Mark Niedringhaus

MMPs and soluble ICAM-5 increase neuronal excitability within in vitro networks of hippocampal neurons.

Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases that are released from neurons in an activity dependent manner. Published studies suggest their activity is important to varied forms of learning and memory. At least one MMP can stimulate an increase in the size of dendritic spines, structures which represent the post synaptic component for a large number of glutamatergic synapses. This change may be associated with increased synaptic glutamate receptor incorporation, and an increased amplitude and/or frequency of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) mini excitatory post-synaptic currents (EPSCs). An associated increase in the probability of action potential occurrence would be expected. While the mechanism(s) by which MMPs may influence synaptic structure and function are not completely understood, MMP dependent shedding of specific cell adhesion molecules (CAMs) could play an important role. CAMs are ideally positioned to be cleaved by synaptically released MMPs, and shed N terminal domains could potentially interact with previously unengaged integrins to stimulate dendritic actin polymerization with spine expansion. In the present study, we have used multielectrode arrays (MEAs) to investigate MMP and soluble CAM dependent changes in neuronal activity recorded from hippocampal cultures. We have focused on intercellular adhesion molecule-5 (ICAM-5) in particular, as this CAM is expressed on glutamatergic dendrites and shed in an MMP dependent manner. We show that chemical long-term potentiation (cLTP) evoked changes in recorded activity, and the dynamics of action potential bursts in particular, are altered by MMP inhibition. A blocking antibody to β1 integrins has a similar effect. We also show that the ectodomain of ICAM-5 can stimulate β1 integrin dependent increases in spike counts and burst number. These results support a growing body of literature suggesting that MMPs have important effects on neuronal excitability. They also support the possibility that MMP dependent shedding of specific synaptic CAMs can contribute to these effects.

Characterization of noradrenergic transmission at the dorsal motor nucleus of the vagus involved in reflex control of fundus tone

Quantitative analysis of innervation to dorsal motor nucleus of the vagus (DMV) fundus-projecting neurons indicates that approximately 17% of input neurons are noradrenergic. To determine whether this small percentage of neurons innervating DMV output to the stomach is physiologically relevant, we evaluated the role of norepinephrine at the DMV in mediating a vagovagal reflex controlling the fundus. A strain gauge was sutured onto the fundus of isoflurane-anesthetized rats to monitor changes in tone evoked by esophageal distension (ED). ED produced a decrease in fundus tone of 0.31 +/- 0.02 g (P < 0.05), which could be reproduced after a 30-min interval between distensions. Bilateral cervical vagotomy and/or pretreatment with intravenous atropine methylbromide prevented the reflex-induced fundus relaxation. In contrast, intravenous N(G)-nitro-L-arginine methyl ester had no effect. Bilateral microinjection of alpha2-adrenoreceptor antagonists (yohimbine and RS-79948) into the DMV also prevented the response. Before microinjection of alpha2-adrenoreceptor antagonists, ED decreased fundus tone by 0.33 +/- 0.05 g (P < 0.05). After antagonist microinjection, ED decreased fundus tone by only 0.05 +/- 0.06 g (P > 0.05). Bilateral microinjection of prazosin into the DMV had no effect on the response. Microinjection of norepinephrine into the DMV mimicked the effect of ED and was also prevented by prior microinjection of an alpha2-adrenoreceptor antagonist. Our results indicate that noradrenergic innervation of DMV fundus-projecting neurons is physiologically important and suggest that norepinephrine released at the DMV acts on alpha2-adrenoreceptors to inhibit activity in a cholinergic-cholinergic excitatory pathway to the fundus.

Brainstem sites controlling the lower esophageal sphincter and crural diaphragm in the ferret: A neuroanatomical study

The lower esophageal sphincter (LES) and the crural diaphragm (CD) surrounding the esophagogastric junction are key components of the gastroesophageal reflex mechanism, which engages the vago-vagal brainstem circuitry. Although both components work in conjunction to prevent gastroesophageal reflux, little is known about the brain area(s) where this integration takes place. The aims of this study were to: (1) trace the brainstem circuitry associated with the CD and the LES, and (2) determine possible sites of convergence. Experiments were done in adult male ferrets. Under isoflurane anesthesia, recombinant strains of the transneuronal pseudorabies virus (PRV-151 or PRV-Bablu) or the monosynaptic retrograde tracer cholera toxin beta-subunit (CTb) were injected into either the CD or the LES. Following a survival period of 5-7 days, animals were euthanized, perfused and their brains removed for dual-labeling immunofluorescence processing. In animals injected with recombinants of PRV into the CD and the LES, distinct labeling was found in various brainstem nuclei including: area postrema, DMV, nucleus tractus solitarius (NTS), medial reticular formation (MRF) and nucleus ambiguous (NA). Double-labeled cells were only evident in the DMV, NTS and MRF. Injections of CTb into the CD or the LES resulted in retrograde labeling only in the DMV. These findings demonstrate the presence of a direct projection from the DMV to the CD. They further suggest that the neuronal connections responsible for CD or LES function are contained in circuitries that, though largely independent, may converge at the level of DMV, NTS and MRF.

Synaptic Potentiation Facilitates Memory-like Attractor Dynamics in Cultured In Vitro Hippocampal Networks

Collective rhythmic dynamics from neurons is vital for cognitive functions such as memory formation but how neurons self-organize to produce such activity is not well understood. Attractor-based computational models have been successfully implemented as a theoretical framework for memory storage in networks of neurons. Additionally, activity-dependent modification of synaptic transmission is thought to be the physiological basis of learning and memory. The goal of this study is to demonstrate that using a pharmacological treatment that has been shown to increase synaptic strength within in vitro networks of hippocampal neurons follows the dynamical postulates theorized by attractor models. We use a grid of extracellular electrodes to study changes in network activity after this perturbation and show that there is a persistent increase in overall spiking and bursting activity after treatment. This increase in activity appears to recruit more “errant” spikes into bursts. Phase plots indicate a conserved activity pattern suggesting that a synaptic potentiation perturbation to the attractor leaves it unchanged. Lastly, we construct a computational model to demonstrate that these synaptic perturbations can account for the dynamical changes seen within the network.

Long-Term Dynamical Constraints on Pharmacologically Evoked Potentiation Imply Activity Conservation within In Vitro Hippocampal Networks

This paper describes a long-term study of network dynamics from in vitro, cultured hippocampal neurons after a pharmacological induction of synaptic potentiation. We plate a suspension of hippocampal neurons on an array of extracellular electrodes and record electrical activity in the absence of the drugs several days after treatment. While previous studies have reported on potentiation lasting up to a few hours after treatment, to the best of our knowledge, this is the first report to characterize the network effects of a potentiating mechanism several days after treatment. Using this reduced, two-dimensional in vitro network of hippocampal neurons, we show that the effects of potentiation are persistent over time but are modulated under a conservation of spike principle. We suggest that this conservation principle might be mediated by the appearance of a resonant inter-spike interval that prevents the network from advancing towards a state of hyperexcitability.

T1337 Local Release of Nitric Oxide Restores Gastric Emptying of Solid Food in STZ-Induced Diabetic Rats

G A A b st ra ct s recovery. Then the rats were fed with 1.5ml phenol red solution mixed with methycellulose and EA was performed for 30min immediately after feeding. Gastric retention of phenol red was calculated by the spectrophotometric method. In the control session, no EA was performed. In another study, a gastric balloon was chronically implanted in 5 rats. Gastric accommodationwasmeasured by barostat with ramp distension before and after the induction of diabetes, and with and .without EA at ST36. Results: 1. EA improved gastric dysrhythmia in diabetic rats. The normal percentage of slow wave was 54.7 ± 4.0% at baseline and significantly increased to 68.4 ± 3.6% with EA (P < 0.01) and sustained at 67.8 ± 2.3% during recovery period and 67.1 ± 5.2 after the test meal (P < 0.01). 2. EA resulted in a 21.4% increase in gastric emptying in diabetic rats. Gastric emptying at 30minwas 75.1±5.0% in the control diabetic animals and 91.2±1.5% in the diabetic animals with EA (P=0.01). 3. EA improved impaired gastric accommodation. The confirmed diabetic rats showed a significantly decreased gastric accommodation (0.85 ± 0.14ml with diabetes vs. 0.69 ± 0.12ml before STZ injection, P = 0.05). EA at ST36 resulted in a striking effect on gastric accommodation with a mean increase of 87.8 ± 25.8% (P < 0.05). 4. EA significantly increased vagal activity and sympathovagal balance in diabetic rats. The high frequency (vagal component) was 0.55±0.05 at baseline and increased to 0.61 ± 0.03 during EA (P = 0.03); the LF/HF (sympathovagal balance) was 0.85 ± 0.13 at baseline and significantly decreased to 0.67 ± 0.08 during EA. Conclusion: EA at ST 36 improves impaired gastric accommodation, gastric dysrhythmia and delayed gastric emptying and the improvement may be mediated via the vagal pathway. EA may have a promising therapeutic potential for diabetic gastroparesis.