Ephaptic interactions in the mammalian olfactory system.

Ephaptic coupling refers to interactions between neurons mediated by current flow through the extracellular space. Ephaptic interactions between axons are considered negligible, because of the relatively large extracellular space and the layers of myelin that separate most axons. By contrast, olfactory nerve axons are unmyelinated and arranged in tightly packed bundles, features that may enhance ephaptic coupling. We tested the hypothesis that ephaptic interactions occur in the mammalian olfactory nerve with the use of a computational approach. Numerical solutions of models of axon fascicles show that significant ephaptic interactions occur for a range of physiologically relevant parameters. An action potential in a single axon can evoke action potentials in all other axons in the fascicle. Ephaptic interactions can also lead to synchronized firing of independently stimulated axons. Our findings suggest that ephaptic interactions may be significant determinants of the olfactory code and that such interactions may occur in other, similarly organized axonal or dendritic bundles.

Ocular oscillations induced by shifts of the direction and depth of visual fixation.

Shifts of the point of fixation between two targets aligned on one eye that are located near and far (Müller paradigm) stimulates a combined saccadic-vergence movement. In normal subjects, this test paradigm often induces saccadic oscillations of about 0.3 degrees at 20 to 30 Hz. We measured eye movements using the magnetic search coil technique in 2 patients recovering from viral opsoclonus-myoclonus syndrome, comparing saccadic-vergence responses to the Müller paradigm with conjugate saccades between distant targets. Both patients exhibited intermittent conjugate ocular oscillations of about 4 to 5 degrees amplitude at about 10 Hz. Combined saccadic-vergence movements induced these oscillations twice as often as did conjugate saccades. One patient also exhibited disjunctive ocular oscillations at 10 Hz while sustaining fixation on the near target. The Müller paradigm provides a useful clinical and experimental technique for inducing saccadic oscillations. The probable mechanism is that pontine omnipause neurons, which normally gate saccades, are inhibited during the sustained vergence movement that follows the saccadic component of the response to the Müller paradigm.

Brain angiotensin type 1 receptor expression and function in the Zucker obese rat.

The Zucker obese rat is a model with predisposition for hypertension. There is evidence that angiotensin II (ANG II) may play a role in the maintenance of this hypertension. However, the potential role of brain ANG II in this regard has been largely unexplored. The aim of the present study was to compare the pressor response produced by i. c.v. injection of ANG II in Zucker obese and lean rats, and to determine if functional differences could be correlated to changes in brain AT1 receptor protein and/or mRNA expression. The Zucker obese rat had a significantly greater increase in blood pressure after i.c.v. injection of ANG II compared to the lean rat. AT1 receptor protein expression was greater in the brainstem, but not the hypothalamus, of the obese rat. These data raise the possibility that increased central responsiveness to ANG II may play a role in the predisposition of the Zucker obese rat to hypertension.

Spin glasses without time-reversal symmetry and the absence of a genuine structural glass transition

We study the three-spin model and the Ising spin glass in a field using the Migdal-Kadanoff approximation. The flows of the couplings and fields indicate no phase transition, but they show even for the three-spin model a slow crossover to the asymptotic high-temperature behavior for large values of the coupling. We have also evaluated a quantity that is a measure of the degree of non-self-averaging, and we found that it can become large for certain ranges of the parameters and the system sizes. For a spin glass in a field the maximum of non-self-averaging follows a line for given system size that resembles the de Almeida-Thouless line. We conclude that non-self-averaging found in Monte Carlo simulations cannot be taken as evidence for the existence of a low-temperature phase with replica symmetry breaking. Models similar to the three-spin model have been extensively discussed in order to provide a description of structural glasses. Their theory at mean-field level resembles the mode-coupling theory of real glasses. At that level the approach via one-step replica symmetry breaking predicts two transitions, the first transition being dynamic and the second thermodynamic. Our results suggest that in real finite-dimensional glasses there will be no genuine transitions at all, but that some features of mean-field theory could still provide some useful insights.

Effect of intracerebroventricular and intravenous insulin on Fos-immunoreactivity in the rat brain.

The potential central neural substrate for the sympathoexcitatory effect of insulin was investigated in conscious rats using Fos expression as an index of neural activity. Fos immunoreactivity in the hypothalamus and brainstem was compared in rats infused with intracerebroventricular (i.c.v.) insulin or saline for 60 min. The insulin had no effect on Fos expression in any brain nuclei. Likewise, a 90-min intravenous infusion of insulin (euglycemic) had no effect on brain Fos immunoreactivity. However, when blood glucose was allowed to decrease, Fos expression did increase in the arcuate nucleus and in the brainstem. These data do not provide any evidence to support the idea that insulin can act within the central nervous system to increase sympathetic nervous outflow.

Vasodilator effect of insulin on the microcirculation of the rat cremaster muscle.

We recently showed that, in conscious rats, acute infusions of insulin (10-15 fold increase in plasma insulin) produced decreases in hindquarter vascular resistance, but only if, changes in sympathetic outflow were prevented with a ganglionic blocker. The aim of the present investigation was to determine if similar effects of insulin could be observed in a preparation that allowed direct visualization of striated muscle (cremaster) microvessels. Initial studies with topical application of insulin showed that third-order arterioles (A3), but not first- or second-order arterioles vasodilated in response to 800 microU/ml and 8 mU/ml of insulin. Systemic (euglycemic) infusion of insulin (6 mU/ml, but not 2 mU/ml) also increased A3 arteriole diameter in animals treated with a ganglionic blocker, but not in control rats. These data show that insulin can have a direct vasodilator effect on striated muscle microvessels if concomitant increases in sympathetic outflow are absent. However, the response was only present with supraphysiological doses of the hormone.