Brain stimulation: effects on memory

Targeting papez circuit for cognitive dysfunction- insights into deep brain stimulation for Alzheimer's disease

Hippocampus is the most widely studied brain area coupled with impairment of memory in a variety of neurological diseases and Alzheimer's disease (AD). The limbic structures within the Papez circuit have been linked to various aspects of cognition. Unfortunately, the brain regions that include this memory circuit are often ignored in terms of understanding cognitive decline in these diseases. To properly comprehend where cognition problems originate, it is crucial to clarify any aberrant contributions from all components of a specific circuit -on both a local and a global level. The pharmacological treatments currently available are not long lasting. Deep Brain Stimulation (DBS) emerged as a new powerful therapeutic approach for alleviation of the cognitive dysfunctions. Metabolic, functional, electrophysiological, and imaging studies helped to find out the crucial nodes that can be accessible for DBS. Targeting these nodes within the memory circuit produced significant improvement in learning and memory by disrupting abnormal circuit activity and restoring the physiological network. Here, we provide an overview of the neuroanatomy of the circuit of Papez along with the mechanisms and various deep brain stimulation targets of the circuit structures which could be significant for improving cognitive dysfunctions in AD.

Deep Brain Stimulation Initiative: Toward Innovative Technology, New Disease Indications, and Approaches to Current and Future Clinical Challenges in Neuromodulation Therapy

Deep brain stimulation (DBS) is one of the most important clinical therapies for neurological disorders. DBS also has great potential to become a great tool for clinical neuroscience research. Recently, the National Engineering Laboratory for Neuromodulation at Tsinghua University held an international Deep Brain Stimulation Initiative workshop to discuss the cutting-edge technological achievements and clinical applications of DBS. We specifically addressed new clinical approaches and challenges in DBS for movement disorders (Parkinson's disease and dystonia), clinical application toward neurorehabilitation for stroke, and the progress and challenges toward DBS for neuropsychiatric disorders. This review highlighted key developments in (1) neuroimaging, with advancements in 3-Tesla magnetic resonance imaging DBS compatibility for exploration of brain network mechanisms; (2) novel DBS recording capabilities for uncovering disease pathophysiology; and (3) overcoming global healthcare burdens with online-based DBS programming technology for connecting patient communities. The successful event marks a milestone for global collaborative opportunities in clinical development of neuromodulation to treat major neurological disorders.

Reporting Guidelines and Issues to Consider for Using Intracranial Brain Stimulation in Studies of Human Declarative Memory

Participants with stimulating and recording electrodes implanted within the brain for clinical evaluation and treatment provide a rare opportunity to unravel the neuronal correlates of human memory, as well as offer potential for modulation of behavior. Recent intracranial stimulation studies of memory have been inconsistent in methodologies employed and reported conclusions, which renders generalizations and construction of a framework impossible. In an effort to unify future study efforts and enable larger meta-analyses we propose in this mini-review a set of guidelines to consider when pursuing intracranial stimulation studies of human declarative memory and summarize details reported by previous relevant studies. We present technical and safety issues to consider when undertaking such studies and a checklist for researchers and clinicians to use for guidance when reporting results, including targeting, placement, and localization of electrodes, behavioral task design, stimulation and electrophysiological recording methods, details of participants, and statistical analyses. We hope that, as research in invasive stimulation of human declarative memory further progresses, these reporting guidelines will aid in setting standards for multicenter studies, in comparison of findings across studies, and in study replications.

Significance and Remembrance: The Role of Neuromodulatory Systems

It is now well known that the retention of newly-acquired information can be modulated by drugs or hormones administered shortly following training. It is generally thought that such treatments influence retention by modifying processes underlying the storage of information. The fact that susceptibility to posttraining memory modulation is seen in many species, including bees, fish, birds, and mammals, argues that some common time-dependent memory storage processes have been conserved in evolution. Recent research findings have provided strong support for the view that such susceptibility to posttraining influences provides opportunity for modulation of memory storage by endogenous neurohormonal systems. In rats and mice, posttraining administration of hormones such as epinephrine that are normally released by training experiences enhances subsequent retention. Comparable effects are found with posttraining administration of opiate receptor antagonists such as naloxone. Findings of recent experiments indicate that these treatments affect memory by influencing the release of norepinephrine within the amygdaloid complex. The endogenous regulation of memory storage appears to involve interaction of neurohormones and transmitters in activating brain systems involved in memory storage.

Memory-enhancing amygdala stimulation elicits gamma synchrony in the hippocampus

Activation of the amygdala either during emotional arousal or by direct stimulation is thought to enhance memory in part by modulating plasticity in the hippocampus. However, precisely how the amygdala influences hippocampal activity to improve memory remains unclear. In the present study, brief electrical stimulation delivered to the basolateral complex of the amygdala (BLA) following encounters with some novel objects led to better memory for those objects 1 day later. Stimulation also elicited field-field and spike-field CA3-CA1 synchrony in the hippocampus in the low gamma frequency range (30-55 Hz), a range previously associated with spike timing and good memory. In addition, the hippocampal spiking patterns observed during BLA stimulation reflected recent patterns of activity in the hippocampus. Thus, the results indicate that amygdala activation can prioritize memory consolidation of specific object encounters by coordinating the precise timing of CA1 membrane depolarization with incoming CA3 spikes to initiate long-lasting spike-timing dependent plasticity at putative synapses between recently active neurons.

Preclinical studies modeling deep brain stimulation for depression

Deep brain stimulation (DBS) is currently being investigated for the treatment of depression. Results of early clinical trials have been very promising, but the mechanisms responsible for the effects of DBS are still unknown. This article reviews behavioral findings of stimulation applied to different brain targets in rodents, with a particular focus on the ventromedial prefrontal cortex. Mechanisms and substrates involved in the antidepressant-like effects of DBS, including the role of local tissue inactivation, the modulation of fiber pathways in the vicinity of the electrodes, as well as the importance of the serotonergic system and brain derived neurotrophic factor are discussed.

Event-specific enhancement of memory via brief electrical stimulation to the basolateral complex of the amygdala in rats

The basolateral complex of the amygdala (BLA) modulates memory for emotional events, and direct activation of the BLA following a learning session can enhance subsequent memory. Yet optimal enhancement of episodic memory during emotional events would likely require that BLA activation occur close in time to the event and to be brief enough to target specific memories if some events are to be remembered better than others. In the present study, rats were given a novel object recognition memory task in which initial encounters with some of the objects were immediately followed by brief electrical stimulation of the BLA, and these objects were remembered better one day later as compared to objects for which the initial encounter was not followed by stimulation. The results indicated that BLA stimulation can enhance memory for individual events, a necessary ability for the BLA to modulate episodic memory effectively.

Anterior thalamus deep brain stimulation at high current impairs memory in rats

Deep brain stimulation (DBS) of the anterior thalamic nucleus (AN), an important relay in the circuitry of memory, is currently being proposed as a treatment for epilepsy. Despite the encouraging results with the use of this therapy, potential benefits and adverse effects are yet to be determined. We show that AN stimulation at relatively high current disrupted the acquisition of contextual fear conditioning and impaired performance on a spatial alternating task in rats. This has not been observed at parameters generating a charge density that approximated the one used in clinical practice. At settings that impaired behavior, AN stimulation induced a functional depolarization block nearby the electrode, increased c-Fos expression in cerebral regions projecting to and receiving projections from the AN, and influenced hippocampal activity. This suggests that complex mechanisms might be involved in the effects of AN DBS, including a local target inactivation and the modulation of structures at a distance. Though translating data from animals to humans has to be considered with caution, our study underscores the need for carefully monitoring memory function while selecting stimulation parameters during the clinical evaluation of AN DBS.

Light and electron microscopic study of cholinergic and noradrenergic elements in the basolateral nucleus of the rat amygdala: evidence for interactions between the two systems

Pharmacological studies have suggested that the cholinergic (ACh) and noradrenergic (NA) systems in the amygdala (AM) play an important role in learning and memory storage and that the two systems interact to modulate memory storage. To obtain anatomical evidence for the interaction, the organization of the ACh and NA fibers in rat AM was investigated by immunocytochemistry for choline acetyltransferase (ChAT) and dopamine-beta-hydroxylase (DBH) in conjunction with light, confocal laser scanning, and electron microscopy (LM, CLSM, and TEM, respectively). LM showed that the ChAT immunoreactivity was densest in the basolateral nucleus (BL), whereas the DBH immunoreactivity was densest in the posterior BL. CLSM demonstrated that the ChAT-immunoreactive profiles in the BL were frequently located in juxtaposition to the DBH-immunoreactive axons. The TEM observations were as follows: The majority of the synapses formed by ChAT-immunoreactive terminals were symmetric, but DBH-immunoreactive axons formed both asymmetric and symmetric synapses. The ChAT-immunoreactive terminals usually established the symmetric synaptic contacts with the DBH-immunoreactive terminals and varicosities. The DBH-immunoreactive terminals formed the asymmetric synapses with the ChAT-immunoreactive dendrites of the intrinsic neurons within the AM. The results provide anatomical substrates for mnemonic functions of the ACh and NA systems and for the interactions between the two systems in the AM.

Does the kindling model of epilepsy contribute to our understanding of multiple chemical sensitivity?

Multiple chemical sensitivity (MCS) is a phenomenon whereby individuals report an increased sensitivity to low levels of chemicals in the environment. Kindling is a model of synaptic plasticity whereby repeated low-level electrical stimulation to a number of brain sites leads to permanent increases in seizure susceptibility. Stimulation that is initially subthreshold for subclinical seizure provocation comes, over time, to elicit full-blown motor seizures. Kindling can also be induced by chemical stimulation, and repeated exposures to some pesticides have been shown to induce signs of behavioral seizure, facilitate subsequent electrical kindling, and induce subclinical electrographic signs of hyperexcitability in the amygdala. Many of the symptoms of MCS suggest that CNS limbic pathways involved in anxiety are altered in individuals reporting MCS. Limbic structures are among the most susceptible to kindling-induced seizures, and persistent cognitive and emotional sequelae have been associated with temporal lobe epilepsy (TLE) in humans and kindling in animals. Thus, a number of parallels exist between kindling and MCS phenomena, leading to initial speculations that MCS may occur via a kindling-like mechanism. However, kindling requires the activation of electrographic seizure discharge and has thus been primarily examined as a model for TLE. Events leading to the initial evocation of a subclinical electrographic seizure have been much less well studied. It is perhaps these events that may serve as a more appropriate model for the enhanced chemical responsiveness characteristic of MCS. Alternatively, kindling may be useful as a tool to selectively increase sensitivity in subcomponents of the neural fear circuit to address questions relating the role of anxiety in the development and expression of MCS.

The amygdala: vigilance and emotion

Here we provide a review of the animal and human literature concerning the role of the amygdala in fear conditioning, considering its potential influence over autonomic and hormonal changes, motor behavior and attentional processes. A stimulus that predicts an aversive outcome will change neural transmission in the amygdala to produce the somatic, autonomic and endocrine signs of fear, as well as increased attention to that stimulus. It is now clear that the amygdala is also involved in learning about positively valenced stimuli as well as spatial and motor learning and this review strives to integrate this additional information. A review of available studies examining the human amygdala covers both lesion and electrical stimulation studies as well as the most recent functional neuroimaging studies. Where appropriate, we attempt to integrate basic information on normal amygdala function with our current understanding of psychiatric disorders, including pathological anxiety.

Spatial working memory over long retention intervals: dependence on sustained cholinergic activation in the septohippocampal or nucleus basalis magnocellularis-cortical pathways?

Previous direct neurochemical studies of the temporal dynamics of cholinergic activation in the septohippocampal and nucleus basalis magnocellularis-cortical pathways at various stages during repeated testing of mice with selective spatial reference or working memory protocols [Durkin and Toumane (1992), Behav. Brain Res. 50, 43-52] showed that the post-test durations of cholinergic activation in each pathway varied as a function of the type of memory tested and the level of task mastery. Since (i) the hippocampal formation is considered to constitute a critical component of a temporary memory buffer, and (ii) working memory items are not thought to be submitted to consolidation and permanent storage, we postulated that the duration of testing-induced cholinergic activation in the septohippocampal pathway may govern the maintenance of the working memory trace over the retention interval. In order to test directly this hypothesis C57 B1/6 mice were extensively trained (one trial/day, 25-30 days) on an identical selective working memory task to attain high levels of retention (> 80% correct), but using either 5 min (Group 1), or 60 min (Group 2) retention intervals. At various times (30 s-75 min) following the initial acquisition phase of the test, cholinergic activity in the hippocampus and frontal cortex was quantified using measures of high-affinity choline uptake. Whereas cholinergic activation was observed in both pathways at 30 s post-acquisition and throughout the 5 min retention interval in Group 1, the situation in Group 2 is different, activation of the septohippocampal pathway being maintained for only 15 min, while activation in the nucleus basalis magnocellularis-cortical pathway is maintained for the totality of the 1 h retention interval. The nucleus basalis magnocellularis-cortical cholinergic pathway, in addition to its role in long-term reference memory storage processes may, thus, via an intervention in the temporal encoding of information, also subsume a complementary intermediate-term buffer storage role in working memory situations requiring retention intervals in excess of 15 min in mice. This secondary, "backup", function of the nucleus basalis magnocellularis-cortical pathway would thus liberate the septohippocampal complex from its primary active role in the temporary maintenance and/or accessibility of the working memory trace in these particular cases requiring long retention intervals.

Noradrenergic and cholinergic interactions in the amygdala and the modulation of memory storage

Numerous studies have reported that, in rats, memory can be affected by manipulations of the amygdala noradrenergic system. Typically, low doses of norepinephrine facilitate while higher doses impair memory storage. Muscarinic cholinergic agonists facilitate, while antagonists impair memory storage. Recent evidence from studies using systemic injection of drugs, indicates that these two systems interact in modulating memory storage. The experiments reported here examined interactions between the amygdala noradrenergic and muscarinic cholinergic systems. The results indicate that activation of muscarinic cholinergic mechanisms in the amygdala enhances retention, and that such activation mediates the facilitatory effects of systemically administered oxotremorine. beta-Noradrenergic agonists appear to exert their effects in the amygdala by activating the release of acetylcholine.

Limbic seizures, but not kindling, reversibly impair place learning in the Morris water maze

We investigated the effects of kindling and kindled seizures in different limbic structures on place and cue learning in the Morris water maze. The triggering of seizures by stimulation of the perforant path, septum, or amygdala prior to daily training impaired place learning, but had little effect on visible platform training or swim speed. Seizures triggered by stimulation of the medial perforant path after daily training also impaired place learning. Conversely, place learning proceeded normally in rats tested 24 h after kindling triggered by stimulation of the perforant path, septum, or amygdala, indicating that kindling per se does not affect place learning. Each group was able to learn the location of a reversed platform when pretraining seizures were discontinued; and perforant path and septal kindled rats, but not amygdaloid kindled rats, were impaired at learning the location of a reversed platform when seizures were triggered before training. The results confirm previous reports that limbic seizures produce amnesia, but they contradict the finding that hippocampal kindling impairs learning on tasks sensitive to hippocampal lesions.

Effects of L-threo- and erythro-3,4-dihydroxyphenylserine on learning performance and concentrations of brain noradrenaline and its metabolites in rats

Effects of L-threo and L-erythro-3,4-dihydroxyphenylserine [DOPS, precursor amino acids for noradrenaline (NA)] on the learning performance in a maze paradigm designed to model on the water maze paradigm using a multicomputerized behavioral analysis system were studied. A marked facilitation of learning performance was observed in rats after an intraventricular injection of 5 micrograms L-threo-DOPS (the s-NA precursor), and this effect was inhibited by a simultaneous administration of 1 or 2 micrograms propranolol (a beta-adrenergic antagonist). As concentrations of brain NA, 3-methoxy-4-hydroxyphenylglycol, and normethanephrine were increased by the injection of 5 micrograms L-threo-DOPS, the effect seemed to be derived from activation of beta-adrenoceptors in the CNS by the formed s-NA. On the other hand, an intraventricular injection of 5 micrograms L-erythro-DOPS (the r-NA precursor) attenuated the learning performance, and this effect was probably caused by the formed r-NA from L-erythro-DOPS.

Increase in excitability of cortical nonspecific structures in the presence of a decreased level of influences from the brainstem reticular formation

The results of the measurement of the threshold of electrical stimulation of the cortex sufficient to elicit the EEG activation reaction in the norm and under conditions of the reduction of influences from the brainstem of the reticular formation are presented. A decrease in the thresholds was observed in those conditions which are regarded as evidence of an increase in the excitability of the nonspecific cortical formations. The increase was uneven: it was maximal in the sensorimotor and minimal in the visual cortex. It is hypothesized that the data obtained in brain preparations with a decreased level of brainstem reticular influences may be applied to the understanding of certain phenomena of neurotic pathology in its neurophysiological aspect.

Functional dissociation of acute and persistent cognitive deficits accompanying amygdala-kindled seizures

The effects of amygdala-kindled seizures on cognitive function were examined using long-delay flavor-aversion and passive-avoidance conditioning paradigms in rats. Experiments were conducted to compare the functional consequences of unilateral and bilateral kindled seizures (transient deficits) with those due to a kindling history only (persistent deficits). Animals with a history of unilateral or bilateral kindling demonstrated flavor-aversion conditioning that varied inversely with the delay separating saccharin (CS) and lithium (US). Unilateral stimulation during the CS-US interval produced an attenuation of flavor-aversion conditioning that was independent of delay value; bilateral stimulation eliminated conditioning all together. The effects of kindling and kindled seizures on passive-avoidance conditioning were functionally identical. Animals with a history of unilateral kindling demonstrated strong evidence of conditioning with no effect of posttraining seizures. In contrast, animals with a history of bilateral kindling were impaired in a passive-avoidance task. The impairment was evident in the presence or absence of seizure induction during training. Electrographic and behavioral indices of epileptiform activity produced by unilateral and bilateral stimulation failed to reveal any differences in seizure duration or severity. The results support the conclusion that cognitive disruption by amygdala-kindled seizures is task-dependent, does not show a temporal dependence, and cannot be explained on the basis of electrographic or behavioral measures of seizure severity alone.

Learning disabilities in epilepsy: neurophysiological aspects

Subclinical generalized spike-wave discharges are often accompanied by transitory cognitive impairment, demonstrable by psychological testing during EEG recording. Transitory cognitive impairment is demonstrated most readily by difficult tasks and during generalized regular spike-wave bursts lasting for more than 3 s, but can also be found during briefer and even focal discharges. That this is not simply a consequence of global inattention is shown by the fact that focal discharges exhibit some specificity: left-sided focal spiking is more likely to produce errors on verbal tasks, for instance, whereas right-sided discharges are more often accompanied by impairment in handling nonverbal material. Both learning difficulties in general and specific abnormal patterns of cognitive functioning are well documented in children with epilepsy and are most pronounced in those with frequent interictal discharges. However, there is now evidence that intermittent cognitive impairment due to the discharges themselves contributes significantly to such neurophysiological abnormalities. The significance of transitory cognitive impairment accompanying subclinical EEG discharges for everyday functioning is uncertain, but there is experimental evidence that subclinical discharges may be accompanied by disruption of educational skills in children or by impairment of driving performance in motorists. In some individuals, suppression of discharges by antiepileptic drugs has demonstrably improved psychological function, but further work is required to determine the indications for such treatment.

Animal models of anxiety based on classical conditioning: the conditioned emotional response (CER) and the fear-potentiated startle effect

Stimuli consistently paired with shock become capable of suppressing ongoing operant or consummatory behavior (the conditioned emotional response--CER) or elevating the amplitude of the startle reflex (fear-potentiated startle). These changes are used to infer a central state of fear which involves the central nucleus of the amygdala and its efferent projections to the brainstem. The present paper reviews how psychoactive drugs affect these measures. Both the CER and fear-potentiated startle are reduced by benzodiazepines, barbiturates and opiates. Advantages and disadvantages of these animal tests of anxiety are discussed.

Behavioral dissociation of anterodorsal and posteroventral hippocampus by subseizure stimulation in mice

BALB/c mice were bilaterally implanted with bipolar electrodes either in anterodorsal (ADH) or posteroventral hippocampus (PVH) in order to compare the effects of postsession electrical stimulation on memory processes. For each experiment, 30 s after the end of the first session, the animals were stimulated during 80 s. For both hippocampal regions, the stimulation intensity was half of the afterdischarge threshold value. Control groups were naive, ADH and PVH implanted non-stimulated animals. Different appetitive and aversive tasks were used. Subseizure stimulation never created a deficit. Depending on the region of the hippocampus stimulated and on the learning task, a retention enhancement was eventually observed. These data are in agreement with the involvement of hippocampus in initial stages of memory consolidation. Further, the subseizure stimulation permitted a functional dissociation between the two hippocampal regions. Both regions seemed involved in the integration of information, but the anterodorsal part would be rather related to behavioral inhibition, while the posteroventral part would have the capacity to induce an arousal state allowing behavioral flexibility.

Separable roles of hippocampal granule cells in forgetting and pyramidal cells in remembering spatial information

To investigate the roles individual hippocampal cell groups play in processing of spatial information for memory, we administered low-intensity electrical stimulation to the granule cells, CA3 and CA1 pyramidal cells of the dorsal hippocampus at selected times before and after acquisition of the solution to a radial maze win-stay task. Stimulation of any of the 3 cells populations yielded a variable duration anterograde disruption of memory performance, while stimulation of dentate gyrus granule cells alone produced a declarative memory-specific retrograde amnesia. The amnestic effect of granule cell stimulation was not associated with after discharges in the hippocampus and was prevented by systemic administration of the opiate antagonist naloxone. Our results support the view that this electrical stimulus does not disrupt, but rather, activates the normal function of the granule cell system, resulting in erasure of information held in declarative memory. In contrast, similar activation of the pyramidal cell system does not yield retrograde amnesia, suggesting a normal role for these cells in promoting memory for spatial information.

Modulating effects of posttraining epinephrine on memory: involvement of the amygdala noradrenergic system

These experiments examined the effects, on retention, of posttraining intra-amygdala administration of norepinephrine (NE), and propranolol. Rats were trained on a one-trial step-through inhibitory avoidance task and tested for retention 24 h later. Injections were administered bilaterally (1.0 microliter/injection) through chronically-implanted cannulae. Low doses of NE (0.1 or 0.3 microgram) administered shortly after training enhanced retention while higher doses (1.0 or 5.0 micrograms) were ineffective. Retention was not affected by NE administered 3 h after training. The effect of intra-amygdala NE on retention is blocked by simultaneous administration of propranolol (0.2 microgram). This finding suggests that the memory-enhancing effect of NE may be mediated by beta-receptors. Posttraining intra-amygdala NE also attenuated the retention deficit produced by adrenal demedullation. Further, intra-amygdala injections of propranolol (0.2 microgram) blocked the enhancing effect, on retention, of posttraining s.c. injections of epinephrine. These findings suggest that activation of noradrenergic receptors in the amygdala may be involved in memory processing and may play a role in the memory-modulating effect of peripheral epinephrine.

Recall deficits produced by afterdischarges in the human hippocampal formation and amygdala

The human amygdala, hippocampus and parahippocampal gyrus were stimulated in 3 epileptic patients with low-level electrical current during a paired associates learning task. Delayed recall of the response-words was severely impaired if, and only if, a unilateral afterdischarge was evoked. Stimulation at the same strength produced no deficit in immediate recall, nor in mental arithmetic, even when auras and afterdischarges were evoked. These data support the association of amnesia with medial temporal lobe paroxysms during complex partial seizures.

Hippocampal stimulation disrupts spatial working memory even 8 h after acquisition

The present experiment used hippocampal stimulation to determine the temporal gradient of consolidation of spatial working memory. Rats were trained to perform a spatial working memory task on a radial maze with 12 arms. Each rat went to the ends of 6 arms to obtain a food reward. After 8 h, the rat chose among all the arms to find the ones not previously chosen (and consequently still having food). During some test sessions, the hippocampus was stimulated electrically either at a current level just high enough to produce an electrophysiological seizure, or at a current level below this seizure threshold. Stimulation occurred at one of five intervals (0 to 8 h) following the completion of the first six choices. During other test sessions, the hippocampus was not stimulated. After seizure stimulation, the number of retroactive errors (returning to arms chosen prior to stimulation) increased at all delay intervals; the number of proactive errors (returning to arms chosen after stimulation) increased only with the delay of 8 h. Subthreshold stimulation had no influence on either type of error. These results indicate that normal hippocampal function is required for the maintenance of spatial information in working memory, and that the time course of consolidation of this information is significantly greater than that seen in other types of memory, or consolidation may not take place at all.

Brain catecholamines and memory modulation: effects of footshock, amygdala implantation, and stimulation

The results of previous studies indicate that the extent of a transient decline in brain norepinephrine (NE) levels shortly after training and administration of any of several memory modulating treatments is correlated with later retention performance. The present experiment assessed such changes after one-trial inhibitory (passive) avoidance training and, in addition, measured concentration changes in 3-methoxy-4-hydroxyphenylglycol (MHPG), the major metabolite of brain NE, as well as dopamine (DA) and epinephrine (EPI) levels. The results indicate that the decreases in brain NE after footshock are accompanied by an increase in MHPG, thus providing additional evidence that brain NE is released after training. DA levels were unchanged after training; brainstem EPI levels increased after the training footshock, but forebrain EPI levels were unchanged. A second experiment examined brain catecholamine levels in animals which received post-training electrical stimulation of the amygdala. The findings of this experiment indicate that the amygdala damage which accompanies electrode implantation apparently results in a chronic change in whole brain NE levels and metabolism. After amygdala, NE concentrations in both brainstem and forebrain samples were reduced by 20% and MHPG was increased by 22-34%. Furthermore, NE levels were not responsive to training in implanted animals. Thus, brain NE levels after training were not predictive of retention performance in amygdala-implanted or -stimulated animals. However, the significance of such findings for understanding the possible role of central NE in memory storage is complicated by the severe modification of the dynamics of brain aminergic systems in animals bearing amygdala electrodes.

An investigation of the factors affecting development of frontal cortex self-stimulation

Intracranial self-stimulation (ICSS) of the medial prefrontal cortex (MFC) is acquired gradually, taking 4 or more days to establish. One explanation for this finding is that the stimulation becomes more rewarding with repetition. Four experiments were conducted to test this hypotheses. In Experiment 1, the MFC ICSS frequency thresholds remained constant over the first 3 weeks of testing while the rate of lever pressing response increased. In Experiment 2, it was found that acquisition of MFC ICSS was much more rapid when a motorically simpler response (nose-poking) was employed. Similarly, Experiments 3 and 4 further demonstrated that response factors such as task complexity may ultimately determine the rate of development of frontal cortex ICSS. Overall, these data suggest that independent of the rewarding effects of MFC stimulation there are other effects that initially interfere with learning of complex operant responses.

State-dependent learning following electrical stimulation of the hippocampus: intact and split-brain rats

In experiment 1, electrical stimulation of the posterior hippocampus was shown to produce state-dependent learning (SDL) for a step-out inhibitory avoidance task in rats. Stimulation sites in either the right or left hippocampus were equally effective in producing this effect. Similarly, the presence or absence of afterdischarge (AD) following the stimulation did not differentially affect performance on the task. In experiment 2, forebrain bisection ameliorated the behavioral deficits in the animals receiving stimulation before testing but not before training (N/S group), while those stimulated before training but not before testing (S/N group) remained impaired; thus, providing a demonstration of asymmetrical SDL. Variations in extent of the commissurotomy differentially affected the laterality of the afterdischarge but not the performance in the SDL task. Speculation as to the mechanisms of this SDL effect was presented.

Memory consolidation: brain region and neurotransmitter specificity

Post-trial injections of norepinephrine, but not dopamine, into the amygdala produce a long-term retention deficit (amnesia) for a 1-trial footshock experience in rats. In contrast, post-trial injections of dopamine, but not norepinephrine, into the caudate produce long-term facilitation. The data provide evidence for brain region-neurotransmitter specificity which supports a multiple component hypothesis.