Involvement of the ipsilateral and contralateral cerebellum in the acquisition of unilateral classical eyeblink conditioning in guinea pigs

Neural correlates of gait adaptation in younger and older adults

Mobility decline is a major concern for older adults. A key component of maintaining mobility with advancing age is the ability to learn and adapt to the environment. The split-belt treadmill paradigm is an experimental protocol that tests the ability to adapt to a dynamic environment. Here we examined the magnetic resonance imaging (MRI) derived structural neural correlates of individual differences in adaptation to split-belt walking for younger and older adults. We have previously shown that younger adults adopt an asymmetric walking pattern during split-belt walking, particularly in the medial-lateral (ML) direction, but older adults do not. We collected T[Formula: see text]-weighted and diffusion-weighted MRI scans to quantify brain morphological characteristics (i.e. in the gray matter and white matter) on these same participants. We investigated two distinct questions: (1) Are there structural brain metrics that are associated with the ability to adopt asymmetry during split-belt walking; and (2) Are there different brain-behavior relationships for younger and older adults? Given the growing evidence that indicates the brain has a critical role in the maintenance of gait and balance, we hypothesized that brain areas commonly associated with locomotion (i.e. basal ganglia, sensorimotor cortex, cerebellum) would be associated with ML asymmetry and that older adults would show more associations between split-belt walking and prefrontal brain areas. We identified multiple brain-behavior associations. More gray matter volume in the superior frontal gyrus and cerebellar lobules VIIB and VIII, more sulcal depth in the insula, more gyrification in the pre/post central gyri, and more fractional anisotropy in the corticospinal tract and inferior longitudinal fasciculus corresponded to more gait asymmetry. These associations did not differ between younger and older adults. This work progresses our understanding of how brain structure is associated with balance during walking, particularly during adaptation.

Medial Prefrontal Cortex-Pontine Nuclei Projections Modulate Suboptimal Cue-Induced Associative Motor Learning

Diverse and powerful mechanisms have evolved to enable organisms to modulate learning and memory under a variety of survival conditions. Cumulative evidence has shown that the prefrontal cortex (PFC) is closely involved in many higher-order cognitive functions. However, when and how the medial PFC (mPFC) modulates associative motor learning remains largely unknown. Here, we show that delay eyeblink conditioning (DEC) with the weak conditioned stimulus (wCS) but not the strong CS (sCS) elicited a significant increase in the levels of c-Fos expression in caudal mPFC. Both optogenetic inhibition and activation of the bilateral caudal mPFC, or its axon terminals at the pontine nucleus (PN) contralateral to the training eye, significantly impaired the acquisition, recent and remote retrieval of DEC with the wCS but not the sCS. However, direct optogenetic activation of the contralateral PN had no significant effect on the acquisition, recent and remote retrieval of DEC. These results are of great importance in understanding the elusive role of the mPFC and its projection to PN in subserving the associative motor learning under suboptimal learning cue.

Spontaneous behavior in noise and silence: a possible new measure to assess tinnitus in Guinea pigs

This study describes two experiments that were conducted in search for a behavioral paradigm to test for tinnitus in guinea pigs. Conditioning paradigms are available to determine the presence of tinnitus in animals and are based on the assumption that tinnitus impairs their ability to detect silent intervals in continuous noise. Guinea pigs have not been subjected to these paradigms yet; therefore, we investigated whether guinea pigs could be conditioned in the two-way shuttle-box paradigm to respond to silent intervals in noise. Even though guinea pigs could be trained relatively easy to respond to the presence of a noise interval, training guinea pigs to silent intervals in noise was unsuccessful. Instead, it appeared that they became immobile when the continuous stimulus was suddenly stopped. This was confirmed by the next experiment, in which we subjected guinea pigs to alternating intervals of noise and silence with a random duration between 30 and 120 s. Indeed, guinea pigs were significantly longer immobile during silence compared to during noise. By interpreting immobility as a signature of perceiving silence, we hypothesized that the presence of tinnitus would reduce immobility in silence. Therefore, we unilaterally exposed one group of guinea pigs to an 11-kHz tone of 124 dB sound pressure level for 1 h. A subset of the exposed animals was significantly more active in silence, but also more active in noise, as compared to the control group. The increased mobility during silent intervals might represent tinnitus. However, the increased mobility in noise of this group implies that the observed behavior could have derived from, e.g., an overall increase in activity. Therefore, conducting validation experiments is very important before implementing this method as a new screening tool for tinnitus. Follow-up experiments are discussed to further elucidate the origin of the increased mobility in both silence and noise.

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.

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

Learning-induced changes of synaptic ultrastructure have long been proposed as a mechanism that may contribute to support memory formation. Although recent studies have demonstrated that the interpositus nuclei (IN) play critical role in acquisition and retention of trace conditioned eyeblink responses (CRs), there is now limited evidence associating trace eyeblink conditioning with changes of synaptic ultrastructure in the IN. Here, we investigated this issue using a transmission electron microscope. Adult guinea pigs were randomly allocated to either a trace-paired, delay-paired, unpaired or exposure-only condition. The IN tissue was taken for morphological analysis 1h after the completion of the tenth training session. Serial section analysis of synaptic ultrastructure revealed that trace eyeblink conditioning induced increases in the thickness of excitatory PSD. Classification of the synapses into shape subtypes indicated that the increased thickness of excitatory PSD was mainly attributable to increase in the concave- and convex-shaped synapses. On the contrary, trace eyeblink conditioning resulted in decreases in the thickness of inhibitory PSD. Specifically, these significant changes of PSD thickness were limited to occur in the animals with good behavioral performance. Further analysis of correlations between the trace CR performance and synaptic ultrastructural modifications showed that the thickness of excitatory PSD within the IN correlated with the peak amplitude of trace CRs, whereas the thickness of inhibitory PSD correlated with the onset latency. The present findings suggest that trace eyeblink conditioning induces structural plasticity in the IN, which may play a crucial role in acquiring and executing adaptive eyeblink movements.

Disrupted topography of the acquired trace-conditioned eyeblink responses in guinea pigs after suppression of cerebellar cortical inhibition to the interpositus nucleus

Trace conditioning of the eyeblink reflex, a form of associative motor learning in which presentations of the conditioned stimulus (CS) and the unconditioned stimulus (US) are separated in time by a silent trace interval, requires intact forebrain structures such as the hippocampus and medial prefrontal cortex. Recently, increased learning-related activities have also been observed in specific cerebellar cortical area such as the lobule of HVI during this conditioning task. To date, however, it remains controversial how the cerebellar cortex contributes to trace eyeblink conditioning. In the present study, we addressed this issue by reversibly suppressing the cerebellar cortical inhibition via microinjections of the GABA(A) receptor antagonist bicuculline methiodide (BICM) into the interpositus nucleus of guinea pigs. We showed that, in the well-trained guinea pigs, the BICM administrations failed to abolish the acquired trace-conditioned eyeblink responses (CRs). Although the acquired trace CRs were mostly retained, their peak latencies were shortened and their peak amplitudes diminished as evidenced by only half of the spared trace CRs preserving the topography of adaptive peak latencies or middle-/high-peak amplitudes. In the same animals, the acquired trace CRs were abolished by microinjections of the GABA(A) receptor agonist muscimol and were unaffected by microinjections of the artificial cerebrospinal fluid. Furthermore, we demonstrated that with concurrent BICM-induced suppression of the cerebellar cortical inhibition and presentations of the tone CSs in the guinea pigs receiving unpaired conditioning training, CR-like eyeblink responses were not generated. Altogether, these results support the hypothesis that GABAergic neurotransmission from cerebellar cortex to the interpositus nucleus may participate in regulating the expression of acquired trace CRs.

A neurocomputational model of classical conditioning phenomena: a putative role for the hippocampal region in associative learning

Some existing models of hippocampal function simulate performance in classical conditioning tasks using the error backpropagation algorithm to guide learning (Gluck, M.A., and Myers, C.E., (1993). Hippocampal mediation of stimulus representation: a computational theory. Hippocampus, 3(4), 491-516.). This algorithm is not biologically plausible because it requires information to be passed backward through layers of nodes and assumes that the environment provides information to the brain about what correct outputs should be. Here, we show that the same information-processing function proposed for the hippocampal region in the Gluck and Myers (1993) model can also be implemented in a network without using the backpropagation algorithm. Instead, our newer instantiation of the theory uses only (a) Hebbian learning methods which match more closely with synaptic and associative learning mechanisms ascribed to the hippocampal region and (b) a more plausible representation of input stimuli. We demonstrate here that this new more biologically plausible model is able to simulate various behavioral effects, including latent inhibition, acquired equivalence, sensory preconditioning, negative patterning, and context shift effects. In addition, the newer model is able to address some new phenomena including the effect of the number of training trials on blocking and overshadowing.