Differential effects of electrical stimulation of amygdala, caudate-putamen or substantia nigra pars compacta on taste aversion and passive avoidance in rats

The Phenomenology of Limbic Kindling

Kindling is a model of epilepsy whereby repeated administration of brief low-intensity trains of electrical stimulation come to elicit electrographic and behavioral manifestations of seizure. In the absence of overt tissue damage, an animal that has been kindled is rendered in a permanent state of increased susceptibility to seizures. A number of persistent biochemical and physiological alterations in function accompany kindling, some of which may impact upon behavior of the organism for a long period of time despite the absence offurther seizure activation. The sensitivity of limbic structures to kindling may contribute to the behavioral categories of cognition and affect that are particularly impacted by the kindling process. The increased proclivity for seizure disorders that characterizes kindling is not restricted to the initial kindling stimulus, but generalizes to other agents with convulsive properties. This paper provides an overview of the phenomenology of kindling, describes some of the conditions necessary for its induction, and some of the functional alterations that accompany its development and endure when overt convulsive behavior has subsided. Finally, a series of studies in our laboratory is presented which provides evidence of chemically induced kindling by repeated low-level exposure to some pesticides, namely those of the chlorinated hydrocarbon class.

Kindling increases aversion to saccharin in taste aversion learning

Kindling is a model in which an initially subconvulsive electrical stimulation of certain brain areas eventually develops a generalized seizure that produces behavioral and long term neuronal changes. In the present study we evaluated if kindling can modify conditioning taste aversion (CTA). In this paradigm animals acquire aversion to saccharin when it is presented as the conditioned stimulus (CS) followed by an injection of lithium chloride (LiCl) that induces a gastric irritation as the unconditioned stimulus (US). Male Wistar rats were implanted with bipolar electrodes aimed at the right amygdala (AMG) or at the right insular cortex (IC). The animals were stimulated daily until they reached stages 2-4 (intermediate) or until kindling was fully established (three consecutive stage 5 seizures). At least two weeks after kindling stimulation had ceased the animals were deprived of water for 24 h and given 10-min drinking sessions twice a day for 4 days. On day 5 (morning session) tap water was replaced by saccharin solution (0.1%), 20 min later the animals were injected with LiCl (7.5 ml/kg i.p., 0.2 M) to induce gastric malaise or taste aversion. After three more days of baseline consumption, water was substituted by a fresh 0.1% saccharin solution to test the aversion. AMG-kindling delayed the extinction of CTA. Animals with kindling in the IC had a higher retention than the sham kindling group; that is, they drank significantly less saccharin solution than the other groups. The results of the present experiment show that local modification of brain function induced by kindling stimulation can prolong the aversive effects of CTA.

Substantia nigra role in fear conditioning consolidation

The substantia nigra (SN) is known to be involved in the memorization of several conditioned responses. To investigate the role of the SN in fear conditioning consolidation this neural site was subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats which had undergone fear training to acoustic CS and context. TTX was stereotaxically administered to different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after TTX administration. In this way there was no interference with normal SN function during either acquisition or retrieval phases, so that any amnesic effect could be due only to consolidation disruption. The results show that SN functional integrity is necessary for contextual fear response consolidation up to the 24-h after-acquisition delay. On the contrary SN functional integrity was shown not to be necessary for the consolidation of acoustic CS fear responses. The present findings help to elucidate the role of the SN in memory consolidation and better define the neural circuits involved in fear memories.

Effects of amygdala lesions on body weight, conditioned taste aversion, and neophobia

Female rats with posterodorsal amygdala (PDA), basolateral amygdala (BLA), or sham lesions were compared regarding ad libitum food intake, weight gain, consumption of a novel food, and acquisition of a conditioned taste aversion (CTA). While only the rats with PDA lesions evidenced substantial weight gains at 10 days after surgery eating standard lab chow (25-45 g more than the other groups), only the rats with BLA lesions demonstrated significant deficits in the CTA and neophobia paradigms. Rats with basolateral lesions, on average, took less than 30 s to begin drinking the novel sweetened condensed milk after pairing with illness while the other groups took approximately 15 min to begin drinking. Also, rats with basolateral lesions ate, on average, 5 g of the novel Froot Loops while the other groups ate approximately 2 g. It is concluded that the changes in food-motivated behavioral tests frequently observed in animals with amygdala lesions do not coexist with the hyperphagia and weight gain of animals with PDA lesions.

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.

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.

Development of amygdaloid cholinergic mediation of passive avoidance learning in the rat. I. Muscarinic mechanisms

Passive avoidance learning was studied in young rats 13-30 days of age following bilateral injections of saline or antimuscarinic and/or muscarinic agents into three amygdaloid nuclei--lateral (L), basolateral (BL), and cortical (CO). While acquisition was not influenced by saline injections into various other cerebral structures, it was significantly altered by similar injections into these amygdaloid nuclei, especially by those into the BL nucleus, suggesting that this nucleus is particularly involved in passive avoidance learning. Atropine induced significant deficits from as early as 13 days on. These deficits increased and were of similar strength after injections into any of the three studied nuclei until day 16; after that age, they diminished slightly following CO and L nuclei administration, while remaining substantial after BL nucleus injections at all ages, even at 30 days. No facilitatory effects could be elicited by arecoline injected alone, while arecoline could antagonize the disturbing effect of atropine, when given in combination, from day 13 on. These results suggest a muscarinic cholinergic mediation of passive avoidance learning through the synaptic elements located in the basal lateral part of the amygdala in the young rat.

Conditioned taste aversion induced by self-administered drugs: paradox revisited

In this paper we have reviewed the literature on Conditioned Taste Aversion (CTA) with specific attention to the "apparent paradox" in this literature. This paradox refers to the fact that drugs which are self-administered (SA) by animals and are therefore presumed to possess positive reinforcing properties are also endowed with the capacity to induce a CTA. We have argued that the CTA literature contains evidence of the existence of two qualitatively distinct types of CTA, one which is mediated by emetic agents and the other induced by SA drugs. We first provided evidence to support the notion that the traditional explanation of CTA as a function of "drug toxicity" and its resultant gastrointestinal distress does not fit the data on the nature of CTA induced by SA drugs. We proposed instead that "drug shyness" or the novelty of the drug state of these psychoactive SA drugs constitutes a better explanation of the CTA of SA drugs. We provided further evidence suggesting a functional relationship between the positive reinforcing and aversive properties of SA drugs. We have based this contention on a review of the behavioral, physiological and neurochemical data concerning the nature of CTA of SA drugs. The examination of these data reveals that the neural mechanisms underlying both the positive and aversive properties of SA drugs are the same and at the same time different from the neural mechanisms underlying the induction of CTA by emetic agents. Finally, we discussed the relevance of this interaction between the positive and aversive properties of SA drugs in the context of their abuse liability and the control they exert on drug-oriented behavior.

Retrograde amnesia induced by post-trial injection of atropine into the caudate-putamen. Protective effect of the negative reinforcer

A series of experiments was performed to test the reliability of previous reports which indicated that cholinergic blockade of the caudate-putamen produces memory deficits of passive avoidance, and to determine whether overtraining of this task protects against such deficits. In the first experiment the effects of different doses of atropine injected into the caudate-putamen of rats shortly after training were assessed, and a dose-dependent retention deficit was found. In two additional experiments it was observed that by increasing the magnitude of the negative reinforcer used in training, a protection against such retention deficit was produced. These results support the hypotheses that (a) cholinergic activity of the caudate-putamen is critically involved in memory processes that mediate passive avoidance behavior, and (b) after overtraining the control of this behavior is transferred from the striatal cholinergic system to other neurochemical systems within, or outside, the striatum.

Is cholinergic activity of the caudate nucleus involved in memory?

A review was made of experiments dealing with the involvement of cholinergic activity of the caudate nucleus in memory processes. Injections of acetylcholine-receptor blockers or of neurotoxins against cholinergic interneurons into the striatum produce marked impairments in acquisition and retention of instrumental tasks while injections of acetylcholine or choline into the caudate produce the opposite effect. However, after a period of overtraining cholinergic blockade or interference with neural activity of the caudate does not produce significant deficits in retention. It is concluded that striatal cholinergic activity is critically involved in memory of recent events and that long-term memory is mediated by different neurochemical systems outside the caudate nucleus.

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.

Implication of amygdaloid muscarinic cholinergic mechanisms in passive avoidance learning in the developing rat

Young rats, 12-20 days of age, received bilateral microinjections of atropine sulfate (1, 5 and 20 micrograms) into the basolateral part of the amygdala, and were trained to learn a cool-draft-stimulus passive avoidance task 17 min later. Twelve-day rats did not perform differently from their controls. In contrast, rats 13-20 days of age exhibited significant age- and dose-related acquisition deficits. Sensitivity to atropine was high until day 17, and decreased progressively thereafter. These results demonstrate that muscarinic cholinergic synaptic elements located in the basolateral part of the amygdala are involved in passive avoidance learning in the young rat and begin to function on postnatal day 13. They also suggest that the number of functioning muscarinic receptor sites increase reliably after day 17.

Differential input from the amygdaloid body to the ventromedial hypothalamic nucleus in the rat

Differential amygdaloid afferents to anterior dorsal, anterior ventral, posterior dorsal and posterior ventral subdivisions of the ventromedial hypothalamic nucleus (VMH) were studied by means of retrograde transport of horseradish peroxidase (HRP). Injections of tracer confined to the VMH subdivisions mentioned, and enhancement of tracer uptake and transport were achieved by iontophoretic delivery of an HRP solution containing poly-L-alpha-ornithine. It was shown that the medial, central, basolateral, basomedial, lateroposterior and intercalated nuclei of the amygdala constitute afferent input sources to the ventromedial nucleus in a topographic pattern related to the various subdivisions of the VMH. This topographically organized amygdala-VMH projection is discussed against the background of the functional role that both amygdala and VMH play in the control of feeding, apart from various other autonomous functions that both brain centers are known to be concerned with.

Neurohypophysial hormones and brain function: the neurophysiological effects of oxytocin and vasopressin

An increasing body of evidence suggests that the neurohypophysial hormones, in addition to their classical actions, may also act as neurotransmitters. They have a widespread but discontinuous distribution in the CNS; apart from their presence in the magnocellular nuclei they may be found in the hippocampus, amygdala, septum, substantia nigra, brainstem and spinal cord. They exert profound effects on behavior, particularly on memory, a function frequently ascribed to the hippocampus, amygdala and septum; on memory consolidation, internal reward and self stimulation functions frequently ascribed to brainstem and diencephalic aminergic systems including the substantia nigra and on sensory and autonomic responses which involve the medulla and spinal cord. When applied to the CNS they alter multiple unit activity in certain regions, particularly the hippocampus and cells which contain neural lobe hormones appear to be able to drive other cells synaptically. Finally application of the hormones can profoundly affect the activity of single nerve cells in just those parts of the CNS where, on the basis of their behavioral actions, they might be expected to act.

Electrophysiological and behavioral evidence of interaction of dopaminergic and gustatory afferents in the amygdala

Electrical stimulation of the pontine parabrachial nucleus (PPN) and the lateral aspect of the ventral tegmental area (VTA) was observed to influence the activity of single neurons in the central nucleus of the amygdala (CNA). For a large proportion of CNA neurons there were convergent inputs from PPN and VTA. Injecting either apomorphine a dopamine agonist or spiroperidol a dopamine antagonist into the CNA significantly increased the consumption of 1.5% NaCl solution with no change of water intake in a two-bottle preference test. There was no change in preference for the hypertonic NaCl solution when a combination of apomorphine and spiroperidol was injected into the CNA. These observations suggest that dopaminergic projections from VTA to CNA may interact with gustatory projections from PPN to influence taste-motivated behavior.

Differential effects of electrical stimulation of amygdala or caudate on inhibitory shock avoidance: a role for state-dependent learning

The present study examined the disruptive effects of low-intensity electrical stimulation in the caudate or amygdala on retention of inhibitory shock avoidance. Caudate stimulation administered during a training trial disrupted inhibitory avoidance on a retest trial. However, animals receiving caudate stimulation during both training and retention trials displayed good retention of this behavior and did not differ significantly from implanted controls. These data suggest a role for state-dependent learning in mediating the disruptive effects of caudate stimulation on inhibitory shock avoidance. No evidence was found for state-dependent effects with amygdaloid stimulation. Animals receiving stimulation only during training or during both training and retest trials showed comparable disruption of inhibitory shock avoidance. These data provide evidence for locus-specific state-dependent effects of electrical brain-stimulation. The implications of these state-dependent effects are discussed with respect to the use of electrical brain-stimulation to study the neural substrates of memory.