Hippocampal lesions and discrimination performance of mice in the radial maze: sparing or impairment depending on the representational demands of the task

The effects of ibotenate hippocampal lesions on discrimination performance in an eight-arm radial maze were investigated in mice, using a three-stage paradigm in which the only parameter that varied among stages was the way the arms were presented. In the initial learning phase (stage 1), animals learned the valence or reward contingency associated with six (three positive and three negative) adjacent arms of the maze using a successive (go/no-go) discrimination procedure. In the first test phase (stage 2), the six arms were grouped into three pairs, so that on each trial, the subject was faced with a choice between two adjacent arms of opposite valence (concurrent two-choice discrimination). In the second test phase (stage 3), the subject was faced with all six arms simultaneously (six-choice discrimination). Hippocampal-lesioned mice acquired the initial learning phase at a near-normal rate but behaved as if they had learned nothing when challenged with the two-choice discriminations at stage 2. In contrast, they behaved normally when confronted with the six-choice discrimination at stage 3. Detailed examination of within- and between-stage performance suggests that hippocampal-lesioned mice perform as intact mice when presentation of the discriminanda encourages the storage and use of separate representations (i.e., in initial learning and six-choice discrimination testing), but that they fail in test situations that involve explicit comparisons between such separate representations (two-choice discriminations), hence requiring the use of relational representations.     

Behaviorally specific limb use deficits following globus pallidus lesions in rats

The effects of bilateral globus pallidus lesions were observed in rats trained to perform a somatosensory/proprioceptive guided forelimb reaching task. Postlesion reaching became very inaccurate, characterized by gross abnormalities in limb and body posture. Postural abnormalities and paw use deficits were not observed in other situations. These results suggest that pallidal damage does not result in a simple motor deficit but interferes with motor performance when movements depend upon somesthetic and proprioceptive feedback.     

Relative Resistance of Striatal Neurons Containing Calbindin or Parvalbumin to Quinolinic Acid-Mediated Excitotoxicity Compared to Other Striatal Neuron Types

To evaluate the relative ability of those striatal neuron types containing calbindin or parvalbumin to withstand a Ca²⁺-mediated excitotoxic insult, we injected the NMDA receptor-specific excitotoxin quinolinic acid (QA) into the striatum in mature adult rats and 2 months later examined the relative survival of striatal interneurons rich in parvalbumin and striatal projection neurons rich in calbindin. To provide standardization to the survival of striatal neuron types thought to be poor in Ca²⁺buffering proteins, the survival was compared to that of somatostatin–neuropeptide Y (SS/NPY)-containing interneurons and enkephalinergic projection neurons, which are devoid of or relatively poorer in such proteins. The various neuron types were identified by immunohistochemical labeling for these type-specific markers and their relative survival was compared at each of a series of increasing distances from the injection center. In brief, we found that parvalbuminergic, calbindinergic, and enkephalinergic neurons all showed a generally comparable gradient of neuronal loss, except just outside the lesion center, where calbindin-rich neurons showed significantly enhanced survival. In contrast, striatal SS/NPY interneurons were more vulnerable to QA than any of these three other types. These observed patterns of survival following intrastriatal QA injection suggest that calbindin and parvalbumin content does not by itself determine the vulnerability of striatal neurons to QA-mediated excitotoxicity in mature adult rats. For example, parvalbuminergic striatal interneurons were not impervious to QA, while cholinergic striatal interneurons are highly resistant and SS/NPY+ striatal interneurons are highly vulnerable. Both cholinergic and SS/NPY+ interneurons are devoid of any known calcium buffering protein. Similarly, calbindin does not prevent striatal projection neuron vulnerability to QA excitotoxicity. Nonetheless, our data do suggest that calbindin may offer striatal neurons some protection against moderate excitotoxic insults, and this may explain the reportedly slightly greater vulnerability of striatal neurons that are poor in calbindin to ischemia and Huntington's disease.     

Executive function following focal frontal lobe lesions: impact of timing of lesion on outcome

While it is generally agreed that outcome following cerebral insult during childhood differs from that seen following similar pathology in adulthood, the specific relationship between timing of cerebral lesion and outcome, and the mechanism associated with observed neurobehavioral changes, remains controversial. Data from children with focal lesions suggests a non-linear relationship between age at injury and language function (e.g., Bates et al., 1999). With respect to executive function, animal models also demonstrate a non-linear relationship, and suggest that outcome is tightly linked to underlying neuronal changes (e.g., Kolb et al., 2000). Whether these models easily translate to humans, where brain morphology, cognitive function and environmental influences are more complex, is not clear. To date, focal lesion research in children has been restricted to individual case studies or, to samples of children with lesions to regions subsuming language function, or those who have undergone hemispherectomy for the treatment of intractable epilepsy. This study aimed to build on current knowledge, investigating executive function in 38 children with focal lesions involving prefrontal cortex. Aetiology and timing of lesions was diverse. Results are consistent with animal research suggesting a non-linear relationship between age at injury and outcome, with "critical periods" during development when the frontal lobes are particularly vulnerable to insult, and others when outcome is more optimal. Our findings indicate that children with prenatal lesions are at greatest risk of neurobehavioral deficits. Children with lesions sustained in middle childhood demonstrate least severe impairments across executive domains, possibly due to a period of peak synaptogenesis and dendritic arborization during this developmental stage, in keeping with animal models and research investigating frontal lobe development.     

On the use of lesions of afferents to localize neurotransmitter receptor sites in the striatum

Rats received lesions of either the cortico-striatal or the nigrostriatal projections. Two to three days after either type of lesion, evidence for degenerating dendritic spines that were postsynaptic to degenerating striatal boutons was often encountered. This type of transsynaptic degeneration of spines indicates that it is inappropriate to conclude that decreases in the number of neurotransmitter receptors after these types of lesions necessarily indicates that these receptors are located presynaptically on striatal afferents.     

The impact of chronic network hyperexcitability on developing glutamatergic synapses

The effects recurring seizures have on the developing brain are an important area of debate because many forms of human epilepsy arise in early life when the central nervous system is undergoing dramatic developmental changes. To examine effects on glutamatergic synaptogenesis, epileptiform activity was induced by chronic treatment with GABAa receptor antagonists in slice cultures made from infant rat hippocampus. Experiments in control cultures showed that molecular markers for glutamatergic and GABAergic synapses recapitulated developmental milestones reported previously in vivo. Following a 1-week treatment with bicuculline, the intensity of epileptiform activity that could be induced in cultures was greatly diminished, suggesting induction of an adaptive response. In keeping with this notion, immunoblotting revealed the expression of NMDA and AMPA receptor subunits was dramatically reduced along with the scaffolding proteins, PSD95 and Homer. These effects could not be attributed to neuronal cell death, were reversible, and were not observed in slices taken from older animals. Co-treating slices with APV or TTX abolished the effects of bicuculline suggesting that effects were dependent on NMDA receptors and neuronal activity. Neurophysiological recordings supported the biochemical findings and demonstrated decreases in both the amplitude and frequency of NMDA and AMPA receptor-mediated miniature EPSCs (mEPSCs). Taken together these results suggest that neuronal network hyperexcitability interferes with the normal maturation of glutamatergic synapses, which could have implications for cognitive deficits commonly associated with the severe epilepsies of early childhood.     

Spread of acetylcholine sensitivity in the neocortex following lesion of the nucleus basalis

Neuronal responses to the iontophoretic application of acetylcholine, carbachol and nicotine were studied in rats, 2 or 3 weeks following bilateral lesions of the nucleus basalis region, and compared to those obtained in normal animals. The percentage of cortical neurons excited by acetylcholine and their individual sensitivity were higher in lesioned animals. Furthermore, the laminar distribution of the responses to acetylcholine and the proportion of responses to carbachol and nicotine were also modified.     

Unipolar brush cells form a glutamatergic projection system within the mouse cerebellar cortex

Unipolar brush cells (UBCs) of the mammalian vestibulocerebellum receive mossy fiber projections primarily from the vestibular ganglion and vestibular nuclei. Recently, the axons of UBCs have been shown to generate an extensive system of cortex-intrinsic mossy fibers, which resemble traditional cerebellar mossy fiber afferents and synapse with granule cell dendrites and other UBCs. However, the neurotransmitter used by the UBC axon is still unknown. In this study, we used long-term organotypic slice cultures of the isolated nodulus (lobule X) from postnatal day 8 mouse cerebella to identify the neurotransmitter and receptors at synapses of the UBC axon terminals, relying on the notion that, in these cultures, all of the cortex-extrinsic fibers had degenerated during the first few days in vitro. Quantification of glutamate immunogold labeling showed that the UBC axon terminals have the same high gold-particle density as the glutamatergic parallel fiber varicosities. Furthermore, UBCs identified by calretinin immunoreactivity expressed the glutamate receptor subunits GluR2/3, NMDAR1, and mGluR2/3, like they do in the mature mouse cerebellum in situ. Evoked excitatory postsynaptic currents (EPSCs), spontaneous EPSCs, and burst discharges were demonstrated in UBCs and granule cells by patch-clamp recording. Both the evoked and spontaneous EPSCs were blocked by ionotropic glutamate receptor antagonists CNQX and D-AP5. We conclude that neurotransmission at the UBC axon terminals is glutamatergic. Thus, UBCs provide a powerful network of feedforward excitation within the granular layer, which may amplify vestibular signals and synchronize activity in clusters of functionally related granule cells which project vertically to patches of Purkinje cells.     

Potentiation of glutamatergic agonist-induced inositol phosphate formation by basic fibroblast growth factor is related to developmental features in hippocampal cultures: neuronal survival and glial cell proliferation

We investigated the modulation by growth factors of phospholipase C (PLC)-linked glutamate receptors during in vitro development of hippocampal cultures. In defined medium, glial cells represent between 3 and 14% of total cell number. When we added basic fibroblast growth factor (bFGF) 2 h after plating, we found: (i) a neuroprotection from naturally occurring death for up to 5 days; (ii) a proliferation of glial cells from day 3; and (iii) a potentiation of quisqualate (QA)-induced inositol phosphate (IP) formation from 1 to 10 days in vitro (DIV) and 1S, 3R-amino-cyclopentane-1,3-dicarboxylate (ACPD) response from 3 to 10 DIV. The antimitotic cytosine-beta,D-arabinofuranoside (AraC) blocked glial cell proliferation induced by bFGF, but not neuroprotection. Under these conditions, the early potentiation of the QA response (1-3 DIV) was not changed, while the ACPD and late QA response potentiations were prevented (5-10 DIV). Epidermal growth factor was not neuroprotective but it induced both glial cell proliferation and late QA or ACPD potentiation. Surprisingly, the early bFGF-potentiated QA-induced IP response was blocked by 6, 7-dinitro-quinoxaline-2,3-dione (DNQX), suggesting the participation of ionotropic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (KA) receptors. The delayed bFGF-potentiated ACPD-induced IP response is inhibited by (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), indicating possible activation of glial metabotropic receptors. These results suggest that, in hippocampal cultures, bFGF modulates AMPA and metabotropic glutamate receptors linked to the IP cascade, possibly in relation to the regulation of neuronal survival and glial cell proliferation, respectively.     

Dynamic alterations in the central glutamatergic status following food and glucose intake: in vivo multimodal assessments in humans and animal models

Fluctuations of neuronal activities in the brain may underlie relatively slow components of neurofunctional alterations, which can be modulated by food intake and related systemic metabolic statuses. Glutamatergic neurotransmission plays a major role in the regulation of excitatory tones in the central nervous system, although just how dietary elements contribute to the tuning of this system remains elusive. Here, we provide the first demonstration by bimodal positron emission tomography (PET) and magnetic resonance spectroscopy (MRS) that metabotropic glutamate receptor subtype 5 (mGluR5) ligand binding and glutamate levels in human brains are dynamically altered in a manner dependent on food intake and consequent changes in plasma glucose levels. The brain-wide modulations of central mGluR5 ligand binding and glutamate levels and profound neuronal activations following systemic glucose administration were further proven by PET, MRS, and intravital two-photon microscopy, respectively, in living rodents. The present findings consistently support the notion that food-associated glucose intake is mechanistically linked to glutamatergic tones in the brain, which are translationally accessible in vivo by bimodal PET and MRS measurements in both clinical and non-clinical settings.     

Arterial spin labeling demonstrates that focal amygdalar glutamatergic agonist infusion leads to rapid diffuse cerebral activation

Munasinghe JP, Banerjee M, Acosta MT, Banks M, Heffer A, Silva AC, Koretsky A, Theodore WH. Arterial spin labeling demonstrates that focal amygdalar glutamatergic agonist infusion leads to rapid diffuse cerebral activation.
Acta Neurol Scand: 2010: 121: 209–216.
© 2009 The Authors Journal compilation © 2009 Blackwell Munksgaard. Objectives – To investigate acute effects of intra‐amygdalar excitatory amino acid administration on blood flow, relaxation time and apparent diffusion coefficient in rat brain. Materials and methods – Several days after MR‐compatible cannula placement in right basolateral amygdala, anesthetized rats were imaged at 7 T. Relative cerebral blood flow (CBF) was measured before and 60 min after infusion of 10 nmol KA, cAMPA, ATPA, or normal saline using arterial spin labeling. Quantitative T₂ and diffusion‐weighted images were acquired. rCBF, T₂ and ADC values were evaluated in bilateral basolateral amygdala, hippocampus, basal ganglia, frontal and parietal regions. Results – KA led to the highest, and ATPA lowest bilateral rCBF increases. Time courses varied among drugs. T₂ for KA and AMPA was higher while ADC was lower for KA. Conclusions – Intra‐amygdalar injection of GluR agonists evoked bilateral seizure activity and increased rCBF, greater for KA and AMPA than selective ATPA GluR5 activation.     

The mating movements of male decorticate rats: evidence for subcortically generated movements by the male but regulation of approaches by the female

The study shows that although many features of copulation in decorticate male rats are normal, copulatory success is importantly dependent upon the control of approaches exerted by the normal female rat. Copulation by neonatally decorticated adult rats and normal adult rats was studied in cohabitation and videotaped tests. Seven of 10 decorticate rats and 6 of 6 normal rats sired pups in the cohabitation test. When initially paired with ovariectomized and primed female rats, in the videotaped tests, all normal rats, but only one decorticated rat, copulated. All decorticate rats made movements indicative of sexual interest including: treading on the female's back, passing over the female, and sniffing the female's genitals. After activating stimulation, 5 of 6 remaining decorticated males copulated. After one successful mount the remaining copulatory patterns proceeded relatively normally. Numbers of mounts, intromissions, ejaculations, postejaculatory songs, and the intromission and ejaculatory patterns were like those of control rats, although the decorticate rats had fewer mount bouts and showed abnormalities in the execution of movements. Precopulatory movements were notated, using the Eshkol-Wachmann system, and compared with copulatory movements. Non-copulatory and copulatory approaches were similar, except that clasping appeared to be the key movement involved in the transition of an approach movement into a copulatory movement. The analysis also showed that the females' movements of hopping, turning, and kicking were important for regulating the males' approaches, and were instrumental in the success achieved by the decorticated males. The study shows that although the cortex, insofar as it facilitates the appearance of certain movements and contributes to their efficiency, is involved in male sexual activity, in its absence well organized sexual activity is possible, although this is dependent, in part, upon the behaviour of the female.     

弥漫性脑损伤后代谢型谷氨酸受体亚型4及其激动剂L-AP 4的作用

目的　研究弥漫性脑损伤 (DBI)后大鼠脑皮质代谢型谷氨酸受体亚型 4 (mGluR 4 )及其激动剂L 2 氨基 4 膦酰基丁酸(L AP 4 )的变化及意义。方法　 16 1只SD大鼠随机分为两组。A组包括正常对照组、假手术组及DBI组。用Marmarou弥漫性脑损伤模型 ,制成DBI模型 ,于伤后不同时间进行mGluR 4mRNA原位杂交。B组包括单纯、DBI后生理盐水治疗及DBI后L AP 4治疗组。所有DBI动物伤前进行行为学训练。伤后 1h、12h脑室内分别给予L AP 4 (10 0mM ,10 μl)或生理盐水。大鼠在伤后 1、3、7、14d分批处死前进行运动和行为学检查 ,处死后检测神经元损伤数。结果　与正常对照组相比 ,假手术组阳性神经元数无改变 (P >0 .0 5 ) ;与假手术组相比 ,单纯DBI组mGluR 4mRNA表达于脑损伤后 1h即有明显增加(P <0 .0 1) ,在 6h达到高峰。与DBI后生理盐水治疗组比较 ,DBI后L AP 4治疗组神经元损伤数减少 ,神经功能检查指数增高。结论　mGluR 4参与了DBI的病理生理过程 ,具有神经保护作用。     

Omega‐3 fatty acids regulate plasticity in distinct hippocampal glutamatergic synapses

Dietary omega‐3 fatty acids accumulate and are actively retained in central nervous system membranes, mainly in synapses, dendrites and photoreceptors. Despite this selective enrichment, their impact on synaptic function and plasticity has not been fully determined at the molecular level. In this study, we explored the impact of omega‐3 fatty acid deficiency on synaptic function in the hippocampus. Dietary omega‐3 fatty acid deficiency for 5 months after weaning led to a 65% reduction in the concentration of docosahexaenoic acid in whole brain synaptosomal phospholipids with no impact on global dopaminergic or serotonergic turnover. We observed reduced concentrations of glutamate receptor subunits, including GluA1, GluA2 and NR2B, and synaptic vesicle proteins synaptophysin and synaptotagmin 1 in hippocampal synaptosomes of omega‐3 fatty acid‐deficient mice as compared to the omega‐3 fatty acid rich group. In contrast, an increased concentration of neuronal inositol 1,4,5‐trisphosphate‐receptor (IP₃‐R) was observed in the deficient group. Furthermore, omega‐3 fatty acid deficiency reduced the long‐term potentiation (LTP) in stratum oriens of the hippocampal CA1 area, but not in stratum radiatum. Thus, omega‐3 fatty acids seem to have specific effects in distinct subsets of glutamatergic synapses, suggesting specific molecular interactions in addition to altering plasma membrane properties on a more global scale.     

Oxytocin Pretreatment Enhances Arginine Vasopressin-lnduced Motor Disturbances and Arginine Vasopressin-lnduced Phosphoinositol Hydrolysis in Rat Septum: A Cross-Sensitization Phenomenon

The recent observation that the central oxytocin (OT) receptor has high affinity for both OT and arginine vasopressin (AVP) raises the possibility that it may be involved in some of the central actions of AVP. Repeated intracerebroventricular (icv) injections of AVP in rats evoke an unusual sensitization phenomenon in that a first exposure to the peptide enhances the sensitivity (sensitization) of the brain to a second exposure. This report investigates the possibility that the OT receptor may be involved in the mediation of the phenomenon of sensitization, using OT, a specific OT receptor agonist, [Thr⁴, Gly⁷]OT, and a specific OT receptor antagonist, d(CH₂)₅, [Tyr(Me)², Thr⁴, Tyr-NH₂⁹]OVT (compound 6; cpd 6), as well as a V1 AVP receptor antagonist, d(CH₂)₅Tyr(Me)AVP. Peptides were injected icv in conscious, adult male Sprague-Dawley rats. The data showed that: 1) a first icv AVP injection (10 pmol/5μl) enhanced the sensitivity of the rat brain to the motor response of a second AVP injection (10 pmol/5 μl) given 24 h later; 2) injection of d(CH₂)₅Tyr(Me)AVP (100 pmol/5 μl icv) but not cpd 6, (100 pmol/5 μl icv) 2 min prior to the first AVP injection, blocked AVP-induced sensitization; 3) a first injection of OT or [Thr⁴, Gly⁷]OT (10 pmol/5 μl) enhanced the sensitivity of the brain to the motor actions of a subsequent AVP injection given 24 h later; 4) the magnitude of this cross-sensitization induced by OT pretreatment varied with dose and appeared to be ten times more potent than the sensitization induced by a first AVP injection; 5) injection of cpd 6 (100 pmol/5 μl) but not d(CH₂)₅Tyr(Me)AVP (100 pmol/5 μl icv) 2 min prior to the first OT injection (1 pmol/5 μl) blocked the cross-sensitization induced by OT; 6) an injection of OT (100 to 1,000 pmol/5 μl) or [Thr⁴, Gly⁷]OT (10 pmol/5 μl) in rats that had been cross-sensitized with OT or [Thr⁴, Gly⁷]OT pretreatment did not evoke enhanced motor responses; 7) OT injected 2 min prior to the second AVP injection in AVP-sensitized rats did not block the enhanced AVP-induced motor responses; 8) AVP-induced [³H]inositol monophosphate accumulation in septal slices was also enhanced in rats cross-sensitized by OT pretreatment. These results suggest that while pre-exposure of the rat brain to both AVP and OT alters the responsiveness of the rat brain to subsequent AVP exposures, AVP sensitization appears to be mediated via the V1 AVP receptor, whereas cross-sensitization by OT may be mediated via the OT receptor. The ability of OT to alter the responsiveness of the rat brain to subsequent AVP injection suggests a role for this peptide in modulating central AVP actions.     

Effect of 6-hydroxydopamine lesions on norepinephrine-induced [H]glycogen hydrolysis in mouse cortical slices

The effects of norepinephrine (NE) on in vitro [³H]glycogenolysis were assessed in slices of cerebral cortex from mice whose cortical noradrenergic innervation had been severely reduced by intracisternal 6-hydroxydopamine (6-OHDA) injections. A supersensitive response to NE was observed, as demonstrated by a decrease in the EC₅₀ of the catecholamine in the lesioned mice from 533 ± 88nM to39.3 ± 7.9nM. This supersensitive response, observed two weeks after the lesion, was post-synaptic since isoproterenol, a β-adrenergic agonist not accumulated by pre-synaptic uptake mechanisms, also gave an equally supersensitive response.     

Long‐term plasticity of glutamatergic input from the subthalamic nucleus to the entopeduncular nucleus

The hyperdirect pathway of the basal ganglia bypasses the striatum, and delivers cortical information directly to the subthalamic nucleus (STN). In rodents, the STN excites the two output nuclei of the basal ganglia, the entopeduncular nucleus (EP) and the substantia nigra reticulata (SNr). Thus, during hyperdirect pathway activation, the STN drives EP firing inhibiting the thalamus. We hypothesized that STN activity could induce long‐term changes to the STN‐>EP synapse. To test this hypothesis, we recorded in the whole‐cell mode from neurons in the EP in acute brain slices from rats while electrically stimulating the STN. Repetitive pre‐synaptic stimulation generated modest long‐term depression (LTD) in the STN‐>EP synapse. However, pairing EP firing with STN stimulation generated robust LTD that manifested for pre‐before post‐as well as for post‐ before pre‐synaptic pairing. This LTD was highly sensitive to the time difference and was not detected at a time delay of 10 ms. To investigate whether post‐synaptic calcium levels were important for LTD induction, we made dendritic recordings from EP neurons that revealed action potential back‐propagation and dendritic calcium transients. Buffering the dendritic calcium concentration in the EP neurons with EGTA generated long term potentiation instead of LTD. Finally, mild LTD could be induced by post‐synaptic activity alone that was blocked by an endocannabinoid 1 (CB1) receptor blocker. These results thus suggest there may be an adaptive mechanism for buffering the impact of the hyperdirect pathway on basal ganglia output which could contribute to the de‐correlation of STN and EP firing.