Enhancement of excitability and inhibitory processes in hippocampal dentate gyrus by noradrenaline: a pharmacological study in awake, freely moving rats

Idazoxan (IDA), an alpha 2 receptor antagonist which increases firing rate of noradrenergic neurons in the locus coeruleus (LC) and release of noradrenaline (NA) in target structures, was used to study the neuromodulatory effects of NA in the hippocampus in awake rats. After IDA the population spike in the dentate gyrus (DG), evoked by a single pulse to the perforant path, was greatly enhanced with no effect of the drug on excitatory postsynaptic potentials (EPSPs). Paired pulses with short interpulse intervals (25-30 ms) produced inhibition of the response to the second pulse which was increased by IDA. This drug effect was independent of its effect on the amplitude of the first spike, since the increase in inhibition was seen at stimulation intensities which did not increase the response amplitude to the initial pulse. Thus both excitability and inhibitory processes can be enhanced in the same population of neurons by an alpha 2 adrenoceptor antagonist.

Memory retrieval enhancement by locus coeruleus stimulation: evidence for mediation by beta-receptors

Rats were trained in a complex food-motivated maze task, then implanted with indwelling stimulating electrodes in the noradrenergic nucleus locus coeruleus (LC). When tested 4 weeks later, they showed significant forgetting. Electrical stimulation of the LC alleviated forgetting in that stimulated rats made no more errors during the test than they did on the last learning trial. Systemic treatment with the beta noradrenergic antagonist propranolol blocked the effect of stimulation, suggesting that the memory facilitation is mediated through a beta-receptor.

Plasticity of sensory responses of locus coeruleus neurons in the behaving rat: implications for cognition

The gating and tuning actions of noradrenaline (NA) at post-synaptic sites have been highly suggestive of an important role for the locus coeruleus (LC) in attention, learning and memory. By recording the activity of single units in the LC in behaving rats in a strictly controlled conditioning paradigm, direct evidence was provided that this nucleus is engaged during specific aspects of learning. The neuronal response to a discrete sensory stimulus was monitored as a function of the changing significance of the stimulus i.e., when it was novel, during habituation, associative learning, reversal and extinction. Both appetitive and aversive paradigms were used. We consistently observed differential conditioned responding with food reinforcement, while when footshock reinforcement was used, there was an increase in response to both CS+ and CS-. In both paradigms, the LC response disappeared when the conditioning was expressed at a behavioral level, to reappear vigorously as soon as the stimulus reinforcement contingencies were changed, i.e., during reversal or extinction. These results suggest that the LC does not mediate specific sensory or associative information necessary for ongoing performance but shows remarkable plasticity of sensory responding as a function of changing cognitive significance of the stimulus.

Activation of the noradrenergic system facilitates an attentional shift in the rat

The noradrenergic system was pharmacologically activated with the alpha 2 receptor antagonist, idazoxan (2 mg/kg i.p.), during the acquisition of a complex appetitive task requiring a shift in attention to stimulus dimension and in response strategy. Rats first learned a fixed path of 6 successive choices in a linear maze. The task was then changed to a visual discrimination task in which the spatial configuration of the correct path was indicated by visual cues and changed on each daily trial. During this part of the task, the rats were injected before each trial with idazoxan, a drug which increases the firing rate of neurons in the locus coeruleus and the release of noradrenaline in the cortex and hippocampus. Two control experiments showed that the drug treatment had no effect on the acquisition of either component of the task - the successive place learning or the visual discrimination. The drug was found to be effective only during the shift phase of the experiment, the idazoxan-treated rats taking fewer trials to reach criterion than the saline. A second experiment showed that idazoxan increased the amount of time spent investigating novel and unexpected objects in a familiar hole board. These results implicate the noradrenergic system in problem-solving which requires an attentional shift or a shift in responding from familiar to novel stimuli.

Idazoxan, an alpha-2 antagonist, facilitates memory retrieval in the rat

The role of the noradrenergic system in cognitive function was studied by using the alpha-2 adrenoceptor antagonist idazoxan to increase noradrenergic activity. Rats were trained in a complex maze task for food reward. They were left undisturbed for a 4-week "forgetting" period and were treated with idazoxan, just before the retention test. The dose of idazoxan used had previously been shown to enhance firing of units of the locus coeruleus and to increase noradrenaline (NE) turnover in the forebrain. This pharmacological treatment effectively alleviated forgetting, while control rats showed significant decrement compared to their performance at the last training trial. A control experiment showed that the facilitative effect was not on learning or on ongoing performance of the task, since there was no effect on simple acquisition. The results are taken as support for the notion that NE plays a role in memory retrieval processes.

Glucose effects on firing rate of neurons of the locus coeruleus: another attempt to put memory back in the brain

After a generation of research into the biological mechanisms of memory, the essential nature of the engram remains as elusive as ever. Many investigators have reorganized their conceptual framework to emphasize the role of physiological responses elicited by a learning experience in modulating memory for that event. This approach has generated a long list of correlations between physiological functions and memory performance. The papers comprising the present section have the common theme of contributing to that list, but the remarkable fact which they emphasize is that these correlations are much greater in the aged rat than in young animals, suggesting that it is a parallel degeneration of several physiological functions which mediate deficits in memory performance.

Priming stimulation of locus coeruleus facilitates memory retrieval in the rat

Rats were trained to run a linear maze for food reinforcement. During the 5-week retention interval, they were implanted under electrophysiological control with fine stimulating electrodes aimed at the nucleus locus coeruleus (LC). When tested 5 weeks after training, control rats showed forgetting in that they made significantly more errors at the test trial than at the last training trial. Low-level stimulation of LC immediately before the test alleviated the forgetting in that this group made significantly fewer errors than the non-stimulated group on two successive days. The results are taken as behavioral evidence of a role for noradrenergic projections from LC in memory and attention.

Persistence of habituation deficits after neurological recovery from severe thiamine deprivation

Rats were fed a thiamine-deficient diet for 4 weeks and injected daily with pyrithiamine during the last two weeks of the diet. This regime induced severe neurological anomalies such as ataxia, loss of righting reflex and visual place reflex, and finally full tonic-clonic seizures. These symptoms are reminiscent of the clinical Wernicke-Korsakoff syndrome. Injection of thiamine dramatically reversed these symptoms within one or two hours as seen in Wernicke patients. Six weeks later these rats showed a marked deficit in habituation of exploratory behavior and of the auditory orienting response. To what extent this chronic deficit in habituation contributes to the cognitive dysfunctions seen in Wernicke-Korsakoff patients is discussed. Histological examination of the brains of 6 of the rats revealed a heterogeneous pattern of damage to the brainstem, including mamillary bodies and several thalamic nuclei, reminiscent of that seen in Korsakoff patients. In addition there were many dark abnormal cells in limited fields of hippocampus neocortex and thalamus in almost all animals.

Haloperidol facilitates memory retrieval in the rat

Rats were trained in a complex maze task for food reward. After a 25-day retention interval, they made more errors than at the last training trials. This forgetting was alleviated by pretest treatment with haloperidol. The same dose of haloperidol had no effect on acquisition or performance when injected during training. The results were replicated, but smaller doses were found to be inactive. The possibility that the effect might be mediated through the noradrenergic system is considered.

Intracerebroventricular apomorphine alleviates spontaneous forgetting and increases cortical noradrenaline

Rats were trained in a 6 unit spatial discrimination maze for food reinforcement; they were then implanted with intracerebroventricular (i.c.v.) cannulae and tested 25 days after training. Control animals displayed significant forgetting; 10 micrograms apomorphine alleviated this forgetting, while a higher dose did not. A hole board activity study revealed that i.c.v. Apomorphine does not produce the shaped dose-response activity curve found with systemic injections. Neurochemical analysis of forebrain structures after injections showed that the dose which facilitated memory retrieval also increased cortical and hippocampal noradrenaline, while the larger dose did not.

Reticular stimulation facilitates retrieval of a 'forgotten' maze habit

Rats tested 25 days after training in a complex maze showed significant forgetting. Stimulation of the mesencephalic reticular formation immediately prior to retention testing facilitated performance in that stimulated rats made fewer errors (but did not run faster) than non-stimulated controls. Rats exposed to a contextual cue as a reminder before testing ran faster and made fewer errors than controls. Results are discussed in terms of forgetting being due to retrieval failure, and the reticular stimulation facilitating retrieval of information concerning the spatial configuration of the maze.

Memory retrieval deficits: alleviation by etiracetam, a nootropic drug

Etiracetam, a nonanaleptic drug related to the nootropic substance piracetam, was found to facilitate memory retrieval in rats in several experimental situations, when injected 30 min prior to retention testing. The drug was active when memory deficits were induced by electroconvulsive shock, undertraining, or by a long training-to-test interval. The behavioral paradigms included a one-trial inhibitory avoidance task and a complex multitrial, spatially discriminated approach task. The clinical interest of drugs which facilitate retrieval processes is also discussed.

Piracetam facilitates retrieval but does not impair extinction of bar-pressing in rats

Rats were trained on a continuously reinforced bar-press response for water reward. Seven days later they were retested for retention, with or without pretest injection of the nootropic drug, piracetam. Drug-treated animals had significantly shorter response latencies than saline-treated animals. The results are interpreted as a facilitation of retrieval processes after forgetting. The experiment was extended under extinction conditions and it was found that after three sessions there was a tendency to facilitate extinction when response latency is used as the extinction index. The clinical interest of a drug which facilitates the retrieval aspect of the memory process without impairing extinction is discussed.

Passive avoidance behavior in rats after electroconvulsive shock: facilitative effect of response retardation

Rats were trained in a one-trial passive avoidance task and then were submitted to electroconvulsive shock (ECS) or to sham ECS. Twenty-four hours later they were tested for retention, with the door opened either immediately or 30 sec after the beginning of the test. Rats initially forced to avoid for 30 sec continued to avoid for the entire test, but the others had the usual low step-through latencies seen with ECS-treated animals. Activity measures for those animals stepping through differentiated groups having received footshock from those not having footshock and ECS. A retest 5--10 min later showed "recovery" in the amnestic animals and continued avoidance behavior for those that avoided on the first test. Results are taken as evidence that ECS effects are not on memory storage but on the capacity of the animal to organize information effectively and quickly in order to produce an adaptive response.