Molecular genetic approaches to the targeted suppression of neuronal activity

Understanding how the diverse cells of the nervous system generate sensations, memories and behaviors is a profound challenge. This is because the activity of most neurons cannot easily be monitored or individually manipulated in vivo. As a result, it has been difficult to determine how different neurons contribute to nervous system function, even in simple organisms like Drosophila. Recent advances promise to change this situation by supplying molecular genetic tools for modulating neuronal activity that can be deployed in a spatially and temporally restricted fashion. In some cases, targeted groups of neurons can be 'switched off' and back 'on' at will in living, behaving animals.     

Alpha-chloralose immobilization of rock doves in Ohio

To improve capture efficacy of rock doves (Columba livia) in nuisance situations, we reevaluated the effectiveness of three dosages (60, 120 and 180 mg/kg) of alpha-chloralose (AC). Responses to immobilization using 180 mg/kg AC also were compared in rock doves deprived of food for 16 hr and not food deprived. Mean (+/- SE) time to first effects (33 +/- 2 min) and mean time to capture (94 +/- 5 min) were significantly less for rock doves receiving 180 mg/kg than for rock doves receiving lower dosages (> or = 53 +/- 3 min and > or = 153 +/- 17 min, respectively). Ten, 10, and eight rock doves immobilized with 60, 120, and 180 mg/kg AC recovered within 24 hr, respectively; all rock doves recovered within 29 hr. Although food-deprived rock doves showed effects of AC immobilization earlier than did rock doves with food, time to capture was similar between these two groups. For capturing rock doves, we recommend treating corn with 3 mg AC/kernel and using 180 mg/kg as the effective dose. This modified formulation and dosage should improve capture success of rock doves substantially and improve the ability to resolve nuisance rock dove problems.     

Behavior-related neuronal reactions and dynamics of neuronal activity

Functionally, behavior-related discharges of associative neurons are an efferent flow of pulses continuously generated over the course of each behavioral act of an animal. However, predominant research approaches are based on the "stimulus - reaction" principle. Analysis of the dynamics of unit activity in monkeys during performance of a multi-step behavioral complex showed that differences related to different behavioral acts consist in composition changes in the active neurons (or their recombination) rather than in a number of responsive cells or involvement of action-specific neurons. Each combination of active neurons ensures the distribution of efferent signals characteristic of the given combination. These findings suggest the addressing coding of the efferent neuronal signals.     

Fueling brain neuronal activity

The energy requirements of the brain are amazingly high. The brain represents about 2% of the body weight, but it receives 15% of the total blood flow provided by the cardiovascular system and consumes at least 25% circulating glucose plus 20% oxygen available in the body at rest. The cornerstone feature of the brain energy metabolism is its tight coupling with neuronal activity. An abnormality in the sequence of events allowing neurons to be adequately supplied with the necessary energy could have dramatic consequences exemplified in the neurodegenerative diseases such as epilepsy and Alzheimer’s disease. In this paper, we review the current views on the main pathways of neuronal energy supply.     

Telemetric recording of neuronal activity

A telemetric system is described which allows the wireless registration of extracellular neuronal activity and vocalization-associated skull vibrations in freely moving, socially living squirrel monkeys (Saimiri sciureus). The system consists of a carrier platform with numerous guiding tubes implanted on the skull. Custom-made microdrives are mounted on the platform, allowing the exploration of two electrode tracks at the same time. Commercially available quartz-insulated platinum-tungsten microelectrodes are used. The electrodes can be moved over a distance of 8-10 mm by turning a screw on the microdrive. Vocalization-associated skull vibrations are recorded with a piezo-ceramic element. Skull vibration signal and the signals from the two microelectrodes are fed into separate transmitters having different carrier frequencies. The signals are picked up by an antenna in the animal cage and are sent to three receivers in the central laboratory. Here, the signals are transferred via an analog/digital interface to a personal computer for data analysis and to a video recorder for long-term storage. The total weight of the head mount including carrier platform, microdrive, electrodes, skull vibration sensor, three transmitters, and protection cap is 32 g. The transmitters are powered with two rechargeable lithium batteries, allowing about 8 h of continuous recording. Reliable signal transmission is obtained over a distance of about 2 m. Recording stability allows to follow the activity of specific neurons up to several hours, with no movement artefacts during locomotion.     

Mechanism of suppression of sustained neuronal spiking under high-frequency stimulation

Using Hodgkin–Huxley and isolated subthalamic nucleus (STN) model neurons as examples, we show that electrical high-frequency stimulation (HFS) suppresses sustained neuronal spiking. The mechanism of suppression is explained on the basis of averaged equations derived from the original neuron equations in the limit of high frequencies. We show that for frequencies considerably greater than the reciprocal of the neuron’s characteristic time scale, the result of action of HFS is defined by the ratio between the amplitude and the frequency of the stimulating signal. The effect of suppression emerges due to a stabilization of the neuron’s resting state or due to a stabilization of a low-amplitude subthreshold oscillation of its membrane potential. Intriguingly, although we neglect synaptic dynamics, neural circuity as well as contribution of glial cells, the results obtained with the isolated high-frequency stimulated STN model neuron resemble the clinically observed relations between stimulation amplitude and stimulation frequency required to suppress Parkinsonian tremor.     

Mechanisms to synchronize neuronal activity

 Temporal aspects of neuronal activity have received increasing attention in recent years. Oscillatory dynamics and the synchronization of neuronal activity are hypothesized to be of functional relevance to information processing in the brain. Here we review theoretical studies of single neurons at different levels of abstraction, with an emphasis on the implications for properties of networks composed of such units. We then discuss the influence of different types of couplings and choices of parameters to the existence of a stable state of synchronous or oscillatory activity. Finally we relate these theoretical studies to the available experimental data, and suggest future lines of research. Received: 20 July 1999 / Accepted in revised form: 23 August 2000     

Stochastic dynamical aspects of neuronal activity

A stochastic model of neuronal activity is proposed. Some stochastic differential equations based on jump processes are used to investigate the behavior of the membrane potential at a time scale small with respect to the neuronal states time evolution. A model for learning, implying short memory effects, is described.     

EGFR-dependent suppression of synaptic autophagy is required for neuronal circuit development

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Hypothalamic neuronal histamine mediates the thyrotropin-releasing hormone-induced suppression of food intake

We examined the involvement of thyrotropin-releasing hormone (TRH) and TRH type 1 and 2 receptors (TRH-R1 and TRH-R2, respectively) in the regulation of hypothalamic neuronal histamine. Infusion of 100 nmol TRH into the rat third cerebroventricle (3vt) significantly decreased food intake (p < 0.05) compared to controls infused with phosphate- buffered saline. This TRH-induced suppression of food intake was attenuated partially in histamine-depleted rats pre-treated with alpha-fluoromethylhistidine (a specific suicide inhibitor of histidine decarboxylase) and in mice with targeted disruption of histamine H1 receptors. Infusion of TRH into the 3vt increased histamine turnover as assessed by pargyline-induced accumulation of tele-methylhistamine (t-MH, a major metabolite of neuronal histamine in the brain) in the tuberomammillary nucleus (TMN), the paraventricular nucleus, and the ventromedial hypothalamic nucleus in rats. In addition, TRH-induced decrease of food intake and increase of histamine turnover were in a dose-dependent manner. Microinfusion of TRH into the TMN increased t-MH content, histidine decarboxylase (HDC) activity and expression of HDC mRNA in the TMN. Immunohistochemical analysis revealed that TRH-R2, but not TRH-R1, was expressed within the cell bodies of histaminergic neurons in the TMN of rats. These results indicate that hypothalamic neuronal histamine mediates the TRH-induced suppression of feeding behavior.     

Cycle-triggered averaging of respiration-related neuronal activity

A computer system is presented which provides off-line computation of cycle-triggered histograms (CTH) of respiration-related neuronal activity. Binwidths of the histograms are freely selectable by software from 10 ms to 100 ms. For special evaluation purposes, CTHs can be standardized in different ways concerning cycle duration as well as amplitude. Time incidence of maximum frequency, center of gravity and expiration-to-inspiration phase transition within the respiratory cycle are computed. The system employs special hardware interfaces to an 8-bit microcomputer which are briefly described. Data acquisition, data manipulation and output handling of the results are performed by chaining 3 compiled BASIC programs. Some comments on peculiarities of the BASIC language concerning combined application of a BASIC interpreter and a BASIC compiler are brought up. The usefulness of the method is demonstrated by examples of CTHs computed from the activity of medullary respiration-related neurons as well as of the corresponding phrenic nerve mass activity.     

Optical excitation and detection of neuronal activity

Optogenetics has emerged as an exciting tool for manipulating neural activity, which in turn, can modulate behavior in live organisms. However, detecting the response to the optical stimulation requires electrophysiology with physical contact or fluorescent imaging at target locations, which is often limited by photobleaching and phototoxicity. In this paper, we show that phase imaging can report the intracellular transport induced by optogenetic stimulation. We developed a multimodal instrument that can both stimulate cells with subcellular spatial resolution and detect optical pathlength (OPL) changes with nanometer scale sensitivity. We found that OPL fluctuations following stimulation are consistent with active organelle transport. Furthermore, the results indicate a broadening in the transport velocity distribution, which is significantly higher in stimulated cells compared to optogenetically inactive cells. It is likely that this label‐free, contactless measurement of optogenetic response will provide an enabling approach to neuroscience.