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

Comparison of the effects of ethanol and chlordiazepoxide on electrophysiological activity in the fascia dentata and hippocampus regio superior

Acute intoxicating doses of ethanol-producing blood alcohol levels of 120-200 mg% increase paired-pulse (PP) inhibition in the dentate gyrus of anesthetized rats suggesting that ethanol increases recurrent inhibitory processes (Wiesner, J.B., and S.J. Henriksen (1987) Ethanol enhances recurrent inhibition in the dentate gyrus of the hippocampus. Neurosci. Lett. 79:169-173). To further understanding of the neuronal mechanisms underlying this phenomenon, the authors studied the effects of the benzodiazepine (BZ), chlordiazepoxide, and acute intoxicating levels of ethanol on extracellular field potential recordings and single-unit activity in the dentate gyrus and area CA1 of the hippocampus. In the dentate, ethanol had no effect on population excitatory postsynaptic potential (pEPSP) amplitudes or slopes; decreased population spike (PS) amplitudes (25%); increased PP inhibition; decreased dentate granule cell (DGC) spontaneous activity (58%); had no effect on putative interneuron spontaneous activity; and markedly increased post field potential-evoked interneuron discharges (IDs, 218%). Chlordiazepoxide had no effect on pEPSP amplitudes or slopes or PS amplitudes; increased PP inhibition; decreased DGC (62%) and interneuron (72%) spontaneous activity; and markedly decreased IDs (89%). In CA1, ethanol had no effect on pEPSP amplitudes or slopes; decreased PS amplitudes (26%); had no effect on PP responses; decreased pyramidal cell (PC) spontaneous activity (39%); had no effect on interneuron spontaneous activity; and markedly increased IDs (97%). Chlordiazepoxide had no effect on pEPSP amplitudes or slopes or PS amplitudes; had no effect on PP responses; decreased PC spontaneous activity (41%); and had no effect on interneuron spontaneous activity or IDs. The results suggest that the BZs decrease principal cell excitability by postsynaptic facilitation of inhibitory processes, whereas ethanol decreases principal cell excitability indirectly by increasing the excitability of inhibitory interneurons.

In vitro studies on the broad spectrum anticonvulsant loreclezole in the hippocampus

In hippocampal slices from guinea-pig a paired-pulse stimulation protocol was used to examine the effects of loreclezole, R-(+)-etomidate, phenobarbital and pentobarbital on orthodromic and antidromic GABAergic neuronal inhibition in the CA1 region. All four compounds increased orthodromic GABAergic inhibition, with R-(+)-etomidate and pentobarbital inducing a quantitatively larger effect than loreclezole and phenobarbital. Only R-(+)-etomidate and pentobarbital increased antidromic GABAergic inhibition. We propose that all four compounds are anticonvulsant by increasing feed-forward dendritic GABAergic inhibition, whilst only the sedative/hypnotic compounds (R-(+)-etomidate, pentobarbital) increase feedback recurrent GABAergic inhibition. Loreclezole was also shown to inhibit 'low Ca2+' and 'low Mg2+' epileptogenesis at similar concentrations to those active on inhibition. Thus loreclezole may possess other pharmacodynamic properties, beyond its ability to increase feed-forward GABAergic neuronal inhibition, which contribute to its antiepileptic action.

Inhibitory effect of morphine on yawning induced by cholinoceptor and dopamine D2 receptor activation in rats

1. Bromocriptine (2, 4 and 8 mg kg-1, i.p.), physostigmine (0.05, 0.1 and 0.2 mg kg-1, i.p.) and pilocarpine (1, 3 and 5 mg kg-1, i.p.) induced dose-dependent yawning in rats. 2. These responses were reduced in a dose-dependent manner by pretreatment with morphine. 3. The inhibitory effect of morphine was reversed by naloxone. 4. Naloxone alone induced slight but significant yawning. 5. The present results suggest that morphine inhibits yawning in rats at an opiate receptor downstream from the sites at which cholinoceptor and dopamine D2 activation induce yawning. The anatomical location of these sites remains to be established.

Serotonin blocks the long-term potentiation induced by primed burst stimulation in the CA1 region of rat hippocampal slices

The effect of 5-hydroxytryptamine on the induction of long-term potentiation by a train of high frequency pulses (100 Hz; 1 s) or by a stimulation consisting of one burst of five pulses at 100 Hz delivered 170 ms after a single pulse (primed burst) was investigated in the CA1 region of the rat hippocampal slice in vitro with extracellular recordings. Superfusion with 5-hydroxytryptamine (3-30 microM) produced a concentration-dependent decrease in amplitude of the population spikes evoked by test stimuli. The presence of 5-hydroxytryptamine (30 microM) did not affect the magnitude of long-term potentiation produced by the high-frequency stimulation but it prevented the long-term potentiation induced by a primed burst. The action of 5-hydroxytryptamine was mimicked by the 5-hydroxytryptamine1A agonist 5-carboxamidotryptamine (0.3 microM) and blocked by the 5-hydroxytryptamine2/5-hydroxytryptamine1A antagonist spiperone (3 microM) or by the 5-hydroxytryptamine1/5-hydroxytryptamine2 antagonist methiothepin (1-10 microM). The selective 5-hydroxytryptamine2 antagonist ritanserin (1 microM) did not antagonize the block of long-term potentiation produced by 5-hydroxytryptamine. The selective 5-hydroxytryptamine3 antagonists (3-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930; 1 nM) and ondansetron (GR-38032; 30 nM) did not affect the reduction in the population spike produced by application of 5-hydroxytryptamine. In contrast, a primed burst delivered at the fifth minute of 5-hydroxytryptamine application in the presence of a 5-hydroxytryptamine3 antagonist induced a long-term potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Pregnanolone (3 alpha-hydroxy-5 alpha-pregnane-20-one), a progesterone metabolite, facilitates inhibition of synaptic transmission in the Schäffer collateral pathway of the guinea pig hippocampus in vitro

Pregnanolone (3 alpha-hydroxy-5 alpha-pregnane-20-one, a metabolite of progesterone) caused a significant depression of the amplitude of the population spike evoked in stratum pyramidale in CA1 of the guinea pig hippocampus in vitro. Local application of pregnanolone on the surface of the slice in stratum oriens depressed the population spike without effects on the presynaptic spike and the population excitatory postsynaptic potential simultaneously recorded in stratum radiatum. The depression was dose-dependent and was observed with a minimum latency of 10 s after application of a 0.5-nl droplet of 3.1 microM pregnanolone. The concentration at the recording site was computed to be 0.2 microM. The duration of the depression was 20-30 min. The depression was significantly reduced during perfusion of the slice bath with 100 microM picrotoxin in artificial cerebrospinal fluid. When pregnanolone was applied locally in stratum radiatum, the amplitudes of the presynaptic spike, the population excitatory postsynaptic potential and the population spike were depressed. The effects on the presynaptic spike and the population excitatory postsynaptic potential vanished with different time courses. It is concluded that the depression of the population spike was caused by GABAA-mediated inhibition of the pyramidal neurones. The role of pregnanolone as a positive modulator of the GABAA receptor and the effect of this modulation on the complex mechanisms underlying catamenial epilepsy are discussed.

Differential effects of mu- and delta-receptor selective opioid agonists on feedforward and feedback GABAergic inhibition in hippocampal brain slices

Previous studies have suggested that opioid receptor activation in the hippocampus increases pyramidal neuron excitability by reducing GABAergic inhibition. This hypothesis has received support with regard to mu-receptor agonists but has not been adequately tested with selective delta-receptor agonists. In the present investigation we compared the effects of the selective mu-opioid receptor agonist [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO) and the delta-receptor agonist [D-Pen2,D-Pen5]-enkephalin (DPDPE) to those of bicuculline methiodide (BMI) on extracellularly recorded feedforward (FFW) and recurrent (feedback; FB) inhibition. It was discovered that the control population spike response, evoked by Schaffer collateral/commissural axon stimulation, increased in response to DAGO, DPDPE, and BMI, while the secondary or test response increased only in the presence of DAGO and BMI. The resulting hypothesis that delta-opioid receptor activation facilitates synaptically evoked responses independently of a reduction of inhibition was investigated by examining the effect of DPDPE on the field EPSP response recorded in stratum radiatum of CA1, or postsynaptically on a burst response activated through antidromic stimulation of pyramidal neurons in low calcium medium. delta-Opioid receptor activation had no effect on either the field EPSP response or the burst response, suggesting that neither synaptic transmission nor postsynaptic excitability were augmented. Finally, the possibility that DPDPE acts to enhance pyramidal cell excitability independently of GABAergic transmission was further investigated by examining responses to both mu- and delta-opioid agonists following treatment with BMI (30 microM). Responses to DPDPE and DAGO were completely blocked by this treatment, supporting the involvement of a GABAergic circuit in the actions of these enkephalins. These results suggest that the delta-opioid receptor agonist DPDPE may mediate a reduction in GABAergic inhibition which is not detectable using paired stimulation techniques designed to examine FFW and FB inhibition in the hippocampal slice.

Parvalbumin-immunoreactive neurons in the cerebral cortex of the lizard Podarcis hispanica

An antibody against the calcium binding protein parvalbumin selectively labels a set of neurons in the cerebral cortex of lizards. Golgi-like immunostained bipolar, multipolar and pyramid-like neurons appear mainly located in the inner plexiform layers. Parvalbumin-immunoreactive (PARV-IR) puncta are concentrated in the cell layer of the dorsal and dorsomedial cortices showing a basket-like distribution. The morphology and distribution of PARV-IR neurons and puncta overlap GABA-immunostaining in the cerebral cortex of lizards. Thus, it is likely that PARV-IR neurons are a subset of the cortical GABAergic neurons of lizards.

Induction of heat shock protein 72-like immunoreactivity in the hippocampal formation following transient global ischemia

Global ischemia was produced in adult rats by combining bilateral carotid artery occlusions with systemic hypotension for 5 or 10 minutes. Induction of the 72 kD heat shock protein (HSP72) in the hippocampus was examined immunocytochemically 18-24 hours later. Several patterns of HSP72-like immunoreactivity (HSP72LI) were observed. Five minutes of ischemia induced HSP72 in isolated columns of CA1a pyramidal neurons, or throughout CA1 pyramidal neurons and dentate hilar neurons. Ten minutes of ischemia induced marked HSP72LI in CA3 pyramidal neurons, moderate HSP72LI in dentate granule cells, and minimal HSP72LI in CA1 pyramidal, dentate hilar neurons, and hippocampal glia. Two hippocampi subjected to 10 minutes of ischemia exhibited marked HSP72LI in capillary endothelial cells but no neuronal or glial HSP72LI. It is proposed that (a) the induction of HSP72 in hippocampal sectors correlates with their vulnerability to global ischemia (CA1 greater than hilus greater than CA3 greater than dentate gyrus); (b) the induction of HSP72 in hippocampal cells correlates with their vulnerability to global ischemia in that mild ischemia induced HSP72 only in neurons, moderate ischemia in neurons and glia, and severe ischemia only in capillary endothelial cells; (c) the failure to induce HSP72 in hippocampal neurons in 2 cases of 10 min ischemia may be related to severe injury causing disruption of protein synthesis in these cells.

Effects of baclofen and bicuculline on inhibition in the fascia dentata and hippocampus regio superior

The effects of microiontophoretically applied baclofen, bicuculline and phaclofen were studied on evoked field responses, paired-pulse (PP) plasticity and single-unit activity of dentate granule cells (DGCs) and CA1 pyramidal cells (PCs) in anesthetized rats. The GABAB agonist, baclofen, increased population spike (PS) amplitudes in the dentate evoked by perforant path stimulation but decreased PS amplitudes in CA1 evoked by Schaffer collateral stimulation, whereas the GABAA antagonist, bicuculline, increased PS amplitudes in both regions. Neither baclofen nor bicuculline had significant effects on dendritically recorded population excitatory postsynaptic potentials (EPSPs) in the dentate or CA1 evoked by stimulation of their respective afferents. Control PP curves in the dentate revealed a triphasic response of inhibition/potentiation/inhibition, whereas control PP curves in CA1 manifested a biphasic response of inhibition/potentiation of test/conditioned PS amplitudes. Baclofen and bicuculline reversed the early and late phases of PP inhibition in the dentate and the early phase of PP inhibition in CA1. The GABAB antagonist, phaclofen, selectively reversed the effects of baclofen on PP inhibition in both the dentate and CA1. Whereas baclofen had no effect, bicuculline incre sed and phaclofen decreased DGC single-unit spontaneous firing rate, while baclofen decreased and bicuculline and phaclofen increased PC firing rate. These results support and extend studies suggesting that GABAergic feedback inhibition of DGCs and PCs is mediated by postsynaptic GABAA receptors and feedback inhibition of PCs is mediated by postsynaptic GABAB receptors. Our results also provide significant new evidence suggesting that postsynaptic inhibition in the dentate is not regulated by GABAB receptors and that feedback and feedforward inhibition of DGCs and PCs is regulated by presynaptic GABAB receptors located on GABAergic interneurons.

Effect of perforant path lesion on pattern of glutamate-like immunoreactivity in rat dentate gyrus

To investigate the relation between perforant path and the pattern of glutamate-like immunoreactivity in its target regions in the rat hippocampal formation, unilateral lesions of various size and location were placed to interrupt certain contingents of these afferent fibers. Postembedding immunohistochemistry at the level of light microscopy yielded the same pattern of immunoreactivity in the hippocampal formation contralateral to the lesion as in untreated animals. On the ipsilateral side, however, extensive transections of the perforant path led to a drastic loss of glutamate-immunoreactive terminal-like elements in the outer part of the dentate molecular layer. More restricted lesions induced a loss of punctate glutamate-like immunoreactivity in narrower bands within this zone. The width and the location of the affected bands appeared to depend on the extent of the transections and their topographical relation to the perforant path fiber system. These results and those obtained using a postembedding immunogold method at the level of electron microscopy strongly indicate that perforant path terminals in the dentate molecular layer of the rat contain high levels of glutamate and, thus, provide further support for an already well-documented role of this excitatory amino acid as neurotransmitter in this system.

Quisqualate- and NMDA-sensitive [3H]glutamate binding in primate brain

Excitatory amino acids (EAA) such as glutamate and aspartate are probably the neurotransmitters of a majority of mammalian neurons. Only a few previous studies have been concerned with the distribution of the subtypes of EAA receptor binding in the primate brain. We examined NMDA- and quisqualate-sensitive [3H]glutamate binding using quantitative autoradiography in monkey brain (Macaca fascicularis). The two types of binding were differentially distributed. NMDA-sensitive binding was most dense in dentate gyrus of hippocampus, stratum pyramidale of hippocampus, and outer layers of cerebral cortex. Quisqualate-sensitive binding was most dense in dentate gyrus of hippocampus, inner and outer layers of cerebral cortex, and molecular layer of cerebellum. In caudate nucleus and putamen, quisqualate- and NMDA-sensitive binding sites were nearly equal in density. However, in globus pallidus, substantia nigra, and subthalamic nucleus, quisqualate-sensitive binding was several-fold greater than NMDA-sensitive binding. In thalamus, [3H]glutamate binding was generally low for both subtypes of binding except for the anterior ventral, lateral dorsal, and pulvinar nuclei. In the brainstem, low levels of binding were found, and strikingly the red nucleus and pons, which are thought to receive glutamatergic projections, had approximately 1/20 the binding observed in cerebral cortex. These results demonstrate that NMDA- and quisqualate-sensitive [3H]glutamate binding are observed in all regions of primate brain, but that in some regions one subtype predominates over the other. In addition, certain areas thought to receive glutamatergic projections have low levels of both types of binding.

Glutamate producing aspartate aminotransferase in glutamatergic perforant path terminals of the rat hippocampus. Cytochemical and lesion studies

The enzyme aspartate aminotransferase was demonstrated cytochemically in the rat hippocampus 4, 7, and 14 days after unilateral entorhinal cortex lesion. At the light microscopic level the enzyme showed a significant activity decrease in the ipsilateral entorhinal terminal field which was similar at all postlesion times investigated. Non-denervated areas, i.e. the inner one-third of the dentate gyrus molecular layer and the radiatum layer of CA2/3, showed an increase of aminotransferase activities. At the electron microscopic level in the entorhinal terminal field of the control (unoperated) side aspartate aminotransferase was localized preferentially in a great number of boutons, containing the cytoplasmic and mitochondrial isoenzymes. Following entorhinal lesion a significant loss of these positively reacting boutons was seen. Most of the degenerating boutons contained reaction product but a small number was negative for aspartate aminotransferase. From 4 to 14 postlesion days the positively reacting boutons of the non-denervated supragranular zone expanded outward into the denervated area according to the known terminal proliferation of the commissural and associational systems. The remaining denervated entorhinal terminal field was reinnervated predominantly by negatively reacting boutons (probably terminal proliferations of septal afferents) and by a small number of positively reacting boutons (probably terminal proliferations of the crossed temporo-dentate pathway). The presence of cytoplasmic aspartate aminotransferase in the terminals of a well-known glutamatergic system is discussed in relation to the possible importance of this enzyme for the production of releasable glutamate.

Zinc-containing neurons in hippocampus and related CNS structures

Recent advances in metallohistochemistry have substantiated the identification of a distinct class of neurons in the brain, the zinc-containing neurons. These neurons sequester peculiar amounts of zinc in their presynaptic boutons and show both high-affinity uptake and calcium- and impulse-dependent release of the cation. It is thought that the zinc may act to stabilize the storage of certain macromolecules in presynaptic vesicles, but there is also mounting evidence that zinc released from vesicles can produce a broad spectrum of neuromodulatory effects upon target cells. Zinc-containing neurons are found predominantly in limbic and cerebrocortical regions, and a possible role of these neurons in the modification of synaptic strength is considered.

GABAergic modulation of yawning behavior

The hypothetical modulation by GABAergic neurons of yawning behavior in the rat was explored with GABA-active drugs. Gamma-acetylenic-GABA, a specific inhibitor of GABA-T, increases yawning frequency when injected at a dose of 7 mg/kg. Baclofen, a GABAB agonist (3 mg/kg), inhibits yawning completely; GABA antagonists, bicuculline and picrotoxin, at subconvulsant doses, also decrease yawning. All drugs were injected intraperitoneally with the exception of apomorphine, which was injected subcutaneously. It is suggested that GABAB receptors play a role in yawning behavior by modulating ACh release, and that GABAA receptors may modify yawning frequency by modulating inhibitory influences on ACh neurons.

The EPSP-spike (E-S) component of long-term potentiation in the rat hippocampal slice is modulated by GABAergic but not cholinergic mechanisms

Long-term potentiation of synaptic efficacy (LTP) can be shown to consist of two components: a synaptic and an excitatory postsynaptic potential (EPSP)-spike (E-S) component. The E-S component is expressed as a leftward shift in the curve relating population spike amplitude as a function of EPSP slope. The participation of cholinergic and GABAergic processes in E-S potentiation was studied in field CA1 of rat hippocampal slices. Atropine, a muscarinic antagonist, did not prevent tetanus-induced E-S potentiation. The cholinergic agonist carbachol and the GABAA antagonist picrotoxin produced a leftward shift in the E-S relation; picrotoxin, but not carbachol, prevented the expression of tetanus-induced E-S potentiation. These observations indicate that an increase in the ratio of evoked excitation to inhibition and/or a reduction in tonic inhibition mediated by the activation of GABAA receptors contribute to E-S potentiation produced by high-frequency stimulation.

Regional and laminar distribution of choline acetyltransferase immunoreactivity in the cat superior colliculus

The pattern of distribution of cholinergic fibers was examined immunohistochemically in the cat superior colliculus by using a monoclonal antibody against choline acetyltransferase (ChAT). In the superficial layers, an obvious immunoreactive zone was found in the rostral two-thirds of the outer portion of the superficial gray layer (SGS), with increasing immunoreactive intensity at the rostral pole of the colliculus. A mesh-like distribution of the immunoreactive fibers was found throughout the deeper portion of this layer with a higher concentration in the caudal levels. In the deeper collicular layers, a number of ChAT-immunoreactive fibers were seen in the outer portion of the intermediate gray layer (SGI) in a patch-like fashion. A few fibers were also immunoreactive in the deeper portion of the SGI and in the medial aspect of the deep gray layer. The density of the immunoreactivity in the deeper layers increased in the caudal levels. After unilateral destruction of the parabigeminal nucleus, the ChAT immunoreactivity was markedly reduced in the rostral aspect of the contralateral SGS, and moderately in the caudal aspect of the ipsilateral SGS.

Direct synaptic contacts of medial septal efferents with somatostatin immunoreactive neurons in the rat hippocampus

Anterogradely labeled projections from the medial septum to hippocampal somatostatin immunoreactive (SOM-i) neurons were studied with double-label immunocytochemistry under light (LM) and electron microscopic (EM) conditions. Medial septal projections were identified after injecting the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) followed by immunohistochemical visualization of PHA-L presynaptic terminal labeling and concurrent immunocytochemical staining of SOM in hippocampal target cell bodies. This double-label procedure yielded blue-black nickel enhanced DAB stained, PHA-L-immunoreactive terminals on light brown SOM-i neurons that were investigated by correlative LM and EM observations. PHA-L-labeled terminal contacts with often basket-like appearance were localized with highest densities on soma and proximal dendrites of SOM-i neurons in stratum oriens of Ammon's horn and hilus of dentate gyrus, and some minor projections to stratum pyramidale and radiatum. Most double-labeled contacts could be identified as symmetric type synapses equally divided over soma and proximal dendrites of several forms of SOM-i neurons. These data indicate monosynaptic regulation of the hippocampal intrinsic SOM system by septal input, which probably represents a peptidergic subpopulation of the hippocampal GABAergic system.

Long-term potentiation: studies in the hippocampal slice

Long-term potentiation (LTP) is an example of activity-dependent plasticity that was discovered in the hippocampal formation. There is growing evidence that LTP not only is a useful model for mnemonic processes, but also may represent the cellular substrate for at least some kinds of learning and memory. The hippocampal slice preparation has proven exceptionally useful in pharmacological studies of possible mechanisms of LTP. A slice remains viable and stable for several hours, and known concentrations of drugs in the bathing medium can be added and then washed out. Drugs can also be applied under visual guidance from micropipettes to discrete neuronal regions, an accomplishment that is aided by the lamellar organization of the hippocampus. Electrical stimulation of the perforant path (PP) in the molecular layer of the dentate gyrus produces a monosynaptic excitatory postsynaptic potential (EPSP) and action potential, which can be recorded extracellularly as a population EPSP and population spike, respectively. Presentation of a high-frequency train (HFT; 100 Hz X 1 s) to the PP results in a long-lasting (greater than 30 min) potentiation of the maximal EPSP slope and of the population spike amplitude. Similarly, exposure of the slice to norepinephrine (e.g. 20 microM for 30 min) results in a long-lasting potentiation (LLP) of both EPSP and population spike (Stanton and Sarvey (1987) Brain Res. Bull., 18: 115). No such LLP was seen in field CA1 following NE application (Stanton and Sarvey (1985) Brain Res., 361: 276). beta-Adrenergic antagonists, such as propranolol, inhibit both LTP and NE-induced LLP in dentate (Stanton and Sarvey, J. Neurosci., 5: 2169 (1985); Stanton and Sarvey (1985) Brain Res., 361: 276). Cyclic AMP levels are increased by either an HFT or NE (Stanton and Sarvey (1985) Brain Res., 358: 343). Thus, NE, acting through a beta-receptor, appears to be both necessary and sufficient to produce long-lasting enhancement of synaptic responses. Finally, inhibitors of protein synthesis, such as emetine, also block both LTP and NE-induced LLP (Stanton and Sarvey, J. Neurosci., (1984) 4: 3080; Stanton and Sarvey (1985) Brain Res., 361: 276). The N-methyl-D-aspartate (NMDA) excitatory amino acid receptor subtype appears to play a role in a number of forms of neuronal plasticity. Bath-application of a 1 microM concentration of the NMDA antagonists D-2-amino-5-phosphonavaleric acid (AVP) or 3-((+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) blocked both LTP and NE-induced LLP in the dentate gyrus.(ABSTRACT TRUNCATED AT 400 WORDS)

Type I and type II pyrethroids increase inhibition in the hippocampal dentate gyrus of the rat

Urethane-anesthetized rats were prepared for stimulation of the perforant path and for recording from the granule cell region of the hippocampal dentate gyrus. Subjects were administered varying doses of allethrin (a prototype type I pyrethroid) or deltamethrin (a prototype type II pyrethroid), and the excitability of the perforant path and granule cells was tested. Both pyrethroids produced a dose-dependent decrease in the responsiveness of granule cells, following stimulation of the perforant path, that lasted up to 100 msec. Analysis suggested that the pyrethroid-induced effects were associated with an increase in interneuronally mediated inhibition. Neither the perforant path axon or terminal nor the granule cell was affected by doses which appeared to affect interneurons. Basal excitability of the granule cells was also decreased by deltamethrin. This effect may have been secondary to an increase in tonic inhibition evoked by the same mechanisms responsible for the increase in phasic inhibition.

Studies into the mechanisms of strain differences in hippocampus-related behaviors

The heterogeneous mouse stocks HS/Ibg and SABRA/HUC and the inbred strains C57BL/6J, CBA/LAC, and BALB/Crgl were employed in an investigation of strain differences in delayed spontaneous alteration (SA) and eight-arm maze performance (EAM). Intact male mice were tested for SA at age 41 days for 2 consecutive days and for EAM at age 50-54 days, under conditions of water deprivation that commenced on day 43. In SA, BALB mice had a lower score than all other strains on day 1 but differed significantly only from SABRA; performance on day 2 was not consistent with that on day 1. In EAM, HS was first, CBA second, C57 third, SABRA fourth, and BALB fifth. HS was superior to the other strains, while BALB fell far below all other strains in both tests. Except for these two strains, the correlation between the two tests across the other strains was low. A study was undertaken to investigate the role of the hippocampus in the deficits in the performance of BALB mice in the two behaviors. Noradrenergic neurons were transplanted to hippocampus or cortex, and cholinergic neurons to their hippocampus. There were no significant differences in performance between the control and transplanted mice. The possibility was discussed that the behavioral differences are the outcome of variability in the neurotransmitters systems of the hippocampus but probably not the noradrenergic system.

The effects of unilateral destruction of fimbria-fornix and supracallosal pathways in the rat

Rats with unilateral lesions of either the supracallosal regions (including the dorsal cingulate cortex) and the fimbria-fornix either on the same (S) or the opposite (O) sides of the brain were studied in a 16-hole open field without pharmacologic intervention and, subsequently, after 0.1 and after 1.0 mg/kg scopolamine HBr. Their performances were compared with those of unoperated control animals subjected to the same testing regime. Certain of their behaviors were compared with those of a larger number of animals with bilateral hippocampal destruction (and their control groups) from prior studies. Unilateral lesions of fimbria-fornix and supracallosal afferents to the hippocampal formation produced a decrease in hole poking activity relative to control animals. A further decrease in hole-poking behavior, coupled with increased locomotion, was observed in rats with fimbria-fornix and cingulate cortex lesions on opposite sides of the brain (group O). The smaller dose of scopolamine accentuated these effects. Indeed, the behavior of group O after scopolamine treatment was similar to animals with large bilateral hippocampal lesions. The large dose of scopolamine induced stereotyped rearing or hole poking in the brain-damaged animals but not in the control group. These findings suggest that both the fimbria-fornix and the supracallosal pathway is necessary for normal hippocampal function and that the behavioral deficit is greater when these structures are damaged on the opposite sides of the brain.

Regional calcium accumulation and kainic acid (KA)-induced limbic seizure status in rats

The sites of calcium accumulation were studied by 45Ca autoradiography during kainic acid (KA)-induced limbic seizure in rats. Two hours after KA injection into unilateral amygdala, calcium accumulated in CA3 of the hippocampus, lateral septal nucleus and thalamic reticular nucleus on KA-injected side. Those sites coincided with the sites where neuronal cell damage appeared 4 h after KA injection. These results suggested that regional calcium accumulation might be responsible for neuronal cell loss induced by seizures.

Age-related alterations in noradrenergic input to the hippocampal formation: structural and functional studies in intraocular transplants

Intrinsic versus extrinsic determinants of age-related alterations in hippocampal noradrenergic transmission were investigated using intraocular allografts in rats. Three groups of animals were examined: young hippocampal transplants in young hosts, old transplants in old hosts and young transplants in old hosts. Postsynaptic sensitivity to noradrenaline (NA) was measured by extracellular recordings of spontaneous activity and superfusion with known concentrations of catecholamines in the anterior chamber of the eye. Hill plots demonstrated that the dose-response relationships of NA-induced depressions were linear and parallel in the 3 groups. Aged hippocampal grafts displayed a highly significant subsensitivity to NA of one order of magnitude. The EC50 for this group was 203.1 microM as compared to 29.2 in young grafts. Young intraocular grafts in old hosts responded similarly to transplants in young hosts, with an EC50 of 32.4 microM for the depressant actions of NA. Collaterals of the host iris sympathetic ground plexus invaded the hippocampal grafts. The density of this noradrenergic innervation was estimated by immunohistochemistry for tyrosine hydroxylase. A slightly increased density and fluorescence intensity of the noradrenergic fibers were observed in the old transplants as compared to the young transplants in young and old hosts. This was correlated with a significantly (P less than 0.01) increased content of NA in old transplants, as measured with high performance liquid chromatography. The old transplants also contained a large number of autofluorescent lipofuchsin granules, which were absent in the young transplants, regardless of the recipient age. Taken together, these results suggest the existence of alterations in pre- as well as postsynaptic noradrenergic mechanisms in the aging hippocampus. These changes were dependent on transplant age rather than host age, thus suggesting an involvement of intrinsic rather than extrinsic determinants in this model system.

Muscarinic inhibition of endogenous glutamate release from rat hippocampus synaptosomes

The effects of acetylcholine (ACh) on the depolarization-evoked release of endogenous glutamic acid (Glu) have been studied using synaptosomes prepared from rat hippocampus and depolarized in superfusion with 15 mM KCl. Acetylcholine inhibited Glu release in a concentration-dependent way. The natural agonist was particularly effective causing 50% inhibition of Glu release at 10 microM in the absence of acetylcholinesterase (AChE) inhibitors. The inhibitory effect of ACh on the K+-evoked release of Glu was antagonized by the selective muscarinic receptor antagonist atropine but not by the nicotinic receptor antagonist mecamylamine. The data represent the first demonstration that muscarinic receptors located on Glu axon terminals in rat hippocampus may modulate the release of Glu.

Intracellular dyes mask immunoreactivity of hippocampal interneurons

The results of several studies have suggested that local circuit neurons, or interneurons, of area CA1 of hippocampus use gamma-aminobutyric acid (GABA) as their neurotransmitter. However, when these cells were labelled by intracellular dye injection, and examined immunocytochemically with antisera raised against GABA, none of the interneurons were immunoreactive. Numerous non-injected interneurons in the same tissue section were clearly immunoreactive. These results suggest that intracellular dyes interfere with immunocytochemical staining of hippocampal interneurons.

Glutamate-like immunoreactivity revealed in rat olfactory bulb, hippocampus and cerebellum by monoclonal antibody and sensitive staining method

Although there is good evidence favoring L-glutamate as a major excitatory amino acid transmitter, relatively little is known about the distribution of nerve terminals using this substance. A method visualizing glutamate-like immunoreactivity at the light microscopic level by means of a monoclonal antibody, mAb 2D7, is described. --The antigen used for immunization was a glutaraldehyde-linked glutamate-BSA conjugate, and hybridomas were differentially screened by ELISA for production of antibodies recognizing glutamate- but not aspartate-BSA. The crossreactivity of 'anti-glutamate' mAb 2D7 as estimated in absorption tests was low even with conjugates closely related to glutamate-BSA.--Semithin sections from rapidly perfusion-fixed, plastic-embedded rat brain tissues were etched and stained by a combination of the peroxidase-antiperoxidase method and silver enhancement of the diaminobenzidine reaction product. Only this amongst several other immunohistochemical methods tried produced labeling patterns which showed terminal-like elements in brain regions such as olfactory bulb, hippocampus and cerebellum, and which were mostly consistent with already available information on systems using glutamate as neurotransmitter. Particularly striking was the staining of elements reminiscent of mossy fiber terminals in hippocampus and cerebellum as well as of cerebellar parallel fiber terminals.

Laminar distribution and morphology of gamma-aminobutyric acid (GABA)-immunoreactive neurons in the medial and dorsomedial areas of the cerebral cortex of the lizard Podarcis hispanica

The morphology and laminar distribution of immunolabeled neurons in the medial and dorsomedial telencephalic cortices of the lizard Podarcis hispanica were examined in vibratome sections after preembedding gamma-aminobutyric acid (GABA)-immunocytochemistry. In both cortical areas and at all rostrocaudal levels, GABA-immunoreactive neurons were found in all cortical layers, with the largest number (74%) of GABA-positive cells in layer 3. GABA-positive neurons were classified into pyramidlike, vertical-fusiform, multipolar, and horizontal neurons. Cells that could be so classified were counted in each cortical lamina. In the medial cortex, multipolar and horizontal-bipolar cells dominated layer 1. Layer 2 displayed mainly horizontal and pyramidlike cells at its outer margin and pyramidlike cells at its inner margin. In layer 3, horizontal cells were the prevalent group. In the dorsomedial cortex, layer 1 mainly contained small multipolar neurons (35% of layer-1 cells) in its outer third and vertical-fusiform neurons (37% of layer-1 cells) in its inner two thirds. In layer 2, 47% of the few GABA-positive perikarya were pyramidlike. The largest population of neurons in layer 3 was that formed by multipolar cells (45% of layer-3 cells). Ultrastructural examination revealed that GABA-immunoreactive neurons possessed indented euchromatic nuclei with a central nucleolus. Their cytoplasm contained numerous mitochondria and a very well-developed granular endoplasmic reticulum. Their somata were contacted by numerous unstained boutons making asymmetric contacts and by a few symmetric synapses of GABA-positive terminals. Dendrites of GABA-immunoreactive cells were thin, with irregular outlines, and generally aspinous. Like the somata, dendrites were contacted by many unstained asymmetric synapses. Some dendritic profiles also received symmetric contacts from GABA-positive boutons. GABA-positive terminal-like puncta were found throughout the layers, with a maximal concentration in layer 2. Electron microscopy confirmed that nearly all of the puncta represent GABA-positive terminal boutons. Comparison of GABA-immunoreactive cells in Podarcis with those found in the mammalian hippocampus suggests that these cells may be inhibitory neurons, as in the hippocampus of mammals.

Selective neuronal death after transient forebrain ischemia in the Mongolian gerbil: a silver impregnation study

An important feature of ischemic brain damage is the exceptional vulnerability of specific neuronal populations and the relative resistance of others. Silver impregnation was used to delineate the extent and time-course of neuronal degeneration produced by 5 min of complete forebrain ischemia in the Mongolian gerbil. Lesions were confined to four brain regions: (1) hippocampal areas CA1, CA2-CA3a and CA4; (2) the dorsomedial portion of the lateral septal nucleus; (3) the dorsolateral portion of the striatum; and (4) the somatosensory neocortex. The ischemic lesion evolved with time in all four regions, but at different rates. Somatic argyrophilia developed rapidly in the striatum and hippocampal area CA4 (maximal in 24 h or less), at intermediate rates in the somatosensory neocortex, hippocampal areas CA1a and CA2-CA3a and the lateral septal nucleus (maximal in 2 days), and slowly in hippocampal area CA1b (maximal in 3 days). These results emphasize that the extent and rate of neuronal degeneration can vary even within a presumably homogeneous neuronal population, as evidenced by the different results in areas CA1a and CA1b. Similar results were obtained from analysis of brain sections stained with Cresyl Violet, hematoxylin-eosin or hematoxylin-eosin/Luxol Fast Blue. Terminal-like silver granules were observed in the projection fields of degenerated neurons. They also appeared, however, in the perforant path terminal zone of the hippocampal dentate molecular layer 1-2 days after transient ischemia and in stratum oriens and stratum radiatum of area CA1b prior to somatic degeneration. These granular deposits could not be clearly related to the degeneration of neuronal somata. Novel findings of this study include the degeneration of some dentate basket cells and lateral septal neurons and the appearance of terminal-like argyrophilia in the hippocampal formation without any obvious relation to somatic degeneration. Some of our results lend support to the hypothesis that ischemic neuronal cell death constitutes an excitotoxic process. Other results, however, suggest that the selective vulnerability of neurons to transient ischemia must involve factors beyond excitotoxicity.

The coupling of neurotransmitter receptors to ion channels in the brain

Recent studies on the action of neurotransmitters on hippocampal pyramidal cells indicate that different neurotransmitter receptors that use either the same or different coupling mechanisms converge onto the same ion channel. Conversely, virtually all of the neurotransmitters act on at least two distinct receptor subtypes coupled to different ion channels on the same cell. The existence of both convergence and divergence in the action of neurotransmitters results in a remarkable diversity in neuronal signaling.

Immunocytochemical localization of glutamate decarboxylase in the rat basolateral amygdaloid nucleus, with special reference to GABAergic innervation of amygdalostriatal projection neurons

Glutamate decarboxylase (GAD) immunohistochemistry was employed at the light and electron microscopic levels to localize GABAergic structures in the basolateral amygdaloid nucleus (BL). The GAD-immunoreactive (GAD-IR) staining pattern consisted of punctate structures and a morphologically diverse group of GAD-IR neurons. At the electron microscopic level many of these punctate structures were found to make symmetrical synaptic contacts with cell bodies as well as distal parts of unlabeled, presumably projection and nonprojection, neurons. In addition, GAD-immunoreactive neurons were identified in the BL, and they had the ultrastructural characteristics of local circuit or intrinsic neurons and were not retrogradely labeled with HRP following ventral striatal injections. Some of these GAD-immunoreactive neurons were contacted by GABAergic boutons, forming symmetrical synaptic contacts. GABAergic innervation of amygdaloid projection neurons in the BL was identified by combining GAD immunohistochemistry with Golgi impregnation and retrograde tracing of horseradish peroxidase (HRP) following injections of the tracer in the olfactory-tubercle-related parts of the ventral striatum. Amygdalostriatal projection neurons in the BL were observed to be in continuity with neurons in the piriform cortex which project to the ventral striatum. The results provide direct evidence for the presence of GAD-IR boutons in the BL making synaptic contacts with identified amygdalostriatal projection neurons. The present study provides direct anatomical evidence for the physiological observation that GABA exhibits a powerful regulation of the amygdaloid projection neurons in the BL and lends further support to the concept of a corticallike functional organization of the basolateral amygdala.

Effects of dithiothreitol, a sulfhydryl reducing agent, on CA1 pyramidal cells of the guinea pig hippocampus in vitro

The radioprotectant, dithiothreitol (DTT) has been shown to increase excitability in the hippocampal slice preparation. In the present study, intracellular recording techniques were used to further examine the actions of DTT. Electrophysiological recordings from CA1 pyramidal cells were obtained prior to, during and after DTT exposure. DTT caused a small depolarization without altering membrane resistance. DTT induced spontaneous firing and occasional burst firing in normally silent neurons. These effects were accompanied by a reduction in spike frequency adaptation but no change in the afterhyperpolarization following a train of action potentials. Following DTT exposure, orthodromic stimulation produced multiple firing. Subthreshold excitatory postsynaptic potentials (EPSPs) were significantly prolonged. Isolating the CA1 subfield, attenuated the prolongation of the EPSP by DTT. Recurrent inhibitory postsynaptic potentials were unaffected by DTT. The actions of DTT are likely to result from DTT-induced reduction of disulfide bonds since the reduced form of DTT does not cause a similar hyperexcitability.

Inhibitory action of serotonin in CA1 hippocampal neurons in vitro

The ionic mechanism of the inhibitory effect of serotonin was investigated in vitro in the CA1 region of the rat hippocampus by extra- and intracellular recordings. Local or bath applications of serotonin induced a long-lasting reduction of extracellularly recorded synaptic potentials and orthodromic population spikes without affecting the afferent volley or the antidromic population spike. Serotonin can also reduce the frequency of occurrence of spontaneous excitatory and inhibitory postsynaptic potentials without any reduction of input resistance of the pyramidal neuron. During the response to serotonin, the conductance increase evoked by GABA, the inhibitory neurotransmitter, was not changed. A direct postsynaptic effect of serotonin was demonstrated: local or bath applications of serotonin induced a tetrodotoxin-resistant hyperpolarization and conductance increase. The conductance change was not reduced by manual clamp of the neurons to the control resting membrane potential; therefore, a possible involvement of the sodium-potassium electrogenic pump is unlikely. When neurons were loaded with chloride, serotonin could still induce a hyperpolarization with an apparent reversal more negative than the resting membrane potential. When neurons were loaded with caesium, the hyperpolarization and the conductance increase evoked by serotonin were blocked. It is therefore concluded that serotonin increases potassium permeability. Similar effects were induced by a 5-HT1A ligand. The slow after hyperpolarization was reduced by serotonin; the calcium spike was reduced at the same time. In caesium loaded neurons, the spike duration was not modified by serotonin. In the presence of extracellular caesium (4-5 mM), the serotonin-induced hyperpolarization and the conductance change were blocked, but the effect of serotonin on calcium spikes persisted. Tetraethylammonium (5-10 mM) or 4-aminopyridine (0.5 mM) had no effect on the response to serotonin. These data indicate that serotonin has a postsynaptic inhibitory action by an activating potassium conductance. The possibility of a regulation of calcium currents is discussed. The possible role of serotonin on intrinsic synaptic transmission is also discussed.

Specific effects of punishment on biogenic monoamine turnover in discrete rat brain regions

Specific effects of punishment on the turnover rates of norepinephrine, dopamine and serotonin (5-HT) in brain regions were investigated in rats exposed to punishment. Two yoked controls were also used in an attempt to separate the non-specific effects of response rate, reinforcement density and direct effects of punisher (foot shock). Punished and unpunished littermate rats had similar response rates, and the reinforcement density was almost identical for both groups. A third group (yoked-shock rats) received food and shock independent of responding whenever these were given to the punished rats. When compared to the unpunished rats, changes in the monoamine turnover rates resulting from the punishment were similar to the effects of yoked-shock with respect to the direction of action in most cases (13 out of 17 changes). These changes may be related to non-specific effects of the shock. Four changes by the punishment were determined to be specific effects of the punishment since the yoked-shock had no effect or changed the turnover to the opposite direction. Among these, increase in 5-HT turnover rate in the frontal cortex (greater than 7-fold) was the largest change. These results and reported effects of drugs which act on serotonergic systems on the punished behavior suggest that the increase in 5-HT neuronal activity in the frontal cortex is involved in the behavioral suppression induced by the punishment.

Long-spined polymorphic neurons of the medial cortex of lizards: a Golgi, Timm, and electron-microscopic study

The morphology, ultrastructure, and principal synaptic input of long-spined neurons located in the inner plexiform layer of the medial cortex in three related species of lizards is described. Golgi impregnations have been used to define the external morphology of these neurons and their axonal trajectories. Their most striking characteristic is the presence of very long spines or "microdendrites" especially abundant on the distal dendritic segments. Axons have ascendent trajectories, pass through the cell layer, and ramify in the outer plexiform layer. Combined Golgi-electron microscopy as well as standard electron microscopy permitted the definition of the ultrastructure of these neurons. Timm and sulfide-osmium methods permitted the detection and characterization of their principal synaptic input (i.e., zinc-containing boutons). Gamma aminobutyric acid (GABA)-immunostained sections in one of the species studied allowed the identification of GABA-immunoreactive somata which had the same morphology and ultrastructure as long-spined neurons; these GABA-immunoreactive somata and their processes were found in the same location as long-spined neurons. This suggests that at least some long-spined polymorphic neurons are GABA-ergic and presumably inhibitory. Finally, the neurobiological significance of these long-spined neurons is discussed and briefly compared with that of similar neurons of the hilus of the fascia dentata of the rat.

Muscarinic inhibitory effect in the guinea pig dentate gyrus in vitro

Various drugs known to affect synaptic transmission were tested for their effects on extracellular field potentials evoked in the dentate granule cell layer of guinea pig hippocampal slices. Presumptive commissural (CF) and perforant path (PP) fibers were stimulated electrically by two separate pairs of stimulating electrodes. CF stimulation elicited a field potential with a current sink and PP stimulation with a current source in the cell layer. Bath-applied carbachol (0.5-10 microM), in an atropine sensitive manner, depressed the CF-evoked field potential, but left the PP-evoked field potential unchanged. Further pharmacological testing revealed that this effect is likely to be mediated by inhibitory neurons in the dentate hilus.

Passive avoidance learning produces focal elevation of bursting activity in the chick brain: amnesia abolishes the increase

Presentation of a bright bead to day-old chicks (Gallus domesticus: Ross 1 Chunky Chicks) elicits spontaneous pecking. If the bead is coated with an aversive substance (e.g., methylanthranilate), they will avoid similar beads subsequently; if it is coated with water, they peck avidly on re-presentation. Formation of a memory for this one-trial passive avoidance task is unaffected by subconvulsive transcranial electroshock when applied 10 min after training in 60% of birds, whereas "immediate" post-training electroshock renders 63% of chicks amnesic. Memory formation and retention is associated with a large bilateral enhancement in trained over control chicks (320 and 350% in left and right hemispheres, respectively; p less than 0.001) of a particular spontaneous multi-unit activity firing pattern, that is, short-duration (15-40 ms) bursts of large-amplitude (greater than or equal to 200 microV, 450 microV max p-p), high-frequency (400-450 Hz) spiking in anesthetized chicks. This effect is observed in data lumped from 1-13 h after training and is restricted to the intermediate medial hyperstriatum ventrale. When chicks are rendered amnesic by electroshock immediately following training, there is a complete abolition of this increase in burst firing; in those chicks where this treatment fails to elicit amnesia, the increase in bursting is still observed. In birds in which the shock is delayed and memory formation occurs, the increase in bursting activity is maintained; however, if the delayed shock produces apparent amnesia, then the increase is once again abolished. The electroshock had no effect on bursting per se in untrained chicks. There was no significant difference in tonic spiking between the chicks. A marked increase in the occurrence of bursting epochs in the IMHV of anesthetized chicks following passive avoidance training is therefore closely associated with memory formation, but not with the nonspecific concomitants of the training procedure.

Local cerebral glucose utilization in the Ammon's horn and dentate gyrus of the rat brain

The local cerebral glucose utilization (LCGU) was measured in different regions and layers of the Ammon's horn and dentate gyrus in the conscious rat. The LCGU was determined by quantitative [14C]2-deoxyglucose autoradiography using a computerized image processing system. In the hippocampus, the various regions and layers exhibited different glucose consumptions, the lowest values being found in the alveus and the highest ones in the lacunosum-molecular layers of the sectors of the Ammon's horn and the molecular layer of the dentate gyrus' external limb. Additionally, in many layers, the LCGU values of the left hemispheres were found to be higher compared with the right hemispheres. The analysis of LCGU changes in rostrocaudal direction revealed, that in sector 1 of Ammon's horn and in the dentate gyrus the glucose consumption decreased from rostral to caudal levels, whereas in sector 3 of Ammon's horn an increase was found.

Pitfalls in the use of brain slices

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

Vasoactive intestinal peptide increases acetylcholine synthesis by rat hippocampal slices

The purpose of this study was to determine whether vasoactive intestinal peptide (VIP) might have a presynaptic modulatory effect at cholinergic terminals in the rat hippocampal formation. The exposure of rat hippocampal slices to VIP increased [3H]acetylcholine ([3H]ACh) synthesis from the precursor [3H]choline when tissue was incubated in normal or in high K+ medium; the maximal effect was apparent at 10(-8) M VIP and 10(-7) M VIP, respectively. Also, 10(-7) M VIP increased the activity of choline acetyltransferase (ChAT) in a hippocampal homogenate system. The increased synthesis by hippocampal slices was not the result of a VIP-induced alteration in either the basal release of ACh or the uptake of choline via the high-affinity uptake system. The increase in ACh synthesis induced by VIP in hippocampal slices was not associated with either adenylate cyclase or protein kinase C second messenger systems. There was no correlation between the effect of VIP on cyclic AMP production with that on ACh synthesis; also, forskolin, an activator of adenylate cyclase that increased cyclic AMP production 3.5-fold, did not mimic the effect of VIP on ACh synthesis. Similarly, there was no effect of the protein kinase C activator, phorbol myristate acetate, on ACh synthesis in hippocampal slices. However, the effect of VIP to increase ACh synthesis was not evident in the absence of extracellular calcium, suggesting that the effect of VIP is mediated by a calcium-requiring mechanism. The results suggest that, in the rat hippocampus, VIP has a presynaptic action at cholinergic terminals that results in enhanced synthesis of ACh, possibly by an action that alters ChAT activity.

Water-maze learning and effects of cholinergic drugs in mouse strains with high and low hippocampal pyramidal cell counts

Morphological differences have been found in inbred strains of mice in the number and volume of pyramidal cells in Ammon's horn of the hippocampus. Among the mouse strains surveyed, NZB/BINJ (NZB) and C57BL/10J (B10) are most divergent in both total volume and total number of neurons. These genetically derived differences were exploited to determine hippocampal involvement in the acquisition of a spatial water maze. Genetic differences in hippocampal cell number were related to the acquisition of this spatial task. Mice with small numbers of hippocampal pyramidal cells, the B10 strain, acquired a water-maze task more slowly than either NZB mice or (NZBxNZW) F1 (NZBWF) animals. In addition, strain differences in responsivity to cholinergic manipulations were found. B10 mice were more sensitive than NZB or NZBWF mice to both the disruptive effects of scopolamine and the facilitory effects of physostigmine on swim maze learning. Although other inherited differences undoubtedly exist between these strains as is apparent in other mouse lines, these data suggest a prominent role for the hippocampus in the learning of spatially oriented behavior. Furthermore, this behavior appears to be responsive to cholinergic manipulations.

Increased inhibition in dentate gyrus granule cells following exposure to GABA-uptake blockers

Rats anesthetized with urethane had stimulating and recording electrodes placed in the perforant pathway and in the dentate gyrus. They were then exposed to increasing doses of either the vehicle control dimethylsulfoxide (DMSO) or one of two gamma-aminobutyric acid (GABA)-uptake blockers (SKF-100330A or SKF-89976A). Analysis of evoked field potentials from dentate granule cells indicated that the only effect of the GABA uptake blockers was to increase the threshold for evoking the field population spikes (PS). No other measure of excitatory postsynaptic potentials (EPSPs) or PS's were significantly affected. The lack of effect on evoked EPSP by these drugs suggests no direct effect on transmitter release at this synapse, while the increase in PS threshold suggests a slight decrease in granule cell excitability. The effects of the two GABA-uptake blockers on synaptically mediated facilitation and inhibition was tested by using paired-pulse paradigms. Both GABA-uptake blockers increased early GABA-mediated inhibition to a greater extent than they reduced synaptically mediated facilitation. Neither GABA uptake blocker appeared to effect the late inhibition seen at paired-pulse intervals of 400-1000 ms which is presumably associated with calcium-activated increases in potassium conductance. These effects on granule cell responses occurred at doses found previously not to be associated with side effects and yet to be anticonvulsant in unanesthetized rats. These data confirm in vivo that SKF-100330A and SKF-89976A increase GABA-mediated inhibition. The effect on granule cell excitability and late inhibition are minimal. Although facilitation was reduced by exposure to these drugs, the mechanism of this reduction (direct or prolongation of early inhibition) cannot be determined.

Hippocampal signal transmission to the pedunculopontine nucleus and its regulation by dopamine D2 receptors in the nucleus accumbens: an electrophysiological and behavioural study

The integrative role of the nucleus accumbens and subpallidal area in relaying hippocampal signals to the mesencephalic locomotor region in the brainstem was investigated electrophysiologically in urethan-anaesthetized rats. A behavioural study of the functional connections was also performed in freely moving rats. In the electrophysiological experiments, subpallidal output neurons to the pedunculopontine nucleus and the adjacent ventral gray were first identified by their antidromic responses to electrical stimulation of the pedunculopontine nucleus. Hippocampal stimulation was then shown to inhibit orthodromically some of these subpallidal neurons. The inhibitory response was attenuated following microinjection of a dopamine D2 agonist (LY 171555), but not a D1 agonist (SKF 38393), into the accumbens. This suggests that signal transmission from the hippocampus to the subpallidal output neurons to the pedunculopontine nucleus is modulated by a D2 receptor-mediated mechanism in the nucleus accumbens. Injections of N-methyl-D-aspartate into the ventral subiculum of the hippocampus resulted in a threefold increase in locomotor responses. Injection of a D2 agonist into the accumbens reduced the hyperkinetic response dose-dependently and suggests that D2 receptors regulate locomotor responses initiated by the hippocampal-accumbens pathway. Injection of nipecotic acid, a GABA uptake inhibitor, into the subpallidal area or of procaine, a neural transmission blocker, into the region of the pedunculopontine nucleus, also reduced significantly the hippocampal-induced hyperkinetic response. These results provide evidence of limbic (e.g. hippocampus) influences on locomotor activity by way of nucleus accumbens-subpallidal-pedunculopontine nucleus connections which may contribute to adaptive behaviour. Signal transmission from the hippocampus may be regulated by a dopamine D2 receptor mechanism in the accumbens, presumably mediated by the converging mesolimbic dopaminergic input from the ventral tegmental area.

Peroxide alters neuronal excitability in the CA1 region of guinea-pig hippocampus in vitro

Effects of peroxidative damage on neuronal excitability were investigated with electrophysiological techniques in CA1 pyramidal cells of the hippocampal slice preparation. Hydrogen peroxide alone or combined with ferrous ions (peroxide/iron) is likely to produce hydroxyl free radicals through the Fenton reaction. Intracellularly recorded excitatory postsynaptic potentials and inhibitory postsynaptic potentials were significantly reduced by exposure to peroxide, while responses to iontophoretically applied GABA and glutamate were unaffected. These results suggest that peroxide has presynaptic actions. Peroxide and peroxide/iron also increased frequency adaptation; after exposure, neurons fired fewer action potentials at a lower frequency in response to the same depolarizing current step. A voltage clamp analysis revealed that the potassium currents were unaffected by peroxide/iron. Calcium current was not obviously altered by exposure to peroxide. Sodium spike threshold was also unaffected. Calcium spike threshold was significantly increased by peroxide. This action of peroxide may underlie its presynaptic actions. It is concluded that peroxide produces both presynaptic and postsynaptic damage. This damage is likely to result from the production of free radicals which have been postulated to underlie a number of pathological states.