory amino acid-mediated synaptic driving may lead to a hyperexcitable condition that is responsible for the epileptic manifestations occurring in Otxl-'-mice. This excess of excitation is not counteracted by well-developed y-aminobutyric acid activity, which seems to be involved in the synchronization of cell discharges. Our ongoing and more extensive comparative analysis of the mutants and controls should help to clarify the way in which the putative rearrangement taking place in 0txl-I-neocortex may lead to the excitatory hyperinnervation of layer V pyramidal neurons.

Knockout Otx1 mice present a microcephalic phenotype mainly due to reduced deep neocortical layers and spontaneous recurrent seizures. We investigated the excitable properties of layer V pyramidal neurons in neocortical slices prepared from Otx1-/- mice and age-matched controls. The qualitative firing properties of the neurons of Otx1-/- mice were identical to those found in wild-type controls, but the proportion of intrinsically bursting (IB) neurons was significantly smaller. This is in line with the lack of the Otx1 gene contribution to the generation and differentiation of neurons destined for the deep neocortical layers, in which IB neurons are located selectively in wild-type rodents. The pyramidal neurons recorded in Otx1-/- mice responded to near-threshold electrical stimulation of the underlying white matter, with aberrant polysynaptic excitatory potentials often leading to late action potential generation. When the strength of the stimulus was increased, the great majority of the Otx1-/- neurons (78%) responded with a prominent biphasic inhibitory postsynaptic potential that was significantly larger than that observed in the wild-type mice, and was often followed by complex postinhibitory depolarizing events. Both late excitatory postsynaptic potentials and postinhibitory excitation were selectively suppressed by NMDA receptor antagonists, but not by AMPA antagonists. We conclude that the cortical abnormalities of Otx1-/- neocortex due to a selective loss of large projecting neurons lead to a complex rearrangement of local circuitry, which is characterized by an excess of N-methyl-d-aspartate-mediated polysynaptic excitation that is counteracted by GABA-mediated inhibition in only a limited range of stimulus intensity. Prominent postsynaptic inhibitory potentials may also act as a further pro-epileptogenic event by synchronizing abnormal excitatory potentials.