Amphetamine sensitization and amygdala kindling: pharmacological evaluation of catecholaminergic and cholinergic mechanisms

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.

Effects of amphetamine and medial septal lesions on acquisition and retention of radial maze learning in rats

Procholinergic drugs have failed to overcome the memory deficit induced by alterations of the cholinergic system because their neurochemical target in the brain is either lacking or disorganised. However, there are many reports on a relative involvement of the dopaminergic system in learning and memory that may compensate for the cholinergic deficit because of the interaction or balance between neurotransmitters and the redundancy of the brain. The aim of our experiments is to examine the activation of the dopaminergic system on the performance of normal and medial septal lesioned rats in the radial maze test involving continuous choices. In the first experiment different groups of normal rats were treated with either 0.5, 1.0 or 2.0 mg/kg of D-amphetamine and tested in the radial maze. In the second experiment, medial septal lesioned rats which had learnt pre-op the radial maze test were retested a month later. Amphetamine had no effect on the memory measures provided by the radial maze test in normal and lesioned rats, but non-memory measures were significantly affected: amphetamine decreased the sequential choice responses and the time taken by the rats to perform the test. The present results show that the activation of the dopaminergic system does not compensate for the alteration of the cholinergic activity inducing amnesia, however, they support the recent data on the improving effect of amphetamine on locomotor activity. The interpretation of drug/lesion interaction effects is discussed in this paper in relation to the literature on the effect of promnesic drugs.

Effects of scopolamine on the rewarding and seizure-inducing properties of amygdaloid stimulation

The relationship between amygdaloid brain-stimulation reward and the evolution of seizure activity was evaluated in this study. Current levels that maintained optimal intracranial self-stimulation (ICSS) rates were found to be lower than the minimal current intensity required to elicit an afterdischarge (AD) from the central nucleus of the amygdala. After the ICSS session, AD thresholds (ADTs) were reduced to the same levels of current used to support ICSS. Assessment of seizure stage development during ICSS testing revealed that the emergence of early-stage epileptiform events following repeated amygdaloid stimulation suppressed ICSS performance. While administration of the antimuscarinic scopolamine did not prevent the stimulation-elicited reduction in AD thresholds, it was observed to inhibit seizure progression and increase ICSS rates. These results are consistent with the excitatory function of acetylcholine in epileptogenesis and were related to the possibility that different mechanisms underlie the rewarding and seizure-inducing properties of amygdaloid stimulation.

Clozapine inhibits limbic system kindling: implications for antipsychotic action

Clozapine and haloperidol were tested for their ability to influence the acquisition of kindled seizures following electrical stimulation of the amygdala and ventral hippocampus. While haloperidol pretreatment did not alter kindling genesis from either limbic region, preexposure to clozapine delayed the rate at which kindling evolved. Analysis of the number of seizure behaviors expressed during epileptogenesis revealed that clozapine produced a relative antagonism of seizure development arresting kindling at the stage-3 level. The kindling inhibition was dependent upon the daily administration of clozapine during the kindling process and was not evident after withdrawal from a chronic schedule of clozapine exposure. A subconvulsive dose of pilocarpine (80.0 mg/kg) produced an overall enhancement of kindling rate, a finding consistent with the excitatory role of acetylcholine (ACh) in kindling. Lower doses of pilocarpine (20.0 and 40.0 mg/kg) that did not alter seizure advancement partially antagonized the clozapine-elicited inhibition of amygdaloid kindling. Pilocarpine, however, did not affect the clozapine-induced increase in the number of stage-3 behaviors exhibited during amygdaloid kindling, suggesting that other neurochemical effects of clozapine, not related to its anticholinergic properties, modulate the kindling suppression. Clozapine's unique actions on limbic system sensitization were discussed in relation to its effectiveness as an antipsychotic agent.

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.

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

The effects of partial kindling of the central amygdaloid nucleus on brain stimulation reward were evaluated. Intracranial self-stimulation (ICSS) rate intensity functions were determined using a two-hole nose-poke discrimination paradigm in rats implanted with electrodes in the A10 dopamine (DA) neuronal region of the ventral tegmental area (VTA), and in the medial forebrain bundle (MFB) at the level of the lateral hypothalamus. The kindling process did not influence ICSS baseline rates from either site. However, following stage 4 kindling, a low dose amphetamine challenge increased ICSS, resulted in a significant shift to the left of the rate intensity functions, and decreased reward thresholds. Analysis of the error scores which consisted of nonreinforced responses made during ICSS testing revealed that the interaction between kindling and amphetamine on ICSS was specific to changes in central reward processes, and could not be attributed to the influence of rate-enhancing performance variables. The kindling-elicited sensitization of mesolimbic DA reward functioning seen after amphetamine challenge was discussed in relation to the role of the central amygdala in the integration of stimulus-reward associations, and in the conditioning of affective emotional states.

The effects of amphetamine preexposure on electrical kindling of the hippocampus and related transfer phenomena

Earlier research has established that repeated amphetamine administration can interact synergistically with the processes responsible for the genesis of kindled seizures after intermittent electrical stimulation of the amygdala. In this study, the effects of amphetamine preexposure on primary hippocampal kindling, secondary kindling of the contralateral amygdala, and rekindling of the original hippocampal focus were evaluated. It was shown that amphetamine treatment did not modify kindling rates when electrodes were situated in the dorsal hippocampus. However, transfer kindling of the contralateral amygdala evolved after fewer afterdischarges, and rekindling of the dorsal hippocampus progressed significantly faster in amphetamine-pretreated animals. The effects of amphetamine on ventral hippocampal kindling were also determined, and it was found that although epileptogenesis developed more rapidly relative to the dorsal region of this structure, kindling rates were not affected by amphetamine preexposure. These results were related to the possibility that the amygdala might have a unique function in the relationship between kindling- and stimulant-induced sensitization effects.