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published_or_final_version / abstract / Zoology / Master / Master of Philosophy

Lacosamide (LCM) is a new antiepileptic drug with a previously unknown mode of action. Using electrophysiological recording techniques in a range of in vitro preparations I have determined a mechanism of action of the new drug. In a 4-aminopyridine model of tonic-clonic seizures in rat visual cortex in vitro, LCM stereoselectively reduced maximal frequency and duration of tonic activity with EC[50�s] of 71 and 41 [mu]M respectively. LCM (100 [mu]M) significantly reduced excitability in whole cell patch clamped neurons producing non-selective reduction in the incidence of excitatory/inhibitory postsynaptic currents (EPSCs; LCM: 46.1 � 15.5 %, P <0.01, n = 4, IPSCs; LCM: 24.9 � 9.6 %, P <0.01, n = 4) and block of spontaneous action potentials (EC₅₀ 61 [mu]M). The inhibitory effects of LCM did not result from changes in passive membrane properties (including resting membrane potential or input resistance) as assessed by application of voltage ramps between -70 to +20 mV. LCM did not mimic the effects of diazepam as an allosteric modulator of GABA[A] receptor currents, nor did it inhibit evoked excitatory currents mediated by AMPA or NMDA receptors. Unlike phenytoin (DPH), carbamazepine (CBZ) or lamotrigine (LTG) that blocked sustained action potential firing evoked by brief depolarising steps (750 ms) or ramps (-70 to 20 mV, 90 mV.sec⁻�), LCM could weakly reduce the frequency of action potentials evoked by brief depolarisation suggesting a potential interaction with VGSCs. In accordance with this, the effect of LCM upon neurotransmission was negated in the presence of tetrodotoxin (200 nM, TTX). The frequency of miniature EPSCs was not altered by the drug (100 [mu]M). These results discounted some crucial potential anticonvulsant targets for LCM but implied a potential interaction with electrogenic VGSCs. When SRF duration was prolonged (10 s) LCM produced significant (P <0.01, n = 4-10, EC₅₀: 48 [mu]M) inhibition, but not within the first second of the burst EC₅₀: 640 [mu]M). Evoked TTX sensitive sodium currents in N1E-115 neuroblastoma cells were significantly reduced by LCM, CBZ, LTG and DPH when V[h]: -60 mV. Hyperpolarizing pulses (500 ms) to -100 mV could reverse block by CBZ, LTG and DPH but not LCM. The V₅₀ for steady state fast inactivation was more hyperpolarized by CBZ (-79.45 � 2.64 mV, n = 5, P < 0.001), LTG (-72.30 � 1.70 mV, n = 6, P <0.05) and DPH (-77.17 � 2.32 mV, n = 6, P <0.05) but not by LCM (-65.02 � 1.75 mV, n = 6, CONTROL: -65.84 � 0.86 mV). In contrast to CBZ, LCM did not slow recovery from fast inactivation or produce frequency dependent facilitation of block of a 3 s, 10 Hz pulse train. LCM (100 [mu]M) did produce a (V₅₀: CONTROL ~64 mV, LCM -57.47 � 4.53 mV, P <0.001, n = 4-8) hyperpolarizing shift in the voltage dependence of slow sodium channel inactivation and promoted channel entry into the slow inactivated state (P <0.001, n = 6) but did not alter the rate of recovery. I therefore conclude that LCM produces inhibition of epileptiform cellular activity, at least in part, via enhancement of voltage gated sodium channel slow inactivation and represents a molecule possessing a unique anticonvulsant mechanism of action.

Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Not available / text

The Vasoactive Intestinal Peptide (VIP) is an agonist of the CFTR chloride channel in the human airways. In the genetic disease Cystic Fibrosis, where CFTR is defective or absent from the apical membrane of epithelial cells, VIP innervations are lost. Our group has demonstrated that VIP increases CFTR membrane stability through PKCε. However, the mechanism remained to be determined. Here we found that VIP stimulation increases the interaction of NHERF1 and P-ERMs with CFTR through PKCε phosphorylation. Moreover a reduction of the interaction between intracellular CFTR and the Golgi associated protein, CAL was observed following VIP stimulation. Silencing either ERMs or NHERF1 with siRNA prevented the VIP ability to increase CFTR surface expression and function, confirming that NHERF1 and P-ERMs are necessary for VIP regulation of the sustained activity of membrane CFTR. This study shows the cellular mechanism by which prolonged VIP stimulation of airway epithelial cells regulates CFTR-dependent secretions.

Current concepts of hippocampal circuitry assume a large population of excitatory principal neurones whose activity is largely governed by a network of local-circuit GABAergic interneurones. The diversity of hippocampal local-circuit neurones and their synaptic control over principal cell activity was investigated in vitro, in order to define their synaptic connections and functional roles. Single and dual intracellular recordings were made from local-circuit neurones and pyramidal cells in area CA1 of the rat hippocampal slice. Interneurones were tentatively distinguished from pyramidal cells based on their firing as well as their membrane properties. Intracellular labelling of recorded cells with the marker biocytin revealed a diversity of cell types based on differential dendritic and axonal morphology and synaptic connections. The physiological data revealed that all types of interneurone tested evoked inhibitory postsynaptic potentials (IPSPs) in simultaneously recorded pyramidal cells. The IPSPs had fast rise and decay kinetics and the ones tested pharmacologically, were mediated by GABA_A receptors. Similarly, individual interneurones were also shown to innervate other local-circuit interneurones in addition to pyramidal cells, the evoked effects being qualitatively similar in both types of postsynaptic targets. The postsynaptic effect and functional role of one type of hippocampal interneurone, the basket cell, was investigated in greater detail. Basket cell-evoked IPSPs were reliable, but showed some frequency-dependent attenuation. Moreover, basket cell IPSPs were found to interact with intrinsic pyramidal cell conductances to elicit rebound depolarisations and facilitate action potential generation. More detailed investigation showed that basket and axo-axonic cells were particularly effective in entraining pyramidal cell firing and sub-threshold membrane potential oscillations. Through these powerfully tuned mechanisms, sub-types of local-circuit interneurone provide a powerful mechanism to synchronise the activity of pyramidal cells. These results demonstrate a remarkable diversity of GABAergic local-circuit neurones in the hippocampal CA1 area and suggest that specific subtypes of cell mediate different functions.

We compared a kinetic method (KM) and a colorimetric method (CM) for measuring erythrocyte aspartate aminotransferase (EAST) activity. Twenty-three healthy college students including 7 men and 16 women, aged from 22 to 40 years, participated in this study. Vitamin B-6 status was assessed by EAST activity coefficient (EAST-AC), the ratio of EAST stimulated activity by adding PLP in vitro (EAST-SA) to basal activity (EAST-BA). These subjects' EAST indices (EAST-BA, EAST-SA and EAST-AC) were compared to their plasma PLP concentration and their dietary intake of vitamin B-6 as determined by the food frequency questionnaire (FFQ) and 3-day dietary record. There was a significant correlation (r=0.59, p<0.01) in EAST-BA obtained by the two methods, while the correlation of EAST-SA values between the two methods was not significant (r=0.40, p=0.06). EAST-AC obtained with KM was linearly associated (r=0.57, p<0.01) to EAST-AC obtained with CM, but was 1.26 times higher (p<0.01) than that with CM. Thus, the method used for the determination of the normal EAST-AC value needs to be noted. None of EAST indices measured were significantly correlated with plasma PLP concentration. There was a high correlation for vitamin B-6 intake (r=0.65, p<0.01) and the ratio of vitamin B-6 to dietary protein (r=0.58, p<0.01) estimated between the FFQ and the 3-day dietary record. The results suggested the high validity of the FFQ for determining vitamin B-6 intake. Neither of these dietary methods was, however, correlated with any EAST activity indices or the plasma PLP concentration. / Graduation date: 1995

Lacosamide (LCM) is a new antiepileptic drug with a previously unknown mode of action. Using electrophysiological recording techniques in a range of in vitro preparations I have determined a mechanism of action of the new drug. In a 4-aminopyridine model of tonic-clonic seizures in rat visual cortex in vitro, LCM stereoselectively reduced maximal frequency and duration of tonic activity with EC[50�s] of 71 and 41 [mu]M respectively. LCM (100 [mu]M) significantly reduced excitability in whole cell patch clamped neurons producing non-selective reduction in the incidence of excitatory/inhibitory postsynaptic currents (EPSCs; LCM: 46.1 � 15.5 %, P <0.01, n = 4, IPSCs; LCM: 24.9 � 9.6 %, P <0.01, n = 4) and block of spontaneous action potentials (EC₅₀ 61 [mu]M). The inhibitory effects of LCM did not result from changes in passive membrane properties (including resting membrane potential or input resistance) as assessed by application of voltage ramps between -70 to +20 mV. LCM did not mimic the effects of diazepam as an allosteric modulator of GABA[A] receptor currents, nor did it inhibit evoked excitatory currents mediated by AMPA or NMDA receptors. Unlike phenytoin (DPH), carbamazepine (CBZ) or lamotrigine (LTG) that blocked sustained action potential firing evoked by brief depolarising steps (750 ms) or ramps (-70 to 20 mV, 90 mV.sec⁻�), LCM could weakly reduce the frequency of action potentials evoked by brief depolarisation suggesting a potential interaction with VGSCs. In accordance with this, the effect of LCM upon neurotransmission was negated in the presence of tetrodotoxin (200 nM, TTX). The frequency of miniature EPSCs was not altered by the drug (100 [mu]M). These results discounted some crucial potential anticonvulsant targets for LCM but implied a potential interaction with electrogenic VGSCs. When SRF duration was prolonged (10 s) LCM produced significant (P <0.01, n = 4-10, EC₅₀: 48 [mu]M) inhibition, but not within the first second of the burst EC₅₀: 640 [mu]M). Evoked TTX sensitive sodium currents in N1E-115 neuroblastoma cells were significantly reduced by LCM, CBZ, LTG and DPH when V[h]: -60 mV. Hyperpolarizing pulses (500 ms) to -100 mV could reverse block by CBZ, LTG and DPH but not LCM. The V₅₀ for steady state fast inactivation was more hyperpolarized by CBZ (-79.45 � 2.64 mV, n = 5, P < 0.001), LTG (-72.30 � 1.70 mV, n = 6, P <0.05) and DPH (-77.17 � 2.32 mV, n = 6, P <0.05) but not by LCM (-65.02 � 1.75 mV, n = 6, CONTROL: -65.84 � 0.86 mV). In contrast to CBZ, LCM did not slow recovery from fast inactivation or produce frequency dependent facilitation of block of a 3 s, 10 Hz pulse train. LCM (100 [mu]M) did produce a (V₅₀: CONTROL ~64 mV, LCM -57.47 � 4.53 mV, P <0.001, n = 4-8) hyperpolarizing shift in the voltage dependence of slow sodium channel inactivation and promoted channel entry into the slow inactivated state (P <0.001, n = 6) but did not alter the rate of recovery. I therefore conclude that LCM produces inhibition of epileptiform cellular activity, at least in part, via enhancement of voltage gated sodium channel slow inactivation and represents a molecule possessing a unique anticonvulsant mechanism of action.

Errata is tipped in between leaf 9 & 10. / Bibliography: leaves 209-235. / 235 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Focusses specifically on the role of acetylcholine in the circadian timing system of mammals, using the rat as an animal model. / Thesis (Ph.D.)--University of Adelaide, Dept. of Obstetrics and Gynaecology, 1999