Stimulus generalization of conditioned eyelid responses produced without cerebellar cortex: implications for plasticity in the cerebellar nuclei

In Pavlovian eyelid conditioning and adaptation of the vestibulo-ocular reflex, cerebellar cortex lesions fail to completely abolish previously acquired learning, indicating an additional site of plasticity in the deep cerebellar or vestibular nucleus. Three forms of plasticity are known to occur in the deep cerebellar nuclei: formation of new synapses, plasticity at existing synapses, and changes in intrinsic excitability. Only a cell-wide increase in excitability predicts that learning should generalize broadly from a training stimulus to other stimuli capable of supporting learning, whereas the alternatives predict that learning should be relatively specific to the training stimulus. Here we show that deep nucleus plasticity, as assessed by conditioned eyelid responses produced without input from the cerebellar cortex, is relatively specific to the training conditioned stimulus (CS). We trained rabbits to a tone or light CS with periorbital stimulation as the unconditioned stimulus (US), and pharmacologically disconnected the cerebellar cortex during a posttraining generalization test. The short-latency conditioned responses unmasked by this treatment showed strong decrement along the dimension of auditory frequency and did not generalize across stimulus modalities. These results cannot be explained solely by a cell-wide increase in the excitability of deep nucleus neurons, and imply that an input-specific mechanism in the deep cerebellar nucleus operates as well.     

A modified counterconditioning procedure prevents the renewal of conditioned fear in rats

Two studies using an ABA design examined the Extinction and renewal of conditioned barpress suppression. Following lights-off and foot shock pairings in Context A, rats were placed in Context B and were given either a standard counterconditioning procedure where the lights-off CS was paired with a novel food US delivered freely or a modified counterconditioning procedure where CS-US pairings only occurred if the rat earned the US by performing a required behavior during the CS. Results indicated that the modified counterconditioning procedure thwarted ABA renewal but the conventional counterconditioning procedure did not reduce ABA renewal any more than nonreinforced exposure to the CS alone. Furthermore, the response required during the modified counterconditioning procedure could be one used as a baseline response during Acquisition of fear or it could be a novel response. Implications of the results for theories of Extinction and renewal of fear are discussed.     

Diagnostic value of the H-reflex index in alcoholic polyneuropathy

We examined the H-reflex-index of triceps surae muscle (HI) and the motor nerve conduction velocity of the tibial nerve (mNLG) of 56 alcohol dependent patients. The results were compared with neurologic disorders, duration of the dependency and the type of alcoholism according to Jellinek. The HI showed significant more pathological changes than the mNLG.     

CAMKII inhibition in the parabrachial nuclei elicits conditioned taste aversion in rats

The conditioned taste aversion (CTA) paradigm was used to assess the role of Ca(2+)/calmodulin-dependent protein kinase (CAMKII) in associative learning. KN62, a specific inhibitor of CAMKII, was injected into the parabrachial nuclei (PBN) either immediately after saccharin drinking (CS) or after saccharin drinking and i.p. injection of LiCl (US). Injection of KN62 into the PBN after saccharin drinking elicited clear CTA (Exp. 1). This effect was dosage-dependent and site-specific (Exp. 2). The results are discussed in relation with an earlier report showing that CTA acquisition is disrupted by injection of Ca(2+)/phospholipid-dependent protein kinase (PKC) inhibitor chelerythrine into the PBN during CS-US interval. It is suggested that the principal serine/threonine kinases play different roles in CTA learning: whereas PKC activity is necessary for the gustatory short-term memory formation, CAMKII acts similarly to the US itself-an unexpected role of CAMKII in associative learning.     

Cognitive processing impairments in a supra-second temporal discrimination task in rats with cerebellar lesion

The role of interpositus nuclei (IN) in timing in the sub-second range is well supported in eyeblink conditioning studies. Timing impairments shown in the seconds range in patients with intermediate cerebellar lesion, and known intermediate cerebellar cortex projection to IN raise the question of a possible involvement of IN in timing in the supra-second range as well. To address this question, we tested rats (Sprague-Dawley) given bilateral lesions of IN with Colchicine in a 2- vs. 8-s temporal discrimination task, followed by three daily sessions of a temporal bisection tests with five added intermediate non-reinforced durations. IN lesioned rats showed normal acquisition of the temporal discrimination, but a transient impairment of temporal sensitivity during the bisection tests. In addition, their response latencies suggested a different behavioral strategy from that of control animals. Our results indicate that the IN of the cerebellum may not be critically involved in temporal processing in the 2–8 s range, but may play a role in the cognitive processes that access temporal information in the mediation of choice behavior.     

Effects of postural anxiety on the soleus H-reflex

Previous research has proposed that spinal reflex modulation may mediate anxiety-related changes in postural control. This study investigated how soleus H-reflex amplitude was influenced by standing at heights that induced different levels of anxiety. H-reflexes were elicited in 15 participants standing at the center and edge of a platform raised from a low to a high height (with and without vision). Increased skin conductance confirmed the anxiety effect of elevated surface heights. When standing at the edge of the platform with vision, H-reflex amplitude was attenuated in the high compared to low height condition. Changes in background muscle activity could not explain observed H-reflex changes, suggesting the potential involvement of pre-synaptic inhibition or fusimotor drive on anxiety-related changes in reflex modulation. This study reveals that healthy participants reduce spinal reflex excitability in the presence of increased postural anxiety and a postural threat imposed by standing at the edge of a raised platform. These findings have implications for understanding control of standing balance in individuals with postural instability and/or fear of falling, such as the elderly or stroke.     

Dietary CDP-choline supplementation prevents memory impairment caused by impoverished environmental conditions in rats

We previously showed that dietary cytidine (5')-diphosphocholine (CDP-choline) supplementation could protect against the development of memory deficits in aging rats. In the present study, younger rats exposed to impoverished environmental conditions and manifesting hippocampal-dependent memory impairments similar to those observed in the aging rodents were given CDP-choline, and its effects on this cognitive deficit were assessed. Male Sprague-Dawley rats reared for 3 mo in impoverished (IC) or enriched environmental (EC) conditions concurrently received either a control diet or a diet supplemented with CDP-choline (approximately 500 mg/kg/d). After 3 mo, rats were trained to perform spatial and cued versions of the Morris water maze, and their rates of acquisition and retention were compared. Impoverished rats exhibited a selective deficit in hippocampal-dependent spatial memory which could be ameliorated by feeding them CDP-choline. The CDP-choline had no memory-enhancing effect in enriched rats, nor did it prevent the memory impairment of impoverished rats if the animals consumed it for the initial or final months instead of for the entire 3-mo period. These findings indicate that long-term dietary CDP-choline supplementation can ameliorate the hippocampal-dependent memory impairment caused by impoverished environmental conditions in rats, and suggest that its actions result, in part, from a long-term effect such as enhanced membrane phosphatide synthesis, an effect shown to require long-term dietary supplementation with CDP-choline.     

H-reflex latency, index and conduction velocity in healthy probands

The HL, HI and HV were measured of 76 healthy persons at the age of 20 to 70 years. The length-dependent variation of the HL can be eliminated most favourable by calculating the HI. Because of the proved linear correlation between age and HI the use of age-specific HI-normal ranges is recommendable.     

The role of amygdala nuclei in the expression of auditory signaled two-way active avoidance in rats

Using a two-way signaled active avoidance (2-AA) learning procedure, where rats were trained in a shuttle box to avoid a footshock signaled by an auditory stimulus, we tested the contributions of the lateral (LA), basal (B), and central (CE) nuclei of the amygdala to the expression of instrumental active avoidance conditioned responses (CRs). Discrete or combined lesions of the LA and B, performed after the rats had reached an asymptotic level of avoidance performance, produced deficits in the CR, whereas CE lesions had minimal effect. Fiber-sparing excitotoxic lesions of the LA/B produced by infusions of N-methyl-d-aspartate (NMDA) also impaired avoidance performance, confirming that neurons in the LA/B are involved in mediating avoidance CRs. In a final series of experiments, bilateral electrolytic lesions of the CE were performed on a subgroup of animals that failed to acquire the avoidance CR after 3 d of training. CE lesions led to an immediate rescue of avoidance learning, suggesting that activity in CE was inhibiting the instrumental CR. Taken together, these results indicate that the LA and B are essential for the performance of a 2-AA response. The CE is not required, and may in fact constrain the instrumental avoidance response by mediating the generation of competing Pavlovian responses, such as freezing.Early studies of the neural basis of fear often employed avoidance conditioning procedures where fear was assessed by measuring instrumental responses that reduced exposure to aversive stimuli (e.g., Weiskrantz 1956; Goddard 1964; Sarter and Markowitsch 1985; Gabriel and Sparenborg 1986). Despite much research, studies of avoidance failed to yield a coherent view of the brain mechanisms of fear. In some studies, a region such as the amygdala would be found to be essential and in other studies would not. In contrast, rapid progress in understanding the neural basis of fear and fear learning was made when researchers turned to the use of Pavlovian fear conditioning (Kapp et al. 1984, 1992; LeDoux et al. 1984; Davis 1992; LeDoux 1992; Cain and Ledoux 2008a). It is now well established from such studies that specific nuclei and subnuclei of the amygdala are essential for the acquisition and storage of Pavlovian associative memories about threatening situations (LeDoux 2000; Fanselow and Gale 2003; Maren 2003; Maren and Quirk 2004; Schafe et al. 2005; Davis 2006).Several factors probably contributed to the fact that Pavlovian conditioning succeeded where avoidance conditioning struggled. First, avoidance conditioning has long been viewed as a two-stage learning process (Mowrer and Lamoreaux 1946; Miller 1948b; McAllister and McAllister 1971; Levis 1989; Cain and LeDoux 2008b). In avoidance learning, the subject initially undergoes Pavlovian conditioning and forms an association between the shock and cues in the apparatus. The shock is an unconditioned stimulus (US) and the cues are conditioned stimuli (CS). Subsequently, the subject learns the instrumental response to avoid the shock. Further, the “fear” aroused by the presence of the CS motivates learning of the instrumental response. Fear reduction associated with successful avoidance has even been proposed to be the event that reinforces avoidance learning (e.g., Miller 1948b; McAllister and McAllister 1971; Cain and LeDoux 2007). Given that Pavlovian conditioning is the initial stage of avoidance conditioning, as well as the source of the “fear” in this paradigm, it would be more constructive to study the brain mechanisms of fear through studies of Pavlovian conditioning rather than through paradigms where Pavlovian and instrumental conditioning are intermixed. Second, avoidance conditioning was studied in a variety of ways, but it was not as well appreciated at the time as it is today; that subtle differences in the way tasks are structured can have dramatic effects on the brain mechanisms required to perform the task. There was also less of an appreciation for the detailed organization of circuits in areas such as the amygdala. Thus, some avoidance studies examined the effects of removal of the entire amygdala or multiple subdivisions (for review, see Sarter and Markowitsch 1985). Finally, fear conditioning studies typically involved a discrete CS, usually a tone, which could be tracked from sensory processing areas of the auditory system to specific amygdala nuclei that process the CS, form the CS–US association, and control the expression of defense responses mediated by specific motor outputs. In contrast, studies of avoidance conditioning often involved diffuse cues, and the instrumental responses used to indirectly measure fear were complex and not easily mapped onto neural circuits.Despite the lack of progress in understanding the neural basis of avoidance responses, this behavioral paradigm has clinical relevance. For example, avoidance behaviors provide an effective means of dealing with fear in anticipation of a harmful event. When information is successfully used to avoid harm, not only is the harmful event prevented, but also the fear arousal, anxiety, and stress associated with such events; (Solomon and Wynne 1954; Kamin et al. 1963). Because avoidance is such a successful strategy to cope with danger, it is used extensively by patients with fear-related disorders to reduce their exposure to fear- or anxiety-provoking situations. Pathological avoidance is, in fact, a hallmark of anxiety disorders: In avoiding fear and anxiety, patients often fail to perform normal daily activities (Mineka and Zinbarg 2006).We are revisiting the circuits of avoidance conditioning from the perspective of having detailed knowledge of the circuit of the first stage of avoidance, Pavlovian conditioning. To most effectively take advantage of Pavlovian conditioning findings, we have designed an avoidance task that uses a tone and a shock. Rats were trained to shuttle back and forth in a runway in order to avoid shock under the direction of a tone. That is, the subjects could avoid a shock if they performed a shuttle response when the tone was on, but received a shock if they stayed in the same place (two-way signaled active avoidance, 2-AA). While the amygdala has been implicated in 2-AA (for review, see Sarter and Markowitsch 1985), the exact amygdala nuclei and their interrelation in a circuit are poorly understood.We focused on the role of amygdala areas that have been studied extensively in fear conditioning: the lateral (LA), basal (B), and central (CE) nuclei. The LA is widely thought to be the locus of plasticity and storage of the CS–US association, and is an essential part of the fear conditioning circuitry. The basal amygdala, which receives inputs from the LA (Pitkänen 2000), is not normally required for the acquisition and expression of fear conditioning (Amorapanth et al. 2000; Nader et al. 2001), although it may contribute under some circumstances (Goosens and Maren 2001; Anglada-Figueroa and Quirk 2005). The B is also required for the use of the CS in the motivation and reinforcement of responses in other aversive instrumental tasks (Killcross et al. 1997; Amorapanth et al. 2000). The CE, through connections to hypothalamic and brainstem areas (Pitkänen 2000), is required for the expression of Pavlovian fear responses (Kapp et al. 1979, 1992; LeDoux et al. 1988; Hitchcock and Davis 1991) but not for the motivation or reinforcement of aversive instrumental responses (Amorapanth et al. 2000; LeDoux et al. 2009). We thus hypothesized that damage to the LA or B, but not to the CE, would interfere with the performance of signaled active avoidance.     

A review of the H-reflex and M-wave in the human triceps surae

The soleus is the most commonly used muscle for H-reflex studies in humans, while limited comparable data have been produced from the gastrocnemii muscles. This article reviews the fundamental differences between the structure and function of the human soleus and gastrocnemii muscles, including recent data published about their complex innervation zones. Protocols for eliciting, recording, and assessing the H-reflex and M-wave magnitude in the human triceps surae are also discussed.     

Damage to the lateral and central, but not other, amygdaloid nuclei prevents the acquisition of auditory fear conditioning

It is well established that the amygdala plays an essential role in Pavlovian fear conditioning, with the lateral nucleus serving as the interface with sensory systems that transmit the conditioned stimulus and the central nucleus as the link with motor regions that control conditioned fear responses. The lateral nucleus connects with the central nucleus directly and by way of several other amygdala regions, including the basal, accessory basal, and medial nuclei. To determine which of these regions is necessary, and thus whether conditioning requires the direct or one of the indirect intra-amygdala pathways, we made lesions in rats of the lateral, central, basal, accessory basal, and medial nuclei, as well as combined lesions of the basal and accessory basal nuclei and of the entire amygdala. Animals subsequently underwent fear conditioning trials in which an auditory conditioned stimulus was paired with a footshock unconditioned stimulus. Animals that received lesions of the lateral or central nucleus, or of the entire amygdala, were dramatically impaired, whereas the other lesions had little effect. These findings show that only the lateral and central nuclei are necessary for the acquisition of conditioned fear response to an auditory conditioned stimulus.     

Lesions in the anterior thalamic nuclei of rats do not disrupt acquisition of stimulus sequence learning

Four experiments examined the effects of encoding multiple standards in a temporal generalization task in the visual and auditory modalities both singly and cross-modally, using stimulus durations ranging, across different experiments, from 100 to 1,400 ms. Previous work has shown that encoding and storing multiple auditory standards of different durations resulted in systematic interference with the memory of the standard, characterized by a shift in the location of peak responding, and this result, from Ogden, Wearden, and Jones (2008), was replicated in the present Experiment 1. Experiment 2 employed the basic procedure of Ogden et al. using visual stimuli and found that encoding multiple visual standards did not lead to performance deterioration or any evidence of systematic interference between the standards. Experiments 3 and 4 examined potential cross-modal interference. When two standards of different modalities and durations were encoded and stored together there was also no evidence of interference between the two. Taken together, these results, and those of Ogden et al., suggest that, in humans, visual temporal reference memory may be more permanent than auditory reference memory and that auditory temporal information and visual temporal information do not mutually interfere in reference memory.     

Postacquisition injection of tetrodotoxin into the parabrachial nuclei elicits partial disruption of passive avoidance reaction in rats.

The recent discovery that post-trial functional blockade of the parabrachial nuclei by intracerebral injection of 10 ng tetrodotoxin (TTX) disrupts acquisition of conditioned taste aversion (CTA) (Ivanova & Bures, 1990a,b) has prompted attempts to ascertain the role of this structure in other types of inhibitory learning. In Experiment 1, rats with implanted parabrachial cannulae were trained in a step-through avoidance task and received bilateral TTX (2 x 10 ng) immediately after the acquisition trial; they displayed significantly weakened avoidance of the shock compartment 2 days later. In Experiment 2, rats were anesthetized with pentobarbital (50 mg/kg) immediately after passive avoidance acquisition and received parabrachial TTX 15 min later; whereas anesthesia alone left the passive avoidance reaction (PAR) unaffected, TTX elicited similar disruption as in unanesthetized animals. In Experiment 3, TTX was injected in anesthetized animals 0, 1, 2, or 4 days after PAR acquisition. The amnesic effect was significant when the acquisition-TTX delay had been prolonged to 24 but not to 48 or 96 h. Since CTA is disrupted by reversible blockade of parabrachial nuclei and of the adjacent reticular formation elicited up to 4 days after acquisition (Ivanova & Bures, 1990b), PAR seems to be impaired to a lesser degree and for a shorter time than CTA by similar TTX treatment.     

Testosterone prevents synaptic loss in the perineal motoneuron pool in the spinal cord in male rats exposed to chronic stress

Chronic stress is known to induce disorders of reproductive neuroendocrine functions. Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in male rats play an important role in copulatory behavior. In the present study, it was examined whether chronic stress would alter synaptic organization of the SNB motoneurons and whether androgen would modify the changes under chronic stress. Five male rats were under restraint stress for 5 days per week for 3 weeks, and five males implanted subcutaneously with Silastic capsules containing testosterone were also exposed to stress. Five males served as unstressed controls. After 3 weeks of restraint stress, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the SNB were dissected, processed with a modified tetramethylbenzidine (TMB) method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 SNB motoneurons (total for three groups) were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts. The mean percentage of somatic membranes covered by synapses in males exposed to chronic stress was significantly less than that in controls or stressed males treated with testosterone. Size and number of synaptic contacts per unit length of somatic membranes in males exposed to stress were also significantly less than those in controls or stressed males treated with testosterone. There was no significant difference in any of the parameters between controls and stressed males treated with testosterone. Changes in plasma levels of testosterone showed the same profile as changes in the synaptic contacts. These results suggest that the SNB motoneurons of male rats exposed to chronic stress retain a considerable synaptic plasticity in response to androgen, and that androgen treatment can rescue the SNB system in male rats when under chronic restraint stress.     

Learning of sensory sequences in cerebellar patients

A possible role of the cerebellum in detecting and recognizing event sequences has been proposed. The present study sought to determine whether patients with cerebellar lesions are impaired in the acquisition and discrimination of sequences of sensory stimuli of different modalities. A group of 26 cerebellar patients and 26 controls matched for age, sex, handedness, musicality, and level of education were tested. Auditory and visual sensory sequences were presented out of different sensory pattern categories (tones with different acoustic frequencies and durations, visual stimuli with different spatial locations and colors, sequential vision of irregular shapes) and different ranges of inter-cue time intervals (fast and slow). Motor requirements were small, with vocal responses and no time restrictions. Perception of visual and acoustic stimuli was generally preserved in patients and controls. The number of errors was significantly higher in the faster tempo of sequence presentation in learning of sequences of tones of different frequencies and in learning of sequences of visual stimuli of different spatial locations and different colors. No difference in tempo between the groups was shown. The total number of errors between the two groups was identical in the sequence conditions. No major disturbances in acquisition or discrimination of various sensory sequences were observed in the group of cerebellar patients. Sequence learning may be impaired only in tasks with significant motor demands.     

Speech motor deficits in cerebellar infarctions

Functional imaging studies demonstrated cerebellar activation during speech movements in the rostral cerebellar region. Ischemic lesions of this area, which is supplied by the superior cerebellar artery, induce dysarthria.