Activation of amygdaloid neurones in reward, eating and drinking elicited by electrical stimulation of the brain

The behavioural, cognitive, and neural corollaries of blunted cardiovascular and cortisol reactions to acute psychological stress

Recent research shows that blunted cardiovascular and cortisol reactions to acute psychological stress are associated with adverse behavioural and health outcomes: depression, obesity, bulimia, and addictions. These outcomes may reflect suboptimal functioning of the brain's fronto-limbic systems that are needed to regulate motivated behaviour in the face of challenge. In support of this, brain imaging data demonstrate fronto-limbic hypoactivation during acute stress exposure. Those demonstrating blunted reactions also show impairments of motivation, including lower cognitive ability, more rapid cognitive decline, and poorer performance on motivation-dependent tests of lung function. Persons exhibiting blunted stress reactivity display well established temperament characteristics, including neuroticism and impulsivity, characteristic of various behavioural disorders. Notably, the outcomes related to blunted stress reactivity are similar to those that define Reward Deficiency Syndrome. Accordingly, some individuals may be characterised by a broad failure in cardiovascular and cortisol responding to both stress and reward, reflecting fronto-limbic dysregulation. Finally, we proffer a model of blunted stress reactivity, its antecedents and sequelae, and identify future research priorities.

The amygdala: securing pleasure and avoiding pain

The amygdala has traditionally been associated with fear, mediating the impact of negative emotions on memory. However, this view does not fully encapsulate the function of the amygdala, nor the impact that processing in this structure has on the motivational limbic corticostriatal circuitry of which it is an important structure. Here we discuss the interactions between different amygdala nuclei with cortical and striatal regions involved in motivation; interconnections and parallel circuitries that have become increasingly understood in recent years. We review the evidence that the amygdala stores memories that allow initially motivationally neutral stimuli to become associated through pavlovian conditioning with motivationally relevant outcomes which, importantly, can be either appetitive (e.g. food) or aversive (e.g. electric shock). We also consider how different psychological processes supported by the amygdala such as conditioned reinforcement and punishment, conditioned motivation and suppression, and conditioned approach and avoidance behavior, are not only psychologically but also neurobiologically dissociable, being mediated by distinct yet overlapping neural circuits within the limbic corticostriatal circuitry. Clearly the role of the amygdala goes beyond encoding aversive stimuli to also encode the appetitive, requiring an appreciation of the amygdala's mediation of both appetitive and fearful behavior through diverse psychological processes.

Do low levels of stress reactivity signal poor states of health?

Studies of cardiovascular disease risk have explored the idea that exaggerated physiological responses to stress may signal increased risk of cardiovascular disease. We describe a neurophysiological model of brain structures and peripheral structures that may contribute to exaggerated reactivity. Level I in this model includes the limbic system and its interactions with the prefrontal cortex that determine stress appraisals and coping responses. Level II addresses the hypothalamus and brainstem that contribute outputs to the body and which also includes brainstem nuclei that feed back to Level I to modulate its functioning. Level III includes the peripheral tissues themselves. We then suggest that stress reactivity ranging from very low to very high has a normative midrange of intensity and present evidence that negative health outcomes may be associated with both exaggerated and diminished stress reactivity since both tendencies imply a loss of homeostatic regulation. In particular, dysregulation at Levels I and II in our heuristic model signify altered motivational function and an attendant alteration in outflow to the periphery and poor behavioral homeostasis. In consequence, poor affective and behavioral regulation would be expected to contribute to poor health behaviors therefore additionally impairing health. In conclusion, diminished as well as exaggerated physiological reactivity should be seen as nonoptimal functioning that can contribute to poor health outcomes. This conceptualization places physical health into the context of behavioral and physiological processes that contribute to homeostasis.

Deep brain stimulation for treatment of obesity in rats

OBJECT

Given the success of deep brain stimulation (DBS) in a variety of applications (for example, Parkinson disease and essential tremor), other indications for which there is currently little effective therapy are being evaluated for clinical use of DBS. Obesity may be one such indication. Studies of the control of feeding and appetite by neurosurgical lesioning have been completed previously. This study was conducted to test the authors' hypothesis that continuous bilateral stimulatory inhibition of the rat lateral hypothalamic nucleus (LH) would lead to significant and sustained decrease in food intake and subsequent weight loss.

METHODS

Sixteen Sprague-Dawley rats were maintained on a high-fat diet. Daily food intake and weight gain were measured for 7 days, at which time the animals underwent stereotactic placement of 0.25-mm-diameter bipolar stimulating electrodes bilaterally in the LH. On postoperative Day 7, eight animals began to receive continuous stimulation of the LH. The remaining eight animals were left unstimulated as the control group. Individual animal weight, food intake, and water intake were monitored daily and continuously throughout the experiment until postoperative Day 24.

RESULTS

There was a decreased rate of weight gain after surgery in all animals, but the unstimulated group recovered and resumed a linear weight gain curve. The stimulated group, however, failed to show weight gain and remained below the mean baseline for body mass. There was a significant weight loss between the stimulated and unstimulated groups. On postoperative Day 24, compared with the day of surgery (Day 0), the unstimulated group had a mean weight gain of 13.8%, whereas the stimulated group had a 2.3% weight loss on average (p = 0.001), yielding a 16.1% weight difference between the two groups.

CONCLUSIONS

Bilateral electrical stimulatory inhibition of the LH is effective in causing significant and sustained weight loss in rats.

Behavioural and physiological effects of electrical stimulation in the nucleus accumbens: a review

Electrical stimulation (ES) in the brain is becoming a new treatment option in patients with treatment-resistant obsessive-compulsive disorder (OCD). A possible brain target might be the nucleus accumbens (NACC). This review aims to summarise the behavioural and physiological effects of ES in the NACC in humans and in animals and to discuss these findings with regard to neuroanatomical, electrophysiological and behavioural insights. The results clearly demonstrate that ES in the NACC has an effect on reward, activity, fight-or-flight, exploratory behaviour and food intake, with evidence for only moderate physiological effects. Seizures were rarely observed. Finally, the results of ES studies in patients with treatment-resistant OCD and in animal models for OCD are promising.

Effects of nucleus basolateralis amygdalae neurotoxic lesions on some spontaneous activities in the rat

Drinking, feeding, locomotion and exploratory activity of male Wistar rats were assessed after bilateral stereotaxic administration of ibotenic acid in the nucleus basolateralis amygdalae. Feeding, drinking and locomotion were measured in an activity cage, while exploratory activity was determined in a multiple Y-maze. In the 24-hour cycle, lesioned animals exhibited unvaried feeding, decreased drinking and increased locomotion. Exploration was also increased. The results show that this nucleus is not involved in quantitative feeding control, while it does exert a significant facilitatory influence on drinking. It also exerts an inhibitory influence on exploration and on locomotion.

Electrophysiological study of the response of medial prefrontal cortex neurons to stimulation of the basolateral nucleus of the amygdala in the rat

The neural connections from the basolateral nucleus of the amygdala (BLA) to the medial prefrontal cortex (MPC) in urethane-anesthetized rats were investigated. Extracellular recordings were made from 200 neurons with spontaneous firing in the MPC, and the BLA was electrically stimulated. The most frequent response to BLA stimulation was inhibition (63.5%). Excitatory responses were found in 17 units (8.5%), while 56 neurons (28%) did not change their spontaneous firing after BLA stimulation. Inhibitory responses showed a wide range of latencies, suggesting the coexistence of mono- and polysynaptic pathways. On the contrary, the excitatory responses seem to be mediated by a monosynaptic pathway. BLA projections to the MPC play a predominantly inhibitory role in the spontaneous activity of prefrontal neurons. This inhibition may modulate central motor systems and motivated behaviors.

The regulation of feeding and drinking in rats with lesions of the lateral hypothalamus made by N-methyl-D-aspartate

Rats bearing excitotoxic lesions of the lateral hypothalamus are hypodipsic and hypophagic, but responses to 24 h food or water deprivation are normal, as are responses to different taste stimuli. The most striking deficit present in lateral hypothalamic-lesioned rats is an inability to respond as controls to dehydrating, dipsogenic or glucoprivic challenges. The present experiments examined the ability of rats bearing bilateral N-methyl-D-aspartate-induced lesions of the lateral hypothalamus to recognize and respond to changes in their internal environments. All of the lesioned rats tested showed mild to moderate hypophagia and hypodipsia, and none responded properly by drinking over 1 h after i.p. injection of hypertonic saline. However, the addition of glucose to the water supply promoted an increase in drinking and a decrease in lab chow consumption to maintain a constant energy intake; the addition of salt to the diet promoted an increase in drinking and no change in eating; 24 h water deprivation induced the same amount of drinking in lateral hypothalamic-lesioned rats as in controls; and injection i.p. of water (but not physiological saline) before drinking water was returned to rats which were 24 h water deprived suppressed drinking. These data suggest that lateral hypothalamic-lesioned rats are in receipt of normal information from their peripheries, and that they can adjust their behaviour over a period of days or minutes to changes in the internal milieu. The most consistent deficit is in responding actively and rapidly to challenging stimuli; the nature of this and the mechanisms which might produce it are discussed. We suggest that the consequences of excitotoxic lesion are better explained by disruption of input to the cortex from the lateral hypothalamus rather than by interference with metabolic processes.

Failure of amygdaloid lesions to increase the threshold for self-stimulation of the lateral hypothalamus and ventral tegmental area

It has been proposed that the directly stimulated axons underlying the rewarding effect of medial forebrain bundle (MFB) stimulation originate in the forebrain and descend at least as far as the ventral tegmentum. However, little is known about the location of the somata that give rise to these axons. Among the nuclei that contribute fibers to the descending component of the MFB and project past the lateral hypothalamus (LH) and ventral tegmental area (VTA) are cell groups within the amygdaloid complex. In this study, the rewarding effectiveness of stimulating the LH and VTA was measured before and after the amygdaloid complex was damaged by electrolytic lesions. Changes in rewarding effectiveness were inferred from shifts in the frequency required to sustain a half-maximal rate of lever-pressing at each of 3 currents. Following the lesions, there was no clear evidence of substantial, sustained decreases in rewarding effectiveness at the 14 stimulation sites, although one subject ceased to self-stimulate reliably. Given that the lesions damaged the principal amygdaloid sources of descending MFB fibers, these results suggest that the amygdaloid complex is not a major source of the directly activated fibers responsible for the rewarding effect of MFB stimulation.

Hedonic reactivity to sucrose in rats: modification by pimozide

Reward summation functions (RSFs) are an important way to dissociate and quantify hedonic and motor effects of neuroleptics. Previously used only with brain stimulation reinforcement, we demonstrate they will also work using sucrose solution reinforcement. Eighteen male rats were trained to lever press on a CRF schedule for 0.01 ml sucrose solution reinforcers of varying concentration (0.025, 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.7, 2.3 M). Making the lever harder to press caused an increase in the asymptote of the function, demonstrating a motor effect. Quinine added to the solutions (0.05%) caused the function to shift right, demonstrating an hedonic effect. Rats injected with 0.2 mg/kg pimozide, a dopamine antagonist, then tested four hours later, showed a right shift in the function with no change in asymptote. Thus, at this dose pimozide affects hedonic and not motor substrates of the CNS. These data demonstrate the generalizability of the RSF method to reinforcers other than brain self-stimulation.

A response-specific conditioned aversion to rewarding hypothalamic stimulation in rats

Rats with hypothalamic electrodes were trained to self-stimulate by nose-poking and by licking at a dry spout. Separate groups of animals were then exposed to pairings of either self-stimulation by licking or by nose-poking with the subsequent administration of LiCl. The licking/LiCl pairing group exhibited a marked attenuation of licking maintained by hypothalamic stimulation on the first retention trial (24 h post LiCl). This effect was enhanced by a further licking/LiCl pairing. In a final cross-over trial with this group, nose-poking maintained by stimulation at the same site and intensity was found to be unaffected. With another group of animals nose-poking/LiCl pairing had no effect on self-stimulation, despite two conditioning trials. In a final cross-over trial with this group licking maintained by stimulation of the same site and intensity was also unaffected. These data indicate that the sensory properties of brain stimulation may be dependent on the behavioural context in which the stimulation occurs.

Limitation of access to highly palatable foods increases the norepinephrine content of many discrete hypothalamic and amygdaloidal nuclei of rat brain

The norepinephrine (NE) concentration of 14 discrete brain nuclei of rats fed with either purina rat chow or highly palatable foods, given ad libitum or in a restricted quantity isocaloric with the Purina rat chow diet, was measured by high performance liquid chromatography with electrochemical detection. The NE level was significantly increased in the n. suprachiasmaticus, n. paraventricularis, n. arcuatus and n. amygdaloideus (medialis, corticalis, centralis, basalis pars lateralis, lateralis pars posterior) of rats fed with palatable foods given in restricted amount. The increased NE level seems to be associated with the limitation of access to highly palatable food whilst the regimen composition and the palatability of foods are not implicated in this phenomenon. Some hypothalamic and amygdaloid NE-containing neurones can be affected by food-restriction stress, suggesting a role of this neuronal system in feeding activity.

Neurophysiological analysis of brain-stimulation reward in the monkey

Neuronal activity related to brain-stimulation reward and to feeding was analyzed in rhesus monkeys and squirrel monkeys as follows. First, self-stimulation of the lateral hypothalamus, orbitofrontal cortex, amygdala and nucleus accumbens was found. Second, a population of single neurones in the lateral hypothalamus was found to be trans-synaptically activated from one or several self-stimulation sites. It was also found to populations of neurones in the orbitofrontal cortex and amygdala were activated from at least some of the self-stimulation sites. Thus, in the monkey, there is evidence for an interconnected set of self-stimulation sites, stimulation in any one of which may activate neurones in the other regions. These sites include the lateral hypothalamus, amygdala, and orbitofrontal cortex. Third, in one sample of 764 neurones in the lateral hypothalamus and substantia innominata which were activated from brain-stimulation reward sites, 13.6% were also activated during feeding, by the sight and/or taste of food. The responses of the neurones with activity associated with taste occurred only while some substances (e.g. sweet substances such as glucose) were in the mouth, depended on the concentration of the substances being tasted, and were independent of mouth movements made by the monkeys. Fourth, the responses of these neurones occurred to food when the monkeys were hungry, but not when they were satiated. Fifth, self-stimulation occurred in the region of these neurones in the lateral hypothalamus and substantia innominata, and was attenuated by satiety. These results suggest that self-stimulation of some brain sites occurs because of activation of neurones in the lateral hypothalamus and substantia innominata activated by the sight and/or taste of food in the hungry animal, and that these neurones are involved in responses to food reward.

Conative regulation of cortical activity by the reticular formation, hypothalamus, and thalamus

Evolution of neural regions suggests the requirement for a common format for information units exchanged among regions. Short-term memory experiments suggest a format of six or seven items. A similar number of configurations of primary cerebral interactions is proposed, associated with arousal, sleep, approach, withdrawal, perseveration, alert scanning, and commanded by the reticular formation. A comparable number of basic states (feeding, mating, grooming, shelter-seeking, fighting, etc.) is proposed as operating within these configurations, under regulation by the hypothalamus. Conative variables from this region are transformed into patterns of regulation for local cortical populations by the thalamus. Elaboration of these configurations and states by higher structures leads to new forms of cortical activity only loosely coupled to brain stem systems.

Psychosurgery and the limbic system

The limbic system can be considered to consist of a central ‘core’ from which three well-defined circuits emerge. These are the Papez or medial circuit, the basolateral circuit and the defence reaction circuit. The functions of the primitive limbic brain are modulated by a higher order control—the frontal lobe. Emotional responses and physiological changes have been obtained by stimulation of fronto-limbic pathways and limbic circuits and these have been used for location of target sites in psychosurgery. For the relief of intractable depression and anxiety, lesions are generally made in the lower medial quadrant or posteroorbital part of the frontal lobe, where there is a concentration of fronto-limbic connections. In primary obsessional neurosis lesions at this site produce less satisfactory results, but these illnesses may be helped by lesions in the cingulate gyrus, which is part of the Papez circuit. Patients with epilepsy and aggressive outbursts may respond to temporal lobotomy, or to operations, such as amygdalotomy, where lesions are placed in the defence reaction circuit.