Neuronal spike activity in rat nucleus accumbens during cocaine self-administration under different fixed-ratio schedules.

Chronic ensemble recording techniques were used to investigate neuronal activity in the nucleus accumbens in freely moving rats during different cocaine self-administration schedules. The issue of concern in this study was the role of nucleus accumbens in initiating and sustaining cocaine self-administration. Specifically, to determine the nature of the neuronal activity, either motor or motivational, which precedes the multiple bar presses required to self-administer cocaine and of the post-lever press neuronal response, we used conventional fixed ratio-5, fixed ratio-10, and modified fixed ratio-3 schedules. In the modified fixed ratio-3 schedule, the first lever press resulted in retraction of the lever for 2 s; the second lever press retracted the lever and turned on a cue light; the third lever press turned off the cue light and delivered cocaine (1.0 mg/kg) intravenously. In the fixed ratio-5 and -10 schedules, rats continuously pressed the lever 5 or 10 times, respectively, to obtain a single infusion of cocaine. Phasic alterations in neural spike activity were observed in 50% of nucleus accumbens neurons before (termed "anticipatory" responses) and after lever pressing for cocaine self-administration. Neurons with anticipatory responses typically exhibited such responses for all lever presses in the modified fixed ratio-3, fixed ratio-5, and fixed ratio-10 schedules, but instances were found when the activity correlate was absent. In addition, some neurons had a prominent alteration in firing rate lasting 1-5 min after cocaine self-administration, and some of these neurons also had anticipatory responses. When cocaine was eliminated during self-administration sessions, the post-lever press inhibitory responses were largely abolished or even reversed, whereas anticipatory responses were not markedly changed when rapid lever presses occurred before behavior ceased. Post-cocaine inhibitory responses compared between self-administered and passively administered cocaine were not significantly different between these two conditions. The results suggest that nucleus accumbens may be involved in initiating general reward-seeking behaviors and action which are not exclusively associated with cocaine self-administration. Moreover, the neuronal responses in the nucleus accumbens to cocaine self-administration may play an essential role in maintaining cocaine reinforcement.

Bistability, switches and working memory in a two-neuron inhibitory-feedback model.

It was reported earlier that an inhibitory-feedback network inspired by neostriatal circuitry may exhibit a bistable character and spontaneous switching phenomenon within the neuronal activity. In the presence of noise and external excitation, a few local neurons switch "on" and generate streams of impulses while other neurons remain quiescent. In time, the existing "on" neurons spontaneously switch "off" and other neurons switch "on". In this paper we examine the nature of the bistability and switching phenomenon using a simple model consisting of two mutually inhibitory neurons. For nonspiking neuron model, described by a system of nonlinear differential equations, we present a simple bifurcation analysis, which follows the birth and annihilation of two stable fixed points when model parameters are varied. We show that both nonspiking and spiking models may have two stable states, but only spiking neurons exhibit switching. The mechanism of switching for model spiking neurons, described by an equivalent RC circuit with a number of currents, is analyzed using computer simulations. It is shown that switching can be described by a two-state Markov chain with one parameter, which depends on the set of model physiological parameters, such as duration of afterhyperpolarization (AHP), maximum and the time duration of inhibitory post-synaptic potentials (IPSP's) and amplitude of the neuron noise input. "On" and "off" states of the model can be rapidly changed by localized excitatory input and the network then sustains the pattern of "on" and "off" states. That is, such a network can be used as a programmable memory device.(ABSTRACT TRUNCATED AT 250 WORDS)

Bifurcation analysis of a neural network model.

This paper describes the analysis of the well known neural network model by Wilson and Cowan. The neural network is modeled by a system of two ordinary differential equations that describe the evolution of average activities of excitatory and inhibitory populations of neurons. We analyze the dependence of the model's behavior on two parameters. The parameter plane is partitioned into regions of equivalent behavior bounded by bifurcation curves, and the representative phase diagram is constructed for each region. This allows us to describe qualitatively the behavior of the model in each region and to predict changes in the model dynamics as parameters are varied. In particular, we show that for some parameter values the system can exhibit long-period oscillations. A new type of dynamical behavior is also found when the system settles down either to a stationary state or to a limit cycle depending on the initial point.

A new statistical method for identifying interconnections between neuronal network elements.

A new method is proposed to analyse dependencies in point processes, which takes into account specific character of neuronal activity. Simulation modelling of neuronal network revealed that the estimated weight of connection depends monotonically on the value of the model synaptic strength. In contrast to the crosscorrelation, the method allows for nonlinear interconnections and does not require point processes to be stationary and samples to be large. Examples are presented of the method's application to neurophysiological data analysis.

[Types of spike activity and nature of neuronal interrelations in the rat neostriatum]. / Tipy impul'snoi aktivnosti i kharakter vzaimosviazi neironov v neostriatume mozga krysy.

Statistical methods were used to study types of background activity (BA) of neostriatum neurons. Dependence of the crosscorrelation function on the BA type and on a distance between the neurons was examined in acute experiments on rats (one group was immobilized with d-tubocurarin, the other was under nembutal anesthesia). It is shown that BA according to the shape of the autocorrelation and crosscorrelation functions can be classified into five types (I.1, I.2, I.3, II.1, II.2). Pronounced differences in the crosscorrelation function were found between the neurons with grouped BA (I.1, I.2) and the neurons with unitary BA (II.1, II.2). Differences in the crosscorrelation function between close and distant (300-500) microns) neurons were also found.