Developmental changes in eyeblink conditioning and simple spike activity in the cerebellar cortex

The activity of neurons in the cerebellum exhibits learning-related changes during eyeblink conditioning in adult mammals. The induction and preservation of learning-related changes in cerebellar neuronal activity in developing rats may be affected by the level of maturity in cerebellar feedback to its brainstem afferents, including the inferior olive. Developmental changes in cerebellar plasticity were examined by recording the activity of Purkinje cells in eye regions of cerebellar cortical lobule HVI (lobulus simplex) in infant rats during eyeblink conditioning. The percentage and amplitude of eyeblink conditioned responses increased as a function of age. Analyses of Purkinje cell simple spike activity revealed developmental increases in the number of units that exhibited stimulus-evoked and learning-related changes in activity. Moreover, the magnitude of these changes exhibited a substantial age-related increase. The results support the view that the emergence of learning-specific cerebellar plasticity and the ontogeny of eyeblink conditioning are influenced by developmental changes in the synaptic interactions within brainstem-cerebellum circuits.

Synapses with a segmented, completely partitioned postsynaptic density express more AMPA receptors than other axospinous synaptic junctions

Axospinous perforated synapses of one morphological subtype exhibit multiple transmission zones, each one being formed by an axon terminal protrusion apposing a postsynaptic density (PSD) segment and separated from others by complete spine partitions. Such segmented, completely partitioned (SCP) synapses have been implicated in synaptic plasticity and postulated to be exceptionally efficacious. The present study explored the validity of this supposition. Postembedding immunogold electron microscopy was used for quantifying the postsynaptic AMPA receptor (AMPAR) expression, which is widely regarded as a major determinant of synaptic efficacy. Various subtypes of axospinous synapses were examined in the rat CA1 stratum radiatum. The results showed that the number of immunogold particles for AMPARs in SCP synapses markedly and significantly exceeded that in other perforated subtypes (by 101% on the average) and in nonperforated immunopositive synapses (by 1086%). Moreover, the particle number per single PSD segment, each of which also contained NMDA receptors, was significantly higher than that per nonperforated PSD (by 485%). SCP synapses also exhibited a higher particle density per unit PSD area, as well as a larger overall PSD area as compared with other synaptic subtypes. Analysis of covariance revealed that the high AMPAR expression in SCP synapses was related to the segmented PSD configuration, not only to the PSD size. Moreover, the subpopulations of SCP and other perforated synapses with either overlapping or equal PSD sizes differed in AMPAR content and concentration, with both measures being significantly higher in SCP synapses. Thus, the elevated AMPAR expression in SCP synapses is associated with the presence of separate PSD segments, not only with their large PSD area. These findings are consistent with the idea that SCP synapses have a relatively greater efficacy and may support maximal levels of synaptic enhancement characteristic of certain forms of synaptic plasticity such as the early LTP phase.

Changes in pharyngeal corticobulbar excitability and swallowing behavior after oral stimulation

Faucial pillar (FP) stimulation is commonly used in swallowing rehabilitation, yet its physiological basis remains uncertain. We investigated the effects of intraoral FP stimulation on human corticobulbar excitability and swallowing behavior, to explore the possibility of a central mechanism for functional change. In 10 healthy subjects, corticobulbar projections to pharynx were investigated with transcranial magnetic stimulation, via intraluminal electrodes, before and up to 1 h after 10 min of electrical FP stimulation with three frequencies (0.2, 1, and 5 Hz) or sham and peripheral (median nerve) stimulation. In a second study, swallowing behavior was assessed with videofluoroscopy before and after FP stimulation. FP stimulation at 5 Hz inhibited the corticobulbar projection (-14 +/- 6%, P < 0.02) and lengthened swallow response time (+114 +/- 24%, P = 0.02). By comparison, FP stimulation at 0.2 Hz facilitated this projection (+60 +/- 28%, P < 0.04), without enhancing swallowing behavior. Neither 1-Hz, sham, nor median nerve stimulation altered excitability. Thus changes in corticobulbar excitability to FP stimulation are frequency dependent with implications for the treatment for neurogenic swallowing dysfunction.

Optimization of Scanning Parameters for Multi-slice CT Colonography: Experiments with Synthetic and Animal Phantoms

AIM

To determine the optimal collimation, pitch, tube current and reconstruction interval for multi-slice computed tomography (CT) colonography with regard to attaining satisfactory image quality while minimizing patient radiation dose.

MATERIALS AND METHODS

Multi-slice CT was performed on plastic, excised pig colon and whole pig phantoms to determine optimal settings. Performance was judged by detection of simulated polyps and statistical measures of the image parameters. Fat and muscle conspicuity was measured from images of dual tube-current prone/supine patient data to derive a measure of tube current effects on tissue contrast.

RESULTS

A collimation of 4 x 2.5 mm was sufficient for detection of polyps 4 mm and larger, provided that a reconstruction interval of 1.25 mm was used. A pitch of 1.5 allowed faster scanning and reduced radiation dose without resulting in a loss of important information, i.e. detection of small polyps, when compared with a pitch of 0.75. Tube current and proportional radiation dose could be lowered substantially without deleterious effects on the detection of the air-mucosal interface, however, increased image noise substantially reduced conspicuity of different tissues.

CONCLUSION

An optimal image acquisition set-up of 4 x 2.5 mm collimation, reconstruction interval of 1.25 mm, pitch of 1.5 and dual prone/supine scan of 40/100 mA tube current is proposed for our institution for scanning symptomatic patients. Indications are that where CT colonography is used for colonic polyp screening in non-symptomatic patients, a 40 mA tube current could prove satisfactory for both scans.

Developmental changes in evoked Purkinje cell complex spike responses

The development of synaptic interconnections between the cerebellum and inferior olive, the sole source of climbing fibers, could contribute to the ontogeny of certain forms of motor learning (e.g., eyeblink conditioning). Purkinje cell complex spikes are produced exclusively by climbing fibers and exhibit short- and long-latency activity in response to somatosensory stimulation. Previous studies have demonstrated that evoked short- and long-latency complex spikes generally occur on separate trials and that this response segregation is regulated by inhibitory feedback to the inferior olive. The present experiment tested the hypothesis that complex spikes evoked by periorbital stimulation are regulated by inhibitory feedback from the cerebellum and that this feedback develops between postnatal days (PND) 17 and 24. Recordings from individual Purkinje cell complex spikes in urethan-anesthetized rats indicated that the segregation of short- and long-latency evoked complex spike activity emerges between PND17 and PND24. In addition, infusion of picrotoxin, a GABAA-receptor antagonist, into the inferior olive abolished the response pattern segregation in PND24 rats, producing evoked complex spike response patterns similar to those characteristic of younger rats. These data support the view that cerebellar feedback to the inferior olive, which is exclusively inhibitory, undergoes substantial changes in the same developmental time window in which certain forms of motor learning emerge.

Long-term retention of the classically conditioned eyeblink response in rats

Retention of the classically conditioned eyeblink response in rats was tested with a conditioned stimulus (CS)-alone extinction test and 2 sessions of reacquisition training. Retention of the eyeblink conditioned response (CR) during both tests was highest 24 hr and 1 month after initial acquisition. Three months after initial acquisition, responding during the CS-alone test was at baseline, but there was significant savings during reacquisition. By 6 months after initial acquisition, the memory for the eyeblink CR was not expressed in either test. The group differences in retention, despite initial acquisition of the eyeblink CR to equal levels, suggest that rat eyeblink conditioning may provide a useful behavioral model for studying the neural processes underlying memory retention and loss.

Addition of inhibition in the olivocerebellar system and the ontogeny of a motor memory

The developmental emergence of learning has traditionally been attributed to the maturation of single brain regions necessary for learning in adults, rather than to the maturation of synaptic interactions within neural systems. Acquisition and retention of a simple form of motor learning, classical conditioning of the eyeblink reflex, depends on the cerebellum and interconnected brainstem structures, including the inferior olive. Here, we combined unit recordings from Purkinje cells in eye regions of the cerebellar cortex and quantitative electron microscopy of the inferior olive to show that the developmental emergence of eyeblink conditioning in rats is associated with the maturation of inhibitory feedback from the cerebellum to the inferior olive. The results are consistent with previous work in adult animals and indicate that the maturation of cerebellar inhibition within the inferior olive may be a critical factor for the formation and retention of learning-specific cerebellar plasticity and eyeblink conditioning.

Ontogeny of eyeblink conditioned response timing in rats

Eyeblink conditioned response (CR) timing was assessed in adult and infant rats. In Experiment 1, adult rats were trained with a 150-ms tone conditioned stimulus (CS) paired with a periorbital shock unconditioned stimulus (US; presented at 200- or 500-ms interstimulus intervals [ISIs]). The rats acquired CRs with 2 distinct peaks that occurred just before the US onset times. Experiments 2 and 3 examined developmental changes in CR timing in pups trained on Postnatal Days 24-26 or 32-34. Experiment 3 used a delay conditioning procedure in which the tone CS continued throughout the ISIs. Pups of both ages exhibited robust conditioning. However, there were age-related increases in the percentage of double-peaked CRs and in CR timing precision. Ontogenetic changes in eyeblink CR timing may be related to developmental changes in cerebellar cortical or hippocampal function.

Differences in the expression of AMPA and NMDA receptors between axospinous perforated and nonperforated synapses are related to the configuration and size of postsynaptic densities

Axospinous synapses are traditionally divided according to postsynaptic density (PSD) configuration into a perforated subtype characterized by a complex-shaped PSD and nonperforated subtype exhibiting a simple-shaped, disc-like PSD. It has been hypothesized that perforated synapses are especially important for synaptic plasticity because they have a higher efficacy of impulse transmission. The aim of the present study was to test this hypothesis. The number of postsynaptic AMPA receptors (AMPARs) is widely regarded as the major determinant of synaptic efficacy. Therefore, the expression of AMPARs was evaluated in the two synaptic subtypes and compared with that of NMDA receptors (NMDARs). Postembedding immunogold electron microscopy was used to quantify the immunoreactivity following single labeling of AMPARs or NMDARs in serial sections through the CA1 stratum radiatum of adult rats. The results showed that all perforated synapses examined were immunopositive for AMPARs. In contrast, only a proportion of nonperforated synapses (64% on average) contained immunogold particles for AMPARs. The number of immunogold particles for AMPARs was markedly and significantly higher in perforated synapses than in immunopositive nonperforated synapses. Although all synapses of both subtypes were NMDAR immunopositive perforated synapses contained significantly more immunogold particles for NMDARs than nonperforated ones. Multivariate analysis of variance revealed that the mode of AMPAR and NMDAR expression is related to the complexity of PSD configuration, not only to PSD size. These findings support the notion that perforated synapses may evoke larger postsynaptic responses relative to nonperforated synapses and, hence, contribute to an enhancement of synaptic transmission associated with some forms of synaptic plasticity.

Medial dorsal thalamic lesions impair blocking and latent inhibition of the conditioned eyeblink response in rats

The effects of lesions of the medial dorsal thalamic nucleus (MD) on blocking and latent inhibition (LI) of the rat eyeblink response were examined in the present study. Previous work has demonstrated that the cingulate cortex and related thalamic areas are involved in processing conditioning stimuli throughout training. The experiments in the present study tested the hypothesis that disruption of cingulothalamic stimulus processing produced by lesions of the MD would impair 2 types of associative learning that involve decremental changes in attention. In Experiment 1, MD lesions severely impaired blocking. In Experiment 2, MD lesions severely impaired LI. The results indicate that lesions of the MD impair incremental, decremental, or both types of changes in stimulus processing during learning.

Neuronal correlates of conditioned inhibition of the eyeblink response in the anterior interpositus nucleus

Conditioned inhibition (CI) of the rat eyeblink response and the neuronal correlates of CI in the cerebellar interpositus nucleus were examined in the present study. In Experiment 1, CI was established with a novel, 3-group design. In Experiment 2, neuronal activity in the anterior interpositus nucleus was recorded during CI training and testing. Each rat was given 2 training phases and then tested for CI with summation and retardation tests. Rats given CI training showed behavioral inhibition compared with rats in 2 control groups. Neuronal activity in the anterior interpositus nucleus correlated with behavioral responding during discrimination training and during the summation test. The results suggest that neurons in the cerebellar anterior interpositus nucleus may participate in the acquisition or expression of CI.

Blockade of GABAA receptors in the interpositus nucleus modulates expression of conditioned excitation but not conditioned inhibition of the eyeblink response

The cerebellum and related brainstem structures are essential for excitatory eyeblink conditioning. Recent evidence indicates that the cerebellar interpositus and lateral pontine nuclei may also play critical roles in conditioned inhibition (CI) of the eyeblink response. The current study examined the role of GABAergic inhibition of the interpositus nucleus in retention of CI. Male Long-Evans rats were implanted with a cannula positioned just above or in the anterior interpositus nucleus before training. The rats were trained with two different tones and a light as conditioned stimuli, and a periorbital shock as the unconditioned stimulus. CI training consisted of four phases: 1) excitatory conditioning (8 kHz tone paired with shock); 2) feature-negative discrimination (2 kHz tone paired with shock or 2 kHz tone concurrent with light); 3) summation test (8 kHz tone or 8 kHz tone concurrent with light); and 4) retardation test (light paired with shock). After reaching a criterion level of performance on the feature-negative discrimination (40% discrimination), 0.5 microl picrotoxin (a GABAA receptor antagonist) was infused at one of four concentrations, each concentration infused during separate test sessions. Picrotoxin transiently impaired conditioned responses during trials with the excitatory stimulus (tone) in a dose-dependent manner, but did not significantly impact responding to the inhibitory compound stimulus (tone-light). The results suggest that expression of conditioned inhibition of the eyeblink conditioned response does not require GABAergic inhibition of neurons in the anterior interpositus nucleus.

Ontogenetic changes in the neural mechanisms of eyeblink conditioning

The rodent eyeblink conditioning paradigm is an ideal model system for examining the relationship between neural maturation and the ontogeny of associative learning. Elucidation of the neural mechanisms underlying the ontogeny of learning is tractable using eyeblink conditioning because the necessary neural circuitry (cerebellum and interconnected brainstem nuclei) underlying the acquisition and retention of the conditioned response (CR) has been identified in adult organisms. Moreover, the cerebellum exhibits substantial postnatal anatomical and physiological maturation in rats. The eyeblink CR emerges developmentally between postnatal day (PND) 17 and 24 in rats. A series of experiments found that the ontogenetic emergence of eyeblink conditioning is related to the development of associative learning and not related to changes in performance. More recent studies have examined the relationship between the development of eyeblink conditioning and the physiological maturation of the cerebellum, a brain structure that is necessary for eyeblink conditioning in adult organisms. Disrupting cerebellar development with lesions or antimitotic treatments impairs the ontogeny of eyeblink conditioning. Studies of the development of physiological processes within the cerebellum have revealed striking ontogenetic changes in stimulus-elicited and learning-related neuronal activity. Neurons in the interpositus nucleus and Purkinje cells in the cortex exhibit developmental increases in neuronal discharges following the unconditioned stimulus (US) and in neuronal discharges that model the amplitude and time-course of the eyeblink CR. The developmental changes in CR-related neuronal activity in the cerebellum suggest that the ontogeny of eyeblink conditioning depends on the development of mechanisms that establish cerebellar plasticity. Learning and the induction of neural plasticity depend on the magnitude of the US input to the cerebellum. The role of developmental changes in the efficacy of the US pathway has been investigated by monitoring neuronal activity in the inferior olive and with stimulation techniques. The results of these experiments indicate that the development of the conditioned eyeblink response may depend on dynamic interactions between multiple developmental processes within the eyeblink neural circuitry.

Developmental changes in eye-blink conditioning and neuronal activity in the inferior olive

Neuronal activity was recorded in the dorsal accessory inferior olive in infant rats during classical conditioning of the eye-blink response. The percentage and amplitude of eye-blink conditioned responses (CRs) increased as a function of age. The magnitude of the neuronal response to the unconditioned stimulus (US) decreased with age. There were also age-specific modifications of US-elicited inferior olive neuronal activity during paired trials in which a conditioned eye-blink response was performed. The results indicate that the development of the conditioned eye-blink response may depend on dynamic interactions between multiple developmental processes within the eye-blink circuitry. Differences in the functional maturity of olivo-cerebellar pathways may limit the induction of plasticity in the cerebellum and thereby limit the development of eye-blink conditioned responses.

Lesions of the perirhinal cortex impair sensory preconditioning in rats

The effects of lesions of the perirhinal cortex on the development of associations between two conditioned stimuli (CSs) were examined with a sensory preconditioning procedure. Rats were given either bilateral electrolytic lesions of the perirhinal cortex or control surgery. They were then given either paired or unpaired presentations of a light CS and a tone CS. All of the rats were then given eyeblink conditioning procedures that involved paired presentations of either the light or tone and a periorbital shock unconditioned stimulus (US). The rats were finally given a test session that consisted of unpaired presentations of the tone and light CSs. Sensory preconditioning was established in the control group, but not in the lesion group. The findings are consistent with the view that the perirhinal cortex is involved in forming associations between neutral stimuli (even in the absence of reinforcement).

Developmental changes in eye-blink conditioning and neuronal activity in the cerebellar interpositus nucleus

Neuronal activity was recorded in the cerebellar interpositus nucleus in infant rats during classical conditioning of the eye-blink response. The percentage and amplitude of eye-blink conditioned responses increased as a function of postnatal age. Learning-specific neuronal activity in the cerebellum emerged ontogenetically in parallel with the eye-blink conditioned response. There were also age-specific changes in neuronal activity after the onset of the conditioned and unconditioned stimuli. The results indicate that the development of the eye-blink conditioned response may depend on the development of stimulus-evoked neuronal responses and learning-specific plasticity in the cerebellum. Functional immaturity in the afferent neural pathways may limit the induction of neural plasticity in the cerebellum and thereby limit the development of the eye-blink conditioned response.

Neuronal activity in the cerebellar interpositus and lateral pontine nuclei during inhibitory classical conditioning of the eyeblink response

Single-unit neuronal activity was recorded from the cerebellar interpositus nucleus and lateral pontine nuclei during conditioned inhibition of the eyeblink response in rats. Conditioned inhibition training sessions included 100 trials/day for 12 days. During each training session, the rats were given 50 presentations of a tone conditioned stimulus (CS) that was paired with a brief periocular shock unconditioned stimulus (US). They were also given 50 presentations of a compound stimulus that included the tone-CS and a light-CS. The compound-CS was not paired with the US. The two types of trials were mixed throughout the session and presented in an irregular sequence. This training procedure resulted in significant inhibition of the eyeblink response during the compound-CS. Neurons in the interpositus and lateral pontine nuclei exhibited significantly less activity during the compound-CS relative to the tone-CS. The suppression of cerebellar and pontine learning-related neuronal activity during the inhibitory CS may be critical for inhibiting the conditioned eyeblink response.

Two cases of seminal vesicle fistula

Two cases of fistulation into the seminal vesicles are described. One related to Crohn's disease and the other following surgery for carcinoma of the rectum. Both cases were diagnosed by CT sinography. This technique is described and is recommended when attempting to demonstrate the internal communications of difficult perineal fistulae when standard techniques of fistulography fail.

The cost effectiveness of metal oesophageal stenting in malignant disease compared with conventional therapy

Expanding metal oesophageal stents are being used more commonly to palliate patients with inoperable oesophageal carcinoma. Many reports have so far documented their clinical effectiveness, however, their high acquisition cost has caused on-going concern when compared with the cost of conventional therapies. We reviewed 64 consecutive patients with inoperable oesophageal carcinoma, half of whom had received our conventional method of palliation using a variety of techniques including, BICAP diathermy, alcohol injection and Atkinson tube insertion. The other half (32 patients) were treated with expandable metal stents -- Gianturco Z stents (Cook UK Ltd) and uncovered Ultraflex stents (Microvasive, Boston Scientific). The physical amount of resources consumed were identified and measured (number of diagnostic and support procedures, days as in patients, number of day cases or outpatient attending) and an average NHS cost was applied to this resource use. All costs were summated over the period of palliation from the date of the first intervention with palliative intent until death. Although the patients in this study were not randomized, the two groups were matched to ensure comparability in clinical manifestation (uncomplicated biopsy proven oesophageal carcinoma) and the average age of patients from each group. A difference was identified between the length of survival in both patient groups and the analysis was corrected for this by estimating a cost per day of palliative support. Patients palliated with metal stents underwent fewer procedures and spent fewer days in hospital during the time period from presentation until death even when corrected for differences in survival. Patient outcome (effectiveness of palliation) was measured by recording mean dysphagia scores which were recorded before and after palliation. Metal stents were found to lead to a significantly higher improvement in dysphagia in comparison to conventional therapy. In addition, the mortality related to metal oesophageal stents was lower than Atkinson tube insertion. The average cost of palliation was much lower in the metal stent group (mean = pound sterling 2817) compared with the cost in those palliated conventionally (mean = pound sterling 4566). However, once this was corrected for survival the difference in the cost of palliation on a per diem basis was reduced (metal stents = pound sterling 60 per day, conventional group = pound sterling 72 per day). The results of our study indicate that the initial high cost of metal stents is more than outweighed by resource savings elsewhere in the hospital by virtue of reduced need for re-intervention and shorter length of hospital in patient stay. Such cost savings taken in combination with the improved clinical effectiveness and low mortality related to metal stents provide significant support for introducing their use into clinical practice.

Recovery of swallowing after dysphagic stroke relates to functional reorganization in the intact motor cortex

BACKGROUND & AIMS

The aim of this study was to determine the mechanism for recovery of swallowing after dysphagic stroke.

METHODS

Twenty-eight patients who had a unilateral hemispheric stroke were studied 1 week and 1 and 3 months after the stroke by videofluoroscopy. Pharyngeal and thenar electromyographic responses to magnetic stimulation of multiple sites over both hemispheres were recorded, and motor representations were correlated with swallowing recovery.

RESULTS

Dysphagia was initially present in 71% of patients and in 46% and 41% of the patients at 1 and 3 months, respectively. Cortical representation of the pharynx was smaller in the affected hemisphere (5 +/- 1 sites) than the unaffected hemisphere (13 +/- 1 sites; P </= 0.001). Nondysphagic and persistently dysphagic patients showed little change in pharyngeal representation in either hemisphere at 1 and 3 months compared with presentation, but dysphagic patients who recovered had an increased pharyngeal representation in the unaffected hemisphere at 1 and 3 months (15 +/- 2 and 17 +/- 3 vs. 9 +/- 2 sites; P </= 0.02) without change in the affected hemisphere. In contrast, thenar representation increased in the affected hemisphere but not the unaffected hemisphere at 1 and 3 months (P </= 0.01).

CONCLUSIONS

Return of swallowing after dysphagic stroke is associated with increased pharyngeal representation in the unaffected hemisphere, suggesting a role for intact hemisphere reorganization in recovery.