Sensitization of mesolimbic brain stimulation reward after electrical kindling of the amygdala

The central amygdala as an integrative hub for anxiety and alcohol use disorders

The central amygdala (CeA) plays a central role in physiologic and behavioral responses to fearful stimuli, stressful stimuli, and drug-related stimuli. The CeA receives dense inputs from cortical regions, is the major output region of the amygdala, is primarily GABAergic (inhibitory), and expresses high levels of prostress and antistress peptides. The CeA is also a constituent region of a conceptual macrostructure called the extended amygdala that is recruited during the transition to alcohol dependence. We discuss neurotransmission in the CeA as a potential integrative hub between anxiety disorders and alcohol use disorder, which are commonly co-occurring in humans. Imaging studies in humans and multidisciplinary work in animals collectively suggest that CeA structure and function are altered in individuals with anxiety disorders and alcohol use disorder, the end result of which may be disinhibition of downstream "effector" regions that regulate anxiety-related and alcohol-related behaviors.

Kindling modifies morphine, cocaine and ethanol place preference

Brailowsky and Garcia (1999) proposed the existence of a relationship between epilepsy and addiction. To prove this hypothesis, pentylenetetrazol kindled rats were tested in the conditioned place preference (CPP) paradigm for their reaction to various addictive drugs with different modes of action (morphine, cocaine and ethanol). In separate experiments, locomotor activity and body temperature after application of the same drugs were tested in kindled and non-kindled rats. In the CPP experiment there were significant differences between both groups of rats. Non-kindled animals showed place preference to morphine (5.0 mg/kg) or cocaine (20.0 mg/kg). This reaction was abolished in the kindled rats. Moreover, control rats demonstrated aversion to 2.0 g/kg ethanol. However, ethanol aversion was not detectable in kindled rats. Moreover, there was no difference between non-kindled and kindled rats in locomotor activity and body temperature after morphine (1.0 and 5.0 mg/kg), cocaine (10.0 and 20.0 mg/kg), or ethanol (0.5 and 2.0 g/kg) application. This suggests alterations in reward systems as a consequence of kindling. It is hypothesised that GABAergic neurones in the ventral tegmental area might play a major role in the alterations found.

Extinction deficit and fear reinstatement after electrical stimulation of the amygdala: implications for kindling-associated fear and anxiety

Generalized seizures produced by electrical kindling of the amygdala in laboratory rats are a widely used animal model of temporal lobe epilepsy. In addition to seizure evolution amygdala kindling enhances emotionality. The relative roles of electrical stimulation and seizure induction in fear responding are unclear. Here we investigate this issue using extinction and reinstatement of fear-potentiated startle. After classical conditioning (light+footshock pairings) laboratory rats were fear extinguished with each light presentation followed by nonepileptogenic amygdala stimulation. In contrast to the normal extinction learning of control subjects, amygdala stimulated animals exhibited conditioned fear after 120 presentations of the nonreinforced conditioned stimulus (CS). In a second experiment electrical stimulation of the amygdala restored extinguished fear responding and the fear reinstatement was specific to extinction context. The reinstatement effect did not involve sensitized fear to the CS produced by amygdala stimulation. The possibility that electrical activation of the amygdala produces unconditioned fear was considered. Animals uniformly failed to demonstrate fear-potentiated startle using electrical stimulation of the amygdala as the unconditioned stimulus. This was the case with a subthreshold afterdischarge stimulus and a stimulation schedule that produced kindled seizures. The extinction deficit and fear reinstatement results were interpreted to suggest that amygdala stimulation activates acquired excitatory stimulus-affect neural connections formed during Pavlovian fear conditioning. Our data supports a model in which excitation of an amygdala-based memory-retrieval system reinforces the expression of learned fear behaviors.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

Cocaine preexposure sensitizes conditioned fear in a potentiated acoustic startle paradigm

The consequences of chronic cocaine administration on fear-potentiated startle were evaluated in two experiments. Cocaine treatment (40 mg/kg) for 7 days prior to fear acquisition (light + shock pairings) had an attenuating influence on the ability of the conditioned stimulus (CS) to increase acoustic startle. When cocaine was administered in the context of the CS, following fear conditioning, a marked enhancement of potentiated startle was observed. In contrast, an extinction of the fear response was seen in saline and procaine animals repeatedly exposed to the nonreinforced CS. The results from control subjects injected with cocaine either in the shock chambers (contextual cues) or in their home cage environment, suggest that the systemic effects of this stimulant served to intensify the fear-eliciting properties acquired by the CS during fear conditioning. These findings demonstrate a cocaine sensitization of conditioned fear, and were related to the emotional and psychological disturbances associated with long-term cocaine use.

Electrical sensitization of the meso-limbic dopaminergic system in rats: a pathogenetic model for schizophrenia

To study whether it was possible to modify mesolimbic dopaminergic activity by intermittent electrical stimulations (IES), 44 rats were either electrically stimulated or sham-stimulated in the ventral tegmental area (VTA) once daily for 70 days. This was done through chronically implanted intracranial electrodes. The intensity of electrical stimulation was determined by the lowest current that elicited a definite motor response. Stimulated rats demonstrated a significantly potentiated behavioral response after 70 stimulations. Seven months after IES rats still demonstrated an increased sensitivity to electrical stimulations in the VTA. A new stimulation period only resulted in a modest additional fall in threshold values. There was a highly significant difference between the current needed to provoke a given response in sensitized rats and in sham-stimulated rats. The behavioral response to stimulation was suppressed both by the dopamine (DA) D2 receptor antagonists haloperidol and raclopride and by the DA D1 receptor antagonist SCH 23390. Furthermore, stimulated rats showed an enhanced response to stimulation with amphetamine and to a lesser extent with apomorphine. Between stimulation periods sensitized animals demonstrated a reduced social interaction. In conclusion intermittent electrical stimulations of the VTA resulted in a syndrome characterized by a hypersensitive response to electrical and pharmacological DA provocation combined with abnormal social interaction. This animal model has points of resemblance with recent interpretations of the DA hypothesis for schizophrenia.

Effects of kindling on subsequent learning, memory, behavior, and seizure susceptibility

To determine the long-term effects of seizures on the developing brain we kindled 20-, 40-, and 60-day-old rats to stage 5 seizures and then elicited an additional 15 seizures using the same kindling stimulation. At age 80 days, all animals that reached stage 5 kindling, and their respective age-matched controls, underwent behavioral testing using the Morris water maze, open field test, and handling test. Prior to euthanasia the animals had seizure threshold tested using flurothyl inhalation. No differences were noted in time to platform in the water maze or activity level in the open field test between the kindled rats and controls in any of the three age groups. Rats kindled at age 20 and 40 were more emotional than the controls in the handling test. In the flurothyl inhalation test, rats kindled at 40 and 60 days of age had a shorter latency to all seizures stages than the controls. These results demonstrate that while kindling results in no alteration of learning, memory, or activity level, it does result in altered emotionality and activity level in immature animals, as well as reduced seizure threshold in pubescent and mature rats. The animal model used appears to be an important variable in determining the long-term effects of seizures.

The effects of amygdaloid stimulation on amphetamine-elicited locomotor sensitization

Systemic injection of d-amphetamine (1.0 mg/kg) resulted in a progressive increase in locomotor activity as a function of repeated daily drug administration. The magnitude of the stimulant-induced sensitization effect was enhanced by low-current electrical stimulation of the central nucleus of the amygdala during open-field testing. Amygdaloid stimulation in the absence of amphetamine treatment did not influence spontaneous locomotor activity, and there was no behavioral evidence of epileptogenesis following amygdaloid stimulation over the course of the experiment. However, with continued stimulation of the amygdala, early-stage convulsive activity was apparent in animals after approximately 40 days of testing, signifying the advancement of kindling evolution. These results suggest that the processes responsible for kindling acquisition, prior to the behavioral expression of epileptiform events, interact with the underlying substrates of amphetamine sensitization.

Amphetamine and antidepressant drug effects on GABA- and NMDA-related seizures

Research has shown a synergistic relationship between amphetamine sensitization and limbic system kindling. To explore the role of GABA and NMDA receptor activity in modulating the positive effects of amphetamine on epileptogenesis, alterations in GABA- and NMDA-related convulsions were examined after acute and chronic amphetamine administration. A single injection of d-amphetamine (7.5 mg/kg) significantly decreased latencies to generalized motor seizures induced 12 h later by the noncompetitive GABAA receptor antagonist picrotoxin (10 mg/kg). The increased sensitivity to clonus was specific to acute amphetamine treatment and was not evident following withdrawal from chronic drug exposure. Seizures induced by NMDLA (1,000 mg/kg), on the other hand, were not modified by acute amphetamine injection; however, the latency to clonus was reduced substantially after NMDLA injection to mice chronically preexposed to amphetamine. The short- and long-term amphetamine effects on GABA- and NMDA-associated convulsive activity were not paralleled by similar drug treatment schedules involving acute (20 mg/kg) and chronic administration of desipramine, zimelidine, and buproprion. These results suggest that amphetamine may be acting on inhibitory and excitatory amino acid systems independently of its monoaminergic properties. The implications of these findings were discussed in relation to amphetamine sensitization of mesolimbic functioning.

Scopolamine increases nonreinforced behavior in an intracranial self-stimulation discrimination paradigm

The effects of several doses of systemic scopolamine administration on brain-stimulation reward from the A10 nucleus of the ventral tegmental area (VTA) were evaluated. The intracranial self-stimulation (ICSS) task involved a two-hole nose-poke procedure allowing for the assessment of both reinforced (correct) and nonreinforced (incorrect) performance levels as a function of varying current intensities. Scopolamine (0.75, 1.5, and 3.0 mg/kg) was found not to alter the rate-intensity functions derived from descending and ascending presentation of seven current levels. However, when nonreinforced behavior was considered significant increases in error responding were evident following scopolamine injection. These results are consistent with the known disinhibitory and perseverative properties of scopolamine, and indicate that the previously reported positive actions of peripheral administration of anticholinergic drugs on ICSS likely involved a drug-induced rate-enhancement of reward-unrelated performance variables.