Changes of synaptic ultrastructure in the guinea pig interpositus nuclei associate with response magnitude and timing after trace eyeblink conditioning

Role of glucocorticoid receptor phosphorylation-mediated synaptic plasticity in anxiogenic and depressive behaviors induced by monosodium glutamate

Psychiatric diseases and metabolic disorders frequently cooccur, yet the mechanisms underlying this interaction remain unknown. The aim of this study was to determine the role of glucocorticoid receptor (GR) phosphorylation in the comorbidity of metabolic and psychiatric disorders. Neonatal Sprague-Dawley rats were subcutaneously injected with monosodium glutamate (MSG) every 2 days for 10 days after birth. Metabolic and behavioral tests were performed 12 weeks later. Golgi staining and transmission electron microscopy (TEM) were performed to evaluate synaptic structural plasticity. Changes in GR phosphorylation and the BDNF/TrkB pathway were evaluated by western blotting and immunofluorescence. We found that MSG-treated rats displayed significant metabolic abnormalities accompanied by anxiogenic and depressive behaviors, an altered synaptic ultrastructure and the loss of dendritic spines. The expression of phosphorylated GR was reduced in the brain. Furthermore, a specific agonist of BDNF/TrkB significantly reversed the reduction in GR phosphorylation, as well as the metabolic and behavioral outcomes. These findings indicate that a decrease in BDNF/TrkB pathway-dependent GR phosphorylation is a long-term effect of MSG treatment that may contribute to metabolic and behavioral disturbances.

Trace eyeblink conditioning is associated with changes in synaptophysin immunoreactivity in the cerebellar interpositus nucleus in guinea pigs

Synaptic plasticity plays a role during trace eyeblink conditioning (TEBC). Synaptophysin (Syn) is a major integral transmembrane protein, located particularly in the synaptic vesicles, and is considered a molecular marker of synapses. In addition, Syn immunoreactivity is an important indicator of synaptic plasticity. In the present study, we used immunohistochemical techniques to assess changes in Syn expression in the cerebellar interpositus nucleus (IN) of guinea pigs exposed to TEBC and pseudoconditioning. Additionally, we analyzed the relationship between Syn immunoreactivity and the percentage of trace-conditioned responses. Guinea pigs underwent trace conditioning or pseudoconditioning. Following two, six, or ten sessions, they were perfused and the cerebellum was removed for Syn immunohistochemical evaluation. After sessions 6 and 10, a significant increase in conditioned response (CR) percentage was observed in the trace-conditioned group, with the CR percentage reaching the learning criteria following session 10. Besides, for trace-conditioned animals, the Syn expression in IN was found significantly up-regulated after session 10 compared with pseudoconditioned ones. Our data suggest that the increase in Syn expression links to synaptic plasticity changes in the cerebellar IN and provides a histological substrate in the IN relating to TEBC training. The changing trend of Syn immunoreactivity in the IN is associated with CR percentage.

Establishment and transfer of classical eyeblink conditioning using electrical microstimulation of the hippocampus as the conditioned stimulus

The present experiment was designed to determine whether classical eyeblink conditioning (EBC) can be established by using electrical microstimulation of the hippocampus as a conditioned stimulus (CS) paired with an air-puff unconditioned stimulus (US). We intended to examine whether EBC transfer could occur when a CS was shifted between microstimulation of the hippocampus as a CS (Hip-CS) and tone as a CS (tone-CS) and to compare the difference in transfer effectiveness between delay EBC (dEBC) and trace EBC (tEBC). Eight groups of guinea pigs, including 4 experimental groups and 4 control groups, were included in the study. First, the experimental groups received either a Hip-CS or a tone-CS paired with a US; then, these groups were exposed to a shifted CS (tone-CS or Hip-CS) paired with the US. The control groups received the corresponding Hip-CS or tone-CS, which was, however, pseudo-paired with the US. The control groups were then shifted to the tone-CS (or Hip-CS) paired with the US. The results show that EBC can be successfully established when using microstimulation of the hippocampus as a CS paired with an air-puff US, and that the acquisition rates of EBC are higher in the experimental groups than in the control groups after switching from the Hip-CS to the tone-CS or vice versa, indicating the occurrence of learning transfer between EBC established with the Hip-CS and tone-CS. The present study also demonstrated that the EBC re-acquisition rates were remarkably higher in dEBC than in tEBC with both types of transfer, which suggests that the saving effect was more evident in dEBC than tEBC. These results significantly expand our knowledge of EBC transfer as well as the functional neural circuit underlying EBC transfer.

Pretrial functional connectivity differentiates behavioral outcomes during trace eyeblink conditioning in the rabbit

Fluctuations in neural activity can produce states that facilitate and accelerate task-related performance. Acquisition of trace eyeblink conditioning (tEBC) in the rabbit is enhanced when trials are contingent on optimal pretrial activity in the hippocampus. Other regions which are essential for whisker-signaled tEBC, such as the cerebellar interpositus nucleus (IPN), somatosensory and prelimbic cortices, may also show optimal connectivity prior to successful performance. Functional magnetic resonance imaging (fMRI) was acquired in nine rabbits during tEBC on the first and tenth days of initial training and once again after a 30-d, training-free hiatus. Data acquired during the intertrial interval was parsed depending on whether or not a conditioned response (CR) occurred on the upcoming trial and seed-based functional connectivity was calculated among the IPN, hippocampus, somatosensory, and prelimbic cortices. Functional connectivity between the left somatosensory cortex and right IPN, regions critical for establishing and producing CRs evoked by right vibrissae vibration and right corneal airpuff, was significantly negative prior to successful, CR trials as compared with unsuccessful, non-CR trials. Differences were not observed for any of the other possible combinations of connectivity. Our results demonstrate that specific pretrial functional connectivity exists within the rabbit brain and differentiates between upcoming behavioral response outcomes. Online analysis of network fluctuations has the potential to be used as the basis for therapeutic interventions to facilitate learning and memory.

Transfer of classical eyeblink conditioning with electrical stimulation of mPFC or tone as conditioned stimulus in guinea pigs

Learning with a stimulus from one sensory modality can facilitate subsequent learning with a new stimulus from a different sensory modality. To date, the characteristics and mechanism of this phenomenon named transfer effect still remain ambiguous. Our previous work showed that electrical stimulation of medial prefrontal cortex (mPFC) as a conditioned stimulus (CS) could successfully establish classical eyeblink conditioning (EBC). The present study aimed to (1) observe whether transfer of EBC learning would occur when CSs shift between central (mPFC electrical stimulation as a CS, mPFC-CS) and peripheral (tone as a CS, tone CS); (2) compare the difference in transfer effect between the two paradigms, delay EBC (DEBC) and trace EBC (TEBC). A total of 8 groups of guinea pigs were tested in the study, including 4 experimental groups and 4 control groups. Firstly, the experimental groups accepted central (or peripheral) CS paired with corneal airpuff unconditioned stimulus (US); then, CS shifted to the peripheral (or central) and paired with US. The control groups accepted corresponding central (or peripheral) CS and pseudo-paired with US, and then shifted CS from central (or peripheral) to peripheral (or central) and paired with US. The results showed that the acquisition rates of EBC were higher in experimental groups than in control groups after CS switching from central to peripheral or vice versa, and the CR acquisition rate was remarkably higher in DEBC than in TEBC in both transfer ways. The results indicate that EBC transfer can occur between learning established with mPFC-CS and tone CS. Memory of CS-US association for delay paradigm was less disturbed by the sudden switch of CS than for trace paradigm. This study provides new insight into neural mechanisms underlying conditioned reflex as well as the role of mPFC.

Prefrontal control of cerebellum-dependent associative motor learning

Behavioral studies have demonstrated that both medial prefrontal cortex (mPFC) and cerebellum play critical roles in trace eyeblink conditioning. However, little is known regarding the mechanism by which the two brain regions interact. By use of electrical stimulation of the caudal mPFC as a conditioned stimulus, we show evidence that persistent outputs from the mPFC to cerebellum are necessary and sufficient for the acquisition and expression of a trace conditioned response (CR)-like response. Specifically, the persistent outputs of caudal mPFC are relayed to the cerebellum via the rostral part of lateral pontine nuclei. Moreover, interfering with persistent activity by blockade of the muscarinic Ach receptor in the caudal mPFC impairs the expression of learned trace CRs. These results suggest an important way for the caudal mPFC to interact with the cerebellum during associative motor learning.

Baseline theta activities in medial prefrontal cortex and deep cerebellar nuclei are associated with the extinction of trace conditioned eyeblink responses in guinea pigs

It has been shown that both the medial prefrontal cortex (mPFC) and the cerebellum are involved in the extinction of trace conditioned eyeblink responses (CR). However, the neural mechanisms underlying the extinction are still relatively unclear. Theta oscillation in either the mPFC or the cerebellum has been revealed to correlate with the performance of trace CRs during the asymptotic acquisition. Therefore, we sought to further evaluate the impacts of pre-conditioned stimulus (CS) spontaneous theta (5.0-10.0Hz) oscillations in the mPFC and the deep cerebellar nuclei (DCN) on the extinction of trace CRs. Albino guinea pigs were given acquisition training for ten daily sessions followed by seven daily sessions of extinction. Local field potential (LFP) signals in the mPFC and the DCN were recorded when the animals received the CS-alone extinction training. It was found that higher mPFC relative theta ratios [theta/(delta+beta)] during the baseline period (850-ms prior to the CS onset) were predictive of fewer CR incidences rather than more adaptive CR performance (i.e., higher CR magnitude and later CR peak/onset latencies). Likewise, the pre-CS DCN theta activity was associated with the faster CR extinction. Furthermore, it was revealed that the power of pre-CS theta activities in the mPFC and the DCN were correlated until the extinction training day 2. Collectively, these results suggest that the mPFC and the DCN may interact with each other, and the brain oscillation state in which baseline theta activities in both areas are present contributes to the subsequent extinction of trace CRs.

Predictive nature of prefrontal theta oscillation on the performance of trace conditioned eyeblink responses in guinea pigs

Stimulus-evoked theta oscillations are observed in the medial prefrontal cortex (mPFC) when executing a variety of learning tasks. Here, we aimed to further determine whether spontaneous theta-band (5.0-10.0 Hz) oscillations in the mPFC predicted the subsequent behavioral performance in trace eyeblink conditioning (TEBC), in which the conditioned stimulus (CS) was separated from the unconditioned stimulus (US) by 500 ms trace interval. By recording local field potentials (LFP) signals in the guinea pigs performing the TEBC task, we found that, a higher mPFC relative theta ratio [theta/(delta+beta)] during the baseline (850-ms period prior to the onset of the CS) was predictive of higher magnitude and more adaptive timing rather than faster acquisition of trace conditioned eyeblink responses (CR). However, the prediction of baseline mPFC theta activity was time-limited to the well-learning stage. Additionally, the relative power of mPFC theta activity did not correlate with the CR performance if the trace interval between the CS and the US was shortened to 100 ms. These results suggest that the brain state in which the baseline mPFC theta activity is present or absent is detrimental for the subsequent performance of trace CRs especially when the asymptotic learning state is achieved.

Aging in the cerebellum and hippocampus and associated behaviors over the adult life span of CB6F1 mice

In the present study we examined the effects of normal aging in the hippocampus and cerebellum, as well as behaviors associated with these substrates. A total of 67 CB6F1 hybrid mice were tested at one of five ages (4, 8, 12, 18 or 25 months) on the context pre-exposure facilitation effect (CPFE) modification of fear conditioning, rotorod, Barnes maze, acoustic startle, Morris water maze (MWM) and 500-ms trace eyeblink classical conditioning (EBCC). Behavioral tasks were chosen to increase the ability to detect age-related changes in learning, as trace EBCC is considered a more difficult paradigm (compared to delay EBCC) and the CPFE has been found to be more sensitive to hippocampus insults than standard contextual fear conditioning. To assess the effects of age on the brain, hippocampus volume was calculated and unbiased stereology was used to estimate the number of Purkinje neurons in the cerebellar cortex. A significant, age-related loss of Purkinje neurons was found-beginning at 12 months of age-and hippocampus volume remained stable over the adult life span. Age-related impairment was found, beginning at 12-18 months in the rotorod, and mice with fewer Purkinje neurons showed greater impairment in this task. CB6F1 mice retained auditory acuity across the life span and mice aged 25 months showed significant age-related impairment in the EBCC task; however, deficits were not associated with the loss of Purkinje neurons. Although the CPFE task is considered more sensitive to hippocampus insult, no age-related impairment was found. Spatial memory retention was impaired in the Barnes maze at 25 months, but no significant deficits were seen in the MWM. These results support the finding of differential aging in the hippocampus and cerebellum.

Functional inactivation of orexin 1 receptors in the cerebellum disrupts trace eyeblink conditioning and local theta oscillations in guinea pigs

The cerebellum plays an essential role in motor learning. Recently, orexins, the newfound lateral hypothalamic neuropeptides, have been found to excite Purkinje cells in the cerebellar cortex and neurons in the deep cerebellar nuclei (DCN). However, little is known about their roles in cerebellum-dependent motor learning. Therefore, the present study was designed to investigate the functional significance of hypothalamic orexinergic system during trace eyeblink conditioning, a tractable behavioral model system of cerebellum-dependent motor learning. It was revealed that the orexin 1 receptors (OXR1) were specifically localized on the soma of Purkinje cells and large DCN neurons. Furthermore, interfering with the endogenous orexins' effects on the cerebellum via the selective OXR1 antagonist SB-334867 disrupted the timing rather than the acquisition of trace conditioned eyeblink responses. In addition to the behavioral effects, the SB-334867 prevented the increase in peak amplitude of cerebellar theta oscillations with learning. These results suggest that the endogenous orexins may modulate motor learning via the activation of cerebellar OXR1.

Classical eyeblink conditioning using electrical stimulation of caudal mPFC as conditioned stimulus is dependent on cerebellar interpositus nucleus in guinea pigs

AIM

To determine whether electrical stimulation of caudal medial prefrontal cortex (mPFC) as conditioned stimulus (CS) paired with airpuff unconditioned stimulus (US) was sufficient for establishing eyeblink conditioning in guinea pigs, and whether it was dependent on cerebellar interpositus nucleus.

METHODS

Thirty adult guinea pigs were divided into 3 conditioned groups, and trained on the delay eyeblink conditioning, short-trace eyeblink conditioning, and long-trace eyeblink conditioning paradigms, respectively, in which electrical stimulation of the right caudal mPFC was used as CS and paired with corneal airpuff US. A pseudo conditioned group of another 10 adult guinea pigs was given unpaired caudal mPFC electrical stimulation and the US. Muscimol (1 μg in 1 μL saline) and saline (1 μL) were infused into the cerebellar interpositus nucleus of the animals through the infusion cannula on d 11 and 12, respectively.

RESULTS

The 3 eyeblink conditioning paradigms have been successfully established in guinea pigs. The animals acquired the delay and short-trace conditioned responses more rapidly than long-trace conditioned responses. Muscimol infusion into the cerebellar interpositus nucleus markedly impaired the expression of the 3 eyeblink conditioned responses.

CONCLUSION

Electrical stimulation of caudal mPFC is effective CS for establishing eyeblink conditioning in guinea pigs, and it is dependent on the cerebellar interpositus nucleus.