Stimulation Of Amygdala Research Articles

Chemical kindling with Met-enkephalin and transfer between chemical and electrical kindling

The role played by Met-enkephalin (ME) in epileptic seizures was investigated, using 57 ME kindled rats and 10 saline injected control rats. Repeated microinjection of 10 μg ME into the right amygdala (AM) of male Wistar rats led to the development of generalized convulsions. One week after the completion of ME kindling, 1 or 2 electrical stimulations (200–400 μA, 60 Hz, 1 sec) of the right AM of ME kindled rats resulted in generalized convulsions in 5 rats. The duration of after-discharge (AD) in the first generalized convulsion induced by electrical AM stimulation in the ME kindled rats was significantly longer than that in the first generalized convulsion induced by electrical stimulation in the saline treated control rats ( P < 0.05). One week after the completion of ME kindling, naloxone (10 or 20 mg/kg, i.p.) given 10 min before the infusion of ME into the other 3 ME kindled rats attenuated both convulsive behavior and electrographical seizures. With the progress of convulsive behavior, the frequency of wet-dog shakes (WDS) tended to decrease and was significantly lower after ME injection in the first stage 5 seizures than after the first ME injection ( P < 0.01). These results strongly suggest that ME has a potent epileptogenic effect on the rat brain which is caused by the opioid receptors. There are some differences between chemical kindling with ME and electrical kindling as indicated by the development of the AD duration and the WDS frequency.

Suppression of amygdaloid kindled convulsion following unilateral injection of 2-amino-7-phosphonoheptanoic acid (2-APH) into the substantia innominata of rats

The comparative effect of intracerebral injection of 2-APH, a selective antagonist for N-methyl- d-aspartate (NMDA) receptors, into the substantia innominata (SI) and the amygdala (AM) of AM-kindled rats was examined. The intra-SI injection (ipsilateral to the kindled AM) induced a transient incoordination followed by immobility with loss of the rightening reflex, beginning at about 5 min following the injection and lasting for about 3 h. When the animals were stimulated at the previously established generalized seizure triggering threshold (GST) 45 min after the injection, the kindled seizure regressed to earlier stages although the afterdischarge (AD) duration remained unchanged. At 24 h, kindled seizure was readily activated at the GST. When 2-APH was injected into the kindled AM, no behavioural change occurred but AM stimulation at the GST failed to produce AD 45 min after the injection. Kindled seizure could be elicited, however, when the stimulus intensity was increased. This elevation of the GST lasted for 1–18 days. The findings suggest that NMDA receptors in the AM and SI play a differential role in AM seizure initiation and propagation, respectively. They also provide further support to the role presumed to be played by the SI in transforming the limbic seizure into motor seizure.

Hippocampal plasticity in the kindling model of epilepsy in rats

Histochemical and autoradiographic techniques were used to study mossy fiber sprouting which occurs in the hippocampus of kindled rats. In rats kindled by daily stimulation of amygdala or entorhinal cortex, the mossy fibers sprout to innervate the supragranular zone of fascia dentata; this synaptic reorganization was associated with a significant increase in the density of high-affinity kainic acid binding sites. Amygdala but not entorhinal kindling also induces sprouting of mossy fibers in the stratum infrapyramidale of CA3.

Restriction of enhanced [2- 14C]deoxyglucose utilization to rhinencephalic structures in immature amygdala-kindled rats

Sixteen-day-old albino rat pups were kindled to varying degrees of seizure severity with amygdala stimulations spaced 15 to 20 min apart. Subsequently, each rat pup was injected (ip) with 10 μCi of [2- 14C]-deoxyglucose, and received several additional kindled seizures at regular intervals throughout the following 80 min, at which time it was killed and processed for deoxyglucose autoradiography. Increased seizure severity was associated with correspondingly increased deoxyglucose utilization in many rhinencephalic limbic structures. However, unlike adults, rat pups did not show discernibly increased neocortical, thalamic, or substantia nigra utilization. We postulate that the apparent confinement of seizure activity to limbic structures in pups is related to their relative lack of postictal seizure refractoriness, as well as to other indices of increased seizure susceptibility in immature animals.

Low doses of accumbens dopamine modulate amygdala suppression of spontaneous exploratory activity in rats

The effect of pharmacological stimulation of the amygdala on spontaneous locomotor activity in the rat and its modulation by accumbens dopamine were investigated. Bilateral injection of N- methyl- d-aspartic acid into the basolateral nucleus of the amygdala produced a dose-dependent suppression of spontaneous locomotor activity in the rat. The suppression of locomotor activity was reversed completely by injection of l-glutamic acid diethyl ester, a putative glutamatergic antagonist, into the nucleus accumbens but partially enhanced by injection of nipecotic acid, a GABA uptake inhibitor, into the ventral pallidum. Furthermore, low doses of dopamine injected into the accumbens, which by itself did not elicit hyperactivity in the animals, completely reversed the suppression of locomotor activity following amygdala stimulation. These results show that the projection from the amygdala to nucleus accumbens has an inhibitory effect on spontaneous locomotor activity in rats and that dopamine in the accumbens attenuated this suppression effect possibly due to its neuromodulatory action as demonstrated in previous electrophysiological experiments.

Suprarhinal cortex response to electrical stimulation of the lateral amygdala nucleus in the rat.

Electrical stimulation of the lateral amygdala nucleus was found to evoke field potentials and influence unitary activity in the suprarhinal cortex of anesthetized rats. Laminar distributions of the field responses consisted of positive waves in superficial layers, that reversed to electronegatives from a depth of 0.4-0.5 mm. This response was followed by a shallow electropositive wave deeper than 0.7-0.8 mm. Extracellularly recorded units were studied in the posterior agranular insular area of the suprarhinal cortex. The data revealed that stimulation of the lateral amygdala produced a train of small amplitude spikes in association with a negative slow potential. Furthermore, such stimulation invariably elicited an inhibition of the spontaneous firing of large amplitude spikes, in association with a positive slow potential. The onset of this inhibitory response always occurred at longer latency than the excitatory one. The small amplitude spikes may well represent the firing of inhibitory interneurons after lateral amygdala stimulation. The study suggests that a feed-forward system of inhibition appears to be present in the connection between lateral amygdala and posterior agranular insular area of the suprarhinal cortex.

Anxiolytic properties of amygdaloid kindling unrelated to benzodiazepine receptors.

We investigated the possibility that repeated electrical stimulation of the basolateral amygdala (kindling) would increase punished behavior, a pre-clinical indicator of anti-anxiety activity. Male Sprague-Dawley rats, prepared with electrodes in the left basolateral nucleus and frontal cortex, were trained to bar-press under a multiple schedule in which either every 30th response produced food (no punishment) or every 10th response produce both food and a brief electric shock (punishment). Rates of punished responding were less than 10% of non-punished values. Brief electrical stimulation of the amygdala occurred biweekly, and the current levels were incremented from subconvulsive values (50 microA) to suprathreshold levels (150 microA). Electrical stimulation increased punished responding without concomitant changes in nonpunished behavior; increases were most pronounced in fully-kindled rats. Fully-kindled animals also demonstrated the largest primary afterdischarge. Electrical stimulation of the frontal cortex induced similar seizure and convulsive patterns but did not increase punished responding. Anxiolytic activity of amygdaloid stimulation was not prevented by the selective benzodiazepine antagonist Ro 15-1788, which antagonized similar behavioral effects of chlordiazepoxide. Thus, kindling of seizures in the amygdala results in behaviorally and neuroanatomically specific antianxiety actions which do not depend upon receptor sites blocked by Ro 15-1788 (benzodiazepine receptors).

The NMDA-receptor antagonist, MK-801, suppresses limbic kindling and kindled seizures

Excitatory amino acid receptors in the mammalian central nervous system (CNS) are divided into 3 receptor subtypes: kainate, quisqualate, and N-methyl- d-aspartate (NMDA). MK-801 is a selective, non-competitive antagonist of excitatory amino acid transmitters at the NMDA receptor site. The role of excitatory amino acid neurotransmission in electrical kindling was examined in animals stimulated daily in the amygdala following i.p. administration of low dosages of MK-801 (0.1 and 0.5 mg/kg). A second experiment evaluated the anticonvulsant properties of MK-801 in rats kindled in the hippocampus and amygdala, and contrasted its efficacy with the antiepileptic agents, diazepam, phenobarbital and phenytoin, and the dissociative anesthetics phencyclidine and ketamine. MK-801 (0.5 mg/kg) retarded the development of amygdala kindling and reduced mean AD duration over the first 10 stimulation sessions. The low dosage reduced total AD accrued during each kindling stage but failed to alter kindling rate. MK-801 blocked motor seizures induced by stimulation of hippocampal or amygdala kindled foci, but was more effective in reducing seizure severity and AD duration resulting from stimulation of the hippocampal focus. All other drugs tested, with the exception of phenytoin, protected against amygdaloid kindled seizures. It was concluded that excitatory amino acid transmission contributes in an important, but non-critical way to amygdala kindling.

Functional [ 14C]2-deoxyglucose mapping of progressive states of status epilepticus induced by amygdala stimulation in rat

Electrical stimulation of rat amygdala induced self-sustained steady-state seizures (status epilepticus (SE)) within 60 min. These SE states varied in behavioral severity from mere alteration of motility to frank clonic convulsions. Four distinct behavioral states were observed: immobility, exploration, mastication and clonus. These SE states were associated with [ 14C]2-deoxyglucose (2-DG) autoradiography anatomic patterns that were correspondingly more extensive and complex. Four distinct 2-DG activation patterns were observed: a restricted pattern involving several discrete limbic nuclei, including amygdala; more extensive patterns involving numerous limbic areas, first unilaterally, then bilaterally; finally the most extensive pattern involving widespread areas of forebrain. These data imply a systematic progression of seizure activity: originating in the amygdala, then spreading to some direct amygdala projection areas, and from there to a restricted network of interconnected ipsilateral limbic nuclei. This restricted network then recruits most of the remaining limbic structures, first ipsilaterally, then contralaterally. Finally, most of the basal ganglia, thalamus and neocortex are recruited.

Intraventricular injection of antiserum to nerve growth factor delays the development of amygdaloid kindling

To investigate the possibility that nerve growth factor (NGF) may play some role in the development of kindling, rabbit anti-NGF serum was injected into the third ventricle on the first 3 days of daily electrical stimulation of the basolateral amygdala. The number of stimulations required to reach full amygdaloid kindling increased significantly in the rat injected with anti-NGF serum compared to that in the rat injected with normal rabbit serum. It was confirmed that anti-NGF serum did not act as an anticonvulsant. The results demonstrate that NGF may be important for the long-lasting neuronal changes induced by daily electrical stimulation of the amygdala.

Distribution and projection of single units in the cat preoptic region responding to stimulation of the medial amygdala

Neuronal responses were recorded extracellularly in the preoptic region of 11 male castrate cats following stimulation in the 'medial' group of amygdalar nuclei (medial, basomedial, basolateral). The latency of responses varied with the site of stimulation and could be explained if connections to the preoptic region were made through both the stria terminalis and the ventral amygdalofugal pathway. Projection of 13% of orthodromically excited preoptic units to the medial basal hypothalamus was established by collision tests following stimulation in these regions. The amygdalar excitation was probably exerted through the amygdalofugal pathway. In 5/6 cats the amygdalar stimulation was successfully used to raise plasma LH levels. In one cat no preoptic units projecting to the medial basal hypothalamus were found and in the cat which failed to increase plasma LH in response to amygdala stimulation, 5 such units were found. We conclude the pathway from the amygdala through the preoptic region to the hypothalamus may not be the way in which amygdalar stimulation affects LH release in the cat. Projection of 10% of orthodromically excited preoptic units in the medial forebrain bundle was established by collision tests in 3 cats. These identified units were driven from amygdalar sites thought to project to the preoptic region by ventral amygdalofugal pathways.

Neurogenic stimuli alter preoptic area and amygdala unit activity: Central effects of olfactory projections on paraventricular nucleus units

Unit responses were recorded in the preoptic area and amygdala of conscious male rats during exposure to stressful neurogenic stimuli. Olfactory stimulation elicited increases in preoptic area activity on all occasions and also increased activity in the intercalating, medial, and basomedial nuclei of the amygdala, but not in other regions. Acoustic stimulation had less specific effects, even inhibiting unit activity in the central amygdala. A separate series of experiments using urethane-anesthetized rats was carried out to examine the effects of electrical stimulation of the medial amygdala and olfactory tubercle on single-unit activity within the hypothalamic paraventricular nucleus. Inhibition was the predominant response following olfactory tubercle stimulation while excitatory responses predominated following stimulation of the medial amygdala. This was the case particularly for those paraventricular nucleus units identified as projecting to the median eminence ( P < 0.005 vs unidentified cells). The results obtained may be related to the neural regulation of adrenocortical activity as well as higher central nervous activity and have been discussed within these contexts.

Paroxysmal “Feeling of Somebody Being Nearby”

Several days after a right temporal lobectomy for an astrocytoma, a patient experienced a paroxysmal feeling that somebody was nearby. This spontaneous phenomenon has only rarely been reported in the epilepsy literature but has been reported to be evoked by electrical stimulation of the right amygdala. Despite the rarity in the literature, such unusual experiential phenomena probably are seen more frequently in clinical practice.

Unitary activity in the suprarhinal cortex of the rat and its modulation after lateral amygdala stimulation

Unitary responses in the suprarhinal cortex were studied in anesthetized rats. Lateral amygdala stimulation gave rise to two types of cortical responses: (i) an excitatory response (59%) characterized either by a single spike discharge, or by a burst of two to four spikes, or (ii) an inhibitory response (41%) affecting cortical neurons which exhibited spontaneous discharge. The duration of inhibition appeared to be related to the intensity of stimulation and the spontaneous discharge frequency.

Enhancement of acoustic startle by electrical stimulation of the amygdala.

The present study demonstrated that electrical stimulation of the amygdala enhanced the acoustic startle response. A 25-ms train of 0.1-ms pulses initiated 5 ms before the onset of a 20-ms noise burst significantly increased startle at currents from 40 to 400 microA. Electrode placements just medial to the amygdala (in the pathway connecting the amygdala to the brain stem) increased startle with the lowest currents. Startle was also increased in all animals with stimulation in the central, medial, and intercalated nuclei of the amygdala. Stimulation in areas surrounding the amygdaloid complex was ineffective. In a second experiment, paired pulses with interpulse intervals between 0.1 and 20.0 ms delivered to the amygdala demonstrated that the stimulated axons had a distribution of refractory periods between 0.6 and 1.0 ms. This suggests that the population of neurons which subserves the enhancement of acoustic startle is fairly homogeneous and has small, myelinated axons.

Temporal characteristics of enhancement of startle by stimulation of the amygdala

A previous study demonstrated that electrical stimulation of the amygdala facilitated the acoustic startle reflex. The purpose of the present study was to determine the temporal relationship between activation of the amygdala and its enhancement of the acoustic startle reflex. A single 0.1 msec pulse was delivered to the central nucleus of the amygdala at various times before or alter the onset of 20 msec noise burst or a 0.1 msec click. Startle was enhanced when the stimulation occurred within 5 msec of the startle stimulus onset. Electromyographic recordings from the neck muscles demonstrated that the short 6 msec latency startle response was facilitated even when amygdala stimulation was presented 1.25 msec after the onset of the startle stimulus. This indicates that the time for the effects of amygdala stimulation to reach the brain stem startle circuit is less than 5 msec, suggesting a very direct pathway from the amygdala to the startle circuit. The similarity of this effect to fear-potentiated startle is also discussed.

Effect of clonidine on amygdala kindling in normal and 6-hydroxydopamine-pretreated rats

The role of the α 2 adrenergic receptor in the development and propagation of amygdala kindled seizures was determined. Male Wistar rats, depleted of norepinephrine with the neurotoxin 6-hydroxydopamine and vehicle controls, received an injection of the α 2 agonist, clonidine (0.001, 0.01, or 0.1 mg/kg, i.p.), or saline, 30 min prior to each amygdala stimulation (every 3 to 4 days). The largest dose of clonidine had a significant retarding effect on the development of kindling in both the vehicle- and 6-hydroxydopamine-treated groups. The majority of this effect was observed as a protracted number of trials with unilateral afterdischarge. When the discharge became bilateral, generalized seizures were soon apparent. There was no effect of clonidine on the amygdala afterdischarge threshold in both groups, or on the subsequent generalized motor seizure and associated afterdischarge durations in the vehicle groups. The largest dose of clonidine, however, reduced the severity of the convulsive response in the 6-hydroxydopamine-treated rats from violent stage 7 or 8 seizures to more moderate stage 5 responses. The significance of these data, and the involvement of the postsynaptic α 2 receptors in the genesis of amygdala kindled seizures, are discussed.

Cardiovascular responses to chemical and electrical stimulation of amygdala in rats

Electrical stimulation of the amygdala has been shown to produce changes in cardiovascular variables. To locate neuronal cell bodies responsible for these changes, responses of arterial pressure (AP) and heart rate (HR) to DL-homocysteate (DLH, 0.15 M, 50-100 nl) microinjected into sites in three amygdaloid nuclei were compared with responses to electrical (90-150 microA) stimulation of the same sites in 35 artificially ventilated, paralyzed, urethan-anesthetized rats. Electrical stimulation resulted in depressor responses in most sites (89%). Changes in AP were accompanied by variable changes in HR. Chemical stimulation produced significantly fewer (25%) depressor responses. Similar results were obtained with injections of 1.0 M DLH. To eliminate the influence of the anesthetic on these responses, AP was recorded in nine conscious rats while stimulating the amygdala. Changes in behavior and AP in these animals could be obtained only by electrical stimulation. These results may be interpreted to indicate either that cell bodies responsible for changes in cardiovascular variables during electrical stimulation are not located in the amygdala or that chemical and electrical stimulation affect different neuronal elements in circuits located in the same anatomic site.

Anticonvulsant effect of intracortical, chronic infusion of GABA in kindled rats: Focal seizures upon withdrawal

The behavioral and electrographic effects of chronic (7 days), localized infusion of GABA (100 μg/gml) into the somatomotor cortex of fully amygdala-kindled rats is reported. The animals were stimulated once daily until a stage 5 (generalized clonic seizure) was obtained for five consecutive days. After determination of a stable seizure triggering threshold, the rats were implanted with osmotic minipumps (1 μl/h for 7 days) connected to previously implanted bilateral cannulae. Amygdala stimulation was continued for 14 successive days. GABA infusion reduced the motor seizure without significantly modifying the limbic afterdischarge. This effect lasted until termination of drug application, with recovery of stage 5 convulsions on the following 3 to 5 days. No effects were observed in saline-infused animals or in rats with unilateral GABA treatment. Upon cessation of GABA treatment (removal of the osmotic devices by day 7 postimplantation), spontaneous epileptic discharges localized to the infusion sites appeared. In some animals, the abnormal activity was accompanied by behavioral signs of myoclonus. This cortical hyperexcitability lasted 2 to 24 h, with complete recovery afterward. These data indicate that two types of focal epilepsy may coexist independently in the same animal and provide confirmation of previous observations in the monkey on the existence of a “GABA-withdrawal syndrome” after chronic, focal infusion of the amino acid.

Acquisition of amygdala-kindled seizures in female rats: Relationship between the effect of estradiol and intra-amygdaloid electrode location

Ovariectomized, adult female rats, with or without estradiol replacement, were kindled by daily amygdala stimulation. Kindling acquisition varied with the intra-amygdala site of stimulation. During stimulation of the medial (AME) or central (ACE) nucleus, the only effect of estradiol replacement (E), compared to non-replaced rats (nE), was to significantly decrease the number of trials with afterdischarge (AD) during early kindling (stage 0). In rats receiving stimulation of the cortical nucleus (ACO) or the baso-lateral group of nuclei (ABL), a similar effect of estradiol was extended through stage 1. In addition, nE rats with ACO or ABL electrodes required significantly more trials with AD and accumulated more than twice the sec of AD during the late stages of kindling, compared to E rats and regressed to lower stage responses between the first stage 4 and last stage 5 responses; regressed responses never occurred in E rats. Estradiol also significantly decreased the prekindling AD threshold of the AME and ACE. These results indicate that estradiol accelerates early stage kindling, likely by proconvulsive properties to increase excitability within immediate amygdala projections. During late kindling stages, estradiol may participate in reinforcing or sustaining the convulsive readiness of kindling circuits established during bilateral recruitment. The site of action for this latter effect of estradiol may reside within circuits accessed by stimulation of the ACO or ABL, and not the AME or ACE.