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Hippocampal afferents to the rat prefrontal cortex: synaptic targets and relation to dopamine terminals.

Afferents to the prefrontal cortex (PFC) from the hippocampal formation and from midbrain dopamine (DA) neurons have been implicated in the cognitive and adaptive functions of this cortical region. In the present study, we investigated the ultrastructure and synaptic targets of hippocampal terminals, as well as their relation to DA terminals within the PFC of adult rats. Hippocampal afferents were labeled either by anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from the ventral hippocampal formation or by anterograde degeneration following fimbria lesion. Hippocampal terminals in the PFC, identified by either method, formed primarily asymmetric axospinous synapses, with a small percentage forming asymmetric axodendritic synapses. Dopamine terminals in the PFC were identified by peroxidase immunocytochemistry for either tyrosine hydroxylase or DA and formed primarily symmetric synapses onto dendritic spines and small caliber dendritic shafts. Spines that received symmetric synaptic contact from DA terminals invariably also received an asymmetric synapse from an unlabeled terminal, forming a triadic complex. Hippocampal and DA terminals in the PFC were not often observed in the same area of the neuropil, and no examples of convergence of hippocampal and DA terminals onto common postsynaptic targets were observed. Further analysis revealed that spines receiving synaptic contact from hippocampal terminals did not receive additional synaptic contact from any other source. However, when localized to the same area of the neuropil, hippocampal and DA terminals were often in direct apposition to one another, without forming axo-axonic synapses. These results suggest that 1) hippocampal terminals primarily form excitatory synapses onto spiny pyramidal neurons, 2) hippocampal afferents are unlikely to be synaptically modulated by DA or non-DA terminals at the level of the dendritic spine, and 3) appositions between hippocampal and DA terminals may facilitate presynaptic interactions between these afferents to the PFC.

Afferent Pathways↗

Increased responsiveness of dopamine to atypical, but not typical antipsychotics in the medial prefrontal cortex of rats reared in isolation.

Dopaminergic hypofunction in the medial prefrontal cortex (mPFC) has been associated with the aetiology of negative symptoms and cognitive dysfunction of schizophrenia, which are both alleviated by clozapine and other atypical antipsychotics such as olanzapine. In rodents, early life exposure to stressful experiences such as social isolation produces a spectrum of symptoms emerging in adult life, which can be restored by antipsychotic drugs. The present series of experiments sought to investigate the effect of clozapine (5-10 mg/kg s.c.), olanzapine (5 mg/kg s.c.), and haloperidol (0.5 mg/kg s.c.) on dopamine (DA) and amino acids in the prelimbic/infralimbic subregion of the mPFC in group- and isolation-reared rats. Rats reared in isolation showed significant and robust deficits in prepulse inhibition of the acoustic startle. In group-reared animals, both clozapine and olanzapine produced a significant increase in DA outflow in the mPFC. Isolation-reared rats showed a significant increase in responsiveness to both atypical antipsychotics compared with group-reared animals. In contrast, the administration of haloperidol failed to modify dialysate DA levels in mPFC in either group- or isolation-reared animals. The results also show a positive relationship between the potency of the tested antipsychotics to increase the release of DA in the mPFC and their respective affinities for 5-HT1A relative to DA D2 or D3 receptors. Finally, isolation-reared rats showed enhanced neurochemical responses to the highest dose of clozapine as indexed by alanine, aspartate, GABA, glutamine, glutamate, histidine, and tyrosine. The increased DA responsiveness to the atypical antipsychotic drugs clozapine and olanzapine may explain, at least in part, clozapine- and olanzapine-induced reversal of some of the major behavioral components of the social isolation syndrome, namely hyperactivity and attention deficit.

Amino Acids↗

Role of the medial prefrontal cortex in cardiovascular responses to acute restraint in rats.

The medial prefrontal cortex (mPFC) modulates neurovegetative and behavioral responses, being involved in memory, attention, motivational and executive processes. There is evidence indicating that mPFC modulates cardiovascular correlates of behavioral responses to stressful stimuli. Acute restraint is an unavoidable stress situation that evokes marked and sustained cardiovascular changes, characterized by elevated blood pressure (BP) and intense heart rate (HR) increase. We presently report effects of mPFC pharmacological manipulations on BP and HR responses evoked by acute restraint in rats. Bilateral microinjection of 200 nl of the unspecific synaptic blocker CoCl2 (1 mM) in the mPFC prelimbic area (PL) increased HR response to acute restraint, without significant effect on the BP response. This result indicates that PL synaptic mechanisms have an inhibitory influence on restraint-evoked HR changes. Injections of the non-selective glutamatergic receptor antagonist kynurenic acid (0.02 M) or the selective N-methyl-d-aspartic acid (NMDA) receptor glutamatergic antagonist (LY235959) (0.02 M) caused effects similar to cobalt, suggesting that local glutamatergic neurotransmission and NMDA receptors mediate the PL inhibitory influence on restraint-related HR responses. Pretreatment with the non-non-N-methyl-D-aspartic acid glutamatergic antagonist glutamatergic antagonist glutamatergic receptor antagonist NBQX (0.02 M) did not affect restraint-related cardiovascular responses, reinforcing the idea that NMDA receptors mediate PL-related inhibitory influence. Pretreatment with the glutamatergic-receptor antagonists did not affect baseline BP or HR values. I.v. pretreatment with the quaternary ammonium anticholinergic drug homatropine methyl bromide (0.2 mg/kg) also increased the restraint-related HR response to values similar to those observed after treatment with kynurenic acid or LY235959, thus, suggesting that PL inhibitory influence on restraint-evoked heart rate increase could be related to increased parasympathetic activity. This dose of homatropine had no significant effects on baseline BP or HR values. Results suggest a PL inhibitory influence on restraint-evoked HR increase. They also indicate that local NMDA receptors involved in parasympathetic activation mediate PL inhibitory influence on restraint-evoked HR increase.

Analysis of Variance↗

Glial fibrillary acidic protein immunoreactivity in the prefrontal cortex distinguishes younger from older adults in major depressive disorder.

BACKGROUND: Recent postmortem studies in major depressive disorder (MDD) provide evidence for a reduction in the packing density and number of glial cells in different regions of the prefrontal cortex; however, the specific types of glia involved in those morphologic changes are unknown. METHODS: The territory occupied by the astroglial marker glial fibrillary acidic protein (GFAP) was measured as an areal fraction in cortical layers III, IV, and V in sections from the dorsolateral prefrontal cortex (dlPFC) of MDD and control subjects. In addition, the packing density of GFAP-immunoreactive somata was measured by a direct three-dimensional cell counting method. RESULTS: The mean areal fraction and packing density of GFAP-immunoreactive astrocytes in the dlPFC of MDD subjects were not significantly different from those in control subjects; however, in MDD there was a significant strong positive correlation between age and GFAP immunoreactivity. When the MDD group was divided into younger (30-45 years old) and older (46-86) adults, in the five younger MDD adults, areal fraction and packing density were smaller than the smallest values of the control subjects. In contrast, among older MDD subjects these parameters tended to be greater than in the older control subjects. CONCLUSIONS: The present results suggest that the GFAP-immunoreactive astroglia is differentially involved in the pathology of MDD in younger compared with older adults.

Adult↗

The effect of orbital prefrontal cortex lesions on performance on a progressive ratio schedule: implications for models of inter-temporal choice.

In a previous experiment [Kheramin S, Body S, Mobini S, Ho M-Y, Velazquez-Martinez DN, Bradshaw CM, et al. Effects of quinolinic acid-induced lesions of the orbital prefrontal cortex on inter-temporal choice: a quantitative analysis. Psychopharmacology 2002;165: 9-17], destruction of the orbital prefrontal cortex (OPFC) in rats altered choice between two delayed food reinforcers, enhancing preference for the larger reinforcer. Theoretical analysis based on a quantitative model of inter-temporal choice [Ho M-Y, Mobini S, Chiang T-J, Bradshaw CM, Szabadi E. Theory and method in the quantitative analysis of 'impulsive choice' behaviour: implications for psychopharmacology. Psychopharmacology 1999;146:362-72] indicated that the lesion had increased the relative value of the larger of the two reinforcers due to a general reduction of absolute reinforcer value. The present experiment tested this hypothesis using a reinforcement schedule that did not entail either explicit choice or delayed reinforcement. Ten rats received quinolinic acid-induced lesions of the OPFC, and ten rats received sham lesions. The rats were trained under a progressive-ratio schedule of food reinforcement for 60 daily sessions. Response rates in successive ratios were a bitonic (inverted-U) function of ratio size. Analysis of the data using a three-parameter equation derived from a quantitative model of ratio schedule performance [Killeen PR. Mathematical principles of reinforcement. Behav. Brain Sci. 1994;17:105-72] revealed that the parameter specifying hypothetical reinforcer value was significantly lower in the OPFC-lesioned group than in the sham-lesioned group, consistent with the hypothesis that destruction of the OPFC resulted in devaluation of the food reinforcer.

Animals↗

Neurons in medial prefrontal cortex signal memory for fear extinction.

Conditioned fear responses to a tone previously paired with a shock diminish if the tone is repeatedly presented without the shock, a process known as extinction. Since Pavlov it has been hypothesized that extinction does not erase conditioning, but forms a new memory. Destruction of the ventral medial prefrontal cortex, which consists of infralimbic and prelimbic cortices, blocks recall of fear extinction, indicating that medial prefrontal cortex might store long-term extinction memory. Here we show that infralimbic neurons recorded during fear conditioning and extinction fire to the tone only when rats are recalling extinction on the following day. Rats that froze the least showed the greatest increase in infralimbic tone responses. We also show that conditioned tones paired with brief electrical stimulation of infralimbic cortex elicit low freezing in rats that had not been extinguished. Thus, stimulation resembling extinction-induced infralimbic tone responses is able to simulate extinction memory. We suggest that consolidation of extinction learning potentiates infralimbic activity, which inhibits fear during subsequent encounters with fear stimuli.

Acoustic Stimulation↗

Delta 9-tetrahydrocannabinol enhances presynaptic dopamine efflux in medial prefrontal cortex.

Acute administration of 1.0-2.0 mg/kg delta 9-tetrahydrocannabinol (delta 9-THC) increased presynaptic dopamine (DA) efflux in the medial prefrontal cortex of rats, as measured by intracerebral microdialysis in awake, behaving rats. These data are congruent with suggestions that (1) marijuana's euphorigenic effects and abuse potential may be related to augmentation of presynaptic DA mechanisms, and (2) the medial prefrontal cortex may be an important site of action for drugs of abuse in general and for delta 9-THC in particular.

3,4-Dihydroxyphenylacetic Acid↗

Relationship between performance on the Stroop test and N-acetylaspartate in the medial prefrontal cortex in deficit and nondeficit schizophrenia: preliminary results.

The aim of this research was to investigate the relationship between performance on the Stroop test and N-acetylaspartate/creatine assessed using proton magnetic resonance spectroscopy in the medial prefrontal cortex (MPFC) of schizophrenia patients. The Schedule for the Deficit Syndrome was used to subdivide the schizophrenia patients into deficit (n=5) and nondeficit (n=17) subtypes. Twenty-one control subjects served as a comparison group. A strong correlation between right-sided N-acetylaspartate/creatine levels and Stroop scores was found in the deficit patients but not in the nondeficit patients and the controls. This result suggests a relationship between a dysfunction of the right medial prefrontal cortex and a deficit in selective attention in schizophrenia patients with the deficit syndrome.

Aspartic Acid↗

Amphetamine increases extracellular concentrations of glutamate in the prefrontal cortex of the awake rat: a microdialysis study.

Using microdialysis, the effect was investigated of intracerebral infusions of different doses of amphetamine (1.25, 2.5, 5, 10, and 20 microg/microl) on the extracellular concentrations of glutamate in the medial prefrontal cortex of the rat. Amphetamine produced a dose-related increase in extracellular concentrations of glutamate. At the highest dose, amphetamine increased extracellular glutamate by 445% of baseline as well as extracellular concentrations of taurine, and reduced extracellular concentrations of glutamine. Amphetamine did not modify other amino acids such as arginine. Increases in extracellular concentrations of glutamate and taurine were independent of calcium in the perfusion medium. This is the first study showing that amphetamine produces a calcium-independent increase in extracellular concentrations of glutamate and taurine in the medial prefrontal cortex of the rat.

Amphetamine↗

Prefrontal cortex and working memory for spatial response, spatial location, and visual object information in the rat.

In the first experiment, rats were trained on a working memory task for a spatial response (right-left turn) information using a delayed matching-to-sample procedure. Following lesions of the medial prefrontal cortex (MPF), which includes anterior cingulate and medial precentral cortex, there was a profound working memory deficit even at the shortest delay. In the second experiment, rats were trained on a working memory task for spatial location information using a delayed matching-to-sample procedure. Following lesions of the MPF, there was only a mild working memory deficit, whereas following dorsal hippocampal lesions there was a profound working memory deficit even at the shortest delay. In the third experiment, rats were trained on a working memory task for visual object information using a delayed nonmatching-to-sample procedure. Following lesions of the MPF, there were no working memory deficits, whereas following lesions of the prelimbic and infralimbic cortex there was a profound working memory deficit even at the shortest delay. The results suggest that different neural subregions of the prefrontal cortex mediate working memory for specific attribute information.

Animals↗

Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex.

1. An oculomotor delayed-response task was used to examine the spatial memory functions of neurons in primate prefrontal cortex. Monkeys were trained to fixate a central spot during a brief presentation (0.5 s) of a peripheral cue and throughout a subsequent delay period (1-6 s), and then, upon the extinction of the fixation target, to make a saccadic eye movement to where the cue had been presented. Cues were usually presented in one of eight different locations separated by 45 degrees. This task thus requires monkeys to direct their gaze to the location of a remembered visual cue, controls the retinal coordinates of the visual cues, controls the monkey's oculomotor behavior during the delay period, and also allows precise measurement of the timing and direction of the relevant behavioral responses. 2. Recordings were obtained from 288 neurons in the prefrontal cortex within and surrounding the principal sulcus (PS) while monkeys performed this task. An additional 31 neurons in the frontal eye fields (FEF) region within and near the anterior bank of the arcuate sulcus were also studied. 3. Of the 288 PS neurons, 170 exhibited task-related activity during at least one phase of this task and, of these, 87 showed significant excitation or inhibition of activity during the delay period relative to activity during the intertrial interval. 4. Delay period activity was classified as directional for 79% of these 87 neurons in that significant responses only occurred following cues located over a certain range of visual field directions and were weak or absent for other cue directions. The remaining 21% were omnidirectional, i.e., showed comparable delay period activity for all visual field locations tested. Directional preferences, or lack thereof, were maintained across different delay intervals (1-6 s). 5. For 50 of the 87 PS neurons, activity during the delay period was significantly elevated above the neuron's spontaneous rate for at least one cue location; for the remaining 37 neurons only inhibitory delay period activity was seen. Nearly all (92%) neurons with excitatory delay period activity were directional and few (8%) were omnidirectional. Most (62%) neurons with purely inhibitory delay period activity were directional, but a substantial minority (38%) was omnidirectional. 6. Fifteen of the neurons with excitatory directional delay period activity also had significant inhibitory delay period activity for other cue directions. These inhibitory responses were usually strongest for, or centered about, cue directions roughly opposite those optimal for excitatory responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

Endogenous dopamine potentiates the effects of glutamate on extracellular GABA in the prefrontal cortex of the freely moving rat.

Using microdialysis, the effects of endogenous dopamine on basal extracellular concentrations of gamma-aminobutyric acid (GABA) and on the increases of GABA produced by glutamate were investigated in the medial prefrontal cortex of the awake rat. The dopamine uptake inhibitor nomifensine (1, 100 and 1000 microM), used to increase extracellular dopamine, produced a dose-related increase of dialysate dopamine (0.1-1 nM) but did not change dialysate concentrations of GABA or glutamate at any dose used. The glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxilic acid (PDC; 0.5 and 2 mM), used to increase extracellular glutamate, produced a dose-related increase of dialysate glutamate (1.5-5.5 microM) and increased dialysate GABA by 125%. When a simultaneous increase of endogenous dopamine and glutamate was produced, the increases of dialysate GABA were significantly higher (185% of baseline) than those produced by glutamate alone. These effects on dialysate GABA were attenuated by the D2 receptor antagonist (-) sulpiride, but not by the D1 receptor antagonist SCH-23390, all of which suggests that extracellular dopamine plays an important role in modulating endogenous glutamate-GABA interactions in the prefrontal cortex of the rat.

Animals↗

The distribution and morphology of prefrontal cortex pyramidal neurons identified using anti-neurofilament antibodies SMI32, N200 and FNP7. Normative data and a comparison in subjects with schizophrenia, bipolar disorder or major depression.

Alterations in the density and size of pyramidal neurons in the prefrontal cortex have been described in schizophrenia and mood disorder. However, the changes are generally modest and have not always been replicated. We investigated the possibility that specific pyramidal neuron sub-populations, defined by their immunoreactivity with the anti-neurofilament antibodies SMI32, N200, and FNP7, are differentially affected in these disorders. First, we assessed the distribution and characteristics of pyramidal neurons labelled by the antibodies in the human dorsolateral prefrontal cortex (Brodmann areas 9, 32, 46), using single and double label immunocytochemistry and immunofluorescence. Three largely separate sub-populations of pyramidal neurons were identified, although with more substantial overlap between SMI32- and FNP7-positive neurons in lamina V. We then determined the density, size and shape of the three pyramidal neuron sub-populations in area 9 in patients with schizophrenia, bipolar disorder, or major depressive disorder, compared to controls (n=15 in each group). We found a lower density of lamina III N200-positive neurons in major depressive disorder than in schizophrenia or bipolar disorder. There were no other overall differences in neuronal density, or in neuronal size or shape, although a planned secondary analysis supported the previously reported decrease of neuronal size in lamina V in bipolar disorder. In summary, our study illustrates a conceptual and methodological approach which may be of value for investigating the differential neuropathological involvement of pyramidal neuron sub-populations. However, we found no clear evidence that the prefrontal neuropathology of schizophrenia or mood disorders preferentially affects SMI32-, N200- or FNP7-immunoreactive pyramidal neurons.

Adult↗

The monkey's prefrontal cortex functions in motor programming.

A new experimental approach is presented which resulted in clarification of the specific functions of the monkey’s prefrontal cortex. Monkeys with chronically implanted transcortical nonpolarizable electrodes were trained on delayed response (DR) and visual delayed matching-to-sample (DMS) tasks. The onset of the trial for each group depended upon on-line computer detection of one of the specified events: FN – surface-negative steady potential shifts (SPS) from principalis cortex; MN – a similar SPS from precentral cortex; FB – near baseline SP from principalis cortex; LEM – rightward eye deviations; and YC – controls, with intertrial intervals yoked to those of other monkeys. Monkeys were trained with 1-s cue presentations. on successive delays of 2 to 12 s. The DR acquisition rate by the FN group was substantially faster than that of any other group, as indicated by its mean error that was only 17.24 percent the YC group’s error. The MN and LEM monkeys acquired the task at the same rates as the YCs, while the FB monkeys were the slowest learners. The correct DR performance transferred to testing with constant intertrial intervals (without preconditions). Subsequent on-line tests with brief (0.1 s) cue duration showed high DR performance by the FN, but not by other groups. No similar rapid learning was found with the DMS task. The findings from this, and other experiments, suggest that the major function of principalis cortex is the selection, or programming of delayed spatial choice responses. The view seems consonant with interpretations for the role of the human prefrontal cortex.

Animals↗

Transient increases in catecholaminergic activity in medial prefrontal cortex and nucleus accumbens shell during novelty.

Voltammetric recordings with electrochemically modified carbon-fiber electrodes were obtained from specific regions of the forebrain in rats given free-choice access to a novel environment. Entry into novelty increased the catechol signal in the medial prefrontal cortex and shell of the nucleus accumbens by more than 100%, but had no consistent effect in either the neostriatum or accumbal core. In both the medial prefrontal cortex and accumbal shell, moreover, the novelty-induced increase in catecholaminergic activity was detectable only during the initial entry into the novel compartment and did not reappear when animals returned to the familiar environment. These results support increasing evidence for a functional distinction between the accumbal core and shell, with the latter having been linked to brain reward mechanisms. The results also indicate that novelty activates, albeit very transiently, some of the same neurochemical systems believed to play a critical role in the reinforcing effects of certain drugs of abuse.

3,4-Dihydroxyphenylacetic Acid↗

Differential expression of c-fos mRNA in rat prefrontal cortex, striatum, N. accumbens and lateral septum after typical and atypical antipsychotics: an in situ hybridization study.

The regional difference in the expression of c-fos mRNA induced by typical and atypical antipsychotics was determined in prefrontal cortex, striatum, N. accumbens and lateral septum in rats by in situ hybridization. Two typical antipsychotics, haloperidol (2 mg/kg) and fluphenazine (2 mg/kg), and three atypical antipsychotics, (-)sulpiride (100 mg/kg), clozapine (20 mg/kg) and OPC-14597 (40 mg/kg), were used. Brains were fixed with 4% paraformaldehyde 45 min after drug administration (i.p.). Brain sections of 30 microns-thickness were made in a cryostat and hybridized with 35S-labelled for c-fos oligonucleotide probe. These sections were apposed to X-ray films and the autoradiograms were semi-quantitatively analysed by computer-assisted densitometry. All antipsychotics used increased c-fos mRNA expression in N. accumbens shell, a region of the forebrain associated with limbic systems. On the other hand, two typical antipsychotics (haloperidol and fluphenazine) that cause a high incidence of acute motor side effects increased the expression of c-fos mRNA in the dorsolateral striatum, an extrapyramidal region primarily involved in motor control. Only clozapine induced c-fos mRNA in the medial prefrontal cortex and lateral septum. These results strongly suggest that the shell region of N. accumbens may be a common site of therapeutic action of antipsychotics.

Animals↗

Fornix, medial prefrontal cortex, nucleus accumbens, and mediodorsal thalamic nucleus: roles in a fear-based context discrimination task.

The goal of the present study was to evaluate the contributions of various brain structures anatomically and functionally linked to the hippocampus and amygdala in a fear-based context discrimination task. The brain areas of interest included the fornix, medial prefrontal cortex, mediodorsal (MD) thalamic nucleus, and nucleus accumbens. Damage to the MD thalamic nucleus and medial prefrontal cortex produced the largest impairment in context-specific fear responses. Damage to the fornix impaired some fear responses (freezing, ultrasonic vocalizations, defecation, and approach/avoidance) while leaving conditioned fear expression of heart rate and urination unaltered. Damage to the nucleus accumbens was also coupled with deficits in the discriminative expression of some (heart rate, urination, and ultrasonic vocalizations) but sparing of context-appropriate freezing, defecation, and approach/avoidance behaviors.

Analysis of Variance↗

Effects of acute and chronic clozapine and haloperidol administration on 3-methoxytyramine accumulation in rat prefrontal cortex, nucleus accumbens and striatum.

The accumulation of 3-methoxytyramine (3-MT), a reflection of dopamine release, was measured in the prefrontal cortex, nucleus accumbens, and striatum following administration of acute and chronic clozapine and haloperidol. Several doses of each drug were used. The effects of chronic drug treatment were measured 1 h (chronic 1 h groups), 24 h (chronic 24 h groups) and 48 h (chronic 48 h groups) after the final dose of each drug. In the prefrontal cortex, clozapine and haloperidol elevated 3-MT more in the acute groups than in the chronic 1 h groups, suggesting that partial tolerance developed. In the striatum and nucleus accumbens, acute and chronic (chronic 1 h) haloperidol produced equal increases in 3-MT above the appropriate baselines, suggesting that no tolerance developed. In the striatum, clozapine reduced 3-MT in the chronic 1 h group after high doses (25 mg/kg), and in the chronic 24 h group. These results suggest that neuroleptics may not produce the reduction in dopamine release that has been predicted with the development of depolarization inactivation. The reduction of striatal dopamine release during chronic clozapine treatment may be related to clozapine not being associated with the development of tardive dyskinesia.

Animals↗