Localization of transmitter candidates in the brain: the hippocampal formation as a model

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Long-term reduction in spontaneous alternations after early exposure to phenobarbital

Spontaneous alternation behavior is related to the integrity of the hippocampus. Our earlier studies demonstrated hippocampal deficits after early phenobarbital (PhB) exposure. In the present study, we examined spontaneous alternation of mice who had been exposed to PhB prenatally or neonatally. Prenatal PhB was administered transplacentally: pregnant females were fed 3 g PhB/kg milled food on gestation days 9-18. Neonates were treated directly with daily injections of 50 mg PhB/kg on postnatal days 2-22. The animals were tested for spontaneous alternation in a T maze at the ages of 22, 28, 35 and 42 days. The test was conducted at each age for two consecutive days. A maximum of four alternations were allowed on the first day, and one alternation on the second day. Animals treated neonatally had reductions in alternation from the control group for every age group. Looking at the mean of the four trials on the first day there was a reduction of 35% at age 22 (P < 0.001), 8% at age 28, 21% at age 35 (P < 0.05) and 36% at age 42 (P < 0.02). On the second day the respective reductions were 32, 19, 24 and 36% (P < 0.05). The differences in alternation between animals treated with PhB prenatally and the control group were too small to reach statistical significance. Subsequently a more sensitive test, delayed spontaneous alternation (30 s), was applied to an additional group of animals at age 42 which had been prenatally exposed to PhB: 31% reduction from the control group was found on day 1 (P < 0.001), and 34% on day 2 (P < 0.02). The greater differences after neonatal as opposed to prenatal administration could be related to the more extensive hippocampal damage that was found in adults after neonatal treatment.

Long term reduction in eight arm maze performance after early exposure to phenobarbital

Performance in the hippocampal eight arm maze was studied in mice after early exposure to phenobarbital (PhB). since previous studies suggested that these animals suffered neural deficits in the hippocampus. For prenatal exposure pregnant mothers were fed 3 g PhB/kg milled food on gestation days 9-18. Neonates were injected daily with 50 mg PhB/kg. on postnatal days 2-21. After a week of water deprivation, the animals were tested at age 50 days for 5 days preceded by 1 day of habituation. Deficits in eight arm maze performance were demonstrated in early treated mice on every testing day. For example, on day 5 of testing the number of correct entries during the first eight attempts in the prenatally treated group were 12% below control level (P<0.01), the respective reduction in the neonatal group was 10% (P< 0.001). The number of trials needed to enter all arms on day 5 was 27% above control level among prenatally treated mice (P< 0.001), and 13% in neonatally treated mice (P< 0.05). It took prenatal PhB animals twice the time to reach criterion than their controls (P< 0.001) and four times as long for neonatally treated mice (P< 0.001).

Mixed neurotransmission in the hippocampal mossy fibers

The hippocampal mossy fibers (MFs), the axons of the granule cells (GCs) of the dentate gyrus, innervate mossy cells and interneurons in the hilus on their way to CA3 where they innervate interneurons and pyramidal cells. Synapses on each target cell have distinct anatomical and functional characteristics. In recent years, the paradigmatic view of the MF synapses being only glutamatergic and, thus, excitatory has been questioned. Several laboratories have provided data supporting the hypothesis that the MFs can transiently release GABA during development and, in the adult, after periods of enhanced excitability. This transient glutamate-GABA co-transmission coincides with the transient up-regulation of the machinery for the synthesis and release of GABA in the glutamatergic GCs. Although some investigators have deemed this evidence controversial, new data has appeared with direct evidence of co-release of glutamate and GABA from single, identified MF boutons. However, this must still be confirmed by other groups and with other methodologies. A second, intriguing observation is that MF activation produced fast spikelets followed by excitatory postsynaptic potentials in a number of pyramidal cells, which, unlike the spikelets, underwent frequency potentiation and were strongly depressed by activation of metabotropic glutamate receptors. The spikelets persisted during blockade of chemical transmission and were suppressed by the gap junction blocker carbenoxolone. These data are consistent with the hypothesis of mixed electrical-chemical synapses between MFs and some pyramidal cells. Dye coupling between these types of principal cells and ultrastructural studies showing the co-existence of AMPA receptors and connexin 36 in this synapse corroborate their presence. A deeper consideration of mixed neurotransmission taking place in this synapse may expand our search and understanding of communication channels between different regions of the mammalian CNS.

Organophosphate-induced brain damage: mechanisms, neuropsychiatric and neurological consequences, and potential therapeutic strategies

Organophosphate (OP)-induced brain damage is defined as progressive damage to the brain, resulting from the cholinergic neuronal excitotoxicity and dysfunction induced by OP-induced irreversible AChE inhibition. This delayed secondary neuronal damage that occurs mainly in the cholinergic regions of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, might be largely responsible for persistent profound neuropsychiatric and neurological impairments (memory, cognitive, mental, emotional, motor and sensory deficits) in the victims of OP poisoning. Neuroprotective strategies for attenuating OP-induced brain damage should target different development stages of OP-induced brain damage, and may include but not limited to: (1) Antidote therapies with atropine and related efficient anticholinergic drugs; (2) Anti-excitotoxic therapies targeting attenuation of cerebral edema and inflammatory reaction, blockage of calcium influx, inhibition of apoptosis program, and the control of seizures; (3) Neuroprotective strategies using cytokines, antioxidants and NMDAR antagonists (a single drug or a combination of drugs) to slow down the process of secondary neuronal damage; and (4) Therapies targeting individual symptoms or clusters of chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may help limit or prevent secondary neuronal damage at the early stage of OP poisoning and attenuate the subsequent neuropsychiatric and neurological impairments, thus reducing the long-term disability caused by exposure to OPs.

Physiology of obliquely striated muscle fibres withinGrillotia erinaceusmetacestodes (Cestoda: Trypanorhyncha)

The tentacular bulb ofGrillotia erinaceusmetacestodes consists of obliquely striated muscle fibres with obvious motor end-plates. In this study isometric tension recordings and intracellular microelectrodes have been used to record mechanical and electrical activity from single isolated bulbs. Bulbs were mechanically quiescent and displayed resting membrane polentials (RMP) in the region of −49 to −64 mV with a mean RMP of −56 mV (n= 60). The membrane potential varied with [K+]oin a manner consistent with the RMP being determined largely by the K+equilibrium potential. High K+solution (> 15 mM) caused membrane depolarization and contraction of the preparation with the contraction showing both phasic and tonic components. L-glutamate caused membrane depolarization, contraction of quiescent preparations and increased the amplitude of electrically evoked responses. In contrast, 5-HT, dopamine, histamine, adrenaline, GABA, noradrenaline and D-glutamate, at concentrations up to and including 10−3M, were without apparent affect, although acetylcholine, at relatively high concentrations (≥ 10−4M) slightly reduced the amplitude of field-evoked contractions.

A quantitative assessment of glutamate uptake into hippocampal synaptic terminals and astrocytes: new insights into a neuronal role for excitatory amino acid transporter 2 (EAAT2)

The relative distribution of the excitatory amino acid transporter 2 (EAAT2) between synaptic terminals and astroglia, and the importance of EAAT2 for the uptake into terminals is still unresolved. Here we have used antibodies to glutaraldehyde-fixed d-aspartate to identify electron microscopically the sites of d-aspartate accumulation in hippocampal slices. About 3/4 of all terminals in the stratum radiatum CA1 accumulated d-aspartate-immunoreactivity by an active dihydrokainate-sensitive mechanism which was absent in EAAT2 glutamate transporter knockout mice. These terminals were responsible for more than half of all d-aspartate uptake of external substrate in the slices. This is unexpected as EAAT2-immunoreactivity observed in intact brain tissue is mainly associated with astroglia. However, when examining synaptosomes and slice preparations where the extracellular space is larger than in perfusion fixed tissue, it was confirmed that most EAAT2 is in astroglia (about 80%). Neither d-aspartate uptake nor EAAT2 protein was detected in dendritic spines. About 6% of the EAAT2-immunoreactivity was detected in the plasma membrane of synaptic terminals (both within and outside of the synaptic cleft). Most of the remaining immunoreactivity (8%) was found in axons where it was distributed in a plasma membrane surface area several times larger than that of astroglia. This explains why the densities of neuronal EAAT2 are low despite high levels of mRNA in CA3 pyramidal cell bodies, but not why EAAT2 in terminals account for more than half of the uptake of exogenous substrate by hippocampal slice preparations. This and the relative amount of terminal versus glial uptake in the intact brain remain to be discovered.

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.

Timing of potential and metabolic brain energy

The temporal relationship between cerebral electro-physiological activities, higher brain functions and brain energy metabolism is reviewed. The duration of action potentials and transmission through glutamate and GABA are most often less than 5 ms. Subjects may perform complex psycho-physiological tasks within 50 to 200 ms, and perception of conscious experience requires 0.5 to 2 s. Activation of cerebral oxygen consumption starts after at least 100 ms and increases of local blood flow become maximal after about 1 s. Current imaging technologies are unable to detect rapid physiological brain functions. We introduce the concepts of potential and metabolic brain energy to distinguish trans-membrane gradients of ions or neurotransmitters and the capacity to generate energy from intra- or extra-cerebral substrates, respectively. Higher brain functions, such as memory retrieval, speaking, consciousness and self-consciousness are so fast that their execution depends primarily on fast neurotransmission (in the millisecond range) and action-potentials. In other words: brain functioning requires primarily maximal potential energy. Metabolic brain energy is necessary to restore and maintain the potential energy.

Hippocampal abnormalities and memory deficits: new evidence of a strong pathophysiological link in schizophrenia

The central goals of this manuscript are (1) to better characterize what appears to be the most parsimonious account of schizophrenic long-term memory impairment in the neuropsychological literature: a contextual binding deficit rooted in the medial temporal lobes; (2) to link this deficit to concrete abnormalities at the level of the hippocampus; and (3) to suggest that this deficit could lead to the functional impairment experienced by schizophrenia patients in their daily lives. As far as long-term memory is concerned in schizophrenia, there seems to be a general agreement to conclude that explicit mechanisms are disturbed compared to relatively spared implicit mechanisms. More precisely, both subsystems of explicit memory (i.e., episodic and semantic) appear to be dysfunctional in this patient population. Errors during the encoding processes could be responsible for this dysfunction even if retrieval per se is not totally spared. Recently, a number of studies have suggested that impairments in conscious recollection and contextual binding are closely linked to episodic memory deficit. Since the hippocampal formation is considered to be the central element in the neural support for contextual binding and episodic memory, we have conducted an extensive review of the literature concerning the hippocampal formation in schizophrenia. Emerging evidence from varying disciplines confirm the coherence of the different anomalies reported concurrently at the neuroanatomical, neurodevelopmental, biochemical, and genetic levels. It seems highly probable that the synaptic disorganization in the hippocampus concerns the regions crucial for encoding and contextual binding memory processes. The consequences of these deficits could result in schizophrenia patients experiencing major difficulties when facing usual events which have not been encoded with their proper context.

Modulation of GABA-mediated Synaptic Potentials by Glutamatergic Agonists in Neonatal CA3 Rat Hippocampal Neurons

Intracellular recordings were made from slices of adult and neonatal hippocampal neurons. During the first 2 weeks of life the majority of pyramidal cells exhibited spontaneous gamma-aminobutyric acid (GABA)-mediated synaptic potentials, which were depolarizing at birth and became hyperpolarizing by the end of the first postnatal week. These synaptic potentials were reduced in frequency or blocked by the N-methyl-d-aspartate (NMDA) receptor antagonist d(-)2-amino-5-phosphonovalerate (AP-5, 50 microM) (13/15 cells). The non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 - 10 microM) abolished the GABA-mediated synaptic potentials in all the cells tested (n=12), Superfusion of l-glutamate (up to 100 microM) increased the frequency of both depolarizing and hyperpolarizing GABA-mediated synaptic potentials. This effect was reduced by AP-5 or dl-2-amino-7-phosphonoheptanoate (AP-7, 50 microM) and fully blocked by concomitant application of AP-5 (50 microM) and CNQX (5 - 10 microM). NMDA (0.5 - 2 microM) increased the frequency of the GABA-mediated synaptic potentials. These effects were blocked by AP-5 (50 microM) and by bicuculline (10 microM). Quisqualate (100 - 300 nM), (RS)-alpha-amino-3-hydroxy-5-methyl-4-izopropionate (AMPA, 100 - 300 nM) and kainate (100 nM) also increased the frequency of the GABA-mediated synaptic potentials. These effects were blocked by CNQX (5 - 10 microM) and by bicuculline (10 microM) but not by AP-5 (50 microM). In the presence of tetrodotoxin (TTX, 1 microM), quisqualate (up to 300 nM), AMPA (up to 500 nM) and kainate (100 nM) had no effect on membrane potential or input resistance. In conclusion, our experiments suggest that, in early postnatal life, NMDA and non-NMDA receptors located on GABAergic interneurons modulate GABAergic synaptic potentials.

Spontaneous, low frequency (approximately 2-3 Hz) field activity generated in rat ventral hippocampal slices perfused with normal medium

This study demonstrates that transverse slices taken from the ventral hippocampus of adult rats perfused with a medium of normal ionic composition sustain spontaneous periodic field potentials due to the synchronous activity of a population of neurons. This ventral hippocampus spontaneous synchronous activity (VHSSA) in CA1 stratum pyramidale consisted of positive potentials (approximately 0.12 mV, 55 ms) occurring at a frequency of 2.8 +/- 0.2 Hz for hours without interruption. VHSSA was most frequently observed in slices taken 1-3 mm from the ventral end of hippocampus, and was absent in slices taken from tissue more than 4.5 mm away from it. Stimulation of Schaffer collaterals primed the appearance of potentials, which were similar to VHSSA and clearly distinguishable from excitatory postsynaptic potentials. In view of the known relative proneness of ventral hippocampus to epilepsy, we perfused ventral slices with high-[K(+)](o) medium (8 mM). Albeit reduced in amplitude, VHSSA persisted during the high-[K(+)](o) induced interictal-like epileptiform activity. We could not document any temporal relationship between the two phenomena. Low concentrations of the antagonist of gamma-amino-butyric acid receptors, type A, bicuculline (2-3 microM), which enhanced the high-[K(+)](o) induced epileptiform activity, reversibly blocked the VHSSA. We conclude that under standard in vitro conditions small circuits in the ventral hippocampus are most often and for long periods of time engaged in synchronous quasi-rhythmic low-frequency activity, generated locally by mechanisms substantially differing from those supporting epileptiform discharges.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Decreased ability of rat temporal hippocampal CA1 region to produce long-term potentiation

Tetanic stimulation of Schaffer collaterals in the CA1 region of transverse slices, taken from the septal (dorsal) part of young rat hippocampus, produced N-Methyl-D-aspartate-dependent long-term potentiation (LTP) of the rising slope of excitatory postsynaptic potential (mean 38%). Under identical conditions of stimulation (100 Hz, 1 s) slices taken from the temporal (ventral) third of hippocampus presented a substantially reduced ability for LTP (mean 5%). The defect appeared to lie with the induction rather than the maintenance phase of LTP. These results suggest that a significant functional differentiation at the local synaptic plasticity level occurs between the two poles of hippocampus, which together with the substantial differences in their extrinsic connections, may help explain the reported differential participation of neurons in these parts of hippocampus during animal memory tests.

A double dissociation within the hippocampus of dopamine D1/D5 receptor and beta-adrenergic receptor contributions to the persistence of long-term potentiation

We compared the effects of the D1/D5 receptor antagonist SCH-23390 with the beta-adrenergic receptor antagonist propranolol on the persistence of long-term potentiation in the CA1 and dentate gyrus subregions of the hippocampus. In slices, SCH-23390 but not propranolol reduced the persistence of long-term potentiation in area CA1 without affecting its induction. The drugs exerted reverse effects in the dentate gyrus, although in this case the induction of long-term potentiation was also affected by propranolol. The lack of effect of SCH-23390 on the induction and maintenance of long-term potentiation in the dentate gyrus was confirmed in awake animals. The drug also had little or no effect on the expression of inducible transcription factors. In area CA1 of awake animals, SCH-23390 blocked persistence of long-term potentiation beyond 3 h, confirming the results in slices. To rule out a differential release of catecholamines induced by our stimulation protocols between brain areas, we compared the effects of the D1/D5 agonist SKF-38393 with the beta-adrenergic agonist isoproterenol on the persistence of a weakly induced, decremental long-term potentiation in CA1 slices. SKF-38393 but not isoproterenol promoted greater persistence of long-term potentiation over a 2-h period. In contrast, isoproterenol but not SKF-38392 facilitated the induction of long-term potentiation. These data demonstrate that there is a double dissociation of the catecholamine modulation of long-term potentiation between CA1 and the dentate gyrus, suggesting that long-term potentiation in these brain areas may be differentially consolidated according to the animal's behavioural state.

Effects of vasopressin and related peptides on neurons of the rat lateral septum and ventral hippocampus

The effects of vasopressin (VP), VP fragments and propressophysin glycopeptide on neuronal activities in the septum-hippocampus complex of rats were studied in vitro and in vivo. The frequency of the hippocampus theta rhythm in Brattleboro rats homozygous for diabetes insipidus was significantly slower than that of heterozygous litter mates and normal rats. Intracerebroventricular micro-injection of des-glycine-amide vasopressin corrected for several hours the frequency deficit of the theta rhythm in the homozygous Brattleboro rats and the centrally administered VP slowed down theta rhythm in normal rats. Microinotophoretically administered VP excited single neurons in the lateral septum of ventral hippocampus, and/or facilitated the responses of these neurons to glutamate and to stimulation of the glutamatergic afferent fibers in the fimbria bundle. The excitatory effects of VP vanished within seconds after termination of the peptide administration, however, the peptide-induced enhancement of glutamate and syntatically induced excitations were sustained for up to 60 min after the peptide administration. In vitro, pM concentrations of VP, VP 4-8 and C-terminus glycopeptide of propresophysin facilitated for 30-60 min the glutamate-mediated EPSPs in neurons of the lateral septum or the ventral hippocampus. The EPSPs increase in the lateral septum neurons was not prevented by pretreatment with antagonist of the V1a type of the vasopressin receptor. The resting membrane potential and input resistance were not affected by the peptides. A low-frequency electrical stimulation in the diagonal Band of Broca or in the Bed nucleus of the stria terminals, sources of the vasopressinergic innervation of the septum, facilitated the negative wave of the filed potentials responses evoked in the lateral septum by stimulating the fimbria bundle fibers in control Long-Evans and Brattleboro rats heterozygous for diabetes insipidus. The field potential increase was sustained for several hours after the stimulation, and it was not occluded by long-term potentiation elicited by high frequency stimulation of the fimbria bundle afferent fibers. Brattleboro rats homozygous for diabetes insipidus failed to show the filed potential increase after the diagonal band stimulation. It is suggested that the long-lasting facilitation of glutamate-mediated excitations might be a physiological action of the propressophysin-derived peptides in the septum-hippocampus complex which, in concert with other forms of synaptic plasticity like the long-term potentiation, facilitates the hippocampus-mediated forms of learning and memory. This action is presumably related to the memory enhancing effect of the propressophysin-derived peptides.

Chronic repetitive transcranial magnetic stimulation alters beta-adrenergic and 5-HT2 receptor characteristics in rat brain

Repetitive transcranial magnetic stimulation (rTMS) has been shown to affect mood in health and disease. Evidence to date has demonstrated an antidepressant potential for low- and high-frequency rTMS treatment. In animal behavioral models of depression magnetic stimulation of the brain induced similar effects to those of electroconvulsive shock (ECS). In this study the effects of repeated rTMS on rat brain noradrenaline, dopamine, serotonin and their metabolites levels, as well as on beta-adrenergic and 5-HT2 receptor characteristics were studied. After 10 days of treatment, beta-adrenergic receptors were significantly up regulated in the frontal cortex, down regulated in the striatum and were unchanged in the hippocampus. 5-HT2 receptors were down regulated in the frontal cortex and were not changed in the other brain areas. No change in benzodiazepine receptors in the frontal cortex and cerebellum were demonstrated. These findings demonstrate specific and selective alterations induced by repeated rTMS, which are distinct from those induced by other antidepressant treatments. TMS therapeutic effects in humans and behavioral and biochemical effects in animal, suggest that TMS has a unique mechanism of action which requires further investigation.

alpha-bungarotoxin- and methyllycaconitine-sensitive nicotinic receptors mediate fast synaptic transmission in interneurons of rat hippocampal slices

This study demonstrates for the first time that alpha7 nicotinic receptors (nAChRs) mediate fast synaptic transmission in conventional hippocampal slices. In the presence of antagonists of muscarinic, AMPA, NMDA, GABAA, ATP, and 5-HT3 receptors, spontaneous and evoked postsynaptic currents (PSCs) recorded from CA1 interneurons were blocked by the alpha7 nAChR antagonists methyllycaconitine and alpha-bungarotoxin and by a desensitizing concentration of the alpha7 nAChR agonist choline. Spontaneous nicotinic PSCs were also accompanied by Na+ transients, indicating that alpha7 nAChR-mediated transmission serves as an excitatory signal to the CA1 interneurons in the hippocampus.

Interactions of cholinergic and glutamatergic neuronal systems in the functional activation of cerebral blood flow response: a PET study in unanesthetized monkeys

The effects of somatosensory stimulation on the regional cerebral blood flow (rCBF) response were studied in unanesthetized monkeys under modulations of the glutamatergic and cholinergic systems using [15O]H2O and positron emission tomography (PET). Under a saline condition, vibrotactile stimulation elicited a significant increase in the rCBF response in the contralateral somatosensory cortex. The systemic administration of scopolamine, a muscarinic cholinergic receptor antagonist, resulted in the dose-dependent reduction of the rCBF response to the stimulation. The rCBF response abolished by scopolamine was recovered by the administration of physostigmine, a cholinesterase inhibitor in a dose-dependent manner. In addition, D-cycloserine, a partial agonist at the glycine site coupled to N-methyl-D-aspartate (NMDA) receptors, also restored the scopolamine-abolished rCBF response. The regional cerebral metabolic rate of glucose (rCMRglc) response, measured with [18F]-2-fluoro-2-deoxy-D-glucose, was not affected by the administration of scopolamine, physostigmine and/or D-cycloserine. The systemic administration of (+)-3-amino-1-hydroxy-2-pyrrolidone (HA-966), an antagonist of the glycine modulatory site on the NMDA receptors, induced the dose-dependent suppression of the rCBF response to the stimulation. The rCBF response abolished by HA-966 was restored by D-cycloserine, but not by physostigmine. The rCMRglc response was partially but significantly reduced by the administration of HA-966, and its reduction was restored by D-cycloserine, but not by physostigmine. These findings provided pharmacological evidence for an interaction between cholinergic and glutamatergic neuronal systems, the latter of which mediates the former by downstream regulation, in the functional rCBF response to somatosensory stimulation.

Lipid-dependent modulation of Ca2+ availability in isolated mossy fiber nerve endings

An enhancement of glutamate release from hippocampal neurons has been implicated in long-term potentiation, which is thought to be a cellular correlate of learning and memory. This phenomenom appears to be involved the activation of protein kinase C and lipid second messengers have been implicated in this process. The purpose of this study was to examine how lipid-derived second messengers, which are known to potentiate glutamate release, influence the accumulation of intraterminal free Ca2+, since exocytosis requires Ca2+ and a potentiation of Ca2+ accumulation may provide a molecular mechanism for enhancing glutamate release. The activation of protein kinase C with phorbol esters potentiates the depolarization-evoked release of glutamate from mossy fiber and other hippocampal nerve terminals. Here we show that the activation of protein kinase C also enhances evoked presynaptic Ca2+ accumulation and this effect is attenuated by the protein kinase C inhibitor staurosporine. In addition, the protein kinase C-dependent increase in evoked Ca2+ accumulation was reduced by inhibitors of phospholipase A2 and voltage-sensitive Ca2+ channels, as well as by a lipoxygenase product of arachidonic acid metabolism. That some of the effects of protein kinase C activation were mediated through phospholipase A2 was also indicated by the ability of staurosporine to reduce the Ca2+ accumulation induced by arachidonic acid or the phospholipase A2 activator melittin. Similarly, the synergistic facilitation of evoked Ca2+ accumulation induced by a combination of arachidonic acid and diacylglycerol analogs was attenuated by staurosporine. We suggest, therefore, that the protein kinase C-dependent potentiation of evoked glutamate release is reflected by increases in presynaptic Ca2+ and that the lipid second messengers play a central role in this enhancement of chemical transmission processes.

Effect of single and repeated administration of fluvoxamine on noradrenaline release in rat brain

In vivo microdialysis in conscious rats was used to evaluate the effect of acute and chronic administration of fluvoxamine on extracellular levels of noradrenaline, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex. A single administration of fluvoxamine (12 mg/kg i.p.) during dialysate collection caused a significant delayed decrease in 5-HIAA and a mild increase in noradrenaline with no change in HVA and DOPAC levels. Chronic administration of fluvoxamine (12 mg/kg i.p. daily for 3 weeks, last dose 24 h prior to microdialysis) or a single dose 24 h prior to microdialysis had no effect on noradrenaline, DOPAC and HVA levels in the frontal cortex; 5-HIAA levels were significantly decreased 24 h after a single dose, but increased following long term treatment. Tissue concentrations of 5-HT and 5-HIAA in the frontal cortex showed a mild (though not significant) increase in rats chronically treated with fluvoxamine. Release of noradrenaline in the frontal cortex may be enhanced initially by a 5-HT uptake inhibitor, but this effect is not seen following drug washout.

Effects of lead exposure on bromocriptine-induced penile erection in rats

In the present work we have studied the effects of lead exposure on penile erection induced by bromocriptine. Intraperitoneal injection of bromocriptine (2, 3, 4 and 8 mg/kg) induced dose-related penile erection in rats. Maximum response was observed with 4 mg/kg of the drug. Lead exposure (as Pb-acetate in drinking water) for periods of 7, 14, 21 and 28 days decreased the bromocriptine-induced penile erection response. Higher concentrations of lead (0.05%) were shown to cause a more prominent decrease of penile erection. The same procedure for lead administration did not significantly alter penile erection induced by physostigmine (0.1 and 0.3 mg/kg, intraperitoneally). In a series of experiments, blood lead concentrations were measured 7 and 21 days after lead exposure. Significant increases of lead concentrations were found after lead exposure. It is concluded that lead can influence bromocriptine-induced penile erection.

Differential effects of scopolamine on neuronal survival in ischemia and glutamate neurotoxicity: relationships to the excessive vulnerability of the dorsoseptal hippocampus

The neurodegeneration in the CA1 subfield of hippocampus exhibited a dorsal-ventral gradient of susceptibility in global ischemia (82% dorsoseptally and only 16% ventrotemporally). Scopolamine (SCOP) did not improve the neuronal damage caused by the global ischemic challenge in rats and did not reduce the infarct area after the focal MCA-occlusion in mice. No differences were observed between saline and SCOP-treated animals in the physiologic parameters, except for a slight increase in rectal temperature. In contrast, treatment of hippocampal cultures with increasing concentrations of SCOP (1 nM to 1 mM) under glutamate incubation had a beneficial effect on neuronal viability. These data show that (1) there is substantial gradient of vulnerability of the hippocampus from dorsal to ventral in global ischemia and (2) that interactions between the NMDA, muscarinic receptors and their corresponding neurotransmitter inputs to hippocampal neurons are evident in vitro and may play a crucial role in neuronal neurodegeneration. However, the mechanisms underlying the high vulnerability of dorsal hippocampus still remain enigmatic.

Increases in thyrotropin-releasing hormone messenger RNA expression induced by a model of human temporal lobe epilepsy: effect of partial and complete kindling

Thyrotropin-releasing hormone and its receptor are differentially distributed throughout the limbic forebrain. In addition to its neuroendocrine function, several non-endocrine central nervous system effects of thyrotropin-releasing hormone and its analogs have been reported, including anticonvulsant effects in animals and humans. Kindling, as a model of temporal lobe epilepsy, produces elevations of endogenous thyrotropin-releasing hormone specifically in seizure-prone limbic regions. The present study used semi-quantitative in situ hybridization to characterize changes in thyrotropin-releasing hormone messenger RNA that occurred during the kindling process (partial kindling), as well as after fully kindled seizures. No significant change in thyrotropin-releasing hormone messenger RNA was detected 1 h postictally, whereas significant elevations were detected in the granule cell layer of the hippocampal dentate gyrus, diffuse nuclei of the amygdala and in layers II and III of piriform and entorhinal cortices from 3 to 48 h after a single generalized seizure in fully kindled rats. Peak messenger RNA expression occurred from 6 to 12 h postictally, with a decline at 24 h, followed by a precipitous return to undetectable levels by 48 h, except in the dentate gyrus. In marked contrast, partial kindling produced no detectable change in thyrotropin-releasing hormone messenger RNA by 6 h after the first occurrence of stage 1-5 seizures. Electrode placement, a single afterdischarge, or a 20-microA stimulation of the amygdala was not associated with accumulation of thyrotropin-releasing hormone messenger RNA. Thus, only full kindled generalized seizures increased thyrotropin-releasing hormone messenger RNA expression in identical limbic regions which also showed postictal elevations in thyrotropin-releasing hormone. However, this enhancement followed a more immediate and shorter lasting time-course than previously demonstrated increases in the tripeptide. These results support the hypothesis that thyrotropin-releasing hormone is an important neuromodulator in epileptic foci.

Neuronal cytotoxicity of inositol hexakisphosphate (phytate) in the rat hippocampus

D-myo-Inositol hexakisphosphate (InsP6, phytate), a normal cellular constituent, was found to be toxic to neuronal perikarya when injected into the rat hippocampus. However, the extrinsic cholinergic innervation of the hippocampus (as estimated by staining for acetylcholinesterase) was unaffected. Its potency as a toxin was approximately equal to that of the excitotoxin quinolinate. Other highly charged derivatives of inositol (inositol hexakissulphate, inositol monophosphate) were not toxic. The cytotoxicity of InsP6 was not due to a high osmolality, or to seizure-induced lesions, but was reduced by calcium. Nevertheless, the toxicity was not due to chelation of brain calcium by InsP6, as another calcium chelator with a higher affinity for calcium, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), produced only a very mild lesion. Thus, abnormal metabolism of InsP6 might possibly contribute to neuronal death in neurodegenerative diseases.

GABA plasma membrane transporters, GAT-1 and GAT-3, display different distributions in the rat hippocampus

This study evaluates the distribution of two high affinity gamma-aminobutyric acid (GABA) transporters (GAT-1 and GAT-3) in the rat hippocampus using immunocytochemistry and affinity purified antibodies. GAT-1 immunoreactivity was prominent in punctate structures and axons in all layers of the dentate gyrus. In Ammon's horn, immunoreactive processes were concentrated around the somata of pyramidal cells, particularly at their basal regions. The apical and basal dendritic fields of pyramidal cells also displayed numerous GAT-1 immunoreactive punctate structures and axons. The zone of termination of the mossy fibers that includes both the hilus of the dentate gyrus and stratum lucidum of the CA3 area was the lightest immunolabeled region of the hippocampal complex. Electron microscopic preparations demonstrated that GAT-1 immunoreactive axon terminals form symmetric synapses with somata, axon initial segments, and dendrites of granule and pyramidal cells in the dentate gyrus and Ammon's horn, respectively. Immunoreactivity was localized to the plasma membrane and the cytoplasm of axon terminals. The somata of previously described local circuit neurons in the dentate gyrus and Ammon's horn contained GAT-1 immunoreactivity associated with the Golgi complex. Light, diffuse GAT-3 immunoreactivity was present throughout the hippocampal formation. Thin, astrocytic glial processes displayed GAT-1 and GAT-3 immunoreactivity. This localization of GAT-1 and GAT-3 indicates that they are involved in the uptake of GABA from the extracellular space into GABAergic axon terminals and astrocytes.

Entorhinal cortex modules of the human brain

Much is known about modular organization in the cerebral cortex, but this knowledge is skewed markedly toward primary sensory areas, and in fact, it has been difficult to demonstrate elsewhere. In this report, we test the hypothesis that a unique form of modules exists in the entorhinal area of the human cortex (Brodmann's area 28). We examined this issue using classic cyto- and myeloarchitectonic stains, immunolabeling for various neurochemicals, and histochemistry for certain enzymes. The findings reveal that the entorhinal cortex in the human is formed by a mosaic of cellular aggregates whose most conspicuous elements are the cell islands of layer II and myelinated fibers around the cell islands, the disposition of glutamic acid decarboxylase-positive neurons and processes, cytochrome oxidase staining, and the pattern of cholinergic afferent fibers. The neuropathology of Alzheimer's disease cases highlights the modules, but inversely so, by destroying their features. The findings are of interest because 1) anatomically defined modules are shown to be present in areas other than the sensory and motor cortices, 2) the modules are morphological entities likely to reflect functions of the entorhinal cortex, and 3) the destruction of entorhinal cortex modules may account disproportionately for the severity of memory impairments in Alzheimer's disease.

Insect neurotransmission: neurotransmitters and their receptors

The roles of acetylcholine, dopamine, octopamine, tyramine, 5-hydroxytryptamine, histamine, glutamate, 4-aminobutanoic acid (gamma-aminobutyric acid) and a range of peptides as insect neurotransmitters are evaluated in terms of the criteria used to identify transmitters. Of the biogenic amines considered, there is good evidence that acetylcholine, dopamine, octopamine, 5-hydroxytryptamine, and histamine should be considered to be neurotransmitters, but the case for tyramine is less convincing at the moment. The evidence supporting neurotransmitter roles for glutamate and gamma-aminobutyric acid at specific insect synapses is overwhelming, but much work remains to be undertaken before the full significance of these molecules in the insect nervous system is appreciated. Attempts to characterise biogenic amine and amino acid receptors using pharmacological and molecular biological techniques have revealed considerable differences between mammalian and insect receptors. The number of insect neuropeptides isolated and identified has increased spectacularly in recent years, but genuine physiological or biochemical functions can be assigned to very few of these molecules. Of these, only proctolin fulfills the criteria expected of a neurotransmitter, and the recent discovery of proctolin receptor antagonists should enable the biology of this pentapeptide to be explored fully.

Acute effects of ethanol on recurrent inhibitory circuits of CA1 neurons in vivo

This study examined the dose-dependent effects of acute ethanol on recurrent inhibitory mechanisms of hippocampal CA1 neurons in anesthetized rats. The effects of micropressure-ejected GABAergic compounds were studied on evoked field responses and recurrent inhibition of CA1 pyramidal neurons. None of the systemic doses of ethanol examined altered recurrent inhibition of CA1 neurons. In contrast, local application of both GABAa and GABAb compounds produced clear changes in evoked field responses and blocked recurrent inhibition. These results suggest that (a) recurrent inhibition in the CA1 region in vivo is resistant to the effects of ethanol, and (b) both GABAergic receptor subtypes modulate recurrent inhibition in the CA1 region in vivo.

Changes in GABAergic and non-GABAergic synapses during chronic ethanol exposure and withdrawal in the dentate fascia of LS and SS mice

Ethanol-sensitive LSIBG and ethanol-insensitive SSIBG mice were exposed to ethanol (23.5% ethanol-derived calories) for 4 months. Half of the animals was sacrificed at this time and the other half was withdrawn from the ethanol diet for 1 month. GABA immunoelectron microscopy was used to study the impact of the treatments on synaptic contacts in the dentate molecular layer. In the LS mice a significant loss of non-GABAergic axospinous synapses (26.7%; p < 0.05) was observed during ethanol exposure which was followed by a loss of GABAergic synapses on dendritic shafts (54.7%; p < 0.01) during withdrawal. In the SS mice there was a significant decrease in the non-GABAergic axospinous synapses (23.5%; p < 0.05) and a significant increase in axodendritic synapses (63.3%; p < 0.05) during ethanol exposure. The observed changes in the GABAergic and non-GABAergic innervation of the dentate fascia induced by ethanol were observed in the projection zone of the perforant path. They could adversely affect the hippocampal physiology with a consequent impairment of mnemonic functions.

Morphine inhibits dopaminergic and cholinergic induced ejaculation in rats

1. Subcutaneous injection (s.c.) of apomorphine (0.1-0.5 mg/kg) and intraperitoneal administration (i.p.) of quinpirole (0.01-0.25 mg/kg), physostigmine (0.05-0.2 mg/kg) and pilocarpine (0.75-3 mg/kg, i.p.) but not neostigmine (0.1-1 mg/kg) induced ejaculation in rats. 2. The responses of drugs were reduced by morphine (1-6 mg/kg, s.c.) pretreatment. 3. The inhibitory effect of morphine was reversed by naloxone (1.5 mg/kg, s.c.). 4. Naloxone (0.75-3 mg/kg, s.c.) alone induced slight but significant ejaculation. 5. Ejaculatory responses induced by apomorphine and quinpirole but not those by physostigmine and pilocarpine were reduced by sulpiride (100 mg/kg, i.p.) pretreatment. 6. Domperidone (1-30 mg/kg, i.p.) did not change the response induced by apomorphine. 7. Pretreatment of animals with the cholinergic antagonist atropine (10 mg/kg, i.p.) decreased the frequency of ejaculation induced by apomorphine, quinpirole, physostigmine or pilocarpine. 8. It may be concluded that D-2 activation induces ejaculation through influence on cholinergic mechanisms and morphine inhibits the ejaculation induced by activation of both cholinergic and dopaminergic systems via opiate receptor sites.

Differential behavioral effects of supracallosal and infracallosal lesions of the septohippocampal pathways: no ameliorative effects of oxotremorine or pilocarpine on radial-maze performance

We examined the effects in young adult female Long-Evans rats of single or combined lesions of the infracallosal and supracallosal septohippocampal pathways on a battery of behavioral tasks over two postoperative periods (14-65 and 75-150 days, respectively). During the first period, rats with lesions of the infracallosal pathways, whether given alone or in combination with lesions of the supracallosal pathways, were more active in the open field and in their home cage, and showed increased reactivity to novel extracage stimuli. Behavioral results during the second postoperative period were similar to those of the first except that rats with lesions of the infracallosal pathways (either alone or in combination with lesions of the supracallosal pathways) were no longer hyperactive in their home cage and rats with the infracallosal lesion alone were no longer hyperactive in the open field. We also observed in rats with lesions of the infracallosal pathways impaired performance in the radial-arm maze task, whether conducted under an uninterrupted protocol (first and second postoperative periods) or with a 1-min intratrial interruption (second postoperative period). Thus, behavioral deficits were observed only in rats with a lesion to the infracallosal component of the septohippocampal pathways, the behavior of rats with the combined lesions being similar to that of rats with single lesions of the infracallosal pathways in most measures. The behavior of rats with lesions of the supracallosal pathways did not differ from that of sham-operated controls in any measure at either postoperative period. Acute, systemic injections of oxotremorine (0.03 or 0.1 mg/kg, ip) or pilocarpine (0.32 or 1.0 mg/kg, ip), two muscarinic agonists, did not affect radial-arm maze performance under either the uninterrupted or interrupted protocol. The use of nonspecific muscarinic agonists does not appear to be sufficient to enhance radial-arm maze performance in rats with infracallosal septohippocampal lesions which, in contrast to supracallosal lesions, were shown to induce a deficit in this task.

Brain lesions induced by specific and non-specific inhibitors of sodium-potassium ATPase

The cytotoxicity in the brain of potent selective and non-selective inhibitors of Na+/K+ ATPase has been assessed. Following injection of cardiac glycosides into the dorsal hippocampus of rats, the extent of neuronal loss roughly paralleled their potencies as inhibitors of the enzyme. Dihydroouabain was less potent than ouabain as a cytotoxin by an order of magnitude, similar to their relative affinities for Na+/K+ ATPase. The non-specific inhibitors, melittin, erythrosin B and zinc (chloride) were less neurocytotoxic than the selective inhibitors having equivalent potencies. The toxicity of a low dose of ouabain appeared to be selective for neuronal perikarya as staining for acetylcholinesterase (present on the nerve terminals of the afferent cholinergic innervation) was unaffected. A higher dose of ouabain caused a non-specific necrosis including damage to the neuropil and a loss of cholinesterase staining. Concurrently, there was an invasion of the tissue by cells resembling foaming macrophages. Other inhibitors of the enzyme caused a mixed pathology with both types of responses evident. It is suggested that the pathological response may depend on the relative degrees to which the glial and neuronal activities of the enzyme are affected.

Lesion-specific pattern of immunocytochemical distribution of growth-associated protein B-50 (GAP-43) in the cerebellum of Weaver and PCD-mutant mice: lack of B-50 involvement in neuroplasticity of Purkinje cell terminals?

The growth-associated protein B-50 (GAP-43) is thought to play a major role in the development and regeneration of neurons. The participation of B-50 in neuronal plasticity is well documented, especially for monoaminergic systems. However, such an important role for B-50 in GABAergic systems has not been substantiated to date. This study was performed to obtain detailed information about the identity of B-50 immunopositive axons and terminals in the cerebellum and to test the involvement of this protein during plastic changes as observed in the projections of GABAergic Purkinje cells to the lateral vestibular nucleus (LVN). For this purpose mutant mice with specific cerebellar cell loss were used. Weaver mutants (B6CBA wv/wv), PCD-mutants (B6C3Fe pcd/pcd), and their corresponding wild-type mice were investigated with immunocytochemical and immunoblot procedures at the age of 8-23 days and 5-6 months using polyclonal and monoclonal antibodies to B-50. Substantial differences in B-50 distribution were detected between normals and mutants and between young and adult animals. These results demonstrate that the labeling of B-50 is mainly related to the out-growth of parallel fibers and to a minor degree on the ingrowth of non-GABAergic cerebellar afferents. There was no immunocytochemical indication that B-50 is related to Purkinje cells or accompanies the plasticity of the GABAergic innervation of the LVN.

Changes in evoked potentials and amino acid content during fluorocitrate action studied in rat hippocampal cortex

Fluorocitrate inhibits the glial tricarboxylic acid cycle and thereby the synthesis of glutamine, which is the main precursor for transmitter glutamate. We investigated the possibility that there is a functional correlate to fluorocitrate action by recording evoked field potentials in rat hippocampal slices. The excitatory postsynaptic potential (field-EPSP) was markedly depressed after 7-8 h of fluorocitrate action. The population spike was also reduced, but a major part of the reduction may be the result of weaker synaptic activation rather than reduced excitability of the postsynaptic cells. The activity of thin unmyelinated fibres was only slightly affected. Preceding the changes in the field-EPSP there was a decrease in the glutamine content in the fluorocitrate treated slices relative to controls. Only a small decrease in tissue glutamate was seen concomitantly with the synaptic failure, probably because the transmitter pool of glutamate in those fibres stimulated makes little contribution to the total tissue glutamate.

Comparison of the immunocytochemical localization of DARPP-32 and I-1 in the amygdala and hippocampus of the rhesus monkey

Dopamine and adenosine 3':5'-monophosphate (cAMP) regulated phosphoprotein of M(r) 32 kDa (DARPP-32) and phosphatase inhibitor 1 (I-1) have been associated with intracellular signal transduction processes and share several biochemical features. Localization of each phosphoprotein in distinct neural structures will aid investigation of their physiologic properties and help identify their unique roles in the nervous system. We have compared the distribution of the two phosphoproteins in the amygdala and hippocampus of the rhesus monkey with the aid of immunocytochemical procedures. Neurons immunoreactive to antibodies raised against the phosphoproteins DARPP-32 and I-1 were noted in the cortical, central, and components of the basal group, including the basomedial, the lateral, and to a lesser extent, the basolateral amygdaloid nuclei. Within the large basal nuclei positive neurons were found preferentially in their medial and ventral subdivisions. By making a direct comparison in the same animals, we observed differences in the distribution of the two phosphoproteins in the amygdala. DARPP-32 and I-1 positive neurons overlapped partially in the basal nuclei, to a lesser extent in the cortical, but were segregated in the central amygdaloid nucleus with neurons positive for DARPP-32 noted laterally, and for I-1 medially. In contrast to the amygdala, where numerous DARPP-32 and I-1 positive neurons were observed, only I-1 had a notable presence in the hippocampus. Moreover, I-1 associated label was found only in neurons in the granule cell layer of the dentate gyrus, their dendritic plexus, and axons which innervate hilar and CA3 neurons. DARPP-32 and I-1 are intracellular messengers associated with signal transduction. Their regional distribution in the amygdala and the hippocampus suggests an involvement in the level of excitability of specific components of these limbic structures. Moreover, our results suggest that I-1 has a unique role in the intrinsic circuitry of the hippocampal formation and indicate a system where the physiologic properties of I-1 may be studied in isolation.

Hippocampal cholinergic alterations and related behavioral deficits after early exposure to ethanol

The present study was designed to ascertain septohippocampal cholinergic alterations and their related behavioral deficits after early exposure to ethanol. Mouse pups were exposed to ethanol, 3 g/kg by daily subcutaneous injection on postnatal days 2-14. At age 50 days, the ethanol-exposed mice had significant reductions from control levels in eight-arm maze performance. For example, on the fourth testing day, the number of correct entries in the ethanol group was 21% below control levels (P < 0.05) and the number of trials needed to enter all arms was 48% above control (P < 0.001). It took the ethanol-exposed mice twice the time to reach criterion than it did control (P < 0.01). A 33% increase from control level in muscarinic receptor number (Bmax) was found in the treated mice of age 22 days and a 64% increase at age 50 days (P < 0.001). However, no differences between control and treated groups could be detected in the presynaptic component of the cholinergic innervation, choline acetyltransferase activity. The results suggest that early ethanol exposure acts on hippocampal function similarly to phenobarbital, probably via alterations in postsynaptic processes in the septohippocampal cholinergic pathways.

Comparison of 5 alpha-pregnan-3 alpha-ol-20-one and phenobarbital on cortical synaptic activation and inhibition studied in vitro

The effects of 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) and phenobarbital (PB) on synaptic excitation and inhibition in rat hippocampal slices in vitro were compared. Stimulations were made orthodromically and antidromically while we recorded extracellularly from the dendritic and the somatic layer of the CA1 region. Perfusion with 5 micrograms/ml of 3 alpha-OH-DHP for 30 min significantly increased the recurrent inhibition evoked by antidromic stimulation. The effect was most pronounced at short interstimulus intervals. The duration of the recurrent inhibition also was prolonged. There was no effect on the conditioned population spike after orthodromic paired-pulse stimulation. Furthermore, no effect was observed on the amplitude of the orthodromic fiber volley, the rate of increase in the field excitatory postsynaptic potential (EPSP) and the latency and amplitude of the CA1 population spike. Qualitative and quantitative similar findings were observed during perfusion with PB 0.1 mg/ml, (i.e., a concentration 20 times higher than that of 3 alpha-OH-DHP). Higher concentrations of PB also affected synaptic excitation. The findings suggest a similar effect of 3 alpha-OH-DHP and PB on recurrent GABA-ergic inhibition; however, 3 alpha-OH-DHP appears to be much more potent.

Ontogenic distribution of muscarinic receptors and acetylcholinesterase in the rabbit hippocampus

Ontogenic development of muscarinic receptors was examined in the hippocampus of rabbits (from P2 to P60) using radioautographic method. Muscarinic sites were labelled with (3H)-quinuclinidyl-benzilate and pharmacologically defined M1 and M2 receptor subtypes with (3H)-pirenzepine and (3H)-oxotremorine, respectively. The distribution of binding sites was compared to acetylcholinesterase (AChE) staining in adjacent hippocampal sections. The two cholinergic components are progressively set up in the hippocampus during the first three postnatal weeks. The AChE staining was very low in all hippocampal fields in P2 rabbits. At P8 and after, the AChE staining was more pronounced in CA3 and CA4 than in CA1 and CA2. On the contrary, the M1 muscarinic binding sites were more abundant in CA1 and CA2 hippocampal fields than in CA3 and CA4 at all ages studied. M2 muscarinic binding sites were only distinguishable at P45 and have a relatively homogeneous distribution. This study shows a differential developmental evolution in the distribution of AChE and muscarinic M1 receptors, and no obvious correspondence between these two cholinergic markers was observed.

Neurochemical changes associated with the action of acute administration of diazepam in reversing the behavioral paradigm conditioned emotional response (CER)

Neurotransmitter turnover of biogenic monoamines (dopamine, norepinephrine, and serotonin) and amino acids (glutamate, aspartate, and gamma-aminobutyric acid) was evaluated in rats exposed to the conditioned emotional response (CER) paradigm in the absence (total suppression) or presence of acute 5 mg/kg i.p. diazepam (which reversed suppression and restored normal responding). Based on previous studies of CER, with controls for shock and stimulus histories, the results with respect to the anxiolytic could be divided into several categories: changes in turnover which are associated only with the CER behavior; changes associated only with the drug, diazepam; changes which augmented the effects of the behavior; or changes which were the reverse of those associated with the behavior. Due to the multitude and complexity of the results, not all observations have clear explanations at this time. However, for the CER behavior per se, it is apparent that a combination of neurotransmitters, including some implications about acetylcholine, act in concert to bring about the behavioral suppression. The action of diazepam is more complex, involving the full spectrum of neurotransmitters to bring about its direct and indirect effects.

Synaptic Activation of GABAA Receptors Causes a Depolarizing Potential Under Physiological Conditions in Rat Hippocampal Pyramidal Cells

Intracellular recordings with K-acetate-filled microelectrodes were performed in slices of the adult rat hippocampus maintained in vitro at 35 - 36 degrees C to analyse the potentials associated with the orthodromic inhibitory sequence generated by CA1 pyramidal cells. In 43 of 72 cells, stimuli that were delivered in the stratum radiatum induced (i) an initial excitatory postsynaptic potential (EPSP), (ii) an early, hyperpolarizing inhibitory postsynaptic potential (IPSP) (peak latency from the stimulus artefact 20 ms), (iii) an intermediate depolarizing component (peak latency=60 - 120 ms; duration=60 - 150 ms, and (iv) a late, long-lasting hyperpolarizing IPSP (peak latency=120 - 160 ms, duration >400 ms). In the remaining cells the orthodromic inhibitory response lacked the intermediate depolarization. The depolarizing component was selectively blocked by local applications of bicuculline or picrotoxin on the apical dendrites of pyramidal cells. This pharmacological procedure induced an increase in the amplitude of the EPSP that was capable of triggering 2 - 3 action potentials, but no reduction of the recurrent IPSP which is caused by GABAA receptors located close to the soma. The amplitude and duration of the depolarizing component was enhanced by lowering the temperature in the tissue chamber to 29 - 31 degrees C or by application of the GABA uptake blocker nipecotic acid, further indicating that the depolarizing component represented an active phenomenon mediated through GABA. Application of the Cl- pump blocker furosemide reduced and eventually blocked the early IPSP and the depolarizing component. These data demonstrate that under physiological conditions rat hippocampal pyramidal cells generate a depolarization that is presumably caused by an outwardly directed Cl- movement due to the activation of GABAA receptors located on the apical dendrites. This novel mechanism might modulate hippocampal excitability in both physiological and pathophysiological conditions.

Endogenous noradrenaline and dopamine in nerve terminals of the hippocampus: differences in levels and release kinetics

The presence and release of endogenous catecholamines in rat and guinea pig hippocampal nerve terminals was studied by fluorimetric HPLC analysis. In isolated nerve terminals (synaptosomes) the levels and breakdown of endogenous catecholamines were determined and the release process was characterized with respect to its kinetics and Ca2+ and ATP dependence. Endogenous noradrenaline and dopamine, but not adrenaline, were detected in isolated hippocampal nerve terminals. For dopamine both the levels and the amounts released were more than 100-fold lower than those for noradrenaline. In suspension, released endogenous catecholamines were rapidly broken down. This could effectively be blocked by monoamine oxidase inhibitors, Ca(2+)-free conditions, and glutathione. The release of both noradrenaline and dopamine was highly Ca2+ and ATP dependent. Marked differences were observed in the kinetics of release between the two catecholamines. Noradrenaline showed an initial burst of release within 10 s after K+ depolarization. The release of noradrenaline was terminated after approximately 3 min of K+ depolarization. In contrast, dopamine release was more gradual, without an initial burst and without clear termination of release within 5 min. It is concluded that both catecholamines are present in nerve terminals in the rat hippocampus and that their release from (isolated) nerve terminals is exocytotic. The characteristics of noradrenaline release show several similarities with those of other classical transmitters, whereas dopamine release characteristics resemble those of neuropeptide release in the hippocampus but not those of dopamine release in other brain areas. It is hypothesized that in the hippocampus dopamine is released from large, dense-cored vesicles, probably colocalized with neuropeptides.

Effects of medial and lateral septal lesions on acquisition of a place and cue radial maze task

Rats with lesions limited to either the medial septal (MS) or dorsolateral septal (LS) nuclei were trained on a place and cue version of the radial maze using a procedure that permits determination of both reference memory (RM) and working memory (WM). Following training the presence or absence of hippocampal theta for MS and LS groups was determined. The results showed that both MS- and LS-lesioned rats were impaired in acquisition. Specifically, the pattern of results indicated a general impairment in working memory that was found on both the place and cue tasks together with impaired acquisition of the place (but not the cue) task. While both septal lesion groups evidenced a similar impairment, rats in the MS group made more errors than LS rats on several components of the tasks. Electroencephalographic recordings revealed that hippocampal theta was not affected in the LS group but was abolished in rats with MS lesions. Thus, it appears that the functional role of the septo-hippocampal system cannot be fully described by a single mechanism or process but rather several processes seem to be affected when the septal area is damaged.

Calcium dependent release of gamma-aminobutyric acid (GABA) from human cerebral cortex

The release of the amino acids GABA, taurine, glycine, glutamine and leucine from human neocortex was investigated in vitro by utilizing brain tissue removed during 8 standard temporal lobectomies for epilepsy or tumor. Slices (0.5 mm thick) were cut from each biopsy and randomly placed in three different chambers. After 90 min preincubation, the three sets of slices were incubated for 60 s in wells containing, respectively, (A) regular ACSF (control), (B) ACSF with 50 mM K+ (to depolarize the cell membrane) and (C) ACSF with 50 mM K+, 0 mM Ca2+ and 4 mM Mg2+ (depolarization during blocked synaptic transmission). The content of amino acids in the wells was determined by high-performance liquid chromatography after pre-column derivatization of the amino acids with o-phthalaldehyde. Membrane depolarization (well B) increased the GABA release to 650% (620 pmol/mg) of control (well A, 95 pmol/mg). Blocking synaptic transmission (well C) reduced the evoked release by 50% (360 pmol/mg). The release of glycine, taurine, glutamine and leucine during membrane depolarization was not significantly different from the control values. The data provide evidence for a Ca(2+)-dependent release of GABA, supporting a possible role of this amino acid as a neurotransmitter in human neocortex.

Inhibitory contacts on dendritic spines of the dentate fascia

GABA-containing axon terminals were observed in the distal two-thirds of the dentate molecular layer to contact spines and dendrites of the granule cells. These contacts have the morphological characteristics of inhibitory synapses: they contain pleomorphic vesicles and have symmetrical junctional specializations. Convergence of an asymmetrical, non-GABAergic and a symmetrical, GABAergic synapse on one spine was often observed.