Discrimination between aversive brain stimulations: effect of stimulation parameters

The relationship between thiamine deficiency and performance of a learning task in rats

We have been investigating the relationship between learning and thiamine. Electrical stimulation of mesencephalic periaqueductal gray matter (PAG) is known to have an aversive effect and elicits spontaneous instrumental escape behavior. We taught rats to press a lever to escape from the pain of electrical stimulation by learning to turn a switch off. Then we examined the relationship between learning and the thiamine concentration in various portions of the brain. (1) One group of rats was given a normal diet and another group was given a thiamine-deficient diet which contained half of the amount of thiamine present in the normal diet. We measured the response time required for each rat to react by moving after an electrical impulse was applied, and the running time during which the rat was moving from the starting point to the end point to press a lever. The rats that were fed the thiamine-deficient diet showed a slower response time and a longer running time than the rats fed the normal diet. (2) We divided the rats fed the normal diet into two groups, one group trained to switch off a lever and the other group not trained for such a task. We found that the thiamine concentration in the blood of the rats in the trained group was significantly higher than that in the group without training.

Discriminative properties of aversive electrical stimulations of the so-called "mesencephalic locomotor region": a parametric study

The classical 2-lever food-reward-reinforced discrimination paradigm, although already successfully applied to study discriminative properties of positively reinforcing electrical brain stimulations (EBS), has not yet been used for negatively reinforcing (aversive) EBS. This study was aimed at assessing if such a discrimination paradigm could be used to demonstrate discriminative properties of aversive EBS of the so-called "mesencephalic locomotor region" (MLR). Fourteen rats were trained for food reward to press one lever in the presence of EBS of the MLR and the other lever in the absence of EBS. Discriminative control by EBS of the MLR was obtained in 11 rats, which were subsequently subjected to a parametric study. It was found that the discriminative properties of EBS of the MLR covaried with changes in the intensity, frequency and pulse duration of the EBS. Our data also suggest that the aversive dimension of the stimulation might constitute a component of the discriminative cue induced by these EBS.

Spike trains in rat periaqueductal gray depend on the stochastic properties of interacting electrical stimulation trains

Spike trains flowing into the periaqueductal gray (PAG) might be discriminated from one another by PAG neurons on the basis of the distribution or sequence of their respective interspike intervals. The various sequences of interspike intervals characteristic of spontaneous PAG unit activities were assessed in a preliminary experiment. These sequences were then simulated by means of appropriate mathematical functions. These functions allowed the production of stimulation trains that were applied to two PAG sites to induce spike trains with similar sequences in order to reveal the sensitivity of PAG neurons to the stochastic structure of afferent spike trains. We placed emphasis on parameters of the spike train that proved to be altered independently of any alteration of the corresponding parameters in the stimulation train. The mean pulse rate is the simplest example of such a parameter as it was never altered in the stimulation train. Alterations of either the distribution or sequence of pulses in the stimulation train were found to affect the mean discharge rate in a number of cases (30-40% of the cases). Despite their moderate degree (20-30% mean rate alteration) such differential effects could correspond to stimulation-induced differential behavioral effects as was shown in a previous study. Furthermore, a specific dependence of the generated spike trains on the sequential structure of the stimulation train was observed in some cases when appropriate stimulation trains were simultaneously applied to another PAG stimulation site. This fact is worth considering in relation to the integrative function of the PAG neuronal network.

A neuropharmacological study of the periventricular neural substrate involved in flight

This paper reviews results obtained in experiments concerning the neurochemical characteristics of the substrate involved in the control of flight reactions and the induction of aversive effects in the rat. These experiments investigated the behavioural effects produced by microinjecting into the periaqueductal grey matter (PAG) or the medial hypothalamus (MH) compounds known to interfere with the functioning of some neurotransmitter systems known to exist in these structures. The data obtained show that: the activity of the substrate involved in the production of flight reactions is tonically inhibited by the release of GABA (gamma-aminobutyric acid); the behavioural reactions produced by microinjecting GABA antagonists can be clearly distinguished, depending on whether such drugs were injected into the PAG or the MH, despite the fact that jumps were produced from either level; behavioural effects, comparable to some extent to those produced by microinjections of GABA antagonists, can be obtained by injecting drugs which act on non-GABAergic neurochemical substrates, namely opioidergic or cholinergic systems; and behavioural effects, comparable to those produced by injecting GABA antagonists into the PAG, can be obtained by injecting such drugs into various sites located in other parts of the tectum such as the inferior colliculus or adjacent structures.

Morphine injected into the periaqueductal gray attenuates brain stimulation-induced effects: an intensity discrimination study

A discrimination situation was used to study the effects of morphine microinjected into the periaqueductal gray (PAG) on the aversive effects induced by PAG or medial hypothalamic (MH) electrical stimulation. Rats were trained in a T-maze to interrupt a high intensity (HI) stimulation inducing a short escape latency by pressing the lever (HI lever) located in one arm of the maze and a low intensity (LI) stimulation inducing a longer escape latency by pressing the lever (LI lever) located in the other arm. Microinjections of 15 nmol (5 micrograms) or 40 nmol (13 micrograms) of morphine both lengthened the escape latencies and shifted towards the LI lever the animal's choice in order to interrupt HI stimulations. This effect of morphine showed a similar time course as regards both escape latency and lever choice; it was more marked on PAG than on MH stimulation-induced aversive effects. The data are discussed in terms of morphine microinjections into PAG lengthening the escape latency by decreasing the aversiveness of PAG or MH stimulation rather than by affecting the animals' ability to respond to such stimulations.