Research reportInvolvement of the amygdala in classical conditioning of eyeblink response in the rat
Introduction
The two-factor theory of learning advocates the idea that classical conditioning proceeds through two separate stages of learning [8], [12]. The two stages are most often demonstrated in the context of the aversive conditioning paradigm, which supports acquisition of emotional responses (emotional CRs) in the first stage and acquisition of discrete motor responses (motor CRs) in the second stage. Acquisition of emotional CRs is fast, typically after just a few presentations of paired CS–US trials. Emotional CRs are defined as ‘preparatory’ and ‘nonspecific’ in the sense that they feature an energized state of the organism but not an actual action that may alleviate the impact of the impending aversive US, e.g. a conditioned suppression of ongoing activity. On the other hand, acquisition of motor CRs is typically slow. The motor CRs are considered to be ‘specific’ since they involve discrete skeletal movements of an organ that is best equipped to antagonize the effects of the noxious US, e.g. a conditioned eyeblink preceding a corneal airpuff [5], [8], [13].
The two-factor theory of conditioning also advocates a possible interaction between the two stages of learning whereby acquisition of motor CRs benefits from the preceding acquisition of emotional CRs [8]. Neurobiological testing of the above hypothesis was made possible after realization that distinct brain sites mediate the two stages of learning. The amygdala, and particularly its lateral and central nuclei, seems to be a key component in acquisition and expression of conditioned fear [1], [7]. Lesions of the central nucleus abolished conditioned heart rate, blood pressure, and freezing responses [1], [4], [14]. The cerebellum is involved in acquisition of discreet motor responses and lesions of the interpositus abolished the acquisition and retention of the eyeblink CRs [5], [6], [11], [16]. These findings point to the amygdala and the cerebellum as prototypical structures involved in emotional and motor conditioning, respectively. Manipulation of these sites may help in revealing the nature of interaction between the two stages of learning.
Weisz et al. [13] showed that lesions of the central amygdala, performed in order to abolish the emotional conditioning, retarded the acquisition of cerebellum-related eyeblink CRs. These findings suggest that the two learning stages do interact in an intact brain, and that this interaction features some beneficial effects of the amygdala on the conditioning process in the cerebellum. Interestingly, this form of interaction between the two stages of learning was revealed only when the CS intensity (65 dB) was set to trigger sub-maximal rates of CR acquisition. Lesions of the amygdala had no effect on the fast acquisition of eyeblink CRs when the tone CS intensity was raised to 85 dB.
The present study attempts to demonstrate that cerebellum-related eyeblink conditioning benefits from the preceding acquisition of amygdala-based fear responses. To achieve this goal, rats were exposed sequentially to two procedures, each preferentially supporting the acquisition of fear and eyeblink CRs, respectively. Contribution of the fear CRs to motor conditioning could then be revealed in comparison to rats in which the fear-inducing procedure was omitted and to rats with amygdaloid lesions. Throughout the two procedures, we relied on findings by Weisz et al. [13] which suggest that amygdala effects could be revealed only when experimental parameters supported sub-maximal rates of eyeblink conditioning. Thus, the CS was set to moderate intensity levels throughout the two procedures and the US was set to low-intermediate levels during the motor conditioning procedure.
Section snippets
Subjects
Subjects were male Wistar rats, weighing 320–380 g at the time of surgery. They were housed individually with free access to food and water and were kept under a reversed dark/light schedule with all experiments performed during the dark phase.
Surgery
Animals were anesthetized with Equithesine (3 ml/kg, i.p.). Standard stereotaxic procedures were used to induce bilateral electrolytic lesions of either the cerebellar interpositus or the central nucleus of the amygdala. An insulated insect pin, exposed
Conditioning
Fig. 1 shows the rate of eyeblink CRs across the four conditioning sessions. Intact controls with emotional preconditioning (intact+EP) showed the highest rate of conditioning, followed by the intact controls with no emotional preconditioning (intact, no-EP). Rats with either amygdaloid or cerebellar lesions showed virtually no acquisition of eyeblink CRs. The rate of CRs was analyzed using MANOVA with between-Ss variables of Group (four groups) and intensity of US shock (0.5 mA or 1.0 mA), and
Discussion
Experimental procedures were designed to support preferential conditioning of fear and eyeblink responses in successive sessions. Fast acquisition of fear CRs was attempted during the first session by exposing rats to tone CS, paired with a loud noise US (EP) [1]. Acquisition of eyeblink CRs was attempted during subsequent sessions by pairing the same tone CS with a novel US in the form of a periorbital electric shock. Among intact rats, those that experienced the EP had a significantly higher
References (16)
- et al.
Effects of bilateral lesions of the dentate and interpositus cerebral nuclei on conditioning of heart-rate and nictitating membrane/eyelid responses in the rabbit
Brain Res.
(1984) Emotional memory systems in the brain
Behav. Brain Res.
(1993)The role of the amygdala in conditioned fear
- et al.
The amygdala central nucleus and appetitive pavlovian conditioning: lesions impair one class of conditioned behavior
J. Neurosci.
(1990) - et al.
A neuroanatomical systems analysis of conditioned bradycardia in the rabbit
- et al.
Electrolytic lesions of the amygdala block acquisition and expression of fear-potentiated startle even with extensive training but do not prevent reacquisition
Behav. Neurosci.
(1993) - et al.
Mammalian brain substrates of aversive classical conditioning
Annu. Rev. Psychol.
(1993) - et al.
Analysis of response systems in Pavlovian conditioning reveals rapidly versus slowly acquired conditioned responses: support for two factors, implications for behavior and neurobiology
Psychobiology
(1992)
Cited by (50)
Emotion in motion: A three-stage model of aversive classical conditioning
2020, Neuroscience and Biobehavioral ReviewsAvoidance learning and classical eyeblink conditioning as model systems to explore a learning diathesis model of PTSD
2019, Neuroscience and Biobehavioral ReviewsInactivation of the interpositus nucleus blocks the acquisition of conditioned responses and timing changes in conditioning-specific reflex modification of the rabbit eyeblink response
2018, Neurobiology of Learning and MemoryCitation Excerpt :Since IP inactivation did not prevent the acquisition of associative changes in the amplitude and area of URs, other possibly non-cerebellar substrates should be considered as sites of plasticity underlying these aspects of CRM. A role for the amygdala in the modulation of cerebellar learning is well-established (Farley, Radley, & Freeman, 2016; Lee and Kim, 2004; Neufeld and Mintz, 2001; Shors and Mathew, 1998; Weisz et al., 1992; Whalen and Kapp, 1991), but our prior work showed that the central nucleus (CE) of the amygdala is crucial for the expression but not acquisition of amplitude, area, and frequency increases in CRM (Burhans and Schreurs, 2008). In agreement with these findings, work in rats has shown that blocking memory consolidation in the CE with protein synthesis inhibitors had no effect on eyeblink conditioning even though CE lesion or inactivation typically impairs or slows the acquisition of eyeblink CRs (Steinmetz, Ng, & Freeman, 2017).
Amygdala central nucleus modulation of cerebellar learning with a visual conditioned stimulus
2018, Neurobiology of Learning and MemoryClassical Conditioning of Timed Motor Responses: Neural Coding in Cerebellar Cortex and Cerebellar Nuclei.
2016, The Neuronal Codes of the CerebellumNeurosubstrates and mechanisms underlying the extinction of associative motor memory
2015, Neurobiology of Learning and Memory