Dopamine-induced pruning in monocyte-derived-neuronal-like cells (MDNCs) from patients with schizophrenia

The long lapse between the presumptive origin of schizophrenia (SCZ) during early development and its diagnosis in late adolescence has hindered the study of crucial neurodevelopmental processes directly in living patients. Dopamine, a neurotransmitter consistently associated with the pathophysiology of SCZ, participates in several aspects of brain development including pruning of neuronal extensions. Excessive pruning is considered the cause of the most consistent finding in SCZ, namely decreased brain volume. It is therefore possible that patients with SCZ carry an increased susceptibility to dopamine's pruning effects and that this susceptibility would be more obvious in the early stages of neuronal development when dopamine pruning effects appear to be more prominent. Obtaining developing neurons from living patients is not feasible. Instead, we used Monocyte-Derived-Neuronal-like Cells (MDNCs) as these cells can be generated in only 20 days and deliver reproducible results. In this study, we expanded the number of individuals in whom we tested the reproducibility of MDNCs. We also deepened the characterization of MDNCs by comparing its neurostructure to that of human developing neurons. Moreover, we studied MDNCs from 12 controls and 13 patients with SCZ. Patients' cells differentiate more efficiently, extend longer secondary neurites and grow more primary neurites. In addition, MDNCs from medicated patients expresses less D1R and prune more primary neurites when exposed to dopamine. Haloperidol did not influence our results but the role of other antipsychotics was not examined and thus, needs to be considered as a confounder.

Fluoroethylnormemantine, A Novel Derivative of Memantine, Facilitates Extinction Learning Without Sensorimotor Deficits

BACKGROUND

Memantine, a noncompetitive N-methyl-D-aspartate receptor antagonist, has been approved for use in Alzheimer's disease, but an increasing number of studies have investigated its utility for neuropsychiatric disorders. Here, we characterized a novel compound, fluoroethylnormemtantine (FENM), which was derived from memantine with an extra Fluor in an optimized position for in vivo biomarker labeling. We sought to determine if FENM produced similar behavioral effects as memantine and/or if FENM has beneficial effects against fear, avoidance, and behavioral despair.

METHODS

We administered saline, FENM, or memantine prior to a number of behavioral assays, including paired-pulse inhibition, open field, light dark test, forced swim test, and cued fear conditioning in male Wistar rats.

RESULTS

Unlike memantine, FENM did not produce nonspecific side effects and did not alter sensorimotor gating or locomotion. FENM decreased immobility in the forced swim test. Moreover, FENM robustly facilitated fear extinction learning when administered prior to either cued fear conditioning training or tone reexposure.

CONCLUSIONS

These results suggest that FENM is a promising, novel compound that robustly reduces fear behavior and may be useful for further preclinical testing.

Selective activation of D1 dopamine receptors exerts antidepressant-like activity in rats

BACKGROUND

Major depressive disorder is a common illness that severely decreases psychosocial functioning. Due to the major limitations of current treatments including response failure, it is crucial to develop better therapy strategies. Evidence suggests that dopamine dysregulation might play a major role in major depressive disorder physiopathology.

AIMS

This study investigates whether the dopamine D1 receptor agonist A77636 modulates antidepressant-like activity in rats.

METHODS

Rats were injected with an acute single dose of A77636 (0.75, 1.5 or 3 mg/kg), a potent and selective dopamine D1-like receptor agonist. Their locomotor activity, social interactions and behavioural response to the forced swim test were analysed 30 min after the injection.

RESULTS

During the forced swim test, the D1 agonist dose dependently reduced the immobility while the time of bursting was increased. Social interactions were significantly increased in the animals exposed to 3 mg/kg of A77636 whereas no significant changes were measured in general motor activity.

CONCLUSIONS

The present results provide evidence that pharmacological modulation of D1 receptor by the selective agonist A77636 induces antidepressant-like effects in rats, which encourages further studies regarding D1-specific modulation in major depressive disorder treatment.

Antidepressants Promote and Prevent Cancers

The review states that antidepressants (ADs) increase brain-derived neurotrophic factor (BDNF) transmission concomitantly in the brain and the blood: ADs increasing BDNF synthesis in specific areas of the central nervous system (CNS) could presumably affect megakaryocyte's production of platelets. ADs increase BDNF levels in the CNS and improve mood. In the blood, ADs increase BDNF release from platelets. The hypothesis presented here is that the release of BDNF from platelets contributes to the ADs effects on neurogenesis and on tumor growth in the cancer disease. Oncological studies indicate that chemicals ADs exert an aggravating effect on the cancer disease, possibly by promoting proplatelets formation and enhancing BDNF release from platelets in the tumor.

Exposure to cannabinoids can lead to persistent cognitive and psychiatric disorders

BACKGROUND

Cannabinoids are proposed in a wide array of medical indications. Yet, the evaluation of adverse effects in controlled clinical studies, following the evidence-based model, has partly been bypassed. On the other hand, studies on the consequences of recreational use of cannabis and experimental studies bring some insights on the potential long-term consequences of cannabinoids use.

RESULTS

Epidemiological studies have consistently demonstrated that cannabis use is associated with a risk of persistent cognitive deficits and increased risk of schizophrenia-like psychoses. These risks are modulated by the dose and duration of use, on top of age of use and genetic factors, including partially shared genetic predisposition with schizophrenia. Experimental studies in healthy humans showed that cannabis and its principal psychoactive component, the delta-9-tetrahydrocannabinol (THC), could produce transient, dose-dependent, psychotic symptoms as well as cognitive effects, which can be attenuated by cannabidiol (CBD). Studies in rodents have confirmed these effects and shown that adolescent exposure results in structural changes and impaired synaptic plasticity, impacting fronto-limbic systems that are critically involved in higher brain functions. The endocannabinoid system plays an important role in brain maturation. Its over-activation by cannabinoid receptor type 1 agonists (e.g., THC) during adolescence and the resulting changes in neuroplasticity could alter brain maturation and cause long-lasting changes that persist in the adult brain.

CONCLUSIONS

Exposure to cannabinoids can have long-term impact on the brain, with an inter-individual variability that could be conveyed by personal and family history of psychiatric disorders and genetic background. Adolescence and early adulthood are critical periods of vulnerability.

SIGNIFICANCE

The assessment of benefice-risk balance of medical use of cannabis and cannabinoids needs to carefully explore populations that could be more at-risk of psychiatric and cognitive complications.

The dynamics of stress: a longitudinal MRI study of rat brain structure and connectome

Stress is a well-established trigger for a number of neuropsychiatric disorders, as it alters both structure and function of several brain regions and its networks. Herein, we conduct a longitudinal neuroimaging study to assess how a chronic unpredictable stress protocol impacts the structure of the rat brain and its functional connectome in both high and low responders to stress. Our results reveal the changes that stress triggers in the brain, with structural atrophy affecting key regions such as the prelimbic, cingulate, insular and retrosplenial, somatosensory, motor, auditory and perirhinal/entorhinal cortices, the hippocampus, the dorsomedial striatum, nucleus accumbens, the septum, the bed nucleus of the stria terminalis, the thalamus and several brain stem nuclei. These structural changes are associated with increasing functional connectivity within a network composed by these regions. Moreover, using a clustering based on endocrine and behavioural outcomes, animals were classified as high and low responders to stress. We reveal that susceptible animals (high responders) develop local atrophy of the ventral tegmental area and an increase in functional connectivity between this area and the thalamus, further spreading to other areas that link the cognitive system with the fight-or-flight system. Through a longitudinal approach we were able to establish two distinct patterns, with functional changes occurring during the exposure to stress, but with an inflection point after the first week of stress when more prominent changes were seen. Finally, our study revealed differences in functional connectivity in a brainstem-limbic network that distinguishes resistant and susceptible responders before any exposure to stress, providing the first potential imaging-based predictive biomarkers of an individual's resilience/vulnerability to stressful conditions.

Transdifferentiation of Human Circulating Monocytes Into Neuronal-Like Cells in 20 Days and Without Reprograming

Despite progress, our understanding of psychiatric and neurological illnesses remains poor, at least in part due to the inability to access neurons directly from patients. Currently, there are in vitro models available but significant work remains, including the search for a less invasive, inexpensive and rapid method to obtain neuronal-like cells with the capacity to deliver reproducible results. Here, we present a new protocol to transdifferentiate human circulating monocytes into neuronal-like cells in 20 days and without the need for viral insertion or reprograming. We have thoroughly characterized these monocyte-derived-neuronal-like cells (MDNCs) through various approaches including immunofluorescence (IF), flow cytometry, qRT-PCR, single cell mRNA sequencing, electrophysiology and pharmacological techniques. These MDNCs resembled human neurons early in development, expressed a variety of neuroprogenitor and neuronal genes as well as several neuroprogenitor and neuronal proteins and also presented electrical activity. In addition, when these neuronal-like cells were exposed to either dopamine or colchicine, they responded similarly to neurons by retracting their neuronal arborizations. More importantly, MDNCs exhibited reproducible differentiation rates, arborizations and expression of dopamine 1 receptors (DR1) on separate sequential samples from the same individual. Differentiation efficiency measured by cell morphology was on average 11.9 ± 1.4% (mean, SEM, n = 38,819 cells from 15 donors). To provide context and help researchers decide which in vitro model of neuronal development is best suited to address their scientific question,we compared our results with those of other in vitro models currently available and exposed advantages and disadvantages of each paradigm.

White matter changes in microstructure associated with a maladaptive response to stress in rats

In today's society, every individual is subjected to stressful stimuli with different intensities and duration. This exposure can be a key trigger in several mental illnesses greatly affecting one's quality of life. Yet not all subjects respond equally to the same stimulus and some are able to better adapt to them delaying the onset of its negative consequences. The neural specificities of this adaptation can be essential to understand the true dynamics of stress as well as to design new approaches to reduce its consequences. In the current work, we employed ex vivo high field diffusion magnetic resonance imaging (MRI) to uncover the differences in white matter properties in the entire brain between Fisher 344 (F344) and Sprague-Dawley (SD) rats, known to present different responses to stress, and to examine the effects of a 2-week repeated inescapable stress paradigm. We applied a tract-based spatial statistics (TBSS) analysis approach to a total of 25 animals. After exposure to stress, SD rats were found to have lower values of corticosterone when compared with F344 rats. Overall, stress was found to lead to an overall increase in fractional anisotropy (FA), on top of a reduction in mean and radial diffusivity (MD and RD) in several white matter bundles of the brain. No effect of strain on the white matter diffusion properties was observed. The strain-by-stress interaction revealed an effect on SD rats in MD, RD and axial diffusivity (AD), with lower diffusion metric levels on stressed animals. These effects were localized on the left side of the brain on the external capsule, corpus callosum, deep cerebral white matter, anterior commissure, endopiriform nucleus, dorsal hippocampus and amygdala fibers. The results possibly reveal an adaptation of the SD strain to the stressful stimuli through synaptic and structural plasticity processes, possibly reflecting learning processes.

Hyper-responsivity to stress in rats is associated with a large increase in amygdala volume. A 7T MRI study

Stress is known to precipitate psychiatric disorders in vulnerable people. Individual differences in the stress responsivity can dramatically affect the onset of these illnesses. Animal models of repeated stress represent valuable tools to identify region-specific volumetric changes in the brain. Here, using high resolution 7T MRI, we found that amygdala is the most significant parameter for distinction between F344 and SD rats known to have differential response to stress. A significant substantial increase (45%) was found in the amygdala volume of rats that do not habituate to the repeated stress procedure (F344 rats) compared to SD rats. This strain-specific effect of stress was evidenced by a significant strain-by-stress interaction. There were no significant strain differences in the volumes of hippocampi and prefrontal cortices though stress produces significant reductions of smaller amplitude in the medial prefrontal cortex (mPFC) (9% and 12%) and dorsal hippocampus (5% and 6%) in both strains. Our data further demonstrate the feasibility and relevance of high isotropic resolution structural ex vivo 7T MRI in the study of the brain effects of stress in small animals. Neuroimaging is a valuable tool to follow up brain volumetric reorganization during the stress response and could also be easily used to test pharmacological interventions to prevent the deleterious effects of stress.

TIMP-1 abolishes MMP-9-dependent long-lasting long-term potentiation in the prefrontal cortex

BACKGROUND

Understanding of the molecular mechanisms of prefrontal cortex (PFC) plasticity is important for developing new treatment strategies for mental disorders such as depression and schizophrenia. Long-term potentiation (LTP) is a valid model for synaptic plasticity. The extracellular proteolytic system composed of matrix metalloproteinases (MMPs) and their endogenous tissue inhibitors (TIMPs) has recently been shown to play major role in the hippocampal plasticity.

METHODS

We tested whether induction of hippocampal-prefrontal LTP results in accumulation of tissue inhibitor of MMP-1, TIMP-1 mRNA, in the PFC of rats and whether adenovirally driven overexpression of TIMP-1 affects LTP. Additional study of slices was done with a specific MMP-9 inhibitor.

RESULTS

The TIMP-1 is induced in the rat medial PFC by stimuli evoking late LTP; its overexpression blocks the gelatinolytic activity of the MMP family; its overexpression before tetanization blocks late LTP in vivo; and MMP-9 inhibitor prevents late LTP in vitro.

CONCLUSIONS

We suggest a novel extracellular mechanism of late LTP in the PFC, engaging TIMP-1-controlled proteolysis as an element of information integration. Our results may also be meaningful to an understanding of mental diseases and development of new treatment strategies that are based on extracellular mechanisms of synaptic plasticity.

Anticonvulsive Effect of a Selective mGluR8 Agonist (S)-3,4-Dicarboxyphenylglycine (S-3,4-DCPG) in the Mouse Pilocarpine Model of Status Epilepticus

PURPOSE

We sought to investigate the anticonvulsive and neuroprotective effect of a selective metabotropic glutamate receptor 8 (mGluR8) agonist (S)-3,4-dicarboxyphenylglycines (S-3,4-DCPG) on pilocarpine-induced status epilepticus (PISE) and subsequent loss of hilar neurons in the dentate gyrus after systemic (intravenous) or local (intracerebroventricular) administration. We compared the difference in granular cell responses after paired-pulse stimulation of the perforant pathway and the sensitivity to local injection of S-3,4-DCPG into the stratum granulosum in the control and mice at 2 months after PISE.

METHODS

We used intravenous, intracerebroventricular, or intrahippocampal administration of S-3,4-DCPG to mice with status epilepticus or temporal lobe epilepsy and neurophysiologic recording of somatic field excitatory postsynaptic potential (sfEPSP) and population spike (PS) of granular cells in response to perforant-pathway stimulation or S-3,4-DCPG treatment.

RESULTS

Intracerebroventricular (1.91 micromol) but not systemic administration of S-3,4-DCPG (at doses of 12.5, 50, 100, 200, 400, 800, and 1,200 mg/kg) could control PISE with no neuroprotective effect. In epileptic mice, mGluR8-mediated inhibition of fEPSPs was reduced significantly in granular cell bodies.

CONCLUSIONS

At doses ranging from 12.5 to 1,200 mg/kg, intravenous administration of S-3,4-DCPG may not be effective in controlling status epilepticus. Down-regulation of mGluR8 may be related to reduced S-3,4-DCPG-mediated inhibition and the subsequent occurrence of spontaneously recurrent seizures.

Peri-pubertal maturation after developmental disturbance: a model for psychosis onset in the rat

Schizophrenia is thought to be associated with abnormalities during neurodevelopment although those disturbances usually remain silent until puberty; suggesting that postnatal brain maturation precipitates the emergence of psychosis. In an attempt to model neurodevelopmental defects in the rat, brain cellular proliferation was briefly interrupted with methylazoxymethanol (MAM) during late gestation at embryonic day 17 (E17). The litters were explored at pre- and post-puberty and compared with E17 saline-injected rats. We measured spontaneous and provoked locomotion, working memory test, social interaction, and prepulse inhibition (PPI). As compared with the saline-exposed rats, the E17 MAM-exposed rats exhibited spontaneous hyperactivity that emerged only after puberty. At adulthood, they also exhibited hypersensitivity to the locomotor activating effects of a mild stress and a glutamatergic N-methyl-D-aspartate receptor antagonist (MK-801), as well as PPI deficits whereas before puberty no perturbations were observed. In addition, spatial working memory did not undergo the normal peri-pubertal maturation seen in the sham rats. Social interaction deficits were observed in MAM rats, at both pre- and post-puberty. Our study further confirms that transient prenatal disruption of neurogenesis by MAM at E17 is a valid behavioral model for schizophrenia as it is able to reproduce some fundamental features of schizophrenia with respect to both phenomenology and temporal pattern of the onset of symptoms and deficits.

Common efficacy of psychotropic drugs in restoring stress-induced impairment of prefrontal plasticity

We recently investigated the effects of stress on synaptic plasticity in the prefrontal cortex, namely the prelimbic area or the apparent homologue of the primate subgenual prefrontal cortex in humans where most of the hippocampal terminal fields are localized. Exposure to an acute stress causes a remarkable and long-lasting inhibition of long term potentiation (LTP) in the frontal cortex evoked by stimulation of hippocampal outflow and this impairment is prevented by the glucocorticoid receptor antagonist mifepristone. Thus, the frontal cortex is also a target for glucocorticoids involved in the stress response. Current data show that antidepressants of various types, i.e., tianeptine and fluoxetine, at doses normally used in antidepressant testing, restore LTP impaired by prior acute stress. Interestingly, clozapine administered in a similar way after stress rapidly reverses the stress-induced impairment of LTP at doses which do not affect LTP alone. This stress paradigm highlights comorbidity for both etiology and treatment of psychiatric disorders like depression and schizophrenia. Restoring appropriate cognitive functions in circuits associated with dysfunctions in coping with stress may be proposed as a new systems-level approach to drug discovery and development. We are presently investigating the involvement of signalling molecules in producing these plastic changes.

Opposite behaviours in the forced swimming test are linked to differences in spatial working memory performances in the rat

Despite consistent evidence of an association between depression and impaired memory performance, only a few studies have investigated memory processes in animal models of depression. The aim of the present study was to determine if rats selected for marked differences in their immobility response in the forced swimming test (FST, i.e. high-immobility, [HI] and low-immobility [LI] rats) exhibit differences in spatial and non-spatial memory performances. In a classic radial maze elimination task, we observed that HI rats made significantly more errors than LI rats, and their first error appeared significantly earlier. In a delayed spatial win-shift procedure where rats have to hold spatially relevant information in working memory across a 30 min delay, HI rats tended initially to perform more poorly than LI rats. HI rats made more across-phase errors, the occurrence of the first error was earlier and by the end of the experiment the differences between the two groups disappeared. Thus, HI rats present more difficulties to learn the rules in a spatial task and show weaker performances in spatial working memory in comparison to LI rats. On the other hand, performances in the two groups of animals were similar in a non-spatial task, the object recognition task. Complementary behavioral data indicate that the differences observed between the two groups are not attributable to opposite locomotor activities or to different levels of anxiety. Overall we can conclude that opposite swimming behavior in the FST could parallel some differences in cognitive performances, more specifically linked to spatial working memory.

Working memory deficits in adult rats after prenatal disruption of neurogenesis

We investigated the cognitive consequences of a prenatal injection of the mitotic inhibitor methylazoxymethanol (MAM) into pregnant rats at embryonic day 15 (E15) or 17 (E17). The male offspring were tested when adult on a version of the radial-arm maze task that assesses spatial working memory with an extended delay, where performance is dependent, in part, on the hippocampal-prefrontal circuit. A major impairment of spatial learning was observed in E15 MAM rats. However, the E17 MAM rats did learn the rule but were impaired selectively in the 30-min delay-interposed task. Morphologically, the E15 MAM rats exhibited dramatic gross brain abnormalities, whereas the E17 MAM animals displayed aberrant cell migration in the hippocampus and a disrupted laminar pattern in the neocortex. These results suggest that late gestational MAM injection (E17) causes a cognitive impairment in a prefrontal cortex-hippocampus-dependent working memory task. This approach could provide a new developmental model of disorders associated with working memory deficits, such as schizophrenia.

Effect of long-term potentiation induction on gamma-band electroencephalograms in prefrontal cortex following stimulation of rat hippocampus in vivo

We examined whether long-term potentiation (LTP) affects cortical gamma-band electroencephalograms (EEG) in the hippocampo-prefrontal cortex (PFC) pathway of anesthetized rats. The LTP induction increased the evoked PFC gamma-band EEG power (40-100 Hz) to 120-135% at 500-700 ms after test stimulation. A simple increment of stimulus intensity, instead of LTP induction, did not reveal this evoked increase. Neither LTP induction nor the intensity increment changed significantly the magnitude of an evoked decrease at around 100 ms or the spontaneous prestimulation gamma-band power. These results indicate that LTP in PFC specifically increases the evoked gamma-band EEG power, which may reflect a phasic mode of plastic neurotransmission through the hippocampo-PFC pathway in vivo.

Plasticity at hippocampal to prefrontal cortex synapses: dual roles in working memory and consolidation

The involvement of the hippocampus and the prefrontal cortex in cognitive processes and particularly in learning and memory has been known for a long time. However, the specific role of the projection which connects these two structures has remained elusive. The existence of a direct monosynaptic pathway from the ventral CA1 region of the hippocampus and subiculum to specific areas of the prefrontal cortex provides a useful model for conceptualizing the functional operations of hippocampal-prefrontal cortex communication in learning and memory. It is known now that hippocampal to prefrontal cortex synapses are modifiable synapses and can express different forms of plasticity, including long-term potentiation, long-term depression, and depotentiation. Here we review these findings and focus on recent studies that start to relate synaptic plasticity in the hippocampo-prefrontal cortex pathway to two specific aspects of learning and memory, i.e., the consolidation of information and working memory. The available evidence suggests that functional interactions between the hippocampus and prefrontal cortex in cognition and memory are more complex than previously anticipated, with the possibility for bidirectional regulation of synaptic strength as a function of the specific demands of tasks.

Essential role of D1 but not D2 receptors in the NMDA receptor-dependent long-term potentiation at hippocampal-prefrontal cortex synapses in vivo

An intact mesocortical dopaminergic (DA) input to the prefrontal cortex (PFC) has been reported to be necessary for long-term potentiation (LTP) to occur at hippocampal-prefrontal cortex synapses. Here, we investigated the role of D1 and D2 receptors in this NMDA receptor-dependent LTP. Local infusion of the D1 agonist SKF81297 at an optimal dose induced a sustained enhancement of hippocampal-PFC LTP, whereas the D1 antagonist SCH23390 caused a dose-related impairment of its induction. The D1 agonist effect was mimicked by infusion of a low dose of the adenylyl cyclase activator forskolin, whereas LTP was severely attenuated with a protein kinase A inhibitor, Rp-cAMPS. To further assess the complex interplay between DA and NMDA receptors, changes in extracellular DA levels in the PFC were estimated during LTP, and a significant increase was observed immediately after tetanus. Taken together, these data suggest that D1 but not D2 receptors are crucial for the DA control of the NMDA receptor-mediated synaptic response on a specific excitatory input to the PFC. The interactions of these receptors may play a crucial role in the storage and transfer of hippocampal information in the PFC.

Excitotoxicity in neurological disorders--the glutamate paradox

Beneficial effects of glutamate-receptor antagonists in models of neurological disorders are often used to support the notion that endogenous excitotoxicity (i.e. resulting from extracellular accumulation of endogenous glutamate) is a major contributor to neuronal death associated with these conditions. However, this interpretation conflicts with a number of robust and important experimental evidence. Here, emphasis is placed on two key elements: (i) very high extracellular levels of glutamate must be reached to initiate neuronal death, far above those measured in models of neurological disorders; and (ii) changes in extracellular glutamate as measured by microdialysis are not related to changes in the synaptic cleft, i.e. the compartment where neurotransmitter glutamate interacts with its receptors. It has become clear that the diversity and complexity of glutamate-mediated processes allow for a wide range of potential abnormalities (e.g. loss of selectivity of glutamate-operated ion channels, abnormal modulation of glutamate receptors). In addition, as neuronal death subsequent to ischemia and other insults is likely to result from multifactorial processes that may be inter-related, inhibition of glutamate-mediated synaptic transmission may be neuroprotective by increasing the resistance of neurons to other deleterious mechanisms (e.g. inadequate energy supply) that are not directly related to glutamatergic transmission.

Integrity of the mesocortical dopaminergic system is necessary for complete expression of in vivo hippocampal-prefrontal cortex long-term potentiation

The prefrontal cortex receives dopaminergic inputs from the ventral tegmental area and excitatory inputs from the hippocampus. Both afferent pathways target in close proximity dendritic spines of pyramidal cells in layer V-VI of the prefrontal cortex. In view of the prominent role of dopamine in cognitive functions we examined the effects of ventral tegmental area stimulation on the induction of long-term potentiation in the hippocampal-prefrontal cortex pathway of anesthetized rats. Stimulation of the ventral tegmental area at a frequency known to evoke dopamine overflow in the prefrontal cortex produces a long-lasting enhancement of the magnitude of the hippocampal-prefrontal cortex long-term potentiation. The role of dopamine was further examined by investigating the effects of prefrontocortical dopamine depletion induced by an electrolytic ventral tegmental area lesion. A significant correlation (r = 0.8; P < 0.001; n = 14) was obtained between cortical dopamine levels and cortical long-term potentiation amplitude, a depletion of more than 50% of cortical levels corresponding to a dramatic decrease in hippocampal-prefrontal cortex long-term potentiation. However, a recovery to normal long-term potentiation was observed 1 h after tetanic stimulation. In contrast to the effects on long-term potentiation, ventral tegmental area stimulation, when applied at low or high frequency, decreases the amplitude of the hippocampal-prefrontal cortex postsynaptic synaptic response. The present study demonstrates the importance of the integrity of the mesocortical dopaminergic system for long-term potentiation to occur in the hippocampal-prefrontal cortex pathway and suggests a frequency-dependent effect of dopamine on hippocampal-prefrontal cortex transmission.

Induction of stable long-term depression in vivo in the hippocampal-prefrontal cortex pathway

We studied excitatory field potentials in the medial prefrontal cortex (mPFC, prelimbic area) to electrostimulation of the ventral hippocampus (CA1/subicular region) in the anaesthetized rat. Nine hundred stimulus trains (5 pulses at 250 Hz) applied at 1 Hz to the ventral hippocampus significantly and persistently depressed the amplitude and maximal slope ( approximately 55% for each index) of the prelimbic field potentials, but did not change the latency of the maximal slope or peak negativity. Twelve stimulus trains (50 pulses at 250 Hz) applied subsequently at 0.1 Hz restored the depression back to control level, and this reversible depression was maintained for at least 13 h. Cumulative depressive effects on the prelimbic field potential amplitude and maximal slope were observed upon addition of stimulus trains in the hippocampus. An important implication of the results is that the direct pathway from the hippocampus to the mPFC in the rat retains long-term depression (LTD) as a neuroplastic form in vivo. This form could cooperate with long-term potentiation (LTP) and such a bi-directional synaptic plasticity in the prefrontal cortex contributes to how cortical neural networks store information.

Long-term potentiation in the dentate gyrus is not linked to increased extracellular glutamate concentration

Long-term potentiation (LTP) of excitatory transmission is a likely candidate for the encoding and storage of information in the mammalian brain. There is a general agreement that LTP involves an increase in synaptic strength, but the mechanisms underlying this persistent change are unclear and controversial. Synaptic efficacy may be enhanced because more transmitter glutamate is released or because postsynaptic responsiveness increases or both. The purpose of this study was to examine whether increased extracellular glutamate concentration was associated with the robust and well-characterized LTP that can be induced in the rat dentate gyrus. To favor the detection of any putative change in extracellular glutamate associated with LTP, our experimental strategy included the following features. 1) Two separate series of experiments were carried out with animals under pentobarbital or urethan anesthesia; 2) changes in extracellular concentration of glutamate were monitored continuously by microdialysis coupled to enzyme amperometry; and 3) dialysate glutamate levels and changes in the slope of excitatory postsynaptic potential evoked by activation of the perforant path were recorded precisely at the same site. Tetanic stimulation of the perforant path increased persistently test-evoked responses in the dentate gyrus (by 19 and 14% in barbiturate and urethan group, respectively), but there was no glutamate change either during or after LTP induction and no indication of increased glutamate efflux when low-frequency stimulation was applied. The results do not rule out a possible contribution of enhanced glutamate exocytosis to LTP induction and/or maintenance because such a presynaptic change may not be detectable extracellularly. However, our findings and other data supporting the notion that neurotransmitter glutamate may hardly leak out of the synaptic cleft conflict with the hypothesis that LTP could also involve a broad synaptic spillover of glutamate.

Rapid increase in PKA activity during long-term potentiation in the hippocampal afferent fibre system to the prefrontal cortex in vivo

The purpose of the present study was to examine whether cAMP-dependent protein kinase (PKA) was implicated in the process of long-term potentiation (LTP) in the hippocampal afferent fibre system to the prefrontal cortex in vivo. Using a biochemical approach, we measured PKA activity at different times after induction of LTP. We show that there is an NMDA receptor-dependent increase in PKA activity in the prefrontal cortex, only at five minutes after LTP induction. These data demonstrate a role of PKA in the induction and not the expression of cortical LTP and suggest that if PKA is involved in the late phase of LTP, it does not appear to be a persistent activation.

Reversal of LTP in the hippocampal afferent fiber system to the prefrontal cortex in vivo with low-frequency patterns of stimulation that do not produce LTD

We examined the efficacy of several patterns of low-frequency stimulation for producing long-term depression (LTD) or depotentiation in the hippocampal fiber pathway to the prefrontal cortex in the anesthetized rat. Field potentials elicited by stimulation of the CA1/subicular region of the ventral hippocampus were recorded in the prelimbic area of the prefrontal cortex. We found no evidence that low-frequency trains (0.5-1 Hz), consisting of either single pulses, paired pulses (35-ms interpulse interval), or two-pulse bursts (5-ms interval), produce LTD in the prefrontal cortex. In contrast, all three stimulus protocols were found to induce a small-amplitude, persistent potentiation of the amplitude of the negative wave of the field response recorded in the prefrontal cortex. We also examined the ability of patterns of low-frequency stimulation to produce depotentiation of previously established long-term potentiation (LTP). Although low-frequency stimulation with single pulses or paired pulses was ineffective, we found that the two-pulse burst protocol selectively produced a rapid reversal of LTP in the hippocampo-prefrontal cortex pathway. Depotentiation is reversible and can be induced >2 h after the induction of LTP. Repeated trains failed to decrease the prefrontal cortex response below the original, unpotentiated level. These findings demonstrate the existence of a depotentiation mechanism that is capable of exerting powerful control over ongoing or recently induced synaptic plasticity in hippocampocortical connections in vivo.

Plasticity of the hippocampal-prefrontal cortex synapses

This paper summarizes experiments showing that the direct excitatory glutamergic pathway projecting from area CA1 and subiculum to the prefrontal cortex in the rat express a long-lasting NMDA receptor-dependent form of LTP that can be induced for several days in the freely-moving rat. Other experiments examined its functional significance in learning and the role of dopamine in the regulation of hippocampal-prefrontal cortex plasticity.

Inhibition of hippocampoprefrontal cortex excitatory responses by the mesocortical DA system

The prefrontal cortex (PFC) receives dopaminergic (DA) afferents from the ventral tegmental area (VTA) and excitatory inputs from the hippocampal formation. The present electrophysiological experiments performed in anaesthetized rats demonstrate that activation of the mesocortical DA system blocked the spontaneous activity of PFC neurones and the excitatory responses induced by hippocampal stimulation. These data indicate that there is a convergence in the effects of hippocampal and DA mesocortical afferents on common PFC neurones. Furthermore, a population of PFC neurones responding to hippocampal stimulation could be identified as projecting to either the nucleus accumbens and/or the VTA, suggesting that the hippocampus exerts an indirect influence on these two structures through the PFC.

NMDA receptor-dependent long-term potentiation in the hippocampal afferent fibre system to the prefrontal cortex in the rat

This study investigated the role of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor in the induction of long-term potentiation (LTP) in the hippocampal-prefrontal cortex pathway in vivo. Field potentials evoked by electrical stimulation of the CA1/subicular region were recorded in the prelimbic area of the prefrontal cortex under continuous perfusion of artificial cerebrospinal fluid in anaesthetized rats. High-frequency stimulation of the CA1/subicular region induced LTP of the evoked response in the prelimbic area of the prefrontal cortex. LTP was completely blocked when the selective NMDA receptor antagonist D-(-)2-amino-5-phosphonopentanoic acid (D-AP5; 200 microM), was perfused during the tetanus. Perfusion of D-AP5 did not affect normal transmission or pre-established LTP. These results demonstrate that induction of LTP in the hippocampal-prefrontal cortex pathway is an NMDA receptor-dependent process.

Anatomical and electrophysiological evidence for an excitatory amino acid pathway from the thalamic mediodorsal nucleus to the prefrontal cortex in the rat

This study was undertaken to identify the neurotransmitter of the projection from the thalamic mediodorsal nucleus (MD) to the prefrontal cortex (PFC) using both retrograde transport of D-[3H]aspartate and electrophysiological approaches in the rat. Unilateral microinjections of D-[3H]aspartate performed into the prelimbic area of the PFC resulted in dense labelling of numerous cells in the ipsilateral MD. Excitatory responses were observed in PFC neurons after electrical stimulation of the MD. However, since cortical neurons project to the MD, these excitatory responses could have resulted either from the activation of the MD-PFC pathway and/or from the activation of recurrent collaterals of antidromically driven cortico-thalamic fibres. The conduction time of each of these two reciprocal pathways was determined by antidromic activation. Short latency excitatory responses resulted from activation of the MD-PFC pathway. They were predominantly observed in PFC neurons located in layer III and evoked at low frequency stimulation (0.3-1 Hz). These excitatory responses disappeared or were replaced by longer latency responses when higher frequency stimulations (3-10 Hz) were used. MD-evoked responses were blocked by the iontophoretic application of the AMPA receptor antagonist CNQX into the PFC. These results indicate that the MD-PFC pathway utilizes glutamate and/or aspartate as the neurotransmitter and that its activation induces excitation in PFC neurons through AMPA receptors. Even though the local application of the NMDA receptor antagonist APV was ineffective, a contribution of these receptors in MD-PFC transmission cannot be excluded.

Distribution of hippocampal CA1 and subicular efferents in the prefrontal cortex of the rat studied by means of anterograde transport of Phaseolus vulgaris-leucoagglutinin

Projections of the hippocampal formation to the prefrontal cortex were visualized in the rat by means of the anterograde tracer Phaseolus vulgaris-leucoagglutinin. These projections distribute only to the prelimbic and the medial orbital cortices and arise exclusively from restricted portions of field CA1 of the Ammon's horn and the subiculum. The most dorsal portion of CA1 does not contribute fibers to this projection. In the subiculum, its origin is restricted to the proximal half, i.e., the portion that directly borders field CA1. Fibers from field CA1 and the subiculum have comparable distribution patterns in the prelimbic and medial orbital cortices. The density and distribution in the prefrontal cortex of the projections from the proximal portion of the subiculum depends on the location of the injections along the dorsoventral axis of the hippocampal formation: the intermediate portion of the subiculum projects more densely and diffusely than its dorsal and ventral portions. In the prelimbic cortex, labeled fibers are present in all layers, showing marked morphological differences in deep versus superficial layers. In layers V and VI, most of the fibers are vertically oriented, while in layers II and III they are short and oriented towards the pial surface. Although no clear differences in terminal distribution were observed along the rostrocaudal extent of the prelimbic cortex, its dorsal and ventral portions show different innervation patterns. In the ventral portion of the prelimbic cortex, varicose fibers and terminal arborizations were present in all cortical layers, deep (V and VI) as well as superficial (II and III). In its dorsal part, the innervation was less dense and mostly present in the deep layers (V and VI). The fiber and terminal distribution in the medial orbital cortex was diffuse in all layers with a slight preference for layers deep to layer II.

Metabolic stability of 3-O-methyl-D-glucose in brain and other tissues

3-O-Methyl-D-glucose (methylglucose) is often used to study blood-brain barrier transport and the distribution spaces of hexoses in brain. A critical requirement of this application is that it not be chemically converted in the tissues. Recent reports of phosphorylation of methylglucose by yeast and heart hexokinase have raised questions about its metabolic stability in brain. Therefore, we have re-examined this question by studying the metabolism of methylglucose by yeast hexokinase and rat brain homogenates in vitro and rat brain, heart, and liver in vivo. Commercial preparations of yeast hexokinase did convert methylglucose to acidic products, but only when the enzyme was present in very large amounts. Methylglucose was not phosphorylated by brain homogenates under conditions that converted 97% of [U-14C]glucose to ionic derivatives. When [14C]methylglucose, labeled in either the methyl or glucose moiety, was administered to rats by an intravenous pulse or a programmed infusion that maintained the arterial concentration constant and total 14C was extracted from the tissues 60 min later, 97-100% of the 14C in brain, greater than 99% of the 14C in plasma, and greater than 90% of that in heart and liver were recovered as unmetabolized [14C]methylglucose. Small amounts of 14C in brain (1-3%), heart (3-6%), and liver (4-7%) were recovered in acidic products. Plasma glucose levels ranging from hypoglycemia to hyperglycemia had little influence on the degree of this conversion. The distribution spaces for methylglucose were found to be 0.52 in brain and heart and 0.75 in liver.

Long-term potentiation in the prefrontal cortex following stimulation of the hippocampal CA1/subicular region

We have examined single cell activity and field potentials in the prelimbic area of the prefrontal cortex of the rat to electrical stimulation of the CA1/subicular region of the temporal hippocampus. Excitatory unit responses were found in 50 out of 120 neurons recorded in the prelimbic area. Paired-pulse facilitation was found for both single cell responses and field potentials. High-frequency, tetanic stimulation of the temporal hippocampus produced a significant and persistent potentiation of prelimbic field potentials. The evidence suggests that the direct pathway from the temporal hippocampus to the prelimbic area of the prefrontal cortex in the rat is excitatory and can undergo long-term potentiation (LTP).

Selectivity of the hippocampal projection to the prelimbic area of the prefrontal cortex in the rat

Afferent connections of the medial and lateral prefrontal cortex of the rat arising from the hippocampal formation were investigated using iontophoretic application of the fluorescent tracer, Fluoro-gold. Our results demonstrate that the projection which originates in the temporal part of the CA1 hippocampal field and in the prosubiculum is restricted to the prelimbic area of the prefrontal cortex.

Local cerebral glucose utilization non-selectively elevated in rapid eye movement sleep of the fetus

The [14C]deoxyglucose method for measuring local cerebral glucose utilization was employed in an effort to identify regions of the brain which participate in the increased neuronal activity of rapid eye movement (REM) sleep. The study was conducted in near term fetal sheep in which REM periods are of sufficient duration to obtain reliable data with this method. Neither the postulated executive centers of REM sleep nor those structures in the brainstem known to participate in the electrical activity peculiar to this sleep phase were found to have selectively elevated rates of glucose utilization. Rather, these regions shared equally with virtually all other structures in having rates higher than those which accompany non-REM sleep.

Measurement of local cerebral blood flow with [14C]iodoantipyrine in the mouse

Local cerebral blood flow was measured in the mouse by means of the [14C]iodoantipyrine method. This method has been previously used in the monkey, dog, cat, and rat, but its application to small mammals such as the mouse requires special attention to potential sources of error. The small size of the mouse brain requires special attention to the rapid removal and freezing of the brain to minimize effects of postmortem diffusion of tracer in the tissue. Because of the relatively low diameter/length ratios of the catheters needed for arterial sampling in small animals, substantial errors can occur in the determination of the time course of the [14C]iodoantipyrine concentration in the arterial blood unless corrections for lag time and dead space washout in the catheter are properly applied. Local cerebral blood flow was measured in seven awake mice with appropriate care to minimize these sources of error. The values were found to vary from 48 ml/100 g/min in the corpus callosum to 198 ml/100 g/min in the inferior colliculus. The results demonstrate that the [14C]iodoantipyrine method can be used to measure local cerebral blood flow in the mouse and that the values in that species are, in general, somewhat higher than those in the rat.

Local cerebral glucose utilization in the free moving mouse: a comparison during two stages of the activity-rest cycle

The 2-deoxy-D[1-14C]glucose ([14C]DG) technique has been applied to the free moving mouse for the quantitative determination of local cerebral glucose utilization (LCGU). Reproducible values for LCGU were obtained indicating that the [14C]DG method had a sufficient resolution power to allow visualization and quantification of very small structures provided that glucose and [14C]DG plasma concentrations were measured on microsamples, autoradiographs prepared from proper tissue sections and suitable techniques used for analysis of the maps thus obtained. LCGU was measured in free moving mice during two stages of the light-dark cycle, one corresponding to a period of rest and the other to a period of high motor activity. In the two groups of animals LCGU was heterogeneous in the grey matter, the highest values being found in the auditory regions, the cerebellar and vestibular nuclei. LCGU was found to be lower in drowsy animals during the day than in active animals during the night and the difference was significant in the 8 following structures: the sensorimotor cortex, the septal nuclei, the nucleus of the olfactory tract, the basal amygdaloid nucleus, the ventral nucleus of the thalamus, the lateral geniculate body, the medial geniculate body and the auditory cortex. On the contrary, the suprachiasmatic nucleus was very active during the day and relatively inactive during the night as previously reported in the rat.