Modelling the anxiety-depression continuum in chicks.

The clinical syndromes of anxiety and depression are now thought to exist along a temporal continuum and this construct has been modelled in a preclinical setting in chicks separated from conspecifics. This research sought to further the validity of the chick anxiety-depression continuum model. Dose-response studies using two classes of anxiolytics (chlordiazepoxide: 2.5, 5.0, 10.0, 15.0 mg/kg, and clonidine: 0.1, 0.15, 0.2, 0.25 mg/kg) and three classes of antidepressants (imipramine: 1.0, 3.0, 10.0, 15.0 mg/kg, maprotoline: 2.5, 5.0, 10.0, 20.0 mg/kg and fluoxetine: 1.0, 5.0, 10.0, 20.0 mg/kg) showed an ability to detect anxiolytic activity of chlordiazepoxide, clonidine, imipramine and maprotoline in the anxiety-like phase of the model and to detect antidepressant effects of imipramine, maprotoline and fluoxetine in the depression-like phase of the model. In addition, blood plasma interleukin-6, a biomarker of stress, was found to be elevated in response to social-separation stress. Collectively, these findings further characterize the model as a simulation of the anxiety-depression continuum and begin to establish the paradigm as a high-utility adjuvant to rodent screening assays for putative anxiolytic and antidepressant compounds.

Anti-inflammatory and anti-hyperalgesic effects of sesquiterpene lactones from Magnolia and Bear's foot.

Sesquiterpene lactones possess a variety of biological activities, including anti-inflammatory activity. Two plants native to the southeastern United States, Magnolia grandiflora (L.) and Smallanthus uvedalius (L.) [syn Polymnia uvedalius (L.)], are novel sources of the sesquiterpene lactones parthenolide and enhydrin, respectively. In this study, the anti-inflammatory and anti-hyperalgesic effects of these isolated lactones from these two plant sources were evaluated in the rat carrageenan inflammation model. Rats received ip injections of either vehicle (propylene glycol), indomethacin (5 mg/kg), 11,13-dihydroparthenolide (20 mg/kg), parthenolide (5 or 20 mg/kg) or enhydrin (5 or 20 mg/kg). A 100-microl injection of 2.0% carrageenan was made into the plantar surface of the right hindpaw. Paw withdrawal latencies and paw volumes in both inflamed and non-inflamed paws were recorded at four test intervals: pre-inflammation baseline (0 time point), and 1, 2 and 4 h post-carrageenan injection. Vehicle-treated animals exhibited a significant time-dependent hyperalgesic and edema response that was greatest at the 4-h test interval. Indomethacin significantly blocked the hyperalgesic response and modestly attenuated the edema response. Parthenolide (20 mg/kg) and enhydrin (20 mg/kg) significantly blocked the hyperalgesic response and significantly attenuated the edema response; 11,13-dihydroparthenolide did not affect either inflammation or hyperalgesia. These findings suggest that parthenolide and enhydrin from these plant sources may be useful in the treatment of inflammatory pain.

Analogs of thyrotropin-releasing hormone in potentiating the spinal monosynaptic reflex in vitro.

The efficacy of thyrotropin-releasing hormone (TRH) and its analogs to potentiate the spinal monosynaptic reflex was studied in isolated cords. The analogs examined were L-pyro-2-aminoadipyl-histidyl-thizolidine-4-carboxyamide (MK-771); pyroglutamyl-histidyl-prolineamide (TRH); pyroglutamyl-L-histidyl-3,3'-dimethyl-prolineamide (RX77368); (3-methyl-His2)TRH(methyl-TRH); gamma-buturolactone-gamma-carbonyl-histidyl-prolineamide citrate (DN-1417); pyroglutamyl-histidyl-proline (TRH-free acid); and histidyl-proline-diketopiperazine (cyclo(His-Pro)). The TRH analogs potentiated the monosynaptic reflex in a dose-dependent manner and the maximal potentiation occurred at about 1 microM. TRH-free acid potentiated the monosynaptic reflex but the maximal potentiation occurred at 100 times the TRH concentration. Cyclo(His-Pro) was totally ineffective. The concentration required to potentiate the monosynaptic reflex by 50% of the maximal response (EC50) was taken as an index for comparing various analogs in relation to TRH. The EC50 values of the analogs did not differ significantly from each other. However, the ratio of the mean value of an analog to that of TRH was of the following order: MK-771 (N- and C-terminally altered) > or = TRH > or = DN-1417 (N-terminal) > or = methyl-TRH > or = RX77368 (C-terminal) >>> TRH-free acid. Cyclo(His-Pro) was ineffective.

Assessment of spinal cord trauma using evoked potentials in a rat model of decompression sickness.

A rapid quantitative technique for assessing spinal cord trauma in a rat model of decompression sickness is described. Evoked potentials are measured from the lower limbs of rats before and after dives with compressed air in a hyperbaric chamber. Under chloral hydrate anesthesia, the sciatic nerve is stimulated at the sciatic notch with needle electrodes and platinum/iridium electrodes are used to record the action potentials from the plantar muscles. Analysis showed that the sensory reflex response was markedly depressed in the rats soon after diving and did not recover for up to 5 days. The motor response was similarly affected although to a lesser degree. The latency of the reflex response also became prolonged after 3 days. The significant and complex pattern of neurological dysfunction shown by this electrophysiologic technique validates the use of the rat model for the study of spinal cord decompression sickness. This technique should aid studies aimed at testing new therapies for this disease.

Thyrotropin-releasing hormone reverses the supersensitively depressed monosynaptic transmission by serotonin in 5,7-dihydroxytryptamine-treated neonatal rats in vitro.

The interaction of serotonin and thyrotropin-releasing hormone (TRH) on monosynaptic reflex (MSR) in isolated neonatal spinal cords was examined. Superfusion of serotonin (1-30 microM) in untreated cords, depressed the MSR in a dose-dependent manner. The depression was about 25% at 10 microM of serotonin, in either control or vehicle-treated groups. While for the same concentration of serotonin, the depression was 97 +/- 2.1% of the control in cords from 5,7 dihydroxytryptamine (5,7-DHT)-treated animals. The inhibition of the reflex seen in cords obtained from 5,7-DHT-treated animals could not be reversed by washing with normal physiological solution (> 60 min) or in presence of serotonin antagonists. TRH (0.03-1.0 microM) reversed the depression in a concentration-dependent manner and complete reversal could be seen with 1 microM of TRH. These observations indicate that, serotonin and TRH act dissimilarly on the spinal synaptic transmission though they are known to coexist in the descending bulbospinal tracts.

Thyrotropin releasing hormone-induced potentiation of spinal monosynaptic reflex in rats in vitro.

Superfusion of thyrotropin-releasing hormone (TRH) in neonatal rat spinal cord in vitro produced dose (0.01-1.00 microM) dependent potentiation of monosynaptic reflex (MSR) which was maximum (44% of control) at 1 microM of TRH. But no ventral root depolarization was observed with TRH (1 microM) although potassium concentration out side ([K+]0) when increased produced a depolarization at the magnitude of 0.2 mV/mM of [K+]0. TRH-induced potentiation of MSR was not altered in spinal cords, obtained from the animals pretreated with 5,7-dihydroxytryptamine or 6-hydroxydopamine. Neither serotonin antagonists (spiperone, ketanserin, cyproheptadine or 3-troponyl-indole-3-carboxylate) nor adrenergic antagonists (phentolamine or haloperidol) could attenuate TRH-induced potentiation. Inhibition of MSR observed in the spinal cord elicited by stimulating the adjacent dorsal root was unaffected by TRH. The results suggest that, TRH potentiates MSR by directly acting on the motoneurons, without involving presynaptic serotonergic or catecholaminergic neuronal systems or the disinhibition of pre- or post-synaptic inhibition in the spinal cord.

Biphasic action of sarin on monosynaptic reflex in the neonatal rat spinal cord in vitro.

The action of sarin, an organophosphorus (OP) compound, was examined in vitro for its effects on the spinal monosynaptic reflex (MSR) in neonatal rats. The effects of sarin were biphasic, i.e. facilitation at lower concentrations (2-20 nM) followed by depression of the MSR at concentrations above 30 nM. Facilitation of MSR was maximal (150% of control) at 20 nM sarin. The depression of MSR was maximal (70% of control) at 200 nM sarin, with half maximal inhibition occurring at 90 nM sarin. Atropine (200-500 nM) effectively reversed the depression caused by sarin, while pretreatment with low concentrations of atropine (10 nM) completely blocked the depression otherwise observed with sarin. Benactyzine was also effective in preventing sarin-induced depression, while pirenzepine was less effective. The nicotinic blocking agents tubocurarine and mecamylamine were, however, ineffective in preventing or reversing sarin-induced depression. The facilitation of MSR seen with lower concentrations (2-20 nM) correlated well with the blockade of late phase inhibition (between 30 and 50 ms conditioning-test interval) elicited in spinal cord by stimulating the adjacent dorsal root at various condition-test intervals, which has been shown elsewhere to be sensitive to bicuculline (Deshpande and Warnick 1988). Thus it is speculated that sarin at lower concentrations blocks GABA transmission, producing facilitation, and at higher concentrations activates the muscarinic receptors producing depression of MSR. The beneficial action of pretreatment with antimuscarinic agents may be attributed to the protection of the muscarinic receptors.

Interaction of thyrotropin-releasing hormone and methysergide on the monosynaptic reflex in isolated mammalian spinal cord.

The interaction between thyrotropin-releasing hormone (TRH) and methysergide (MeSG) on reflex activity was examined in spinal cords from neonatal rats. MeSG depressed the monosynaptic reflex (MSR) by nearly 90% at 0.03 microM but had no effect on the dorsal root reflex at 0.003-3.0 microM. Neither spiperone, ketanserin, cyproheptadine nor ICS 205-930 (3-tropanyl-indole-3-carboxylate) depressed the MSR nor did they affect the potentiation elicited by TRH. TRH reversed the depression of the MSR by MeSG in a concentration-dependent manner without affecting the dorsal root reflex. MeSG-induced depression of the MSR was also reversed by 3,4-diaminopyridine which simultaneously increased the magnitude and duration of both reflexes. It appears that neither MeSG-induced depression nor TRH-induced potentiation of the MSR involves the spinal serotonergic system or blockade of K+ channels.

Selective depression of the segmental polysynaptic reflex by phencyclidine and its analogs in the rat in vitro: interaction with N-methyl-D-aspartate receptors.

The differential sensitivity of monosynaptic and polysynaptic reflexes to phencyclidine (PCP) and its analogs was examined in a Mg+(+)-free physiological solution using an in vitro spinal cord preparation of neonatal rats. Whereas the monosynaptic reflex was relatively resistant to N-methyl-D-aspartate antagonists [Mg++, 2-amino-5-phosphonovalerate (APV) and 2-amino-7-phosphonoheptanoate (AP7)], the polysynaptic reflex was markedly reduced in a concentration-dependent manner. The magnitude of the monosynaptic reflex only decreased 20 to 30% at concentrations of Mg++ (1.3 mM), APV (10 microM) and AP (10 microM), which completely depressed the polysynaptic reflex. PCP and its analogs also selectively depressed the polysynaptic reflex in a concentration-dependent manner and had relative potencies consistent with those for the PCP receptor [i.e., 1-(1-m-amino-phenylcyclohexyl)piperidine = MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate] greater than 1-[1-(2-thienyl)cyclohexyl]piperidine greater than or equal to PCP much greater than (+)-N-allylnormetazocine much greater than 1-(1-m-nitrophenylcyclohexyl)piperidine. The latter compounds depressed the monosynaptic reflex to the same extent as Mg++, APV and AP7 at concentrations which completely depressed the polysynaptic reflex. Furthermore, the depression of the reflexes by PCP was unaffected by haloperidol and methiothepin precluding the involvement of sigma and serotonin receptors in PCP-induced depression of the polysynaptic reflex. Our results suggest that PCP and its analogs selectively depressed the polysynaptic reflex through PCP receptors associated with the N-methyl-D-aspartate receptor complex.

Presynaptic activation of the spinal serotonergic system in the rat by phencyclidine in vitro.

Electrophysiological studies were carried out to investigate the mechanism of action of phencyclidine [PCP; 1-(phenylcyclohexyl)piperidine] on a segmental monosynaptic reflex using isolated spinal cord preparations from neonatal rats. PCP and its related compounds produced a concentration-dependent depression of the monosynaptic reflex with a relative potency as follows: PCP = 1-[1-(2-thienyl)cyclohexyl]piperidine greater than 1-(1-m-aminophenylcyclohexyl)piperidine much greater than 1-(1-m-nitrophenylcyclo-hexyl)piperidine approximately MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] much greater than (+)-N-allylnormetazocine. The N-methyl-D-aspartate receptor antagonists 2-amino-5-phosphonovalerate and 2-amino-7-phosphonoheptanoate had no effect on the monosynaptic reflex. The depression of the monosynaptic reflex by PCP was antagonized by serotonin (5-HT) receptor antagonists (methiothepin, spiperone and ketanserin) but unaffected by noradrenergic (phentolamine and timolol), dopaminergic (chlorpromazine and pimozide) and cholinergic antagonists (atropine and mecamylamine). Whereas 5-HT and a putative 5-HT releaser, p-chloroamphetamine, also depressed the monosynaptic reflex, the blockade of monoamine uptake by imipramine did not. Furthermore, pretreatment of rats with desipramine and 5,7-dihydroxytryptamine largely diminished the depression of the monosynaptic reflex by PCP and p-chloroamphetamine while enhancing the depressant action of 5-HT. These results suggest that PCP acts at sites located on presynaptic terminals of spinal serotonergic neurons, enhancing 5-HT release and thereby depressing the monosynaptic reflex.

Interaction of reversible and irreversible cholinesterase inhibitors on the monosynaptic reflex in neonatal rats.

The ability of physostigmine (PHY) and pyridostigmine (PYR) to protect against the segmental synaptic depression caused by sarin was examined in isolated spinal cords from neonatal rats. The monosynaptic reflex was unaffected at concentrations up to 0.1 microM PHY or 0.3 microM PYR but raising the concentrations of either drug produced a concentration-dependent depression of the monosynaptic reflex which could be completely antagonized by atropine. The monosynaptic reflex was depressed by 50% at 0.45 microM PHY and 2 microM PYR with maximal depression occurring at 1 microM PHY (to about 10% of control) and 10 microM PYR (to about 35% of control). Pretreating the cords with 0.1 microM PHY and PYR for 30 min failed to protect against the depressant effects of sarin even though they inhibited total cholinesterase (ChE) by 27 and 21%, respectively. Both PHY and PYR depressed total ChE activity of the spinal cord in a concentration-dependent manner with 50% inhibition of ChE occurring at 0.8 microM. These results suggest that the carbamates affect segmental transmission by activation of a muscarinic receptor, that protective carbamylation of ChE is ineffective against organophosphorus-induced segmental depression, and that inhibition of ChE is unrelated to both carbamate- and organophosphorus-induced depression of the monosynaptic reflex.

Protirelin (thyrotropin-releasing hormone) in amyotrophic lateral sclerosis. The role of androgens.

Protirelin (thyrotropin-releasing hormone) appears to be a neuromodulator in the extrahypothalamic nervous system and has been suggested as an adjunct in the treatment of amyotrophic lateral sclerosis (ALS). Clinical studies have been divided on the efficacy of protirelin (TRH) despite strong experimental findings that are consistent with a role for the peptide in ALS. Recent findings provide evidence of a gender-related specificity in the ability of protirelin to potentiate the monosynaptic reflex. While castration in male neonatal rats lowered the sensitivity to protirelin, testosterone treatment restored that sensitivity. An examination of the clinical studies reveals a failure either to identify patients' sex or to separate the results on the basis of sex. These findings provide convincing evidence for the potential efficacy of protirelin in ALS if the patient's sex and underlying hormonal status are taken into account.

Prevention of phencyclidine-induced depression of the segmental reflex by L-3,4-dihydroxyphenylalanine in the rat spinal cord in vitro.

The interaction between phencyclidine (PCP) and the catecholamine precursor L-3,4-dihydroxyphenylalanine (DOPA) was studied in the isolated spinal cord from neonatal rats. PCP decreased the magnitude of the dorsal-ventral reflex and enhanced frequency-dependent depression of the reflex in a concentration-dependent manner. Although DOPA and DL-threo-3,4-dihydroxyphenylserine (a direct precursor for norepinephrine) had no effect on the reflex by themselves, DOPA, but not DL-threo-3,4-dihydroxyphenylserine prevented the depression of the reflex response by PCP in a concentration-dependent manner. Inhibition of aromatic-L-amino-acid decarboxylase (EC 4.1.1.2A) by m-hydroxybenzylhydrazine markedly attenuated the action of DOPA in preventing the depression caused by PCP. The dopamine receptor antagonists haloperidol and chlorpromazine blocked the action of DOPA, but the alpha and beta adrenergic receptor antagonists phentolamine and timolol, respectively, did not. In addition, prior treatment of neonatal rats with 6-hydroxydopamine diminished the ability of DOPA to prevent the depressant effect of PCP whereas partially attenuating the depressant effect of PCP alone. These results suggest that DOPA attenuated PCP-induced depression of spinal cord transmission through its conversion to dopamine rather than norepinephrine.

Temperature-dependence of reflex transmission in the neonatal rat spinal cord, in vitro: influence on strychnine- and bicuculline-sensitive inhibition.

The amplitude, latency and rise time of the monosynaptic reflex, recorded from the spinal cord of the neonatal rat in vitro, was temperature-dependent with optimal conditions occurring at 25 degrees C. The reflex was inhibited when conditioning stimuli were applied to an adjacent dorsal root at 1-150 msec before the monosynaptic reflex was evoked; the inhibition had both an early and a late phase. The early phase of inhibition occurred at conditioning-test intervals of 1-20 msec with peak inhibition occurring at 7 msec. Raising the temperature of the bath to 31 degrees C or exposing the cord to strychnine blocked the early phase of inhibition. The late phase of inhibition occurred at conditioning-test intervals greater than 20 msec, was unaffected by temperature and was blocked by bicuculline. The early phase of inhibition therefore appears to be postsynaptic in origin and mediated by glycine while the late component of inhibition may be presynaptic in origin and mediated by gamma-aminobutyric acid (GABA).

Effects of thyrotropin-releasing hormone on phencyclidine- and ketamine-induced spinal depression in neonatal rats.

The effects of thyrotropin-releasing hormone (TRH) were examined on the monosynaptic reflex and on the activity of motoneurons in the spinal cord of the neonatal rat to elucidate the mechanism of action of TRH and its ability to reverse the spinal depression caused by phencyclidine and ketamine. Phencyclidine (2-32 microM) and ketamine (25-200 microM) produced a concentration-dependent depression of the monosynaptic reflex with a 50% inhibitory concentration (IC50) of 8.4 and 77.5 microM, respectively. This depression, however, was significantly attenuated by simultaneous application of TRH (0.03-1 microM) in a concentration-dependent manner. The dose-response curves for phencylidine and ketamine were shifted to the right in the presence of TRH (0.1 microM) with an increase in the respective IC50 of 2- to 3-fold. The incidence of generation of motoneuron spikes, elicited by dorsal root stimulation, was significantly increased by TRH in each neuron tested, concomitantly with an increase in the spontaneous firing rate. These results suggest that TRH directly excites alpha-motoneurons to reverse the spinal depression produced by phencylidine and ketamine.

Segmental synaptic depression caused by diisopropylphosphorofluoridate and sarin is reversed by thyrotropin-releasing hormone in the neonatal rat spinal cord.

The organophosphorus compounds diisopropylphosphorofluoridate (DFP) and isopropylmethylphosphonofluoridate (sarin) depressed the monosynaptic reflex (MSR) in spinal cords from 7- to 9-day-old male rats. The concentrations of DFP and sarin which depressed the MSR by nearly 50% were 100 microM and 100 nM, respectively. Simultaneous superfusion of the cords with thyrotropin-releasing hormone (TRH) with either DFP or sarin resulted in a reversal of the depression. The depression caused by DFP was reversed to 95% of control by 100 nM TRH whereas similar reversal of sarin-induced depression required a 10-fold greater concentration of TRH. The potentiating effect of TRH was not affected by atropine even at a high concentration (1 microM) although atropine easily reversed organophosphorus-induced depression of the MSR. It appears that reversal of organophosphorus-induced depression by TRH might occur through a noncholinergic, TRH-sensitive receptor mechanism and may be unrelated to acetylcholinesterase activity. This action represents a possible utility of TRH as an adjunct in organophosphorus toxicity.

Macromolecular sites for specific neurotoxins and drugs on chemosensitive synapses and electrical excitation in biological membranes.

The present review deals with the molecular mechanisms and elementary phenomena underlying the activation of the voltage- and chemo-sensitive membrane macromolecules: sodium- and potassium-ion channels and nicotinic ACh receptors and their associated ion channel. To achieve an understanding of their various kinetics and conformational states, a number of novel alkaloids, BTX, HTXs, gephyrotoxins, and certain psychotomimetic drugs such as phencyclidine, and many other pharmacologically active agents have been used. Biochemical assays and various electrophysiological techniques have been used in a number of biological preparations--e.g., Torpedo membranes, brain synaptosomes, amphibian and mammalian neuromuscular preparations--to describe the action of such agents. The availability of BTX and scorpion toxins together with aconitine and veratridine as activators and TTX and STX as antagonists of the voltage-sensitive sodium channels, made possible the identification and the physiological and pharmacological characterization of these channels. These studies provided the basis for understanding the mechanisms underlying electrical excitability and culminated, more recently, in the purification and reconstitution of sodium channels from rat brain and in the successful cloning of these channels with the elucidation of their primary structure. We now know that the sodium channel has a molecular mass of 316,000 daltons, consists of five subunits, and has multiple sites for various ligands. In contrast to sodium channels, various classes of potassium channels (inward and outward rectifier potassium channels and Ca(2+)-activated potassium channels) have been described. Unlike the sodium channels, there are no known specific activators for potassium channels. However, a number of potassium channel blockers such as 4-aminopyridine, HTX, histamine, and norepinephrine have been identified which complement the varying types of potassium channels in different neurons. One class of potassium channel blockers with profound medical and social implications comprises PCP and its analogues. The blockade of the potassium-induced 86Rb+ efflux from brain cells, the resulting prolongation of muscle and nerve action potentials, and the increase in transmitter release observed with PCP and some analogues are all highly suggestive of a role for the potassium channel in the behavioral effects of these drugs and its potential involvement in schizophrenia. A number of toxic principles of both plant and animal origin played a significant role in the development of our knowledge about the nAChR.(ABSTRACT TRUNCATED AT 400 WORDS)

Gender-specific action of thyrotropin-releasing hormone in the mammalian spinal cord.

Thyrotropin-releasing hormone (TRH) potentiated the monosynaptic reflex in isolated spinal cords obtained from 7- to 9-day-old rats. A concentration-dependent increase in the monosynaptic reflex was observed in spinal cords obtained from male but not from female or castrated male rats. In contrast, the magnitude of potentiation in cords from ovariectomized control female rats and in ovariectomized female rats treated with testosterone approached that seen in intact males. The results provide evidence that gender plays a prominent role in the variability of response both of humans with amyotrophic lateral sclerosis and of animal tissues to TRH. Furthermore, exposure to androgen during the neonatal period may determine the responsiveness of motoneurons to TRH. Thus the use of TRH in the treatment of amyotrophic lateral sclerosis may be more effective in males than in females.

Pharmacology of the alkaloid pumiliotoxin-B. II. Possible involvement of calcium and sodium-dependent processes in nerve and skeletal muscle.

The mechanism of the twitch potentiating action of pumiliotoxin-B (PTX-B), an indolizidine alkaloid from the skin of the frog Dendrobates pumilio, was studied on frog skeletal muscles. In the presence of PTX-B, a single stimulus to the muscle produced either a burst of repetitive action potentials superimposed on a depolarizing afterpotential or a single potential with a prolonged afterpotential at junctional as well as extrajunctional regions of the frog skeletal muscle fibers. The alkaloid did not cause repetitive activity in quiescent cells or spontaneous contractions. The duration of the burst of action potentials was related inversely and the amplitude and duration of postburst depolarizing after-potential was related directly to the concentration of PTX-B. The typical pattern of repetitive action potentials and postburst depolarization induced by PTX-B could be mimicked by depolarizing the muscle membrane with current pulses of long duration (150-470 ms). Lowering the external calcium or sodium concentration reduced the ability of PTX-B to initiate repetitive action potentials, whereas a low external chloride concentration had no effect. The frequency of MEPPs evoked by potassium, but not the spontaneous MEPP frequency, was increased by PTX-B, suggesting a selective effect on evoked transmitter release. PTX-B evoked repetitive EPPs in response to a single stimulus applied to the nerve, which was dependent upon the external calcium ion concentration. The amplitudes of EPPs in the train were facilitated, and their amplitude increased linearly at the lowest calcium concentration, but not at concentrations from 0.45 to 1.8 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminoglycoside-induced blockade of reflex activity in the isolated spinal cord from the neonatal rat.

The subarachnoid injection of gentamicin into rats causes a transient flaccid paralysis of the hindlimbs lasting 1 to 5 hrs followed by a permanent flaccid paralysis which develops after 24 to 36 hrs. Although the early transient paralysis could be attributed to a pharmacologic blockade of central synaptic transmission, the mechanism of the blockade was not apparent. This study examines the effects of gentamicin and two other aminoglycoside antibiotics, kanamycin and neomycin, on reflex transmission in the isolated, hemisected spinal cord of the neonatal rat and the interaction with calcium. Gentamicin produced a concentration-dependent depression of reflex activity with a 50% inhibitory concentration of 1.6 mM at an external calcium concentration ([Ca2+]o) of 2.5 mM. Reducing the [Ca2+]o by half (i.e., to 1.25 mM) lowered the 50% inhibitory concentration of gentamicin to 0.22 mM. Gentamicin also increased the magnitude of homosynaptic depression of reflex activity in a manner qualitatively similar to that of decreasing [Ca2+]o. Lowering the [Ca2+]o potentiated the effect of gentamicin on homosynaptic depression. The actions of neomycin, kanamycin and magnesium on reflex transmission were nearly identical to those of gentamicin. These findings demonstrate that the early paralysis seen after subarachnoid injection of gentamicin may result from a central blockade of transmission. It is most likely that the site for blockade of reflex activity by gentamicin is presynaptic.