Glutamate as transmitter of hippocampal perforant path

A metabolomic study on the gender-dependent effects of maternal exposure to fenvalerate on neurodevelopment in offspring mice

BACKGROUND

The general population is widely exposed to fenvalerate. However, the effects of maternal exposure to fenvalerate on neurodevelopment in offspring and the underlying metabolic mechanism are largely unknown.

METHODS

Pregnant mice were exposed to fenvalerate for 11 consecutive days. The forced swimming test (FST) was performed in 35 day-old offspring to investigate the effects of fenvalerate on neurobehavioral responses. Blood serum free T4 and free T3 concentrations were measured using commercial ELISA. Blood and thyroid samples were used for metabolomic analyses with UPLC Q-Exactive. The expression levels of neurotransmitter metaolite receptors were determined in the frontal cortex of offspring using real-time PCR.

RESULTS

The immobility time, free T4 and free T3, and expression levels of Htr1a and Htr2a were statistically changed in offspring male mice. Metabolomic analysis revealed that the pentose phosphate pathway, starch and sucrose metabolism, glutamic acid metabolism were the key changed pathways in the blood, and thiamine metabolism was the key changed pathway in the thyroid.

CONCLUSION

Prenatal exposure to fenvalerate affected neurodevelopment in male offspring mice both via the changed abundances of metabolites involved in glycolysis related metabolism and medium-chain fatty acid metabolism, and the changes in 5-HT receptor expression.

Stimulation of entorhinal cortex-dentate gyrus circuitry is antidepressive

Major Depressive Disorder (MDD) is considered a “circuitopathy”, and brain stimulation therapies hold promise for ameliorating MDD symptoms, including hippocampal dysfunction. It is unknown if stimulation of upstream hippocampal circuitry, such as the entorhinal cortex (Ent), is antidepressive, although Ent stimulation improves learning and memory in lab animals and humans. Here we show molecular targeting (Ent-specific knockdown of a psychosocial stress-induced protein) and chemogenetic stimulation of Ent neurons induce antidepressive-like effects in mice. Mechanistically, we show that Ent stimulation-induced antidepressive-like behavior relies on the generation of new hippocampal neurons. Thus, controlled stimulation of Ent hippocampal afferents is antidepressive via increased hippocampal neurogenesis. These findings emphasize the power and potential of Ent glutamatergic afferent stimulation - previously well known for the ability to influence learning and memory - for MDD treatment.

Olfactory and cortical projections to bulbar and hippocampal adult-born neurons

New neurons are continually generated in the subependymal layer of the lateral ventricles and the subgranular zone of dentate gyrus during adulthood. In the subventricular zone, neuroblasts migrate a long distance to the olfactory bulb where they differentiate into granule or periglomerular interneurons. In the hippocampus, neuroblasts migrate a short distance from the subgranular zone to the granule cell layer of the dentate gyrus to become granule neurons. In addition to the short-distance inputs, bulbar interneurons receive long-distance centrifugal afferents from olfactory-recipient structures. Similarly, dentate granule cells receive differential inputs from the medial and lateral entorhinal cortices through the perforant pathway. Little is known concerning these new inputs on the adult-born cells. In this work, we have characterized afferent inputs to 21-day old newly-born neurons. Mice were intraperitoneally injected with bromodeoxyuridine. Two weeks later, rhodamine-labeled dextran-amine was injected into the anterior olfactory nucleus, olfactory tubercle, piriform cortex and lateral and medial entorhinal cortices. One week later, animals were perfused and immunofluorescences were carried out. The data show that projection neurons from the mentioned structures, establish putative synaptic contacts onto 21-day-old neurons in the olfactory bulb and dentate gyrus, in some cases even before they start to express specific subpopulation proteins. Long-distance afferents reach middle and outer one-third portions of the molecular layer of the dentate gyrus and granule and, interestingly, periglomerular layers of the olfactory bulb. In the olfactory bulb, these fibers appear to establish presumptive axo-somatic contacts onto newly-born granule and periglomerular cells.

What does the anatomical organization of the entorhinal cortex tell us?

The entorhinal cortex is commonly perceived as a major input and output structure of the hippocampal formation, entertaining the role of the nodal point of cortico-hippocampal circuits. Superficial layers receive convergent cortical information, which is relayed to structures in the hippocampus, and hippocampal output reaches deep layers of entorhinal cortex, that project back to the cortex. The finding of the grid cells in all layers and reports on interactions between deep and superficial layers indicate that this rather simplistic perception may be at fault. Therefore, an integrative approach on the entorhinal cortex, that takes into account recent additions to our knowledge database on entorhinal connectivity, is timely. We argue that layers in entorhinal cortex show different functional characteristics most likely not on the basis of strikingly different inputs or outputs, but much more likely on the basis of differences in intrinsic organization, combined with very specific sets of inputs. Here, we aim to summarize recent anatomical data supporting the notion that the traditional description of the entorhinal cortex as a layered input-output structure for the hippocampal formation does not give the deserved credit to what this structure might be contributing to the overall functions of cortico-hippocampal networks.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

The perforant path: projections from the entorhinal cortex to the dentate gyrus

This paper provides a comprehensive description of the organization of projections from the entorhinal cortex to the dentate gyrus, which together with projections to other subfields of the hippocampal formation form the so-called perforant pathway. To this end, data that are primarily from anatomical studies in the rat will be summarized, complimented with comparative data from other species. The analysis of the organization of any of the connections of the hippocampus, including that of the entorhinal cortex to the dentate gyrus, is severely hampered because of the complex three-dimensional shape of the hippocampus. In particular in rodents, but to a lesser extent also in primates, all traditional planes of sectioning will result in sections that at some point or another do not cut through the hippocampus at an angle that is perpendicular to its long axis. To amend this, we will describe own unpublished tracing data obtained in the rat with the use of the so-called extended preparation. A number of issues will be addressed. First, data will be summarized which will clarify the laminar origin of the perforant pathway within the entorhinal cortex. Second, we will discuss whether or not a radial organization, along the proximo-distal dendritic axis of granule cells, characterizes the entorhinal-dentate projection. Third, we will discuss whether this projection is governed by any transverse organization, and fourth, we will focus on the organization along the longitudinal axis. Finally, the synaptic organization and the contralateral entorhinal-dentate projection will be described briefly. Taken together, the available data suggest that the projection from the entorhinal cortex to the dentate gyrus is a fairly well conserved connection, present in all species studied, exhibiting a grossly similar organization.

Entorhinal projections terminate onto principal neurons and interneurons in the subiculum: A quantitative electron microscopical analysis in the rat

The synaptic organization of projections to the subiculum from superficial layers of the lateral and medial entorhinal cortex was analyzed in the rat, using anterograde neuroanatomical tracing followed by electron microscopical quantification. Our aim was to assess the synaptic organization and whether the two projection components (lateral, medial) within the perforant pathway are qualitatively and quantitatively similar with respect to the types of synapses formed and with respect to the postsynaptic targets of these entorhinal projections. The tracer biotinylated dextran amine (BDA) was injected into the lateral and medial entorhinal cortex, respectively, and resulting anterograde labeling in the subiculum was studied. For each of the two projection components, we analyzed in four animals (2 x 2) a total of 100 synapses/animal with respect to features of the synapse type, i.e. asymmetrical or symmetrical, as well as regarding their postsynaptic target, i.e. dendritic shaft or spine. No clear differences were observed between the two pathways. The majority of the synapses were of the asymmetrical type, making contact with spines (78%) or with dendritic shafts (14%). A low percentage of symmetrical synapses targeted dendritic shafts (4.2%) or spines (1.3%). About 2.5% of the synapses remained undetermined. The findings indicate that the majority of entorhinal fibers reaching the subiculum exert an excitatory influence primarily onto principal neurons, with a much smaller feed forward inhibitory component. Only a small percentage of entorhinal fibers in the subiculum appears to be inhibitory, largely influencing interneurons.

Donepezil reverses a mnemonic deficit produced by scopolamine but not by perforant path lesion or transient cerebral ischaemia

The purpose of these studies were threefold. Firstly, to further characterize the effect of perforant path transection on a test of short-term memory: delayed matching (or nonmatching)-to-position [D(N)MTP]. Secondly, to evaluate the effect of a transient cerebral ischaemia in the same task. Both surgical procedures were chosen as they produce a CNS lesion similar to that described in Alzheimer's Disease (AD). Thirdly, the effect of the acetylcholinesterase inhibitor, donepezil (Aricept(R), E2020), on the resulting cognitive impairment was studied. Perforant path transection produced a robust, delay-dependent impairment of choice accuracy in rats performing either a delayed matching- or nonmatching-to-position task. Sample latency was also reduced following lesion, yet the lesion-induced impairment was not affected by increasing the response requirement at the sample stage. An 11-min period of transient ischaemia (two-vessel occlusion model) resulted in almost complete loss of hippocampal CA1 pyramidal cells and a delay-dependent impairment in DMTP performance. However, unlike perforant path lesions, this deficit was unstable and declined in magnitude over the experimental period. Increasing the delay interval restored this deficit. Donepezil, at doses that robustly attenuated a scopolamine (0.06 mg/kg s.c.)-induced DMTP accuracy impairment in naïve, unoperated rats, had no effect against either lesion-induced impairment. The results are considered in terms of the effectiveness of acetylcholinesterase inhibitors in noncholinergic-based preclinical cognitive models.

Amyloid beta-peptide inhibits Na+-dependent glutamate uptake

The purpose of this review is to summarize much of the work on the inhibition of the astroglial glutamate transporter in relation to excitotoxic neurodegeneration, in particular, inhibition of uptake by the beta-amyloid peptide (A beta) found in the Alzheimer's disease (AD) brain. There is evidence for oxidative stress in the AD brain, and A beta has been found to generate reactive oxygen species (ROS), thus adding to the stress or possibly initiating it. The oxidative inhibition of the glutamate transporter protein by A beta increases the vulnerability of glutamatergic neurons, and by rendering them susceptible to the excitotoxic insult that results from impaired glutamate uptake, A beta can be directly connected to the neurodegeneration that follows.

Bilateral 6-OHDA lesions to the hippocampus attenuate the facilitatory effect of CCK-8 us and caerulein on memory in rats

The involvement of dopaminergic projection to the hippocampus in the facilitatory effect of cholecystokinin-unsulphated octapeptide (CCK-8 us) and caerulein (CER) on memory motivated affectively was investigated in male rats. CCK-8 us and CER were given subcutaneously at the doses of 10 microg kg(-1)and 0.5 microg kg(-1), respectively, immediately after a single learning trial in a passive avoidance situation, after bilateral 6-OHDA lesions to the dentate gyrus of the hippocampus. In order to protect noradrenergic neurones against destruction by neurotoxin, 30 min before surgery rats were pre-treated intraperitoneally with 25 mg kg(-1)of desmethylimipramine, an inhibitor of noradrenaline uptake. Bilateral 6-OHDA lesions to the hippocampus significantly attenuate the facilitatory effect of CCK-8 us and CER on retention of passive avoidance behaviour evaluated 24 h after the learning trial. Neither, destruction of dopaminergic endings in the hippocampus, nor application of CCK-8 us and CER changed the spontaneous psychomotor activity of rats estimated in an 'open field' test. These results may indicate that the facilitatory effect of CCK-8 us and CER on memory motivated affectively is, in part, mediated by dopaminergic projection from the ventral tegmental area to the dentate gyrus of the hippocampus.

Group II selective metabotropic glutamate receptor agonists and local cerebral glucose use in the rat

The novel mGluR agonist LY354740 and a related analogue LY379268 are selective for mGluR2/3 receptors and are centrally active after systemic administration. In this study, rates of local cerebral glucose use were measured using the [14C]2-deoxyglucose autoradiographic technique to examine the functional consequences of their systemic administration in the conscious rat. Both LY354740 (0.3, 3.0, 30 mg/kg) and LY379268 (0.1, 1.0, 10 mg/kg) produced dose-dependent changes in glucose use. After LY354740 (3.0mg/kg), 4 of the 42 regions measured showed statistically significant changes from vehicle-treated controls: red nuclei (-16%), mammillary body (-25%), anterior thalamus (-29%), and the superficial layer of the superior colliculus (+50%). An additional 15 regions displayed significant reductions in function-related glucose use (P < .05) in animals treated with LY354740 (30 mg/ kg). LY379268 (0.1, 1.0, 10 mg/kg) produced changes in glucose metabolism in 20% of the brain regions analyzed. Significant increases (P < .05) in glucose use were evident in the following: the superficial layer of the superior colliculus (+81%), locus coeruleus (+57%), genu of the corpus callosum (+31%), cochlear nucleus (+26%), inferior colliculus (+20%), and the molecular layer of the hippocampus (+14%). Three regions displayed significant decreases: mammillary body (-34%), anteroventral thalamic nucleus (-28%), and the lateral habenular nucleus (-24%). These results show the important functional involvement of the limbic system together with the participation of components of different sensory systems in response to the activation of mGluR2 and mGluR3 with LY354740 and LY379268.

Muscarinic receptor subtypes involved in hippocampal circuits

Muscarinic receptors modulate hippocampal activity in two main ways: inhibition of synaptic activity and enhancement of excitability of hippocampal cells. Due to the lack of pharmacological tools, it has not been possible to identify the individual receptor subtypes that mediate the specific physiological actions that underlie these forms of modulation. Light and electron microscopic immunocytochemistry using subtype-specific antibodies was combined with lesioning techniques to examine the pre- and postsynaptic location of m1-m4 mAChR at identified hippocampus synapses. The results revealed striking differences among the subtypes, and suggested different ways that the receptors modulate excitatory and inhibitory transmission in distinct circuits. Complementary physiological studies using m1-toxin investigated the modulatory effects of this subtype on excitatory transmission in more detail. The implications of these data for understanding the functional roles of these subtypes are discussed.

Characterization of perforant path lesions in rodent models of memory and attention

Early stage Alzheimer's disease (AD) pathology is associated with neurodegeneration of systems within the temporal cortex, e.g. the entorhinal cortex, perforant pathway and hippocampus. The perforant pathway provides the major neuronal input to the hippocampus from the entorhinal cortex and thus relays multimodal sensory information derived from cortical zones into the hippocampus. The earliest symptoms of AD include cognitive impairments, e.g. deficits in short-term memory and attention. Consequently, we have investigated the effect of bilateral knife cut lesions to the perforant path on cognition in rats using models measuring primarily short-term memory (operant delayed match to position task), attention (serial five-choice reaction time task) and spatial learning (Morris water maze). Rats receiving bilateral perforant path lesions showed normal neurological function and a mild hyperactivity. The lesion produced little effect on attention assessed using the five-choice task. In contrast, animals with equivalent lesions showed a robust delay-dependent deficit in the delayed match to position task. Spatial learning in the water maze task was also severely impaired. The delay-dependent deficit in the match to position task was not reversed by tacrine (3 mg/kg) pretreatment. The present data support a selective impairment of cognitive function following perforant path lesions that was confined to mnemonic rather than attentional processing. These findings complement primate and human studies identifying a critical role of the perforant pathway and associated temporal lobe structures in declarative memory. Degeneration of the perforant pathway is likely to contribute to the mnemonic deficits characteristic of early AD. The failure of tacrine to ameliorate these deficits may be relevant to an emerging clinical literature suggesting that cholinomimetic therapies improve attentional rather than mnemonic function in AD.

Systemic administration of N omega-nitro-L-arginine methyl ester and indomethacin reduces the elevation of brain PGE2 content and prevents seizures and hippocampal damage evoked by LiCl and tacrine in rat

Administration of tacrine (5 mg/kg i.p.), an anticholinesterase agent, in rats pretreated (24 h beforehand) with lithium chloride (LiCl; 12 mEq/kg i.p.) enhances the expression of neuronal nitric oxide (NO) synthase (NOS), increases NO, and causes seizures and hippocampal damage. Here we report immunohistochemistry evidence showing that in rat LiCl and tacrine enhance the expression of cyclooxygenase type 2 (COX-2) enzyme protein in the dorsal hippocampus and elevate brain PGE2 content during the preconvulsive period. The latter effect, but not enhanced COX-2 expression, is inhibited by previous (30 min before tacrine) administration of N omega-nitro-L-arginine-methyl ester (L-NAME; 10 mg/kg i.p.), an inhibitor of NO synthesis, thus implicating NO in the mechanism of stimulation of COX activity leading to elevation of brain PGE2 content. Indomethacin (10 mg/kg given i.p. 30 min before tacrine), an inhibitor of COX activity, prevented brain PGE2 elevation and abolished the expression of seizures and hippocampal damage thus supporting a role for this metabolite of the arachidonic acid cascade in the mechanisms of LiCl and tacrine-evoked neurotoxicity in rat.

Seizures and hippocampal damage produced by dendrotoxin-K in rats is prevented by the 21-aminosteroid U-74389G

The epileptogenic and neurodegenerative effects of dendrotoxin K (DTx-K), from Dendroaspis polylepsis, a specific blocker of a noninactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats pretreated with the 21-aminosteriod U-74389G, a scavenger of free oxygen radicals. Administration of 35 pmol DTx-K elicited in all of the treated animals (n = 6) motor seizures and bilateral electrocortical (ECoG) discharges after a latent period of approximately 5 min. At 24 h, histological examination of brain (n = 6) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation. Quantitation of damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer and dentate gyrus granule cell layer relative to the corresponding brain regions of rats (n = 6) injected with bovine serum albumin (300 ng), which per se was ineffective in all respects. DTx-K (35 pmol) also caused a significant loss of CA3 pyramidal neurons ipsilateral to the site of toxin injection. Systemic (i.p.) administration of U-74389G (5 mg/kg given 30 min beforehand) delayed the onset of motor and ECoG seizures and reduced the number of epileptogenic discharges typically observed in rats receiving an injection of DTx-K (35 pmol) alone. Similarly, this treatment prevented the damage inflicted to the hippocampus by the toxin and in no instance was significant neuronal loss observed. At variance with these results, pretreatment with U-74389G (up to 10 mg/kg i.p.) failed to prevent seizures and CA1 hippocampal damage evoked by intra-hippocampal injection of alpha-DTx (35 pmol), a DTx-K homologue which preferentially inhibits a slowly inactivating, voltage-dependent K+ conductance in nerve cells. In conclusion, the present data support a role for free oxygen radicals in mediating hippocampal damage induced by DTx-K, but not alpha-DTx, and confirm the original deduction that these DTx homologues are complementary neurobiological tools to study mechanisms of seizures and neuronal death.

Involvement of cAMP-dependent protein kinase in mu-opioid modulation of NMDA-mediated synaptic currents

We have previously reported dual effects of mu-opioids on N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic events in the hippocampal dentate gyrus: an indirect facilitating effect via suppression of GABAergic interneurons (disinhibition) and a direct inhibitory effect in the presence of gamma-aminobutyric acid-A (GABA(A)) antagonists. The cellular mechanism underlying the inhibitory effect of mu-opioids remains to be determined. In the present study we examine the role of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) in mu-opioid-induced inhibition of NMDA currents in rat hippocampal slices. NMDA-receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) were evoked by stimulating the lateral perforant path and were recorded from dentate granule cells with the use of whole cell voltage-clamp techniques in the presence of the GABA(A) antagonist and a non-NMDA type of glutamate receptor antagonist. Two selective mu-agonists, [N-MePhe3, D-Pro4]-morphiceptin and [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin, induced dose-dependent inhibition of NMDA EPSCs in a concentration range of 0.3-10 microM. This inhibitory effect could be completely reversed by the opioid antagonists naloxone or prevented by a selective mu-antagonist cyprodime, but was not affected by removal of Mg2+ from the external perfusion medium. Intracellular application of pertussis toxin (PTX) into the granule cell via whole cell recording pipettes completely prevented mu-opioid-induced reduction in NMDA currents, suggesting that a postsynaptic mechanism involving PTX-sensitive G proteins might be responsible for the inhibitory action of mu-opioids. Further studies were conducted to identify the intracellular messengers that coupled with G proteins and transduced the effect of mu-opioids in granule cells. The adenylate cyclase activator forskolin was found to enhance NMDA-receptor-mediated synaptic responses and to reverse the inhibitory effect of mu-opioids. Sp-cAMPS, a specific PKA activator, also enhanced NMDA EPSCs, whereas the PKA inhibitor Rp-cAMPS reduced NMDA EPSCs and occluded further inhibition of the current by mu-opioids. These findings strongly suggest that NMDA receptor function is subject to the modulation by PKA, and that mu-opioids can inhibit NMDA currents through suppression of the cAMP cascade in the postsynaptic neuron. Combined with our previous findings, the present results also indicate that mu-opioids can modulate NMDA-receptor-mediated synaptic activity in a complex manner. The net effect of mu-opioids in the dentate gyrus may depend on the interplay between its disinhibitory action, which facilitates NMDA-receptor-mediated responses, and its inhibitory action on the cAMP cascade.

Glutamate release correlates with brain-derived neurotrophic factor and trkB mRNA expression in the CA1 region of rat hippocampus

Synthesis of the neurotrophic factor brain-derived neurotrophic factor (BDNF) and its receptor TrkB in the hippocampus have been proposed to be influenced by endogenous glutamate. To test this hypothesis we have investigated if increases in BDNF and trkB mRNAs are associated with changes in the synaptic release of glutamate in the dorsal hippocampus in the conscious rat by combining the technique of in vivo microdialysis with in situ hybridization histochemistry. A 35% and 66% increase in extracellular levels of glutamate in the dorsal CA1 region was detected following injection into the lateral entorhinal cortex of 2.4 and 9.6 microg of the non-NMDA glutamate receptor agonist quisqualate, respectively. The increase in glutamate was attenuated by local administration of tetrodotoxin (TTX) indicating neuronal origin. Levels of BDNF and trkB mRNAs were increased in the hippocampus in a dose-dependent fashion following the stimulations. The extracellular levels of glutamate in individual animals correlated to the levels of BDNF and trkB mRNAs in the dorsal CA1 region of the hippocampus. This study provides for the first time evidence of an entorhinal cortex influenced concentration-dependent relationship between the release of endogenous glutamate in vivo and neuronal expression of mRNAs for BDNF and its receptor trkB in the hippocampus.

Comparing the role of metabotropic glutamate receptors in long-term potentiation and in learning and memory

1. Neuronal plasticity has been suggested to be the physical substrate for changes underlying the expression of memory. One model which has attracted wide attention as a possible candidate of such neuronal plasticity is long-term potentiation (LTP), mainly investigated in the hippocampus of rodents. Moreover, various processes with different time constants may underlie LTP, and these phases show striking correspondence to different phases of memory. 2. Pharmacological evidence strongly implicates that the neurotransmitter glutamate plays a major role in LTP. Although the involvement of ionotropic glutamate receptors has been proven, the role of the newly discovered metabotropic glutamate receptors is still uncertain. 3. Metabotropic glutamate receptors (mGluRs) comprise a whole family with currently eight members grouped into three classes according to their amino acid sequence identity and pharmacological profile. They are G-protein coupled, either positively linked to phospholipase C (class I) or negatively linked to adenylate cyclase (class II and III), and among other effects are known to induce phosphorylation of ionotropic glutamate receptors as well as modulate the excitability of neurons. Finally, they are heterogeneously distributed throughout the brain. 4. In hippocampal slice preparations, mGluRs have been shown to be involved in the induction of LTP in CA1 and dentate gyrus by some investigators, but others have failed to reproduce such experiments, leaving the question: what are the appropriate conditions for mGluR-mediated LTP? 5. In vivo, metabotropic receptor antagonists have been shown to block, and agonists to facilitate, induction and maintenance of LTP, mainly at perforant path/dentate granule cell synapses. As demonstrated in behavioral investigations, mGluRs apparently play an important part in hippocampus-dependent learning paradigms. As in LTP, antagonists block memory formation; in contrast to LTP, agonists also prevent memory formation. In memory recall metabotropic receptors seem to play no role. 6. Based on current information the authors develop models for a role of mGluRs in both LTP and memory formation. Activation of metabotropic receptors plays a particular modulatory role when high frequency stimulation is weak. Strong tetanization may bypass mGluRs by stimulating other systems leading to, at least phenomenologically, similar LTP, Behaviorally, mGluRs possibly set the signal to noise ratio of the hippocampal circuit.

Change in neurotrophins and their receptor mRNAs in the rat forebrain after status epilepticus induced by pilocarpine

We studied the effects of status epilepticus (SE) induced by lithium chloride/pilocarpine treatment on gene expression of neurotrophins of the nerve growth factor (NGF) family and of their high-affinity receptors of the tyrosine protein kinase (trk) family in the forebrain. Using in situ hybridization (ISH), we demonstrated an early (3 h after treatment) increase in brain-derived neurotrophic factor (BDNF) and trkB mRNA expression in the dentate gyrus, amygdala, and piriform cortex, as well as widespread increases in the cerebral cortex. NGF mRNA, but not the mRNA of its receptor trkA, was increased in the dentate gyrus. In contrast, 12 h after treatment, neurotrophin-3 (NT-3) decreased, and its receptor trkC mRNA increased. There was no change in NT-4 mRNA levels. All changes were blocked by pretreatment with scopolamine, a muscarinic antagonist. The noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine blocked NGF, BDNF, and trkB mRNA increases in the hippocampus and cerebral cortex, but not in the amygdala and piriform cortex. In contrast, ketamine did not affect NT-3 and trkC changes. These results provide a complete description of changes in mRNA levels of neurotrophins and their receptors in the forebrain after SE and supply additional data supporting the view that neurotrophin gene expression is related to abnormal neuronal activity.

Increased levels of statin, a marker of cell cycle arrest, in response to hippocampal neuronal injury

Injured neurons in the CNS are known to synthesize high levels of proliferation related oncogene products and heat shock proteins without dividing. Statin is a cell cycle regulated nuclear phosphoprotein, selectively associated with the non-proliferative state in a wide variety of cell types. In the present study, neuronal statin was examined following lethal or sublethal neuronal injuries in the hippocampus of Alzheimer's disease patients, in rats receiving kainate lesions to the dorsal hippocampus and in entorhinal cortex lesioned rats. Immunolabelling of nuclear statin showed that statin immunoreactivity increased preferentially in CA1 pyramidal neurons of the hippocampus in Alzheimer's disease. In kainate lesioned rats, statin immunoreactivity was markedly induced in the CA3 hippocampal region in association with neuronal loss. Entorhinal cortex lesioned rats showed a transient induction of statin between 2 and 6 days post lesion in CA1 neurons. However, cell counts in entorhinal cortex lesioned rats remained unaltered in the CA1 and granule cell layers during the entire 30 day time course, indicating that increased statin levels are not secondary to neuronal degeneration and are not necessarily accompanied by irreversible neuronal death. It is concluded that, in addition to proliferation related gene products, neuronal injury induces an increase in levels of statin, a nuclear marker of cell cycle arrest. Furthermore, statin may be a potentially useful marker of injurious neuronal stress, even under conditions that do not necessarily lead to irreversible cell death.

Distribution of metabotropic glutamate receptor 7 messenger RNA in the developing and adult rat brain

The large number of metabotropic glutamate receptor subtypes suggests diverse roles in brain function, although specific distribution patterns can give clues to subtype-specific functions [Hayashi Y. et al. (1993) Nature 366, 687-690; Nakajima Y. et al. (1993) J. biol. Chem. 268, 11868-11873; Nomura A. et al. (1994) Cell 77, 361-369; Ohishi H. et al. (1993), 1009-1018]. The metabotropic glutamate receptor mGluR7 is sensitive to the agonist L-2-amino-4-phosphonobutyric acid, a presynaptic inhibitor of neurotransmitter release. We examined the anatomic distribution of mGluR7 messenger RNA expression by in situ hybridization in the developing and adult rat central nervous systems. Our results demonstrate that mGluR7 messenger RNA is among the most widely distributed of metabotropic glutamate receptors in both the developing and adult rat nervous system and that mGluR7 messenger RNA is expressed in most neuronal groups known to respond to L-2-amino-4-phosphonobutyric acid, including mitral cells of the olfactory bulb, granule cells of the dentate gyrus and neurons of the entorhinal cortex and dorsal root ganglion. mGluR7 exhibits preferential expression in sensory afferent pathways and is highly represented in the periventricular zone of the hypothalamus, the latter implying a modulatory role for mGluR7 in neuroendocrine pathways. Most strikingly, the majority of neurons at all levels of olfactory circuitry are among the areas of highest mGluR7 messenger RNA content. The anatomic distribution of mGluyR7 messenger RNA suggests that mGluR7 activation may participate in the processing of hippocampal, sensory and olfactory information.

AMPA-selective glutamate receptor subtype immunoreactivity in the aged human hippocampal formation

It has been hypothesized that, in Alzheimer's disease, glutamate-mediated excitotoxicity contributes to the degeneration of selected populations of neurons. In the present study, immunocytochemical techniques were used to determine the distribution and anatomical features of GluR1- and GluR2/3-immunolabeled cell bodies and processes within the hippocampal formation of normal (i.e., no pathology) elderly humans. The results of this study provide an essential baseline with which to compare the expression and distribution of glutamate receptor subunits within the brains of patients with Alzheimer's disease. With respect to GluR1 immunoreactivity, the molecular layer of the dentate gyrus displays the most intense immunolabeling of any hippocampal structure. Contributing to this intense labeling are apical dendrites that arise from neurons within the adjacent granule cell layer. Interestingly, GluR1-labeled neurons account for a relatively small percentage of the total number of neurons as revealed by Nissl staining in the granule cell layer. In contrast, GluR2/3-labeled neurons are densely distributed throughout the granule cell layer, yet they provide relatively few processes to the adjacent molecular layer compared to GluR1-positive processes. GluR1 labeling is also prominent within the CA fields of Ammon's horn, with CA2 > CA3 > CA1 > or = CA4. Most prominent within the CA fields are the labeled dendrites of pyramidal neurons. In many instances, apical dendrites can be traced into the adjacent stratum radiatum, where they impart a deep striated appearance to this region of the hippocampus. Robust GluR2/3 labeling is also observed within the pyramidal layer of Ammon's horn, with an order of staining intensity similar to that observed for GluR1. However, unlike GluR1 labeling, which is localized predominantly along dendrites, GluR2/3 labeling is observed primarily in association with cell bodies. Collectively, these data suggest that the molecular composition of the AMPA receptor complex may differ between the dendrite and soma of granule and pyramidal neurons within the hippocampal formation, so functionally we may predict that these two regions of the neuron would respond differently following glutamate receptor stimulation.

Cannabinoids selectively decrease paired-pulse facilitation of perforant path synaptic potentials in the dentate gyrus in vitro

Perforant path synaptic potentials recorded from the outer molecular layer of the dentate gyrus were tested for paired-pulse potentiation and stimulus sensitivity in the presence and absence of the potent cannabinoid receptor ligand, WIN 55,212-2. Extracellular perforant path synaptic potential amplitudes were increased by 51% in 2 mM Ca2+ medium and 60% in 3 mM Ca2+ medium at a conditioning-test (C-T) interval of 10 ms, decreasing to 10-15% facilitation at an 80 ms C-T interval. Exposure to the potent cannabinoid receptor ligand WIN 55,212-2 produced a marked and dose-dependent reduction in the amplitude of the facilitated perforant path synaptic potentials. Maximum paired-pulse facilitation was reduced to 35% and 25% in 2.0 and 5.0 microM WIN 55,212-2 respectively. The effect was selective for potentials facilitated at C-T intervals of 10-60 ms. Input/output (I/O) curves of perforant path field potentials were shifted to the right in a dose-dependent (2.0 and 5.0 microM) manner by WIN 55,212-2. Significant differences in peak amplitudes of perforant path potentials were obtained at all suprathreshold stimulus intensities. A comparison of WIN 55,212-2 (5 microM) with the GABAB receptor agonist baclofen (200 microM) showed that when both drugs were administered independently each produced similar decreases in perforant path paired-pulse potentiation. However when administered together at these concentrations baclofen and WIN failed to potentiate each other, suggesting nonadditivity due to effects on a common process.(ABSTRACT TRUNCATED AT 250 WORDS)

Connection matrix of the hippocampal formation: I. The dentate gyrus

The hippocampal formation presents a special opportunity for realistic neural modeling since its structure, connectivity, and physiology are better understood than that of other cortical components. A review of the quantitative neuroanatomy of the rodent dentate gyrus (DG) is presented in the context of the development of a computational model of its connectivity. The DG is a three-layered folded sheet of neural tissue. This sheet is represented as a rectangle, having a surface area of 37 mm2 and a septotemporal length of 12 mm. Points, representing cell somata, are distributed in the model rectangle in a roughly uniform fashion. Synaptic connectivity is generated by assigning each presynaptic cell a spatial zone representing its axonal arbor. For each postsynaptic cell, a list of potential presynaptic cells is compiled, based on which arbor zones the given postsynaptic cell falls within. An appropriate number of presynaptic inputs are then selected at random. The principal cells of the DG, the granule cells, are represented in the model, as are non-principal cells, including basket cells, chandelier cells, mossy cells, and GABAergic peptidergic polymorphic (GPP) cells. The neurons of layer II of the entorhinal cortex are included also. The DG receives its main extrinsic input from these cells via the perforant path. The basket cells, chandelier cells, and GPP cells receive perforant path and granule cell input and exert both feedforward and feedback inhibition onto the granule cells. Mossy cells receive converging input from granule cells and send their output back primarily to distant septotemporal levels, where they contact both granule cells and non-principal cells. To permit numerical simulations, the model must be scaled down while preserving its anatomical structure. A variety of methods for doing this exist. Hippocampal allometry provides valuable clues in this regard.

AMPA-selective glutamate receptor subtype immunoreactivity in the hippocampal formation of patients with Alzheimer's disease

Immunocytochemical techniques were employed in order to examine the distribution and relative intensity of the AMPA receptor subunits GluR1 and GluR2/3 within the hippocampal formation of normal controls and Alzheimer's disease (AD) cases. Throughout our investigation we examined cases exhibiting a wide range of pathologic severity, thus allowing us to correlate our immunohistochemical data with the extent of pathology. Specifically, we investigated the distribution of these receptor subunits in hippocampal sectors that are particularly vulnerable to AD pathology (i.e., CA1 and subiculum) and compared these findings with those obtained following examination of sectors that are generally resistant to pathologic change (i.e., CA2/3, dentate gyrus). Within vulnerable sectors we observed a variable loss of GluR1 and GluR2/3 immunolabeling. The degree to which these proteins were reduced appeared to correlate with the extent of neurofibrillary pathology and cell loss. Despite the loss of labeled cells, the intensity of immunolabeling within the remaining neurons was comparable with, and in many instances even greater than, that observed in control cases. Within resistant sectors, the distribution of immunoreactive elements was comparable in both case groups yet the intensity of immunolabeling was markedly increased in AD cases, particularly in the molecular layer of the dentate gyrus and in the stratum lucidum of CA3 (i.e., the termination zones of perforant pathway and mossy fibers). In addition, within AD cases dramatic increases were observed within the supragranular and polymorphic layer of the dentate gyrus (i.e., the terminal zones of sprouting mossy fiber collaterals). The increase in GluR1 and GluR2/3 immunolabeling is hypothesized to occur in response to the deafferentation of selected glutamatergic pathways. Moreover, our data support that hippocampal plasticity is preserved, even in severe AD cases, and suggest a critical role for AMPA receptor subunits in this plasticity and in maintaining hippocampal functioning.

Distribution of effects of the kappa-opioid agonist CI-977 on cerebral glucose utilization in rat brain

The effects of the kappa-opioid agonist CI-977 upon local cerebral glucose utilization have been examined in conscious, lightly restrained rats to gain insight into the potential adverse effects of this neuroprotective agent. Cerebral glucose utilization was assessed quantitatively in 45 anatomically discrete brain regions by means of [14C]2-deoxyglucose autoradiography. The i.v. administration of CI-977 (0.03-3 mg/kg) induced relatively homogeneous patterns of altered cerebral glucose utilization with moderate statistically significant reductions (approximately 25%) being observed in 29 brain regions, and a statistically significant increase (approximately 40%) in one brain region, the lateral habenular nucleus. Glucose use throughout the entire neocortex and inferior colliculus was particularly sensitive to reduction (approximately 35%) following CI-977 administration, although there was only a limited dose dependency to the response. Minimal alterations in glucose use were observed in 15 of the 45 brain regions, particularly in the lower brain stem (e.g. superior olives, cochlear nucleus and median raphe) and forebrain limbic regions (e.g. septal nucleus, nucleus accumbens and mediodorsal thalamus). These data demonstrate that CI-977 produces widespread, anatomically organized alterations in function-related glucose use which contrast those seen previously with the NMDA receptor antagonists, thereby suggesting that CI-977 may be intrinsically safer as an in vivo neuroprotective agent.

Evidence for the colocalization of parvalbumin and glutamate, but not GABA, in the perforant path of the gerbil hippocampal formation: a combined immunocytochemical and microquantitative analysis

Gerbils (Meriones unguiculatus) are known for their seizure sensitivity, which is dependent on an intact perforant path from the entorhinal cortex to the hippocampus. In contrast with other species, the perforant path in gerbils contains parvalbumin, a cytosolic high-affinity calcium-binding protein. Parvalbumin is known to be present in a subpopulation of GABA-containing neurons and is thought to be responsible for their physiological characteristics of fast spiking activity and lack of spike adaptation. Therefore, the question arose of whether this projection in gerbils is GABAergic or glutamatergic as in other species. In a first approach to this question, the effect of lesioning the origin of the perforant path, the entorhinal cortex, on levels of GABA and glutamate was determined by enzymatic-luminometric assay in single layers of the dentate gyrus of lyophilized brain sections. Parallel sections were cryofixed using an acidified acetone-formaldehyde mixture at -20 degrees C for 48 h, and subsequently stained for parvalbumin immunocytochemistry. Seven days after ablation of the entorhinal cortex, parvalbumin staining was undetectable in the termination zone of the perforant path, the outer two-thirds of the stratum moleculare. In parallel, glutamate content was reduced to 80% of controls (and of the unoperated contralateral side) but unchanged in the inner third of the stratum moleculare and in stratum granulare. GABA content was not significantly altered by the lesion. From these results, we conclude that in the gerbil as in other species, the perforant path contains glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

A theoretical and neurophysiological consideration on the pathogenesis of positive symptoms of schizophrenia: implications of dopaminergic function in the emotional circuit

The implications of the emotional circuit and the gating mechanism by dopamine (DA) proposed by Maeda in the pathogenesis of positive symptoms of schizophrenia were reconsidered based upon recent advances and findings in the fields of neurophysiology and neuropharmacology and in biological studies of schizophrenia. The gating mechanism by DA was partly supported by new evidence that glutamatergic or GABAergic neurotransmission, which mediates the hippocampo-lateral septal or the piriform cortico-amygdaloid neuronal connections, is likely to be modulated by DA. The compensation-facilitating or gating functions of DA was considered again to play an important role in producing positive symptoms in schizophrenics, who have been suggested to have morphological abnormalities in the limbic system or in the prefrontal cortex prior to the appearance of positive symptoms.

Divergence of hippocampal mossy fibers

By connecting the fascia dentata with the hippocampus proper, the axons of the granule cells, the mossy fibers, represent an important element of the main excitatory, trisynaptic pathway of the hippocampal formation. In this review the various synaptic connections of the mossy fibers are discussed. It turns out that the mossy fibers do not only establish synapses with the pyramidal neurons of regio inferior as traditionally assumed, but a variety of local circuit neurons as well as projection cells are also contacted by the mossy fibers. Thus there is an underestimated divergence of the impulse flow within the "trisynaptic" pathway at the level of the mossy fibers. Similarly, the pattern of afferent input to the granule cells, especially that of GABAergic neurons, is more complex than previously assumed. In this respect the concept of a unidirectional "trisynaptic" pathway certainly is an oversimplification. In particular, the hilus of the fascia dentata, that the mossy fibers traverse on their way to regio inferior, is often neglected in this concept. The hilar region comprises a large variety of morphologically and functionally distinct neuronal types that, to a large extent, are targets of hilar mossy fiber collaterals. By focusing on the mossy fiber system, an attempt is made in this review to summarize new data on hippocampal circuitries that have been accumulated since the original description of the trisynaptic pathway. This concept, which originally comprised the synapses of the perforant path fibers on dentate granule cells, the mossy fiber synapses on CA3 pyramidal neurons, and the synapses of the Schaffer collaterals on CA1 pyramidal cells, has been of great heuristic value but needs to be modified in view of recent morphological and physiological data.

Daily variation in active glycogen phosphorylase patches in the molecular layer of rat dentate gyrus

A larger number of discrete patches of active glycogen phosphorylase (alpha GP) were found in the molecular layer of the hippocampal dentate gyrus in rats sacrificed during the daily dark phase (mean = 28.7/section) than during the light phase (mean = 7.8/section). Light-dark differences in the patterns of alpha GP may reflect circadian differences in metabolic demand in the hippocampus. Patch sizes were consistent with increased activation of single astrocytes or perisynaptic astrocyte clusters by focal input at night.

Characteristics of spontaneous and evoked EPSPs recorded from dentate spiny hilar cells in rat hippocampal slices

1. Excitation of the spiny subtype of hilar neurons in the fascia dentata was characterized by intracellular recording from hilar cells in hippocampal slices. Stimulation of the outer molecular layer was used to activate the perforant path. Evoked responses were examined, as well as the large spontaneous excitatory potentials that are a distinctive characteristic of spiny hilar cells. 2. Excitatory potentials that occurred spontaneously, as well as those that occurred in response to outer molecular layer stimulation, were similar among the cells that were sampled, regardless of morphological variations such as the presence or absence of thorny excrescences. Spontaneous and evoked excitatory postsynaptic potentials (EPSPs) were complex depolarizations that often had several discrete peaks. Spontaneous EPSPs increased in amplitude slightly with hyperpolarization, and evoked EPSPs clearly increased with hyperpolarization. 3. Applications of selective antagonists of excitatory amino acid receptors were used to determine which excitatory amino acid receptor mediates EPSPs of these cells. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was used to block the receptor subtype selective for the agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid (the "AMPA/kainate" receptor). 2-amino-5-phosphonovaleric acid (APV) was used to block receptors specific for the agonist N-methyl-D-aspartate (NMDA; the "NMDA" receptor). Perfusion with CNQX (5-25 microM) completely blocked all spontaneous and evoked excitation, even when activity was examined at relatively depolarized membrane potentials and a low concentration of extracellular magnesium (0.5 mM) was used. Under these conditions, APV (25-50 microM) had no detectable effect on spontaneous activity but did increase the stimulus strength required to elicit responses to outer molecular layer stimulation. 4. When extracellular magnesium was lowered to 0 mM (nominally), there was strong evidence for a contribution of NMDA receptors to spontaneous and evoked EPSPs. Thus, when cells were perfused with 0 mM extracellular magnesium and 5 microM CNQX, spontaneous depolarizations were present and EPSPs could be triggered by stimulation of the outer molecular layer. Both the spontaneous and evoked EPSPs were blocked by 25 microM APV. 5. Because gamma-aminobutyric acid (GABA)A receptors can cause depolarizations in hippocampal neurons, the GABAA receptor antagonist bicuculline was used to determine whether some of the EPSPs were mediated by GABAergic neurons that are normally activated by spontaneous release of excitatory amino acids. Bicuculline (5-25 microM) had no effect on spontaneous depolarizations, and led to an enhancement of evoked depolarizations. Therefore it does not appear that GABAA receptor-mediated depolarizations contribute to hilar cell depolarizations.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic activation in guinea-pig dentate area: dependence on the stimulation site

A negative-going and a positive-going field potential were evoked in the granule cell layer by electrical stimulation of a region near the cell layer (commissural/associational fibres, cf) and of the lateral perforant path (perforant-path fibres, pp). The cf-evoked field potential was more strongly reduced by (-)baclofen (2-5 mumol l-1) and carbachol (2-5 mumol l-1) than was the pp-evoked field potential. By simultaneous intra- and extracellular recordings, the elements activated from the two stimulation sites were determined. Stimulation of cf-fibres elicited concurrently excitatory and inhibitory postsynaptic potentials, but inhibition predominated. pp stimulation elicited excitation followed by inhibition, and excitation predominated. Responses evoked from both stimulation sites were affected by antagonists for glutamatergic excitation. Inhibition blockade revealed that excitation elicited from the cf-stimulation site was largely shunted by inhibition. Presumed inhibitory neurons in the dentate hilus were driven at latencies consistent with the latencies at which inhibition was elicited in granule cells from the two stimulation sites. We conclude that the sequence of synaptic events produced by extracellular stimulation can vary substantially by slightly differing electrode placements. However, it is difficult to decide, simply on the basis of an extracellular field potential, what elements have been activated.

Muscarinic depression of synaptic transmission and blockade of norepinephrine-induced long-lasting potentiation in the dentate gyrus

Bath application of the muscarinic receptor agonist, muscarine, produced a concentration-dependent depression of synaptic activity in the dentate gyrus of hippocampal slices. A concentration of 10 microM muscarine produced a reversible depression that could be competitively antagonized by the muscarinic receptor antagonist pirenzepine. However, other muscarinic receptor subtype (M1-M3) antagonists could also block the effects of muscarine. The rank order of antagonist potency was: 4-diphenylacetoxy-N-methyl-piperidine methiodide (M3/M1 antagonist) > pirenzepine (M1) > AFDX-116 (M2). The depression produced by 10 microM muscarine was not affected by in vivo pretreatment with pertussis toxin, and therefore was not mediated by a pertussis toxin-sensitive G-protein. In addition, high concentrations of muscarine did not affect either basal or isoproterenol-stimulated accumulation of cyclic AMP from slices of dentate gyrus. Muscarine also produced a concentration-dependent blockade of the induction of norepinephrine-induced long-lasting potentiation in the dentate gyrus. Norepinephrine-induced long-lasting potentiation is a form of long-lasting plasticity induced in medial perforant path synapses by beta-adrenergic agonists such as isoproterenol. The muscarinic blockade of norepinephrine-induced long-lasting potentiation was also prevented by pretreatment with pirenzepine. Based on these pharmacological data, we conclude that muscarinic depression of evoked responses, as well as blockade of norepinephrine-induced long-lasting potentiation, involves activation of either M3 or M1, but not M2, muscarinic receptors. These data also demonstrate that in addition to modulating normal synaptic transmission, muscarinic receptors may also play an important role in modulating synaptic plasticity.

Beta-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs: autoradiographic and immunohistochemical studies

Species differences in the distribution of beta-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs were examined using in vitro autoradiographic techniques. beta 1-receptors were found in the hippocampal area CA1 of both species, although guinea pigs had significantly lower receptor densities in comparison to rats. In guinea pigs, beta 2-adrenergic receptors were predominant in hippocampal area CA1. Hippocampal area CA3 had very low levels of beta 1- and beta 2-receptors in both species. The retrohippocampal area was also found to have a distinct topographic distribution of beta-receptors. In rats, the subiculum and parasubiculum (layers II-III) were heavily labeled for beta 1-receptors; in contrast, guinea pigs had few receptors in these regions. beta 2-receptors were particularly prominent in the parasubicular region in rats. The entorhinal cortex laminae was found to contain beta-receptors in both rats and guinea pigs. Immunohistochemical techniques were used to compare the pattern of catecholaminergic innervation with the receptor distribution within each hippocampal subregion. Despite the general lack of beta-receptors in area CA3, abundant catecholamine immunoreactive fibers were observed in CA3 of rat and guinea pig hippocampus. Significant species differences were found in the distribution of hippocampal beta-adrenergic receptor subtypes, and moreover, in both species the distribution of beta-adrenergic receptors did not coincide with the pattern of hippocampal adrenergic innervation.

Cortical transynaptic activation of tyrosine kinase receptor trkB messenger RNA expression in rat hippocampus

The trkB gene encodes a tyrosine kinase receptor which is an essential component of the high-affinity cell surface receptor for the neurotrophin brain-derived neurotrophic factor. In this report we have used quantitative in situ hybridization to study the expression of trkB messenger RNA in the rat hippocampus following stimulation of afferents in the entorhinal cortex. A bilateral three-fold increase of trkB messenger RNA levels in the hippocampus was seen 4 h after quisqualate injection into the left entorhinal cortex. The increase was confined to the granule layer of the dentate gyrus. A small increase, however, was also seen bilaterally in the pyramidal cell layer. The increases in all hippocampal areas were completely prevented by pretreatment of the animals with systemic injection of diazepam but not with scopolamine. We suggest that glutamate release from cortical afferents to the hippocampus has the capacity to increase neuronal expression of trkB messenger RNA within the hippocampus. The results from the present study extend the interpretation of our previous evidence of cortical transynaptic activation of brain-derived neurotrophic factor messenger RNA and indicate the presence of a concomitant activation of trkB messenger RNA expression in the hippocampus.

Production of seizures and brain damage in rats by alpha-dendrotoxin, a selective K+ channel blocker

alpha-Dendrotoxin (Dtx), a snake polypeptide, increases neuronal excitability by blocking certain fast-activating, voltage-dependent K+ channels. Thus, the behavioural, electrocortical (ECoG) and neuropathological effects of Dtx, injected into rat brain areas, were studied. A unilateral injection of 35 pmol of Dtx into the CA1 hippocampal area or the dendate gyrus (DG; upper blade) immediately produced motor and ECoG seizures, followed at 24 h by multi-focal brain damage and significant neuronal loss. Whilst brain damage was seen bilaterally, significant neuronal loss occurred only in regions (CA1, CA3, CA4 and DG) ipsilateral to the site of injection. A lower dose (3.5 pmol) of toxin elicited motor and ECoG seizures but failed to produce brain damage. Seizures were observed 50 min after injecting Dtx (35 pmol) into the amygdala, though significant neuronal loss was not evident. 4-Aminopyridine (100 nmol), given into the CA1 area elicited a similar motor and ECoG pattern to that of Dtx except no brain damage could be seen at 24 h. Systemic pretreatment with antagonists of N-methyl-D-aspartate receptors (MK-801 or CGP 37849) did not protect against the effects typically evoked by injecting Dtx into the CA1 area.

Long-term Potentiation and Field EPSPs in the Lateral and Medial Perforant Paths in the Dentate Gyrus In Vitro: a Comparison

The entorhinal cortex projects monosynaptically to the granule cells in the dentate gyrus via the lateral and medial perforant paths. These two subdivisions of the perforant path differ with respect to synaptic properties, and recent studies suggest that they also differ with respect to long-term potentiation (LTP). In the present study, using the in vitro slice preparation of the guinea-pig hippocampus, field excitatory postsynaptic potentials (EPSPs) and LTP in the lateral and medial perforant paths were compared. The two pathways were distinguished on the basis of their different termination in the dendritic layer, their different pharmacology and short-term synaptic facilitation. The field EPSP [obtained in the presence of gamma-aminobutyric acid (GABA) A and B receptor antagonists] consisted of a non-N-methyl-d-aspartate (NMDA) component with different time characteristics in the two pathways, the decay being monoexponential in the lateral perforant path and biexponential in the medial one. In addition, the field EPSP in both pathways contained a small NMDA-mediated component that could also be observed after complete blockade of the non-NMDA one. LTP induction in both lateral and medial perforant paths was facilitated by blockade of GABAA inhibition, showed associative properties, and was blocked by NMDA receptor antagonists. Following the induction event, LTP in both pathways developed to a peak value within 30 - 40 s, and the stability of LTP was correlated with the amount of postsynaptic, but not presynaptic, activity during the induction event. During blockade of GABAA inhibition the opioid receptor antagonist naloxone and the beta-adrenergic antagonist timolol had no effect on the magnitude or stability of LTP. It is concluded that LTP in the lateral and medial perforant paths does not differ with respect to induction mechanisms and early temporal characteristics.

bFGF promotes the survival of entorhinal layer II neurons after perforant path axotomy

Infusion of basic fibroblast growth factor (bFGF) prevents the loss of cholinergic neurons in the septum/diagonal band of broca following fimbria-fornix transection. However, an in vivo test of whether bFGF will also rescue injured non-cholinergic or cortical neurons has not been carried out. Previous studies have shown that the majority of layer II stellate neurons utilize an excitatory amino acid as their neurotransmitter. In order to determine if bFGF acts on non-cholinergic cortical neurons, a paradigm was developed to examine whether or not bFGF could spare layer II entorhinal stellate cells from axotomy induced death or atrophy. Axotomy of the medial entorhinal cortex fibers projecting to the dentate gyrus of the hippocampal formation via the perforant path lead to retrograde cell loss in entorhinal cortex. Fourteen or thirty days after a unilateral knife cut axotomy of the perforant path, layer II of medical entorhinal cortex showed a 28% decrease in large stellate neurons as well as many weakly stained, hollow cells compared to the non-lesioned side or naive controls. Layer IV neurons, however, which do not project via the perforant path, showed little detectable change in the number of cells ipsilateral to the knife-cut as compared to the contralateral side. Intraventricular infusion of bFGF over a period of 14 days reduced the 28% cell loss to less than 6%. Thus, bFGF is capable of preventing cortical neuronal loss and/or atrophy associated with retrograde degeneration of non-cholinergic neurons following axotomy.

Release of endogenous amino acids, including homocysteic acid and cysteine sulphinic acid, from rat hippocampal slices evoked by electrical stimulation of Schaffer collateral-commissural fibres

This study examined the release of endogenous amino acids from acute hippocampal slices, upon stimulation of the Schaffer collateral-commissural fibres. One-minute samples of superfusate were collected via a cannula placed over the CA1 stratum radiatum, and were analysed by reversed-phase high performance liquid chromatography. Evoked potentials were recorded to ascertain stimulation efficacy. Four minutes of continuous 50 Hz stimulation produced a tetrodotoxin-sensitive release of aspartate and glycine in the second minute of stimulation, as well as a tetrodotoxin-sensitive release of cysteine sulphinic acid, during stimulation and of homocysteic acid, following stimulation. Such 50 Hz stimulation also produced a tetrodotoxin-insensitive decrease in methionine levels, but no significant changes in any of the other 15 amino acids measured. Four minutes of continuous 1 Hz stimulation produced no changes in the levels of any of the amino acids measured, but four 600-ms trains of 100 Hz stimulation, which, unlike the 1 Hz stimulation, produced long-term potentiation, resulted in significant increases in levels of cysteine sulphinic acid and homocysteic acid, but not of any of the other amino acids measured. These results suggest that aspartate, glycine, homocysteic acid, and cysteine sulphinic acid play a role in synaptic transmission in the Schaffer collateral-commissural fibres, and that cysteine sulphinic acid and homocysteic acid may be released specifically by high-frequency stimulation.

Neurons of origin of zinc-containing pathways and the distribution of zinc-containing boutons in the hippocampal region of the rat

Recent methods allow the study of neurons that contain zinc in synaptic vesicles of their boutons (Timm-stainable boutons) by the intravital precipitation (local or throughout the CNS) of the vesicular zinc with selenium compounds and its subsequent retrograde transport to the parent neurons, where the precipitate can be silver enhanced. The present study is a description of the distribution of zinc-containing neurons, their possible connections and their terminal fields within the hippocampal region of the rat. Problems inherent to the methods are addressed. Finally, based on the results and a review of literature, the possible function of zinc in the hippocampal region is considered. Neurons which contain silver-enhanced precipitates were observed in layers II, V and VI of the lateral entorhinal area and in layers V and VI of the medial entorhinal area. In the parasubiculum, labeled cells were seen in layer II/III of the parasubiculum a and in layer V. Labeled cells in the presubiculum were concentrated in layers III and V, in the hippocampal pyramidal cell layer and the dentate granule cell layer, but neurons containing precipitates were largely absent from the subiculum. Zinc-containing axonal boutons defined subpopulations within principal hippocampal neuron populations. Within layer II of the lateral entorhinal cortex and the pyramidal cell layer for regio inferior deeply situated neurons were labeled, whereas superficially placed pyramidal cells were labeled in regio superior. The neuropil staining described in the present study corresponded to that found in earlier studies. However, glial and vascular staining or unspecific background were largely absent, and the neuropil staining could unequivocally be identified light microscopically. Methodological problems are most prominently reflected in unstained mossy fibers in some animals. Based on series from animals treated with decreasing doses of sodium selenite and increased survival times, this problem can be related to small amounts of circulating reactive selenium and a competition of zinc compartments (vesicles) for the selenium. Staining will fail where the competition prevents individual compartments from reaching a threshold amount of zinc precipitate for silver amplification. A guide to evaluate histological material is provided. The distribution of zinc-containing boutons and their cells of origin indicate that zinc-containing and zinc-negative projections are not organized as parallel pathways. The mossy fibers provide an example of a pure zinc-containing pathway. Projections from regio superior to the dorsal presubiculum are likely to be zinc-negative while projections from the same area to the subiculum are zinc-containing.(ABSTRACT TRUNCATED AT 400 WORDS)

Astroglia in Alzheimer's disease

Amyloid deposits are characteristic of Alzheimer's Disease (AD) and there is growing evidence that amyloid may play an important role in the genesis of this neurodegenerative disease. This review discusses data which suggests that reactive astrocytes and microglia may be a necessary concomitant with amyloid to produce the neuropathology which manifests as AD. Several hypotheses and supporting data for mechanisms by which reactive astrocytes may mediate this neuropathology are presented. These include the possibility that amyloid induces excitotoxicity by interferring with astrocytic glutamate uptake, the possibility that amyloid has this effect via an action on a tachykinin-related receptor and the possibility that proteoglycans released by astrocytes may facilitate the deposition of amyloid plaques. Both symptomatic treatment to enhance cognitive function and treatment to stop the progression of AD are needed. It is hoped that answers to some of the unique questions raised here may provide new insight into the etiology and treatment of AD.

Mapping rat brain structures activated during ethanol withdrawal: role of glutamate and NMDA receptors

Brain structures activated during ethanol withdrawal have been mapped by visualizing c-fos mRNA expression. The regional distribution of c-fos mRNA in brain during ethanol withdrawal can be mimicked by acute injection of N-methyl-D-aspartic acid (NMDA) and is stereospecifically blocked by the NMDA receptor antagonist, MK-801. The findings reveal that the dentate gyrus and piriform cortex are selectively activated during ethanol withdrawal and suggest that this may be mediated by glutamate activation of NMDA receptors.

Septal cholinergic afferents regulate expression of brain-derived neurotrophic factor and beta-nerve growth factor mRNA in rat hippocampus

In situ hybridization was used to study the expression of members of the nerve growth factor family of trophic factors in rat hippocampus following stimulation of afferent cholinergic and glutamatergic pathways with quisqualate. A transient increase in brain-derived neurotrophic factor (BDNF) and beta-nerve growth factor (NGF) mRNA expression in the hippocampus was seen 4 h after a quisqualate injection into the medial septal nucleus. Both BDNF and NGF mRNA levels increased more than 4-fold in the granule layer of the dentate gyrus and for BDNF mRNA also in the pyramidal cells of CA1, while the levels of BDNF mRNA in CA3 increased 2-fold. The increase in BDNF and NGF mRNA levels were completely prevented by pretreatment with systemic injections of either scopolamine or diazepam. A quisqualate injection into the entorhinal cortex, containing glutamatergic afferents to the hippocampus, resulted in a 15-, 5- and 17-fold increase in the expression of BDNF mRNA in the ipsilateral granule cells, CA3 and CA1 pyramidal cells, respectively. Similar increases were also seen in the hippocampus contralateral to the injections. In contrast, the level of NGF mRNA did not increase significantly in any of the subfields in the hippocampus. The increase in BDNF mRNA after cortex injections was attenuated by diazepam but not by scopolamine. These findings imply that increased activity in afferent cholinergic and glutamatergic pathways to the hippocampus differentially regulate expression of the NGF family of neurotrophic factors in the hippocampus.

Firing relations of medial entorhinal neurons to the hippocampal theta rhythm in urethane anesthetized and walking rats

The firing of neurons from layers II and III of medial entorhinal cortex (MEC) was examined in relation to the hippocampal theta rhythm in urethane anesthetized and walking rats. 1) MEC neurons showed a significant phase relation to the hippocampal theta rhythm in both walking and urethane anesthetized rats, suggesting that this region contributes to the generation of both atropine-resistant and atropine-sensitive theta rhythm components. 2) The proportion of phase-locked cells was three times greater in walking rats (22/23 cells) as compared to anesthetized rats (8/23 cells), indicating that MEC cells made a greater contribution during walking theta rhythm. This difference was also manifest in the greater mean vector length for the group of phase-locked MEC cells during walking: 0.39 +/- 0.13 versus 0.21 +/- 0.08. Firing rate differences between walking and urethane conditions were not significant. 3) In walking rats, MEC cells fired on the positive peak of the dentate theta rhythm (group mean phase = 5 degrees; 0 degrees = positive peak at the hippocampal fissure). This is close to the reported phases for dentate granule and hippocampal pyramidal cells. The distribution of MEC cell phases in urethane anesthetized rats was broader (group mean phase = 90 degrees), consistent with the phase data reported for hippocampal projection cells. These findings suggest that medial entorhinal neurons are the principal determinant of theta-related firing of hippocampal neurons and that their robust rhythmicity in walking as compared to urethane anesthesia accounts for EEG differences across the two conditions.