Null cell adenomas of the pituitary gland: an institutional review of their clinical imaging and behavioral characteristics.

The aim of the study was to establish if the null cell adenoma (NCA) forms a distinct subgroup with unique clinicopathological characteristics within the nonfunctioning pituitary adenoma group particularly in relation to the silent gonadotroph adenomas (SGAs). We identified 31 patients with the pathological diagnosis of NCA verified by routine histology and immunohistochemistry with distinct differentiation from SGAs by an established negative testing for SF-1 at the Toronto Western Hospital between December 2004 and August 2010. We reviewed their demographic data, clinical features, magnetic resonance imaging, and the histologic variables: MIB-1, FGFR4, and P27. We compared these to 63 SGAs identified within the same period. All the NCAs were macroadenomas with diameter ranging from 15-57 mm and tumor volumes between 1.95-53.5 mm(3). Preoperative cavernous sinus tumor growth was able to predict the presence of a residual after surgery (p = 0.023). Furthermore, preoperative cavernous sinus extension (p = 0.002) and negative P27 expression (p = 0.035) were able to independently predict the subsequent growth of the postoperative tumor residual. Comparing the NCA to SGA, we found that MIB-1 was higher in NCA (mean ± SD = 3.43 ± 2.76 %) compared to SGAs (mean ± SD = 2.49 ± 1.41 %) (p = 0.044). The preoperative and postoperative tumor volume doubling times (TVDTs) displayed a negative correlation in the SGA (r = -0.855, p = 0.002) while in the NCA, a positive correlation was evident (r = 0.718, p = 0.029). Our study suggests that the NCAs are a distinct group with differing behavioral characteristics from the SGAs. It also appears that the finding of cavernous sinus extension on preoperative imaging and a negative P27 expression on immunohistochemistry in NCAs may be valuable tools in predicting residual tumor growth which may impact on postoperative care.

The diagnosis and treatment of pseudoprogression, radiation necrosis and brain tumor recurrence.

Radiation therapy is an important modality used in the treatment of patients with brain metastatic disease and malignant gliomas. Post-treatment surveillance often involves serial magnetic resonance imaging. A challenge faced by clinicians is in the diagnosis and management of a suspicious gadolinium-enhancing lesion found on imaging. The suspicious lesion may represent post-treatment radiation effects (PTRE) such as pseudoprogression, radiation necrosis or tumor recurrence. Significant progress has been made in diagnostic imaging modalities to assist in differentiating these entities. Surgical and medical interventions have also been developed to treat PTRE. In this review, we discuss the pathophysiology, clinical presentation, diagnostic imaging modalities and provide an algorithm for the management of pseudoprogression, radiation necrosis and tumor recurrence.

Enhancing memory formation by altering protein phosphorylation balance.

In Lymnaea, aerial respiration can be operantly conditioned and depending on the training procedure employed two forms of memory can result: intermediate-term (ITM) and long-term memory (LTM). ITM, which persists for 3h, is dependent on de novo protein synthesis whilst LTM, which persists for at least 24 h, is dependent on both de novo protein synthesis and altered gene activity. A single 0.5 h training session (i.e. ITM-training) leaves behind a residual molecular memory trace, which a second bout of ITM-training can activate and boost it to a LTM. Here we extend this finding to show that either inhibiting protein phosphatase activity with okadaic acid (1 microM), or increasing protein kinase C (PKC) activity and therefore protein phosphorylation with bryostatin (0.25 ng/mL) treatment prior to ITM-training, results in a LTM. However, following right pedal dorsal 1 (RPeD1) soma ablation neither of these treatments are effective in producing LTM following ITM-training, indicating transcription is a necessity. These findings suggest that the balance between phosphorylation and dephosphorylation in neurons is a key factor for LTM formation.

One-trial conditioning of aerial respiratory behaviour in Lymnaea stagnalis.

Repeated spaced training sessions of contingent tactile stimulation to the pneumostome as it opens are required to cause long-term memory (LTM) formation of aerial respiratory behaviour making if difficult to determine exactly when memory forms. We have devised a single-trial aversive operant conditioning training procedure in Lymnaea to be better able to elucidate the causal mechanisms of LTM formation. Observations of baseline breathing behaviour in hypoxia were first made. Twenty-four hours later the snails were trained using the single trial procedure, by placing them in a small Petri dish containing 4 ml of 25 mM KCl for 30-35s as soon as the first pneumostome opening in hypoxia was attempted. LTM was present if (1) breathing behaviour following training was significantly less than before; and (2) breathing behaviour post-training was significantly less in experimental groups than in yoked control groups. LTM persisted for 24 h but not 48 h. Yoked controls that received an aversive stimulus not contingent with pneumostome opening had no evidence of memory. Cooling directly after, but not at any other time, blocks LTM formation. LTM formation was also prevented by removal of the cell body of the neuron RPeD1 before training.

Stressful stimuli modulate memory formation in Lymnaea stagnalis.

Stress has been shown to be a strong modulator of learning and memory in animals. We employ operant training of aerial respiratory behaviour in our model system, the pond snail Lymnaea stagnalis, to show that application of an acute consistent physical stressor enhances memory formation. A single 30 min operant conditioning training session, which normally results in intermediate-term memory (ITM) persisting 3h, results in long-term memory (LTM) persisting 24h if immediately preceded or followed by a stressor, for example a 30s exposure to 25 mM KCl. Other physical stressors (0.3% quinine-HCl or quick cooling and warming) similarly enhance memory formation. The memory is context specific and is not seen after the application of too much or too little stress. The memory can be extinguished by exposing snails to the hypoxic training environment and withholding reinforcing stimuli. The LTM that results from 30 min of training and stressor exposure is dependent on de novo protein synthesis and gene transcription in a single neuron, RPeD1. Because the soma of RPeD1 must be present for memory augmentation by the application of a stressor we are well placed for future investigations to directly determine the specific molecular alterations by which stress primes the formation of LTM.

Canadian Association of Neurosciences Review: learning at a snail's pace.

While learning and memory are related, they are distinct processes each with different forms of expression and underlying molecular mechanisms. An invertebrate model system, Lymnaea stagnalis, is used to study memory formation of a non-declarative memory. We have done so because: (1) We have discovered the neural circuit that mediates an interesting and tractable behaviour; (2) This behaviour can be operantly conditioned and intermediate-term and long-term memory can be demonstrated; and (3) It is possible to demonstrate that a single neuron in the model system is a necessary site of memory formation. This article reviews how Lymnaea has been used in the study of behavioural and molecular mechanisms underlying consolidation, reconsolidation, extinction and forgetting.

A context-specific single contingent-reinforcing stimulus boosts intermediate-term memory into long-term memory.

Following operant conditioning of aerial respiration in Lymnaea, memory forms. Depending on the training procedure either intermediate memory (ITM, < 3 h) or long-term memory (LTM, > 6 h) results. ITM is dependent on de novo protein synthesis whilst LTM is dependent on both transcription and de novo protein synthesis. LTM formation requires the soma of RPeD1 (one of the central pattern generator neurons) to be present. Following activation of a memory, it re-enters a labile state and undergoes a reconsolidation process to restabilize it. During reconsolidation, memory may be updated and/or changed. We add here another consequence of memory reactivation: a single contingent-reinforcing stimulus (SCRS), given in the same context as previous ITM training, boosts a residual memory trace to LTM. Separate cohorts of snails first received the ITM training procedure. In the cohort that received the SCRS 24 h after the last ITM training session, LTM was observed on the following day. LTM was not observed in cohorts that were: (i) given a single noncontingent stimulus; (ii) given the SCRS in a context other than the ITM training; (iii) given a 48-h gap between the last ITM training session and the context-specific SCRS; (iv) cooled immediately after the last ITM training session; (v) cooled immediately after the delivery of the context-specific SCRS; (vi) had the soma of RPeD1 ablated before the presentation of the context-specific SCRS; (vii) received a yoked control procedure. These data lead us to conclude that the context-specific SCRS reactivates a residual molecular memory trace in RPeD1 and boosts it into becoming the substrate for LTM.

Modulation of aerial respiratory behaviour in a pond snail.

Aerial respiratory in Lymnaea is driven by a three-neuron CPG whose sufficiency and necessity has been directly demonstrated. While this CPG is 'hard-wired' it displays a tremendous amount of plasticity. That is, it is possible by employing specific training procedures to alter how it functions in a specific hypoxic environment. Thus, it is possible to study directly the causal mechanisms of long-term memory formation, forgetting, and modulation of the memory at a single cell level. Thus, it is possible to use a relatively simple three-neuron CPG to study not only important questions concerning regulation of important homeostatic mechanisms but to also use it to study how learning and non-declarative memory are mediated at a cellular level.

Boosting intermediate-term into long-term memory.

Aerial respiration in the pond snail Lymnaea stagnalis can be operantly conditioned. Depending on the specific training procedure used (i.e. a 0.5 h vs a 1.0 h interval between training sessions) either intermediate (ITM) or long-term memory (LTM) is formed. ITM, which persists for 2-3 h, is dependent only on de novo protein synthesis, whilst LTM persists for up to 4 weeks and is dependent on both transcription and de novo protein synthesis. We found that although the behavioural phenotype of ITM was not apparent 24 h after the last training session, a residual memory trace was present that serves as a foundation upon which a subsequent ITM-training-procedure builds on to form LTM (i.e. a "changed memory"). This residual memory trace could be perturbed by cooling, the behavioural process of context-specific extinction and by increasing the interval between the training procedures. Furthermore in preparations where the somata of RPeD1 (one of three interneurons in the central pattern generator required for aerial respiratory behavior) had been ablated before training, LTM could not be observed following a second bout of ITM-training. These data support the concept that a molecular memory trace is established as a consequence of ITM-training, which serves as a "permissive substrate", when the ITM memory is made active, sufficient to permit the necessary transcription and translation processes that are causal for LTM formation.

Memory, reconsolidation and extinction in Lymnaea require the soma of RPeD1.

The central pattern generator (CPG) that drives aerial respiratory behaviour in Lymnaea consists of 3 neurons. One of these, RPeD1--the cell that initiates activity in the circuit, plays an absolutely necessary role as a site for memory formation, memory reconsolidation, and extinction. Using an operant conditioning training procedure that results in a long-term non-declarative memory (LTM), we decrease the occurrence of aerial respiratory behaviour. Since snails can still breathe cutaneously learning this procedure is not harmful. Concomitant with behavioural memory are changes in the spiking activity of RPeD1. Going beyond neural correlates of memory we directly show that RPeD1 is a necessary site for LTM formation. Expanding on this finding we show that this neuron is also a necessary site for memory reconsolidation and 'Pavlovian' extinction. As far as we can determine, this is the first time a single neuron has been shown to be a necessary site for these different aspects memory. RPeD1 is thus a key neuron mediating different hierarchical aspects of memory. We are now in a position to determine the necessary neuronal, molecular and proteomic events in this neuron that are causal to memory formation, reconsolidation and extinction.

A molluscan model system in the search for the engram.

A 3-neuron central pattern generator, whose sufficiency and necessity has been directly demonstrated, mediates aerial respiratory behaviour in the pond snail, Lymnaea stagnalis. This behaviour can be operantly conditioned, and this associative learning is consolidated into long-lasting memory. Depending on the operant conditioning training procedure used the learning can be consolidated into intermediate term (ITM) or long-term memory (LTM). ITM persists for only 2-3 h, whilst LTM persists for days to weeks. LTM is dependent on both altered gene activity and new protein synthesis while ITM is only dependent on new protein synthesis. We have now directly established that one of the 3-CPG neurons, RPeD1, is a site of LTM formation and storage. We did this by ablating the soma of RPeD1 and leaving behind a functional primary neurite capable of mediating the necessary synaptic interactions to drive aerial respiratory behaviour by the 3-neuron CPG. However, following soma ablation the neuronal circuit is only capable of mediating learning and ITM. LTM can no longer be demonstrated. However, if RPeD1's soma is ablated after LTM consolidation memory is still present. Thus the soma is not needed for the retention of LTM. Using a similar strategy it may be possible to block forgetting.