Organic dust exposures from work in dairy barns

Environmental surveys were conducted in 85 barns, predominantly dairy, in central Wisconsin to characterize exposures to organic dusts and dust constituents from routine barn work. Environmental analytes included airborne dusts (total, inhalable inlet, and respirable), particle size distributions, endotoxins, total spore and bacteria counts, viable bacteria and fungi, histamine, cow urine antigen, mite antigen, ammonia, carbon dioxide, and hydrogen sulfide. The geometric mean (GM) concentration of airborne dusts include area total, 0.74 mg/m3; personal inhalable inlet, 1.78 mg/m3, and area respirable, 0.07 mg/m3. Viable bacteria and fungi, spores, endotoxins, histamine, cow urine antigen, and mite antigen were quantifiable constituents of these organic dusts and potential respiratory exposure hazards from routine dairy barn work. Endotoxin concentrations from the inhalable inlet samples ranged from 25.4 endotoxin units per cubic meter of air (EU/m3) to 34,800 EU/m3. The GM endotoxin concentration from these samples, 647 EU/m3, exceeds estimated threshold exposure levels for respiratory health effects. Ammonia was a common irritant quantified in most dairy barns. There were significant correlations between the concentrations of organic dusts and certain dust constituents, although in most instances these correlations were not strong. These sampling results demonstrate the complex nature of organic dusts and provide quantitative description of the exposures to toxic and immunogenic dust constituents during routine barn work.

WIN 63480, a hydrophilic TCP-site ligand, has reduced agonist-independent NMDA ion channel access compared to MK-801 and phencyclidine

NMDA channel blockers are potentially advantageous therapeutic agents for the treatment of ischemia and head trauma, which greatly elevate extracellular glutamate, because they should most effectively inhibit high levels of receptor activation. A novel high affinity TCP site ligand, WIN 63480, does not produce MK-801- or PCP-like behavioral activation at anti-ischemic doses. While WIN 63480, MK-801 and PCP were all observed to be effective blockers of open NMDA channels, WIN 63480 had much less access to closed NMDA channels. This difference may be due to the fact that WIN 63480 is hydrophilic (logD = -4.1) while MK-801 and PCP are lipophilic (logD = +1.8). In vivo, closed channel access may result in a non-competitive profile of antagonism for MK-801 and PCP compared to a more uncompetitive profile for WIN 63480. Release of glutamate, and depolarization, are likely to produce a high level of NMDA receptor activation in ischemic areas compared to normal tissue. Consequently, at anti-ischemic doses, WIN 63480 may produce less inhibition of physiological NMDA-mediated processes in neural systems involved in behavioral regulation than MK-801 or PCP, leading to an improved side effect profile.

Novel benzo[b]quinolizinium cations as uncompetitive N-methyl-D-aspartic acid (NMDA) antagonists: the relationship between log D and agonist independent (closed) NMDA channel block

A series of permanently charged benzo[b]quinolizinium cations having lower lipophilicity than MK-801 or phencyclidine (PCP) were synthesized. Data relating agonist independent block of N-methyl-D-aspartic acid (NMDA) ion channels to log D are described. Closed channel access is predicted to result in a more noncompetitive profile of antagonism compared to selective open channel blockers, which are uncompetitive inhibitors. Reduced closed channel block may underlie the absence of PCP or MK-801-like behavioral side effects observed for benzo[b]-quinolizinium cations.

AMPA (amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptors in human brain tissues

AMPA receptors may play an important role in acute and chronic neurodegenerative diseases. An assay for the specific binding of [3H]-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) to receptors in membranes from post-mortem human brain is described, which can be used in screening for selective AMPA receptor antagonists. Membranes were prepared from frozen human adult hippocampus and whole fetal brain tissues. [3H]-AMPA binding to human hippocampus was saturable; Scatchard analysis of equilibrium binding data indicated high and low affinity sites with affinity binding constants (KD) of 3.4 +/- 0.5 nM and 65 +/- 9 nM (n = 7) respectively. Biphasic association and dissociation rate constants for [3H]-AMPA binding were consistent with the biphasic Scatchard analysis. Inhibition of [3H]-AMPA binding revealed a rank order of potency as quisqualate = AMPA > BOAA > L-glutamate = DNQX = CNQX > kainate > L-aspartate = NMDA. AMPA receptors in human fetal brain had a comparable pharmacology. AMPA/kainate receptors were expressed in frog oocytes following injection of RNA prepared from human fetal brain. Human brain tissues may therefore be utilized for screening and functional analysis of AMPA receptor antagonists.

Novel NMDA antagonists: replacement of the pyridinium ring of 6,11-ethanobenzo[b]quinolizinium cations with heteroisoquinolinium cations

Replacement of the pyridinium ring of 6,11-ethanobenzo[b]quinolizinium cations with thiazolium (4a and 4b) and N-methylimidazolium (4c and 4d) resulted in equipotent compounds in the [3H]TCP binding assay. The corresponding N-methyl-1,2,4-triazolium analogs were less potent in this assay. The thiazolium derivative 4b, with a Ki = 2.9 nM, is being evaluated as a possible neuroprotective N-methyl-D-aspartic acid (NMDA) antagonist.

Discovery of 6,11-ethano-12,12-diaryl-6,11-dihydrobenzo[b]quinolizinium cations, a novel class of N-methyl-D-aspartate antagonists

6,11-Ethano-12,12-diaryl-6,11-dihydrobenzo[b]quinolizinium cations 8, a novel class of N-methyl-D-aspartate (NMDA) antagonists acting at the phencyclidine site, have been identified. Structure-activity relationship studies around the lead compound 8a led to the identification of 12g (WIN 67870-2), one of the most potent compounds in this series. Compound 12g has a Ki = 1.8 +/- 0.2 nM vs [3H]TCP binding, has 700-fold selectivity for binding to the open state of the NMDA receptor-ionophore, and was devoid of MK-801- and PCP-like behavioral effects in rats. Compound 12g was neuroprotective in cultured mouse cortical neurons and exhibited antiischemic activity in a rat middle cerebral artery occlusion/reperfusion model of focal ischemia.

Identification, synthesis, and characterization of a unique class of N-methyl-D-aspartate antagonists. The 6,11-ethanobenzo[b]quinolizinium cation

A series of novel N-methyl-D-aspartate antagonists acting at the phencyclidine site has been identified. Compound 2 has a Ki = 8 +/- 1 nM (vs [3H]thienylcyclidine, [3H]TCP) as a mixture of enantiomers. Resolution and further testing indicate that (-)-2, Ki = 4 +/- 0.7 nM, is a potent and selective TCP site ligand with neuroprotective activity in cultured neurons in the presence of excitotoxic concentrations of NMDA (IC50 = 26 nM). Compound (-)-2 is > 1000-fold selective for the TCP site vs a panel of receptor types including opiate, adrenergic, serotonergic, dopamine, adenosine, dihydropyridine, and benzodiazepine and displays increased selectivity for the activated (open) NMDA receptor-ion channel complex vs PCP and MK801 as measured by patch recordings in cultured, voltage-clamped neurons. Highly enhanced "open-channel" selectivity leads to tentative classification of these ligands as uncompetitive vs NMDA. Ligands with these characteristics may enable deconvolution of the pharmacologic effects associated with typical noncompetitive NMDA antagonists. We report here on the identification, synthesis, and activity of compounds of this structural class.

Glutamate receptor gene expression in spinal cord of arthritic rats

Injury to peripheral tissue leads to hyperalgesia that appears to be partly mediated by functional changes at the level of the spinal cord. Glutamate receptors are thought to play a role in acute and short-term (minutes to hours) spinal cord nociceptive responses and may be involved in prolonged or chronic pain (hours to days). We used in situ hybridization to examine AMPA/kainate (GluR1, GluR2, and GluR3) and NMDA (NR1) receptor gene expression in spinal cord following induction of prolonged inflammation by a unilateral intraarticular injection of lipopolysaccharide (LPS; 10 micrograms) into the hindpaw. In control rats, GluR1 expression was prominent throughout the layers of the gray matter of the spinal cord. Microscopic examination revealed labeling of neuronal cell somata in all major nuclei. GluR2 was abundant in substantia gelatinosa and motor nuclei; emulsion-dipped sections exhibited intense labeling over densely packed neurons in the superficial laminae of dorsal horn and individual motoneurons of ventral horn. GluR3 and NR1 were expressed at low levels throughout spinal cord gray matter. One day after LPS injection, when joint swelling was maximal, GluR1 expression was bilaterally decreased by 25% in the substantia gelatinosa at the level of the lumbar cord. In contrast, no significant change was apparent in GluR2, GluR3, or NR1 expression in any nucleus of the cord. At 72 hr after injection, when joint diameter approached control values, all four transcripts were expressed at near control levels. These findings provide evidence for a specific decrease in GluR1 expression in the cord in response to joint inflammation.

GABAA receptor-mediated excitation of nociceptive afferents in the rat isolated spinal cord-tail preparation

Algogens such as capsaicin, bradykinin, acetylcholine, 5-hydroxytryptamine and potassium ions applied to exposed tail skin of the rat isolated spinal cord-tail preparation evoke a ventral root response consisting of depolarization and spiking activity. L-glutamate and kainate also evoke similar reflexes. All these compounds evoke depolarization of afferent axons or dorsal root ganglion cells. Since GABA depolarizes unmyelinated afferent fibers, the ability of GABA receptor agonists to activate cutaneous nociceptive afferents has been examined. GABA superfused over exposed tail skin evoked a ventral root reflex essentially identical to that produced by capsaicin (3 microM). The EC50 was 27 microM. Muscimol, 3-aminopropane sulphonate, isoguvacine and beta-alanine had effects comparable to GABA, with EC50 values of 9.6, 26, 56 and 870 microM respectively. Baclofen (100 microM) or glycine (10 mM) had no effect. Bicuculline applied to the tail competitively antagonized GABA (Schild slope = -1.03) with a pA2 of 5.8. Spinal application of 1 microM morphine blocked the actions of GABA and capsaicin. These data indicate that GABAA receptors can depolarize and excite nociceptive afferents. GABA could be involved in nociception by contributing to firing of C-fibres, or by analogy to presynaptic inhibition in the spinal cord, may act to decrease neuropeptide transmitter release in cutaneous tissue.

Long-term transplants of mouse trisomy 16 hippocampal neurons, a model for Down's syndrome, do not develop Alzheimer's disease neuropathology

Hippocampal tissue from embryonic day 15-17 fetal mice, euploid or trisomic for chromosome 16, was transplanted into the striatum or the lateral ventricle of 6-8 week old female C57B1/6 mice. After 6-14 months of survival, host brains were sectioned and the grafts were examined by histochemical techniques and by immunocytochemistry for antigens present in pathological brain structures of Alzheimer's disease (AD) patients. Nissl-stained grafts contained aggregations of neurons similar to the pyramidal or the granule cell layers of the normal adult mouse hippocampus. No obvious morphological difference was detected between trisomic and control transplants. The monoclonal antibody Alz-50, which recognizes the paired helical filaments characteristic of AD, or an antibody raised to beta-amyloid peptide, did not reveal neurodegeneration in these grafts. Antibodies against ubiquitin, 200 kDa subunit of neurofilament, alpha 1-antichymotrypsin and tau also did not demonstrate AD-type immunoreactivity in the trisomic or control grafts. Thioflavin S- or silver stained-sections were also negative. We conclude that transplanted hippocampal tissue from the trisomy 16 mouse does not represent an animal model for AD-type neurodegeneration. These results differ from those of Richards et al., EMBO J. (10) (1991) 297-303, who reported AD-type degeneration in trisomy 16 hippocampal transplants.

Activation of nociceptive reflexes by peripheral kainate receptors

The rat isolated spinal cord-tail preparation was used to examine the ability of peripherally applied excitatory amino acids to stimulate ventral root reflexes. Bolus applications (3 sec) of domoate (0.1-10 microM) or kainate (KA; 10-300 microM) to exposed skin of the rat tail stimulated ventral root responses comparably to those evoked by capsaicin (0.3-10 microM) or bradykinin (0.1-10 microM). The ventral root potential evoked by both capsaicin and KA is thought to be a nociceptive response because both compounds selectively activate peripheral C-fibers. L-Glutamate (0.01-10 mM), quisqualate (0.1-1 mM) and amino-5-methyl-4-isoxazole-propionic acid (0.1-1.0 mM) were of lower potency than KA, whereas N-methyl-D-aspartate (1 mM) and L-aspartate (10 mM) were inactive. Responses evoked by prolonged application of KA faded rapidly. Capsaicin-evoked responses partially faded during prolonged application, but residual spiking activity was recorded for at least 30 min. KA did not evoke any observable response during application of capsaicin. The effects of KA and capsaicin were blocked by spinal application of morphine (0.3-1.0 microM) in a naloxone-reversible manner, consistent with activation of peripheral nociceptive afferents. The action of KA, but not capsaicin, was competitively inhibited by the amino-5-methyl-4-isoxazole-propionic acid/KA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (10-100 microM) applied to the rat tail (pA2 = 5.9). Conversely, ruthenium red (1 microM) selectively blocked capsaicin-evoked nociceptive reflexes. The existence of functional KA receptors on nociceptive afferents suggests that peripherally released glutamate or other excitatory amino acids could be involved in nociception or neurogenic inflammation.

L-glutamate activates peripheral nociceptors

The chemosensitivity of nociceptive afferents has been investigated using the isolated spinal-cord-tail preparation of the neonatal rat. L-glutamate applied to exposed rat tail skin stimulated nociceptive reflexes (ED50 = 136 microM). The action of L-glutamate was selective, since other constitutive L-amino acids or D-glutamate were inactive at a concentration of 10 mM. The known algogens bradykinin, acetylcholine, serotonin and potassium ions also stimulated nociceptive reflexes. Histamine (1 mM) and substance P (0.1 mM) had no observable effect. L-glutamate, which is released from macrophages, could therefore contribute to inflammation-related nociception by stimulation of cutaneous nociceptive nerve endings.

NMDA receptor-mediated depolarizing action of proline on CA1 pyramidal cells

This study investigated the actions of proline on CA1 hippocampal pyramidal cells with use of slice preparations. Bath-applied L-proline first induced these cells to fire multiple orthodromic population spikes in response to a single stimulus and then blocked their response to both orthodromic and antidromic stimulation. These effects could be explained by postsynaptic depolarization followed by depolarization block. Grease-gap studies confirmed that L-proline depolarizes CA1 pyramidal cells. D-Proline was inactive in these tests. Excitatory amino acid antagonists reduced depolarizing responses to proline and N-methyl-D-aspartate (NMDA) in parallel. Mn2+ failed to attenuate proline-evoked depolarizations at concentrations that substantially inhibited synaptic transmission, but at a higher concentration it reduced responses to both proline and NMDA. These results suggest that proline depolarized CA1 pyramidal cells mainly by activating postsynaptic NMDA receptors. The neuroexcitatory and neurotoxic actions of proline in the hippocampus may contribute to the seizures and mental retardation associated with hyperprolinemia.

cis and trans elements differ among mouse strains with high and low extrahepatic complement factor B gene expression

Factor B (Bf), an enzyme of the alternative pathway of complement activation, is one of four major histocompatibility complex (MHC) class III genes. To ascertain the genetic mechanism for tissue-specific constitutive and regulated expression of Bf, we sequenced the regulatory regions 5' of the gene from mice of different H-2 MHC haplotypes and assessed trans-acting factors, specific DNA binding nucleoproteins, in liver and kidney. Striking tissue-specific differences in constitutive expression of Bf were demonstrated in mice of H-2f or H-2z haplotypes when compared with H-2d or H-2u (kidney and intestinal Bf in H-2d or H-2u much greater than H-2f or H-2z). These differences correlated with a point nucleotide substitution 3 bp downstream of the upstream Bf initiation site that affects interaction with a DNA binding protein. This and additional cis differences localize the sequence substitutions responsible for previously identified restriction fragment length polymorphisms among inbred mouse strains and also reveal two previously unrecognized polymorphisms generated by SmaI and HinfI digestion. Evidence for differences in trans was found in a comparison of DNA binding nucleoproteins from kidney, but not liver, of B10.PL when compared with B10.M. These data, together with the high degree of sequence homology between human and mouse Bf 5' flanking regions, should prompt a search for polymorphic restriction sites and cis binding elements in the Bf promoter that could serve as markers of human MHC-associated renal pathology and variants in local MHC class III gene expression.

Effects of nerve growth factor on electrical membrane properties of cultured dorsal root ganglia neurons from normal and trisomy 21 human fetuses

Trisomy 21 (Down syndrome) results in abnormalities of electrical membrane properties of cultured human fetal dorsal root ganglion (DRG) neurons; namely, faster rates of depolarization and repolarization of the action potential, and a shortened spike duration. A possible role of nerve growth factor (NGF) in the expression of abnormal electrical membrane properties fetal human DRG neurons from trisomy 21 subjects was examined. DRG neurons obtained from normal and trisomy 21 abortuses of 16-20 weeks gestation were cultured in the presence or absence of 40 nM 7S NGF. After 1 week in culture, action potentials were recorded using the whole cell patch-clamp technique, in current clamp mode. At the resting membrane potential, normal (diploid) neurons grown without NGF showed reduced maximal rates of depolarization (-41.3%) and of repolarization (-31.4%), a decreased spike amplitude (-14.2%) and a prolonged action potential (+49.2%), when compared to normal cells cultured with NGF. Trisomy 21 neurons showed similar changes, but had a greater relative decrease in the rates of action potential depolarization and repolarization. These changes were evident at different membrane potentials. Normal and trisomic DRG neurons cultured without NGF showed differences in action potential parameters similar to those previously described using NGF-supplemented culture medium. These data indicate that NGF can regulate electrical membrane properties in cultured human fetal DRG neurons, but apparently is not responsible for the abnormalities observed in trisomy 21 neurons.

Mouse trisomy 16 neurons, a model of human trisomy 21 (Down syndrome), can be maintained by intracerebral transplantation

The trisomy 16 mouse is considered to be a model of human trisomy 21 (Down syndrome) due to genetic homology between parts of human chromosome 21 and mouse chromosome 16. Additionally, and because older Down syndrome individuals develop neuropathology indistinguishable from that of Alzheimer's disease, trisomy 16 tissue may provide a model of some pathological processes occurring in Alzheimer's disease. However, trisomy 16 fetuses die in utero or shortly after birth, preventing exploitation of this model. We therefore sought to examine trisomy 16 brain tissue over an extended period of time. We report that neural transplantation to normal hosts allows the maintenance of cortical and hippocampal neurons for at least 8 months, thus providing a model in which to examine pathological processes related to Down syndrome, and perhaps to Alzheimer's disease.

Replating improves whole cell voltage clamp recording of human fetal dorsal root ganglion neurons

The whole cell patch clamp technique allows recording of membrane currents in an entire cell under voltage clamp conditions. However, technical difficulties arise in large cells bearing extensive processes, such as human fetal dorsal root ganglion (DRG) neurons in culture. In order to improve space clamp conditions, human fetal DRG neurons cultured for 1-2 weeks were enzymatically detached and replaced in new dishes, yielding round or oval cells with absent or short processes at 24 h in culture. Current clamp recordings demonstrated no difference in action potential parameters of the replated cells compared to control non-replated cells. Analysis of passive properties showed a reduction of 40% in mean specific membrane capacitance and a 57% increase in mean specific membrane resistance in the replated cells, consistent with the decrease of cell membrane surface area. Whole cell voltage clamp studies demonstrated great improvement of the space clamp, indicating that more efficient voltage clamp conditions can be achieved in neurons in culture by eliminating neurites through replating.

Electrical membrane properties of cultured dorsal root ganglion neurons from trisomy 19 mouse fetuses: a comparison with the trisomy 16 mouse fetus, a model for Down syndrome

Because of synteny between mouse chromosome 16 and human chromosome 21, murine trisomy 16 (Ts16) has been considered an animal model for Down syndrome. Indeed, previous investigations have demonstrated that action potentials of cultured dorsal root ganglion (DRG) neurons from human trisomy 21 (Down syndrome) or mouse Ts16 fetuses show increased depolarization and repolarization rates, and decreased spike duration, compared to control neurons. In order to determine the specificity of these changes, we studied the electrical membrane properties of DRG neurons in culture from trisomy 19 (Ts19) and control fetal mice, using the whole cell patch-pipette recording technique. We found no significant differences in action potential parameters and passive membrane properties between Ts19 and control neurons. These findings support the notion that the alterations previously reported in Ts16 DRG neurons are specific, and not a general consequence of genetic imbalance imposed by autosomal trisomies.

The role of altered sodium currents in action potential abnormalities of cultured dorsal root ganglion neurons from trisomy 21 (Down syndrome) human fetuses

Trisomy 21 (Down syndrome) results in abnormalities in electrical membrane properties of cultured human fetal dorsal root ganglion (DRG) neurons. Action potentials have faster rates of depolarization and repolarization, with decreased spike duration, compared to diploid neurons. In order to analyze the faster depolarization rate observed in trisomic neurons, we examined sodium currents of cultured human fetal DRG neurons from trisomy 21 and control subjects, using the whole-cell patch-clamp technique. The neurons were replated in culture to reduce dendritic spines. Two components of the sodium current were identified: (1) a fast, tetrodotoxin (TTX)-sensitive current; and (2) a slow, TTX-resistant component. The inactivation curves of both current types in trisomic neurons showed a shift of approximately 10 mV towards more depolarized potentials compared to control neurons. Thus, whereas essentially all of the fast sodium channels were inactivated at normal resting potentials in control neurons, approximately 10% of these channels were available for activation in trisomy 21 cells. Furthermore, the fast current showed accelerated activation kinetics in trisomic neurons. The slow sodium current of trisomic neurons showed slower deactivation kinetics than control cells. No differences were observed between trisomic and control neurons in the maximal conductance or current densities of either fast or slow current components. These data indicate that the greater rate of depolarization in trisomy 21 neurons at resting potentials is primarily due to activation of residual fast sodium channels that also have a faster time course of activation.

Electrophysiological analysis of cultured fetal mouse dorsal root ganglion neurons transgenic for human superoxide dismutase-1, a gene in the Down syndrome region of chromosome 21

Our recent whole cell patch-pipette studies have shown that human trisomy 21 (Down syndrome) cultured fetal dorsal root ganglion (DRG) neurons have accelerated rates of action potential depolarization and repolarization, with reduced spike duration, compared to control neurons. Similar observations were made using DRG neurons from the trisomy 16 mouse, an animal model of trisomy 21. In this study we have used transgenic mice in order to investigate the relationship between excess gene dosage and neurophysiological abnormalities. DRG neurons which possessed additional copies of the gene for human superoxide dismutase-1 (SOD), a gene from the Down syndrome region of chromosome 21, were compared to normal neurons. No electrophysiological differences were found between the two groups of neurons, indicating that increased dosage of the SOD gene alone is not causal to action potential dysfunction found in trisomy 21 and trisomy 16 neurons.

Neurophysiological abnormalities in cultured dorsal root ganglion neurons from the trisomy-16 mouse fetus, a model for Down syndrome

The trisomy-16 mouse is considered to be a model of human trisomy-21 (Down syndrome). We have examined the electrical membrane properties of cultured dorsal root ganglion (DRG) neurons from normal and trisomy-16 fetuses. Trisomy-16 neurons had significantly accelerated rates of action potential depolarization and repolarization compared to diploid neurons, resulting in decreased spike duration. These changes match those reported in human trisomy-21 DRG neurons. Such abnormalities may contribute to the mental retardation characteristic of Down syndrome.

Pro-convulsant actions of theophylline and caffeine in the hippocampus: implications for the management of temporal lobe epilepsy

The pro-convulsant actions of theophylline and caffeine have been investigated using the hippocampal slice preparation and rats administered kainic acid or Metrazol. Both theophylline and caffeine induced the generation of epileptiform activity in the CA3 region of the hippocampal slice with convulsive dose50 (CD50) values of 3 microM respectively. Kainic acid-induced bursting in hippocampal slices was enhanced by theophylline (0.3-30 microM) and caffeine (1-100 microM). Theophylline induced burst firing in response to electrical stimulation in hippocampal area CA3 but not area CA1. Theophylline (50 mg/kg) strongly potentiated the effect of the limbic convulsant kainic acid in vivo whilst a dose of 200 mg/kg was necessary to significantly lower the threshold dose of Metrazol required to induce generalized convulsions. We conclude that alkylxanthines, probably by antagonizing the effect of endogenous adenosine, exert a pro-convulsant action in the hippocampus which preferentially promotes limbic seizures.

L-proline depolarizes rat spinal motoneurones by an excitatory amino acid antagonist-sensitive mechanism

1 Isolated spinal cords prepared from neonatal rats were used to examine the effects of L-proline (L-Pro). 2 L-Pro (1-8 mM) depolarized ventral and dorsal roots in a dose-dependent manner with one sixth of the potency of L-glutamate (L-Glu). L-Pro was four times more potent than D-Pro. Prolonged application of L-Pro produced a plateau depolarization of motoneurones with no apparent fade. 3 Omission of calcium ions from the medium potentiated the depolarizing actions of L-Pro, L-Glu and quisqualate. 4 L-Pro was antagonized by concentrations of 2-amino-5-phosphonovalerate (25 microM), gamma-D-glutamylglycine (100 microM) and Mg2+ ions (1 mM) that depressed responses to N-methyl-D-aspartate (NMDA). The NMDA receptor-mediated component of the response to L-Pro was estimated to be 60-70%. 5 These data suggest that L-Pro should be considered as a possible excitatory neurotransmitter and that, because L-Pro is a neutral compound, excitatory amino receptors may not require an agonist to possess two anionic groups and one cationic group.

Efficacy of baclofen and phenobarbital against the kainic acid limbic seizure-brain damage syndrome

Baclofen and phenobarbital were tested for anticonvulsant efficacy against limbic seizures produced by i.c.v. infusion of kainic acid (KA) in unanesthetized rats. All rats treated with KA alone developed a prolonged status epilepticus associated with extensive neuronal degeneration. When administered immediately after the KA infusion, baclofen (5 mg/kg i.p.) protected five of six animals against the development of status epilepticus and did not alter the behavioral expression of the residual discrete electrographic seizures. Phenobarbital (40 mg/kg i.p.) given 15 min before KA also prevented the development of status epilepticus in five of six rats, but blocked the behavioral expression of the residual electrographic seizures. In two of five additional rats, baclofen prevented or reversed status epilepticus when administered 50 to 60 min after the end of the KA infusion. The ability of these drugs to prevent KA-induced neuronal degeneration correlated with their anticonvulsant action.

Comparison of seizures and brain lesions produced by intracerebroventricular kainic acid and bicuculline methiodide

Intracerebroventricular kainic acid produces in rats brain lesions similar to Ammon's horn sclerosis in humans. To test the hypothesis that these lesions result indirectly from prolonged seizure activity and not from a direct action of kainic acid on the neurons that are destroyed, the effects of intracerebroventricular kainic acid and bicuculline methiodide were compared. Although bicuculline methiodide seizures differed dramatically from kainic acid seizures, both electrographically and behaviorally, the resulting brain lesions were similar for a given total limbic seizure duration. These results, in combination with other data, support the view that lesions made by intracerebroventricular administration of convulsants are indeed caused by prolonged limbic seizures. The total duration of seizure activity appears to be one important variable.

Baclofen suppresses bursting activity induced in hippocampal slices by differing convulsant treatments

Epileptiform activity was induced in area CA3 of hippocampal slices by superfusion of medium containing 50 microM bicuculline and 3.5 mM K, 50 microM bicuculline and 5 mM K, 50 nM kainic acid and 3.5 mM K, or 7 mM K. Burst potentials were recorded at rates between 5 and 44/min, depending on the convulsant treatment. Baclofen reduced the frequency of burst firing in all slices tested in a dose-dependent manner, with little change in the morphology of individual bursts. Thus baclofen primarily affected the initiation of epileptiform discharges. IC50 values varied between 27 and 500 nM and were positively correlated with the rate of bursting. These experiments indicate that baclofen, at concentrations present in the CSF of patients treated for spasticity, has an anticonvulsant-like effect in the hippocampal formation and suggest that its mode of action is to reduce the excitability of pyramidal cells.

Baclofen suppresses hippocampal epileptiform activity at low concentrations without suppressing synaptic transmission

Baclofen is used clinically to treat spasticity, but has received little attention as a potential antiepileptic agent. To explore the antiepileptic potential of baclofen further, we tested its effect on stimulus train-induced bursting, an in vitro model of hippocampal epileptiform activity. In hippocampal slices prepared from male rats, extracellular field potentials were recorded in stratum pyramidale of CA3, and electrical stimuli were delivered to s. radiatum of CA3. After stable responses to single stimuli were established, stimulus trains were delivered every 5 min until stable triggered and spontaneous population bursting were elicited. (+/-)-Baclofen was bath-applied to the slices at varying concentrations to study its ability to suppress synaptic transmission and epileptiform activity. EC50 values for suppression of orthodromic population spike amplitude, of triggered burst duration and of spontaneous burst frequency were 2300, 355 and 26.9 nM, respectively; all statistically significantly different. These findings suggest that baclofen suppresses epileptiform electrical activity in the hippocampus at concentrations well below those which suppress normal synaptic transmission, and support renewed consideration of baclofen as an antiepileptic agent.

Adenosine inhibits epileptiform activity arising in hippocampal area CA3

The ability of adenosine and structurally-related compounds to inhibit epileptiform activity induced by bicuculline in the CA3 region of the hippocampal slice of the rat was examined. Bath application of all purinoceptor agonists tested reduced the frequency of generation of burst potentials. Analysis of dose-response curves yielded the following IC50 values: adenosine, 1.5 microM; 2-chloroadenosine, 0.144 microM; 5'-(N-ethyl)carboxamidoadenosine, 30.2 nM; L-phenylisopropyladenosine, 12.1 nM; cyclohexyladenosine, 7.9 nM. Theophylline (30 microM) increased the rate of bursting and antagonized the effect of exogenous adenosine. Dipyridamole (0.03-1 microM) reduced the occurrence of burst firing. In slices untreated with bicuculline, theophylline (30 microM) and adenosine deaminase (10 micrograms ml-1) induced bursting activity. These results demonstrate that purinoceptor agonists can suppress epileptiform activity in the hippocampus and suggest that adenosine may act as an endogenous anticonvulsant.

Effects of baclofen on synaptically-induced cell firing in the rat hippocampal slice

The effects of baclofen on the synaptically-induced firing of pyramidal and granule cell populations were tested in the rat hippocampal slice. Population spikes were evoked by stimulating excitatory pathways in the presence and absence of bath-applied drug. (+/-)-Baclofen (20 microM) completely blocked the firing of CA1 or CA3 hippocampal pyramidal cells subsequent to stimulation of projections that originate in area CA3. In contrast, the firing of dentate granule cells evoked by stimulation of the perforant path fibres was depressed by only 46% and baclofen did not affect the monosynaptic firing of CA3 pyramidal cells evoked by mossy fibre stimulation. These results are consistent with the effects of baclofen on the corresponding extracellularly-recorded excitatory postsynaptic potentials (e.p.s.ps). The Schaffer collateral-commissural population spike in area CA1 was depressed by (-)-baclofen (EC50 = 2.8 microM), GABA (EC50 = 2.2 mM) and 3-aminopropanesulphonic acid (3-APS) (EC50 = 0.34 mM). (-)-Baclofen was 180 times as potent as (+)-baclofen. Bicuculline methiodide (100 microM) did not reverse the depressant action of (-)-baclofen. GABA-induced depressions were antagonized to only a small degree, whilst the effect of 3-APS was readily reversed. Raising the concentration of bicuculline from 100 microM to 500 microM did not further reverse the action of GABA. The effects of (-)-baclofen and 3-APS on the relationship between extracellular e.p.s.p. and population spike were tested by stimulation of the Schaffer collateral-commissural fibres in area CA1. (-)-Baclofen shifted the 'input/output' curve to the right at a concentration of 1 microM, but less or not at all at 3 microM. In contrast, increasing the concentration of 3-APS shifted this curve farther to the right.

Anticonvulsant-like actions of baclofen in the rat hippocampal slice

1 The effects of baclofen were tested on epileptiform discharge in the rat hippocampal slice. Slices were superfused with bicuculline methiodide (100 microM) and maximal periods of afterdischarge were evoked by stimulating the Schaffer collateral-commissural pathway in area CA1, mossy fibres in area CA3 or perforant path fibres in the fascia dentata or by antidromic stimulation of CA1 pyramidal cells. 2 (-)-Baclofen attenuated the afterdischarge evoked by stimulating all three sets of fibres in areas CA1 and CA3. In each case, a threshold effect was observed at a concentration of 0.25 or 0.5 microM, and complete suppression was usually attained with a concentration of 5 microM. EC50 values ranged between 1 and 2 microM. (-)-Baclofen attenuated hippocampal afterdischarge with 120 times the potency of (+)-baclofen. It did not, however, affect the repetitive firing of dentate granule cells in response to stimulation of perforant path fibres. 3 (-)-Baclofen also reduced the amplitude of the initial population spike evoked by stimulation of Schaffer collateral-commissural fibres, but did not affect the antidromic population spike nor the initial population spike evoked by stimulation of the mossy fibres. 4 Recurrent inhibition in area CA1 was abolished by 1 microM (-)-baclofen. Thus baclofen, unlike many anticonvulsants, does not suppress afterdischarge by potentiating GABAergic inhibition. 5 These results suggest that baclofen attenuates hippocampal afterdischarge by a combination of pre- and postsynaptic mechanisms.

Baclofen selectively inhibits transmission at synapses made by axons of CA3 pyramidal cells in the hippocampal slice

The effects of baclofen, an antispastic drug, on excitatory transmission were tested by bath application to the hippocampal slice preparation. (+/-)-Baclofen (20 microM) strongly depressed extracellularly recorded synaptic responses to stimulation of projections that originate from CA3 hippocampal pyramidal cells. Responses to stimulation of four other excitatory pathways were little affected and the amplitudes of presynaptic fiber potentials and antidromic responses were unaltered. When tested on the Schaffer collateral-commissural-CA1 pyramidal cell synapse. (-)-baclofen depressed the amplitude of the extracellular excitatory postsynaptic potential with an IC50 of 3.7 microM and was 180 times more potent than (+)-baclofen. gamma-Aminobutyric acid, 3-aminopropanesulfonic acid and imidazole-4-acetic acid also inhibited transmission at this site. Baclofen could suppress the response completely, and its action was unaffected by bicuculline. In contrast, imidazole-4-acetic acid could suppress the response by a maximum of only 75%, and its action was highly sensitive to bicuculline. gamma-Aminobutyric acid and 3-aminopropanesulfonic acid could suppress the response completely, and their actions were relatively weakly antagonized by bicuculline. These results are consistent with the hypothesis that baclofen inhibits excitatory transmission by interacting with a bicuculline-insensitive gamma-aminobutyric acid receptor. These receptors may be located on one type of glutamatergic/aspartergic synaptic terminal, exemplified by axon terminals of CA3 hippocampal pyramidal cells. Synapses made by these axons may therefore serve as models for studying the mechanism of action of baclofen.

The depressant action of baclofen on the isolated spinal cord of the neonatal rat

Neurotransmission in isolated hemisected spinal cord preparations from immature rats was depressed by micromolar levels of baclofen (threshold 0.5 microM). The depressant action of baclofen was not antagonised by bicuculline and baclofen, unlike GABA, did not depolarize primary afferent fibres. Neurotransmission in isolated vas deferens, anococcygeus muscle and superior cervical ganglion of the rat was unaffected by baclofen (0.1-1 mM). Depolarization of motoneurones, as recorded in ventral roots of tetrodotoxin-blocked spinal cord preparations, induced by excitant amino acids, substance P, noradrenaline or carbachol was unaffected by baclofen (250 microM or higher). The depressant action of baclofen on spinal cord preparations was similar to that produced by the excitant amino acid antagonist alpha,epsilon-diaminopimelic acid. A structure-activity study showed that the (--)-isomer of baclofen was over 20 times more potent than the (+)-isomer as a spinal depressant. Also the position and nature of the halogen substitutent in the ring is critical with baclofen giving optimal activity. It is concluded that the depressant action of baclofen from depression of the presynaptic release of excitant amino acid transmitter(s).

Skin testing in farmers' lung disease

Intradermal skin tests with a culture filtrate antigen of Micropolyspora faeni grown on a synthetic medium were performed on patients with farmers' lung disease (FLD) and well farmers with and without antibodies to a panel of FLD antigens. Seventy-five percent of the FLD patients, 79% of the well farmers with M. faeni antibody, and 5% of well farmers without M. faeni antibody had a 2+ or greater intradermal immediate skin-test reaction. Prausnitz-Küstner (P-K) reactions were positive using serum of M. faeni immediate skin test-positive FLD patients. IgG-rich fractions from a staphylococcal protein A-Sepharose column of such serum contained the sensitizing factor whereas IgG-depleted fractions did not. M. faeni-specific IgE could not be detected in serum by a polystyrene radioimmunoassay. Positive late-onset (6-hr) skin tests occurred only in FLD patients and farmers with precipitating antibody. Biopsy specimens of the 6-hr reactions revealed a generalized dermal and perivascular polymorphonuclear infiltrate with deposits of immunoglobulin and complement about blood vessels. The skin-sensitizing factor noted in FLD patients and well farmers with antibody is not disease specific. This factor appears to be associated with the IgG-rich fraction of serum, and its role in the pathogenesis of FLD is unclear.

Bronchial hyperreactivity to methacholine in farmers' lung disease

Methacholine inhalation challenges were performed in five groups: 14 patients with history of farmers' lung disease (FLD), nine well farmers with precipitating antibodies to Micropolyspora faeni antigen, II normal subjects, 12 asthmatic patients, and 10 patients with interstitial lung disease (ILD) other than hypersensitivity pneumonitis. After calculating the mean areas under the dose-response curves, FLD patients had a significantly greater degree of methacholine sensitivity than did well farmers with M. faeni antibody and normals. However, the methacholine sensitivity of FLD patients was indistinguishable from a group of patients with ILD and significantly less than the asthmatic group. Fifty percent of patients with FLD had positive methacholine challenges (greater than or equal to 20% decrease in their 1-sec forced vital capacity [FEV1] from the control FEV1). Hyperirritable airways was a common finding in FLD.

Selective depression of excitatory amino acid induced depolarizations by magnesium ions in isolated spinal cord preparations

1. The depressant actions of Mg2+ and a range of other divalent ions on synaptic excitation and on responses produced by excitatory amino acids and other putative transmitters have been investigated in hemisected isolated spinal cords of frogs and neonatal rats. Some comparative studies were also made using the rat isolated superior cervical ganglion. 2. At concentrations above 10 microM, Mg2+ selectively antagonized N-methyl-D-aspartate (NMDA)-induced motoneurone depolarization as recorded from ventral roots of tetrodotoxin-blocked spinal cords. Depolarization evoked by quisqualate (unaffected by 20 mM-Mg2+) was resistant to the depressant action of these ions, while depolarizations evoked by other excitant amino acids were depressed to intermediate degrees. 3. Mn2+, Co2+ and Ni2+ had qualitatively similar actions to Mg2+; Mn2+ was somewhat less potent and Co2+ and Ni2+ more potent than Mg2+. The alkaline earth metal ions, Ca2+, Sr2+ and Ba2+, had very weak Mg2+-like actions. Ca2+ and Mg2+ acted additively in depressing amino acid-induced responses. 4. Mg2+ also depressed motoneurone responses evoked by noradrenaline, substance P and carbachol in the neonatal rat isolated spinal cord. However, none of these effects were as marked as the depression of NMDA-induced responses by Mg2+ in this preparation. Mg2+ did not depress motoneurone depolarization produced by 5-HT in the rat spinal cord or the depolarizing action of GABA on primary afferent terminals of the isolated frog spinal cord. 5. At concentrations producing marked depression of NMDA-induced responses, Mg2+ also depressed synaptic transmission in spinal cords in the absence of an effect on ganglionic transmission. At the same concentrations, Mn2+, Co2+ and Ni2+ depressed synaptic transmission in both preparations. 6. From the similarity in action between Mg2+ and the D-alpha-aminoadipate group of NMDA antagonists, it is suggested that the central depressant action of low concentrations of Mg2+ involves predominantly a postsynaptically mediated interference with the action of an excitatory amino acid transmitter.