Antagonism of histamine H4 receptors exacerbates clinical and pathological signs of experimental autoimmune encephalomyelitis

BACKGROUND AND PURPOSE

The histamine H4 receptor has a primary role in inflammatory functions, making it an attractive target for the treatment of asthma and refractory inflammation. These observations suggested a facilitating action on autoimmune diseases. Here we have assessed the role of H4 receptors in experimental autoimmune encephalomyelitis (EAE) a model of multiple sclerosis (MS).

EXPERIMENTAL APPROACH

We induced EAE with myelin oligodendrocyte glycoprotein (MOG35-55 ) in C57BL/6 female mice as a model of MS. The histamine H4 receptor antagonist 5-chloro-2-[(4-methylpiperazin-1-yl)carbonyl]-1H-indole (JNJ7777120) was injected i.p. daily starting at day 10 post-immunization (D10 p.i.). Disease severity was monitored by clinical and histopathological evaluation of inflammatory cells infiltrating into the spinal cord, anti-MOG35-55 antibody production, assay of T-cell proliferation by [(3) H]-thymidine incorporation, mononucleate cell phenotype by flow cytometry, cytokine production by elisa assay and transcription factor quantification of mRNA expression.

KEY RESULTS

Treatment with JNJ7777120 exacerbated EAE, increased inflammation and demyelination in the spinal cord of EAE mice and increased IFN-γ expression in lymph nodes, whereas it suppressed IL-4 and IL-10, and augmented expression of the transcription factors Tbet, FOXP3 and IL-17 mRNA in lymphocytes. JNJ7777120 did not affect proliferation of anti-MOG35-55 T-cells, anti-MOG35-55 antibody production or mononucleate cell phenotype.

CONCLUSIONS AND IMPLICATIONS

H4 receptor blockade was detrimental in EAE. Given the interest in the development of H4 receptor antagonists as anti-inflammatory compounds, it is important to understand the role of H4 receptors in immune diseases to anticipate clinical benefits and also predict possible detrimental effects.

Activation of cannabinoid receptors prevents antigen-induced asthma-like reaction in guinea pigs

In this study we evaluated the effects of the CB₁/CB₂ cannabinoid receptor agonist CP55, 940 (CP) on antigen-induced asthma-like reaction in sensitized guinea pigs and we tested the ability of the specific CB2 receptor antagonist SR144528 (SR) and CB1 receptor antagonist AM251 (AM) to interfere with the effects of CP. Ovalbumin-sensitized guinea pigs placed in a respiratory chamber were challenged with the antigen given by aerosol. CP (0.4 mg/kg b.wt.) was given i.p. 3 hrs before ovalbumin challenge. Sixty minutes before CP administration, some animals were treated i.p. with either AM, or SR, or both (0.1 mg/kg b.wt.). Respiratory parameters were recorded and quantified. Lung tissue specimens were then taken for histopathological and morphometric analyses and for eosinophilic major basic protein immunohistochemistry. Moreover, myeloperoxidase activity, 8-hydroxy-2-deoxyguanosine, cyclic adenosine monophosphate (cAMP) and guanosine monophosphate (cGMP) levels, and CB₁ and CB₂ receptor protein expression by Western blotting were evaluated in lung tissue extracts. In the bronchoalveolar lavage fluid, the levels of prostaglandin D₂ and tumour necrosis factor-α TNF-α were measured. Ovalbumin challenge caused marked abnormalities in the respiratory, morphological and biochemical parameters assayed. Treatment with CP significantly reduced these abnormalities. Pre-treatment with SR, AM or both reverted the protective effects of CP, indicating that both CB₁ and CB₂ receptors are involved in lung protection. The noted treatments did not change the expression of cannabinoid receptor proteins, as shown by Western blotting. These findings suggest that targeting cannabinoid receptors could be a novel preventative therapeutic strategy in asthmatic patients.

Histamine H3 receptor-mediated impairment of contextual fear conditioning and in-vivo inhibition of cholinergic transmission in the rat basolateral amygdala

We investigated the effects of agents acting at histamine receptors on both, spontaneous release of ACh from the basolateral amygdala (BLA) of freely moving rats, and fear conditioning. Extensive evidence suggests that the effects of histamine on cognition might be explained by the modulation of cholinergic systems. Using the microdialysis technique in freely moving rats, we demonstrated that perfusion of the BLA with histaminergic compounds modulates the spontaneous release of ACh. The addition of 100 mm KCl to the perfusion medium strongly stimulated ACh release, whereas, 0.5 microm tetrodotoxin (TTX) inhibited spontaneous ACh release by more than 50%. Histaminergic H3 antagonists (ciproxifan, clobenpropit and thioperamide), directly administered to the BLA, decreased ACh spontaneous release, an effect fully antagonized by the simultaneous perfusion of the BLA with cimetidine, an H2 antagonist. Local administration of cimetidine alone increased ACh spontaneous release slightly, but significantly. Conversely, the administration of H1 antagonists failed to alter ACh spontaneous release. Rats receiving intra-BLA, bilateral injections of the H3 antagonists at doses similar to those inhibiting ACh spontaneous release, immediately after contextual fear conditioning, showed memory consolidation impairment of contextual fear conditioning. Post-training, bilateral injections of 50 microg scopolamine also had an adverse effect on memory retention. These observations provide the first evidence that histamine receptors are involved in the modulation of cholinergic tone in the amygdala and in the consolidation of fear conditioning.

Interactions between histaminergic and cholinergic systems in learning and memory

The aim of this review is to survey biochemical, electrophysiological and behavioral evidence of the interactions between the cholinergic and histaminergic systems and evaluate their possible involvement in cognitive processes. The cholinergic system has long been implicated in cognition, and there is a plethora of data showing that cholinergic deficits parallel cognitive impairments in animal models and those accompanying neurodegenerative diseases or normal aging in humans. Several other neurotransmitters, though, are clearly implicated in cognitive processes and interact with the cholinergic system. The neuromodulatory effect that histamine exerts on acetylcholine release is complex and multifarious. There is clear evidence indicating that histamine controls the release of central acetylcholine (ACh) locally in the cortex and amygdala, and activating cholinergic neurones in the nucleus basalis magnocellularis (NBM) and the medial septal area-diagonal band that project to the cortex and to the hippocampus, respectively. Extensive experimental evidence supports the involvement of histamine in learning and memory and the procognitive effects of H(3) receptor antagonists. However, any attempt to strictly correlate cholinergic/histaminergic interactions with behavioral outcomes without taking into account the contribution of other neurotransmitter systems is illegitimate. Our understanding of the role of histamine in learning and memory is still at its dawn, but progresses are being made to the point of suggesting potential treatment strategies that may produce beneficial effects on neurodegenerative disorders associated with impaired cholinergic function.

Cortical acetylcholine release elicited by stimulation of histamine H1 receptors in the nucleus basalis magnocellularis: a dual-probe microdialysis study in the freely moving rat

Perfusion of the nucleus basalis magnocellularis (NBM) with histamine agonists and antagonists modulates the spontaneous release of cortical acetylcholine (ACh) in freely moving rats. Perfusion of the NBM with Ringer solution containing 100 mM K+ strongly stimulated the spontaneous release of cortical ACh in freely moving rats, whereas perfusion with 1 microM tetrodotoxin reduced cortical ACh spontaneous release by more than 50%. Administration of histamine to the NBM concentration-dependently increased the spontaneous release of cortical ACh. Administration of H1 (methylhistaprodifen) but not H2 (dimaprit) or H3 (R-alpha-methylhistamine) receptor agonists to the NBM mimicked the effect of histamine. Perfusion of the NBM with either H1 (mepyramine or triprolidine) or H2 (cimetidine) receptor antagonists failed to alter ACh spontaneous release from the cortex, however, H1 but not H2 receptor antagonists antagonized the releases of cortical ACh elicited by histamine and methylhistaprodifen. Local administration of H3 receptor antagonists (clobenpropit and thioperamide) to the NBM increased the spontaneous release of ACh from the cortex; this effect was antagonized by H1 receptor antagonism. Conversely local administration of MK-801, a noncompetitive receptor antagonist of the N-methyl-D-aspartate receptor, to the NBM failed to alter ACh spontaneous release from the cortex and to antagonize ACh release elicited by histamine. This study demonstrates that activation of histamine H1 receptors in the NBM increases ACh spontaneous release from the cortex.

Central histaminergic system and cognition

The neurotransmitter histamine is contained within neurons clustered in the tuberomammillary nuclei of the hypothalamus. These cells give rise to widespread projections extending through the basal forebrain to the cerebral cortex, as well as to the thalamus and pontomesencephalic tegmentum. These morphological features suggest that the histaminergic system acts as a regulatory center for whole-brain activity. Indeed, this amine is involved in the regulation of numerous physiological functions and behaviors, including learning and memory, as indicated by extensive research reviewed in this paper. Histamine effects on cognition might be explained by the modulation of the cholinergic system. However, interactions of histamine with any transmitter system, and/or a putative intrinsic procognitive role cannot be excluded. Furthermore, although experimental evidence indicates that attention-deficit hyperactivity disorder symptoms arise from impaired dopaminergic and noradrenergic transmission, recent research suggests that histamine is also involved. The possible relevance of histamine in disorders such as age-related memory deficits, Alzheimer's disease and attention-deficit hyperactivity disorder is worth of consideration, and awaits validation with clinical trials that will prove the beneficial effects of histaminergic drugs in the treatment of these diseases.

Cognitive implications for H3 and 5-HT3 receptor modulation of cortical cholinergic function: a parallel story

Evidence reviewed in this paper suggests that interactions of histamine with H3 receptors decrease the cholinergic tone in the frontal cortex and the hippocampus. These interactions may be important in learning and memory. Both H3 and 5-HT3 receptors represent targets for pharmacological intervention by exogenous compounds acting as antagonists. Thus, the use of compounds with such properties as either thioperamide (H3 receptor antagonist) or ondansetron (5-HT3 antagonist) may represent a potential therapy to correct the deficits resulting from cholinergic hypofunction.

Effect of the selective 5-HT1A receptor antagonist WAY 100635 on the inhibition of e.p.s.ps produced by 5-HT in the CA1 region of rat hippocampal slices

1. The actions of N-(2-(-4(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635), a novel and selective 5-hydroxytryptamine1A (5-HT1A) antagonist, on excitatory postsynaptic potentials (e.p.s.ps) were investigated by use of intracellular recordings in pyramidal cells of the CA1 region of rat hippocampal slices. 2. WAY 100635 (10 nM) did not affect any of the investigated parameters of cell excitability such as membrane potential, total input resistance (Rin), firing threshold, action potential amplitude, action potential frequency adaptation, and slow afterhyperpolarization (sAHP) which follows repetitive firing of action potentials. WAY 100635 did not have any effect on either the slope or the amplitude of e.p.s.ps evoked by stimulation of the CA1 stratum radiatum. 3. Bath application of either 5-hydroxytryptamine (5-HT, 10-30 microM) or 5-carboxamidotryptamine (5-CT, 300 nM) hyperpolarized the membrane potential (deltaVm = -4.1 +/- 0.9 and -6.0 +/- 0.9 mV, respectively), and reduced Rin (-25 +/- 8% and -18 +/- 1%, respectively). 5-HT blocked the action potential frequency adaptation and significantly reduced the amplitude of the sAHP that follows repetitive firing of action potentials. 4. 5-HT significantly decreased the amplitude of evoked e.p.s.ps (-14 +/- 6%). This effect was greater in the presence of the GABA(A) receptor antagonist bicuculline (10 microM, -45 +/- 12%) and was mimicked by 5-CT (-49 +/- 5%). Both AMPA and NMDA components of e.p.s.ps were significantly reduced in amplitude by 5-HT (-38 +/- 8%, n = 6, and -29 +/- 12%, n = 3, respectively; P < 0.05). 5. WAY 100635 fully antagonized the hyperpolarization, the reduction of Rin, and the decrease in amplitude of e.p.s.ps elicited by 5-HT, while it did not affect the action of 5-HT on the action potential frequency adaptation. In the presence of WAY 100635, 5-HT elicited a depolarization which was blocked by 10-30 microM RS 23597-190, a selective 5-HT4 receptor antagonist. 6. Our data demonstrate that WAY 100635 is devoid of direct effects on CA1 pyramidal cell excitability and on evoked e.p.s.ps, while it fully antagonizes the effects of 5-HT on excitatory synaptic transmission and on hyperpolarization, without affecting the 5-HT4 receptor-mediated response. Since WAY 100635 selectively antagonizes 5-HT1A receptor-mediated actions of 5-HT, our data also demonstrate that the inhibitory action of 5-HT on excitatory synaptic transmission in CA1 is mediated by 5-HT1A receptors.

Intercellular communication in normal and aberrant crypts of rat colon mucosa

An original system was developed to detect intercellular communication between epithelial cells of rat colon mucosa. Cell-to-cell communication was tested both in normal and in azoxymethane (AOM)-induced aberrant crypts in an attempt to identify chemically-induced modifications of cell properties. Stripes of unstained live tissue were superfused and oxygenated at room temperature and single cells at the top of the crypt were injected with fluorescent dyes. The bottom cells were filled in isolated crypts. Dyes injected into cells at the surface of the mucosa failed to diffuse to adjacent ones, whereas cells at the base of the crypts were dye-coupled. Surface cells from aberrant crypt foci (ACF) did not transfer the dye, therefore behaving like normal crypts. These results indicate that the pattern of intercellular communication between colon crypt cells changes as these cells differentiate and migrate to the top of the crypts and that the pattern of dye transfer between surface cells is maintained in ACF.

Felbamate decreases synaptic transmission in the CA1 region of rat hippocampal slices

The antiepileptic drug felbamate (FBM) is known to block N-methyl-D-aspartate receptor-mediated responses and to decrease voltage-sensitive Na+ and Ca+2 channels. The present work was aimed at investigating the actions of FBM on synaptic potentials in the hippocampus, a region frequently involved in epileptic discharges. In rat hippocampal slices, application of FBM (100-1300 microM, 10 min) elicited a concentration-dependent, fully reversible decrease in amplitude of electrically evoked population spikes recorded extracellularly from the CA1 pyramidal cell layer. In intracellular recordings, FBM (50-300 microM) decreased the amplitude of excitatory postsynaptic potentials and reduced the probability of firing action potentials upon synaptic activation. Action potential frequency adaptation (accommodation), which typically limits repetitive firing in CA1 pyramidal cells, was increased. By using a paired-pulse protocol, FBM (300 microM) depressed the amplitude of paired excitatory postsynaptic potentials, without affecting the facilitation of the second response. In nominally Mg(+2)-free solution, FBM (100 microM) blocked N-methyl-D-aspartate receptor-mediated synaptic excitatory postsynaptic potentials isolated by the presence of 10 microM 6-nitro-7-sulfamoylbenzo(f)quinoxaline hydrochloride, a selective alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, and 10 microM bicuculline or 25 microM picrotoxin. This effect was not reversed by the addition of 300 microM Gly. All these effects contribute to decrease excitatory synaptic transmission and are likely to limit neuron recruitment and propagation of epileptic discharges.

Interactions during a critical period inhibit bilateral projections in embryonic neurons

The anterior pagoda (AP) neurons in the CNS of the medicinal leech are found as homologous pairs in 20 of the 21 midbody ganglia. Each AP is the mirror image of its mate, extending its main axon across the midline of the CNS and eventually into the contralateral body wall, thereby attaining a unilateral pattern of innervation. Certain features of the adult AP morphology are known to arise through interactions among homologs early in development (Gao and Macagno, 1987b), but it is not known whether the contralateral nature of the projection pattern is due to intrinsic "one-sidedness" or rather to cell-cell interactions that inhibit the formation of a second, ipsilateral projection. In the experiments described in this report, we tested the possibility that an AP's contralateral homolog itself inhibits the formation of bilateral projections. One AP was photoablated in the intact embryo early in development and then the response of the remaining AP was examined. We found that an AP can extend bilaterally symmetrical projections when its homolog is missing, but only during a critical period that, interestingly, begins when an AP's interactions with other specific neurons come to an end. To determine whether synaptic communication between AP homologs could be responsible for the timing of this critical period, we recorded electrophysiologically from pairs of embryonic AP neurons. Although no detectable chemical signaling was observed, AP cells were electrically coupled throughout the entire critical period. Further, the junctions between these neurons were permeated by 5-HT, whereas larger molecules such as carboxyfluorescein were impermeant. This dye coupling decreased with age even while electrical coupling persisted, suggesting but not proving that the properties of the gap junctions between AP neurons may also change with time. We conclude that unilateral AP cells possess the intrinsic ability to project bilaterally, but are inhibited from doing so by age-dependent interactions with homologous neurons, possibly mediated by gap-junctional communication.

EPSP-spike potentiation during primed burst-induced long-term potentiation in the CA1 region of rat hippocampal slices

Long-term potentiation induced by high-frequency stimulation in the CA1 region of the hippocampus exhibits EPSP-spike potentiation. This consists of an increase in population spike amplitude exceeding that predicted by EPSP potentiation alone. This phenomenon is apparently due to an increase in pyramidal cell excitability. Patterns of afferent stimuli which activate pyramidal cells to reproduce the theta rhythm observed in the hippocampus under physiological conditions, have been shown to induce LTP-like enhancement of synaptic responses in vitro. The aim of this study was to investigate the presence of EPSP-spike potentiation and/or changes in pyramidal cell excitability during the long-term potentiation induced in the CA1 region of rat hippocampal slices by theta-like patterns of stimuli: the primed burst and the patterned stimulation. Using extracellular recording, a significant leftward shift in the EPSP-spike relationship was found 30 min after primed burst or patterned stimulation. The magnitude of EPSP-spike potentiation induced by patterned stimulation was similar to that produced by high-frequency stimulation. Both were significantly greater than that induced by a primed burst, indicating that only a subset of pyramidal cells were potentiated by this kind of afferent activation. Modifications in synaptic efficacy and cell excitability brought about by a primed burst were investigated in 25 intracellularly recorded pyramidal cells. Consistent with extracellular results, it was found that only 11 out of 25 neurons receiving a primed burst were potentiated. In these cells the increase in probability of firing action potentials elicited by synaptic activation with test shocks was accompanied by enhanced cell excitability, but not by an increase in EPSP slope. High-frequency stimulation delivered 40 min after a primed burst invariably increased the EPSP slope, the probability of firing upon synaptic stimulation, and the excitability of cells. The presence of EPSP-spike potentiation and of increased excitability of potentiated cells during the primed burst-induced long-term potentiation strengthen the suggestion that theta pattern-induced synaptic potentiation can be considered similar to high-frequency stimulation and long-term potentiation and supports the notion that the EPSP-spike potentiation is a constitutive characteristic of long-term potentiation.

Effects of DAU 6215, a novel 5-hydroxytryptamine3 (5-HT3) antagonist on electrophysiological properties of the rat hippocampus

1. The aim of the present study was to test the effects of DAU 6215 (endo-N-(8-methyl-8-azabicyclo-[3.2.1]-octo-3-yl)-2,3-dihydro-2-ox o-1H- benzimidazole-1-carboxamide carboxamide hydrochloride), a newly synthesized, selective 5-hydroxytryptamine3 (5-HT3) antagonist, on the cell membrane properties and on characterized 5-HT-mediated responses of pyramidal neurones in the hippocampal CA1 region. 2. Administration of DAU 6215, even at concentrations several hundred fold its Ki, did not affect the cell membrane properties of pyramidal neurones, nor modify extracellularly recorded synaptic potentials, evoked by stimulating the Schaffer's collaterals. 3. Micromolar concentrations (15-30 microM) of 5-HT elicited several responses in pyramidal neurones that are mediated by distinct 5-HT receptor subtypes. DAU 6215 did not antagonize the 5-HT1A-induced membrane hyperpolarization and conductance increase, a response that was blocked by the selective 5-HT1A antagonist NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phtalamido)butyl- piperazine). Similarly, DAU 6215 did not affect the membrane depolarization and decrease in amplitude of the afterhyperpolarization, elicited by the activation of putative 5-HT4 receptors. 4. 5-HT increased the frequency of spontaneous postsynaptic potentials (s.p.s.ps) recorded in pyramidal neurones loaded with chloride. In agreement with previous observations, most of the s.p.s.ps were reversed GABAergic events, produced by the activation of 5-HT3 receptors on interneurones, because they persisted in the presence of the glutamate NMDA and non NMDA antagonists, D-aminophosphonovaleric acid (APV; 50 microM) and 6,7-dinitroquinoxaline-2,3-dione (DNQX; 25 microM), and were elicited by the selective 5-HT3 agonist, 2-methyl-5-HT (2-Me-5-HT, 50 microM). 5. The increase in frequency of s.p.s.ps induced by 5-HT was significantly antagonized by DAU 6215 in 70% of the cases, whereas the 5-HT3 antagonist always suppressed the effect of 2-Me-5-HT, at concentrations as low as 60 nM.6. The antagonistic effect of DAU 6215 was also tested on the 5-HT3-mediated block of induction of long-term potentiation (LTP), elicited by a primed burst (PB) stimulation. Extracellular recordings showed that low concentrations (60 nM) of DAU 6215 suppressed the inhibitory action of 5-HT onPB-induced LTP, without affecting the 5-HTlA-induced reduction in the amplitude of the population spike.7. These results provide evidence that DAU 6215 is an effective antagonist of the 5-HT3-mediated responses in the central nervous system and may offer a cellular correlate for the pharmacological effects of DAU 6215 as an anxiolytic and cognition enhancer.

Growth cone "collapse" in vivo: are inhibitory interactions mediated by gap junctions?

In the leech embryo, oppositely directed axons of homologous anterior pagoda (AP) neurons overlap with each other extensively within the nerves that link adjacent ganglia, and inhibit each other's further growth (Gao and Macagno, 1987b). During this 5-8 d period of inhibition, the axons begin to grow thin, and eventually they retract completely. However, deletion of one overlapping AP cell results in the renewed growth of the remaining AP cell's axon, which then innervates territory vacated by the killed cell. Thus, each neuron can detect the presence of the other, and adjust its branching pattern accordingly. To begin to explore how these neurons detect and inhibit each other, we tested for direct communication between them. Dye fills with fluorescent chromophores suggested direct contact between their axons at the light level, and this was confirmed by serial-section electron microscopic analysis. Morphological features resembling aspects of gap junctions were observed where the projections were closely apposed, and subsequent electrophysiological recordings demonstrated electrical coupling between the mutually inhibited axons. Confirmation that these projections communicate via gap junctions was obtained using intracellular injection of 5-HT as a tracer, followed by anti-5-HT immunohistochemistry. The tracer passed selectively between AP neurons. We propose that the gap junctions formed between the transient projections of the developing AP neurons may mediate the exchange of the signals that permit homologs to recognize each other and to inhibit the further forward progress of these projections.

Normally unused positional cues guide ectopic afferents in the leech CNS

Central projections from peripheral sensory neurons segregate into distinct, ventrally positioned longitudinal tracts within the segmental ganglia of the leech Hirudo medicinalis. As documented here, there is an additional tract in the neuropils of the fifth and sixth body ganglia, located at the lateral margin and formed by afferent axons (the "sex afferents") originating from sensory neurons located in the male and female genitalia. Ablation of the genitalia results in the complete absence of this additional tract. We asked (1) whether segmental differences exist in the distribution of pathway cues available to the sex afferents, and (2) whether central pathway selection by these axons is specific. We transplanted the primordia of the male genitalia to several ectopic positions posterior to the sixth body segment and labeled the ectopic sex afferents in order to examine their paths in the CNS. In about 50% of the experimental animals, afferent axons originating in the transplanted tissue segregated into a distinct lateral fiber bundle within the neuropil of a nearby ganglion, in a position corresponding to their normal one in the sex ganglia. The sex afferents therefore find their normal pathways even in the ganglia of inappropriate segments, although these pathways are not used by any other afferents in these ganglia. We propose, therefore, that the positional cues employed by afferent axons to select appropriate pathways in the ganglionic neuropil are expressed in all segments of the leech CNS, regardless of whether such cues are normally used by afferent axons in each segment.

Eye movements in Daphnia magna. Regions of the eye are specialized for different behaviors

Three types of behavior of the compound eye of Daphnia magna are characterized: 'flick', a transient rotation elicited by a brief flash of light; 'fixation', a maintained eye orientation in response to a stationary light stimulus of long-duration; 'tracking', the smooth pursuit of a moving stimulus. The magnitudes of the flick and fixation responses vary with stimulus position and are generally proportional to stimulus intensity, although at high intensities there is an attenuation of both behaviors. When the stimulus is placed at a position approximately 80 degrees dorsal to the eye axis, there is no response; this area is called the null region. For stationary stimuli in other positions, the direction of the response is such as to bring the stimulus closer to the null region. During tracking, the relative positions of the eye and stimulus change; the eye velocity is approximately half that of the moving stimulus. The regions of the eye in which these behaviors may be induced are different, being largest for flick and smallest for tracking. It is proposed that flick and fixation responses are a means for rotating the eye so that the stimulus is within the area surrounding the null region which is used for tracking.

Competition or inhibition? Developmental strategies in the establishment of peripheral projections by leech neurons

Leech neurons, like those of other invertebrates and those of vertebrates, undergo specific interactions during development which serve to define their adult morphologies and synaptic connections. We review here several observations and experiments that illustrate these interactions. In particular, we consider how they shape and constrain peripheral arborizations and whether the evidence favors competition or inhibition as their mode of action.

Adenosine decreases aspartate and glutamate release from rat hippocampal slices

The effect of adenosine and related compounds on the release of endogenous aspartate and glutamate from isolated, superfused rat hippocampal slices was studied at rest and during electrical stimulation of the stratum radiatum in the CA3/CA2 region, using a sensitive mass-spectrometric technique. Evoked extracellular potentials were recorded from the CA1 region. Adenosine, at 3 X 10(-4) M concentration, inhibited the stimulation-evoked potentials and prevented the stimulation-induced release of aspartate and glutamate. Similarly, 1-phenylisopropyladenosine (10(-6) M) and cyclohexyladenosine (10(-6) M) depressed both electrical and neurochemical responses to stimulation of the stratum radiatum. 8-Phenyltheophylline (5 X 10(-6) M) increased the release of aspartate and glutamate and antagonized the cyclohexyladenosine-induced inhibition of amino acid release. Our results support the hypothesis that adenosine modulates the electrophysiological responses to stimulation of stratum radiatum through a reduction of the release of the excitatory amino acids aspartate and glutamate.

8-Phenyltheophylline potentiates the electrical activity evoked in hippocampal slices

The effects of 8-phenyltheophylline (8PT) were studied on coronal slices of rat hippocampus. 8PT was more potent than theophylline in enhancing the pyramidal cell responses evoked by stimulation of the stratum radiatum. 8PT dose dependently antagonized the depression of the excitatory postsynaptic potentials induced by cyclohexyladenosine and did not change the amplitude of antidromically evoked responses of pyramidal cells. These findings suggest that 8PT is a potentially powerful tool for studying adenosine neuromodulation in the CNS.