Comparison of the actions of phencyclidine and sigma ligands on CA1 hippocampal pyramidal neurons in the rat

Inhibition of hyperpolarization-activated cation currents by phencyclidine and some sigma ligands in rat hippocampal CA1 pyramidal neurons in vitro

Using whole-cell voltage-clamp recordings, hyperpolarization-activated cation currents (Ih) were elicited with hyperpolarizing voltage jumps in CA1 pyramidal neurons of rat hippocampal slices, and the effects of phencyclidine (PCP) and some sigma ligands on Ih were studied. PCP concentration-dependently (0.1-100 microM) suppressed Ih and shifted the activation curve of Ih to the negative direction. D-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 20 microM) and MK-801 (30 microM), competitive and non-competitive NMDA blockers, respectively, failed to mimic the inhibitory effect of PCP on Ih, and suppression of Ih by PCP was unaffected in the presence of these blockers. To explore the involvement of sigma1 receptors in the reduction of Ih, the effects of representative sigma1 ligands were studied. SKF10047 (100 microM), a sigma1 agonist, attenuated the maximal Ih and shifted the half-activation potential of Ih to the hyperpolarized direction. In the presence of the sigma1 antagonist NE-100 (1 microM), which alone did not affect Ih, the effect of SKF10047 on Ih was unaltered. By contrast, a higher concentration of NE-100 (10 microM) mimicked the effect of SKF10047. Again, no antagonism of Ih suppression by SKF10047 was obtained with rimcazole (100 microM), a sigma1 receptor antagonist that is structurally distinct from NE-100. This concentration of rimcazole alone resulted in a slight but significant reduction of Ih. Thus these major sigma1 ligands appear to suppress Ih independently of their agonistic or antagonistic properties. The results of this study suggest that PCP and some sigma ligands could modulate cell excitability partly through their action on Ih.

Sensitivity of the N-methyl-D-aspartate receptor channel to butyrophenones is dependent on the epsilon2 subunit

The effects of three kinds of butyrophenones, haloperidol, droperidol and spiperone, on the N-methyl-D-aspartate (NMDA) receptor channel were examined on the epsilon1/zeta1, epsilon2/zeta1, epsilon3/zeta1 and epsilon4/zeta1 heteromeric NMDA receptor channels, expressed in Xenopus oocytes. Micromolar concentrations of haloperidol selectively inhibited the epsilon2/zeta1 channel, whereas the epsilon1/zeta1, epsilon3/zeta1 and epsilon4/zeta1 channels were enhanced or minimally affected by higher concentrations of haloperidol. Similarly, droperidol and spiperone inhibited the epsilon2/zeta1 channel more strongly than the other epsilon/zeta channels, although sensitivities of the epsilon2/zeta1 channel to droperidol and spiperone were lower than those to haloperidol. These results suggest that the sensitivities of the NMDA receptor channels to butyrophenones are dependent on the epsilon2 subunit. Furthermore, the replacement with glutamine of the conserved asparagine residue in segment M2, which constitutes the Mg2+ block sites, of the epsilon2 and zeta1 subunits (the mutations epsilon2-N589Q and zeta1-N598Q, respectively) reduced the sensitivities to haloperidol. The mutation zeta1-N598Q reduced the sensitivities to haloperidol more effectively than the mutation epsilon2-N589Q. These results, together with previous findings, suggest that the haloperidol block sites of the NMDA receptor channel partially overlap the Mg2+ block sites.

Uncompetitive inhibition of [3H]1,3-di-o-tolyl-guanidine-defined sigma binding sites by desipramine, propranolol and alprenolol in rat brain

Desipramine, imipramine, clomipramine, (-)-propranolol, (-)-alprenolol, (+/-)-pentazocine and risperidone caused a concentration-dependent inhibition of 6 nM [3H]DTG (1,3-di-o-tolylguanidine)-defined sigma (sigma) binding with Ki values of about 0.5-2.5 microM in well-washed homogenates obtained from rat cerebral cortex. The saturation studies revealed that the inhibition by desipramine (1-4 microM), (-)-propranolol (1 microM) and (-)-alprenolol (3 microM) resulted from a reduction of the Bmax value without alteration of the Kd of [3H]DTG binding to the cortex or hippocampus. In contrast, imipramine, (+/-)-pentazocine, clomipramine and risperidone competitively attenuated the cortical or hippocampal [3H]DTG binding. These findings demonstrate the uncompetitive inhibition of [3H]DTG binding by neuroactive drugs, thereby providing further support for the possible multiple regulation of cerebral sigma receptors.

Evidence that the NH2-terminus of substance P modulates N-methyl-D-aspartate-induced activity by an action involving sigma receptors

Behaviors induced in mice by intrathecal injections of either N-methyl-D-aspartate (NMDA) or kainic acid are modulated by NH2-terminal fragments of substance P, such as substance P-(1-7). The action of substance P-(1-7) on kainic acid depends on sigma receptor activity. The present study was designed to test the hypothesis that sigma receptor activity is also necessary for modulation of NMDA by substance P-(1-7). Intrathecal injection of mice with NMDA results in a brief burst of biting and scratching behaviors which decrease in intensity when NMDA is injected repeatedly at 2 min intervals. Pretreatment with 1,3-di-O-tolylguanidine (DTG), a ligand at both sigma 1 and sigma 2 sites, converted NMDA-induced desensitization to sensitization, thereby enhancing tonic NMDA receptor activity. Although haloperidol (30 min) alone was without effect, the potentiation of NMDA-induced activity by DTG was abolished by haloperidol but unaffected by an equimolar dose of either spiperone or thiothixine, two dopamine receptor antagonists. When mice received substance P-(1-7), NMDA-induced behaviors were initially inhibited but then potentiated. Pretreatment with haloperidol prevented both inhibitory and potentiative effects of substance P-(1-7) whereas thiothixine did not, suggesting inhibitory as well as potentiative modulation of NMDA by sigma receptor activity. Endogenous sigma 1 receptor activity may enhance NMDA receptor activity as a treatment regimen that down-regulates sigma 1 binding also inhibited responses to NMDA. In contrast, pretreatment with haloperidol just 5 min prior to challenge, which blocks both sigma 1 and sigma 2 receptor activity, increased responses to NMDA suggesting an inhibitory effect of sigma 2 receptor activity. In summary, modulation of NMDA by substance P-(1-7) appears to depend on activity at sigma sites as substance P-(1-7) mimicked the potentiative effects of DTG, while haloperidol inhibited the effects of both DTG and substance P-(1-7).

Modulation of NMDA and dopaminergic neurotransmissions by sigma ligands: possible implications for the treatment of psychiatric disorders

Sigma (sigma) receptors, improperly classified as belonging to the opiate receptor family when discovered in 1976, were subsequently confused with phencyclidine binding sites for several years. It's only recently, with the emergence of new selective ligands that their functional significance could be meaningfully addressed. Several subtypes of sigma receptors are present in high densities in the limbic structures as well as in motor-related areas of the CNS. Different lines of evidence suggest that a major role for sigma receptors might be to regulate the activity of the glutamatergic system via the modulation one of its subtype of receptor, the NMDA receptor. This modulation of the glutamatergic system could in turn interfere with the dopaminergic neurotransmission with which, however, sigma ligands could also interact directly. The potential involvement of sigma receptors in schizophrenia has been considered ever since their discovery. The initial suggestion to this respect emerged from the observation that several of the earliest sigma ligands induced psychotomimetic symptoms such as delusions, hallucinations and depersonalization. This link was later reinforced with the demonstration that several neuroleptics, such as haloperidol, have a high affinity for sigma receptors, whereas, some new molecules with a high affinity for sigma receptors, but a low affinity for dopaminergic receptors demonstrated a "neuroleptic-like" pharmacological profile. However, the therapeutic efficacy of selective sigma ligands in schizophrenia has not yet been established and it has even been suggested that sigma receptors might be responsible for some side effects of the classical neuroleptics. The possible implication of sigma receptors in affective disorders has also been suggested by reports showing that some antidepressant drugs have a high affinity for sigma receptors and that long-term treatments with anti- depressant drugs, even with those devoid of affinity for sigma receptors, modify their binding characteristics. In conclusion, indirect evidence suggests possible etiological and/or therapeutic roles for sigma receptors in some psychiatric disorders. However, despite several attempts, no clear indications of a therapeutic efficacy of sigma ligands has yet emerged. More selective ligands and fundamental studies on the respective role of the different subtypes of sigma receptors are needed before clear concepts can be formulated. p3

Role of calcium in sigma-mediated neuroprotection in rat primary cortical neurons

Since unique calcium dynamics have been reported for toxic (40-80 M) and non-toxic (5-10 microM) concentrations of glutamate, we evaluated the effect of neuroprotective sigma ligands on glutamate and potassium chloride (KCl)-stimulated changes in [Ca2+]i using 12-15 day old primary rat neuronal cortical cultures. In approximately 80% of the neurons tested, 80 microM glutamate caused a sustained calcium flux previously shown to be associated with neurotoxicity. The majority of sigma ligands that were evaluated altered glutamate-induced calcium flux. For example, the primary effect of maximally neuroprotective concentrations of the sigma ligands dextromethorphan, (+)-pentazocine, (+)-cyclazocine, (+)-SKF 10047, carbetapentane and haloperidol was a shift from a sustained, to either a biphasic or a monophasic transient calcium response indicative of neuroprotection. (+)-3-PPP, previously shown not to be neuroprotective in this model system, failed to alter glutamate-induced calcium flux. In contrast to glutamate, KCl (50 mM) produced changes in [Ca2+]i which were not neurotoxic to the neurons as measured by LDH release. The primary response observed in 59% of the neurons treated with 50 mM KCl alone was an initial spike in [Ca2+]i which abruptly declined then plateaued above basal levels throughout the 12 min of analysis (modified sustained response). The highly selective sigma ligands produced a shift from the modified sustained response to a monophasic transient calcium response. Again, (+)-3-PPP had no effect on KCl-induced calcium dynamics. Of the PCP-related sigma ligands only (+)-SKF-10047 consistently attenuated the KCl-induced calcium flux. Collectively, these results indicate that modulation of [Ca2+]i through receptor and voltage-gated calcium channels contributes significantly to sigma mediated neuroprotection.

3-(3-Hydroxyphenyl)-N-(1-propyl)piperidine elicits convulsant effects in mice

1. The behaviour and EEG effects of the dopamine and sigma (sigma) ligands (+) 3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)3-PPP) were studied in mice. 2. (+) 3-PPP dose-dependently (60-100 mg/kg i.p.) produced behavioural and electrical tonic-clonic seizures. 3. The incidence of the tonic seizures elicited by 100 mg/kg of the drug was significantly (P < 0.05) prevented by spiperone (0.5 mg/kg i.p.) and haloperidol (0.5 mg/kg i.p.). 4. The results show an influence on the behavioural and electrical threshold of convulsions by (+) 3-PPP depending on a prevalent interference on dopamine receptors.

Distinct neuroprotective profiles for sigma ligands against N-methyl-D-aspartate (NMDA), and hypoxia-mediated neurotoxicity in neuronal culture toxicity studies

Substantiating evidence has raised the possibility that sigma ligands may have therapeutic potential as neuroprotective agents in brain ischemia. It has been suggested that the neuroprotective capacity of sigma ligands is related primarily to their affinity for the NMDA receptor complex and not to any selective action at the sigma binding site. However, sigma specific ligands, devoid of significant affinity for the NMDA receptor, are also neuroprotective via an inhibition of the ischemic-induced presynaptic release of excitotoxic amino acids. In the present study, we have investigated the potential neuroprotective effect of a comprehensive series of sigma ligands, with either significant (sigma/PCP) or negligible (sigma) affinity for the PCP site of the NMDA receptor, in order to delineate a selective sigma site-dependent neuroprotective effect. For this aim, we have employed two different neuronal culture toxicity paradigms implicating either postsynaptic-mediated neurotoxicity, (brief exposure of cultures to a low concentration of NMDA or Kainate) or pre- and postsynaptic mechanisms (exposure to hypoxic/hypoglycemic conditions). Only sigma ligands with affinity for the NMDA receptor [(+) and (-) cyclazocine, (+) pentazocine, (+) SKF-10047, ifenprodil and haloperidol] were capable of attenuating NMDA-induced toxicity whereas the sigma [(+)BMY-14802, DTG, JO1784, JO1783, and (+)3-PPP] and kappa-opioid [CI-977, U-50488H] ligands, with very low affinity for the NMDA receptor, were inactive. The rank order of potency, based on the 50% protective concentration (PC50) value, of sigma/PCP ligands against NMDA-mediated neurotoxicity correlates with their affinity for the PCP site of the NMDA receptor, and not with their affinity for the sigma site. In addition sigma/PCP, sigma or kappa-opioid ligands failed to attenuate kainate-mediated neurotoxicity. On the other hand, sigma/PCP, sigma and kappa-opioid ligands were potent inhibitors of hypoxia/hypoglycemia-induced neurotoxicity, although their neuroprotective potency did not correlate with their affinity for either the sigma or PCP binding sites. In conclusion, the ability of sigma and kappa-opioid ligands to attenuate hypoxia/hypoglycemia, but not NMDA or kainate-induced toxicity, suggests that these drugs exert their neuroprotective role by a predominantly presynaptic mechanism possibly by inhibiting ischemic-mediated glutamate release.

Sigma receptor-mediated neuroprotection against glutamate toxicity in primary rat neuronal cultures

The role of the putative sigma receptor in mediating neuroprotection against glutamate-induced neuronal injury was examined in mature cultured rat cortical neurons. With the exception of the selective sigma 1 ligand (+)-3-PPP, all of the sigma ligands tested were neuroprotective, preventing glutamate-induced morphological changes and increases in LDH release. Their rank order of neuroprotective potency (and EC50 values) was as follows: (+)-SKF 10,047 (0.81 microM) > (+)- cyclazocine (2.3 microM) > dextromethorphan (3.1 microM) = haloperidol (3.7 microM) > (+)-pentazocine (8.5 microM) > DTG (42.7 microM) = carbetapentane (46.3 microM). When corrected for relative sigma versus PCP binding affinity, it appears that a positive correlation exists between neuroprotective potency and sigma 1 site affinity. However, there does not appear to be a significant correlation between neuroprotective potency and the sigma 2 site. Critically, none of the sigma ligands were neurotoxic when tested alone at concentrations at least 5-30 times their respective neuroprotective EC50 values. Results from preliminary experiments with the selective sigma 1 ligand (+)-pentazocine indicated that sigma-mediated neuroprotection may involve the buffering of glutamate-induced calcium flux. Collectively, the results of these in vitro experiments demonstrate that sigma ligands are neuroprotective and therefore deserve further exploration as potential therapeutic agents in in vivo models of CNS injury and neurodegenerative disorders.

A role of sigma receptors on hypoxia/hypoglycemia-induced decrease in CA1 presynaptic fiber spikes in rat hippocampal slices

Effects of sigma receptor agonists or antagonists on hypoxia/hypoglycemia-induced decrease in CA1 presynaptic fiber spikes elicited by the stimulation of Schaffer collaterals were investigated using rat hippocampal slices. Treatment with sigma receptor antagonists such as haloperidol, NE-100 and rimcazole produced a concentration-dependent attenuation of the hypoxia/hypoglycemia-induced decrease in CA1 presynaptic fiber spikes. The order of potency of protection against hypoxia/hypoglycemia-induced reduction in CA1 presynaptic potential was: NE-100 = haloperidol > rimcazole. Treatment with sigma receptor agonist DTG potentiated the hypoxia/hypoglycemia-induced decrease in the CA1 presynaptic potential, whereas SKF10047 which possesses an affinity for phencyclidine site attenuated the decrease of potential. NE-100 antagonized a functional deficit induced by DTG, but unaffected the improving effect induced by SKF10047. The present results suggest a facilitatory role of sigma receptor stimulation in hypoxia/hypoglycemia-induced an impairment of neurophysiological functions in CA1 presynaptic regions of hippocampal slices.

The effects of sigma ligands and of neuropeptide Y on N-methyl-D-aspartate-induced neuronal activation of CA3 dorsal hippocampus neurones are differentially affected by pertussin toxin

1. The in vivo effects of the high affinity sigma ligands 1,3-di(2-tolyl)guanidine (DTG), (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1- ethyl-but-3-en-1-ylamine hydrochloride (JO-1784), (+)-pentazocine and haloperidol, as well as of those of neuropeptide Y (NPY), on N-methyl-D-aspartate (NMDA)- and quisqualate (Quis)-induced neuronal activations of CA3 pyramidal neurones were assessed, using extracellular unitary recording, in control rats and in rats pretreated with a local injection of pertussis toxin (PTX), to evaluate the possible involvement of Gi/o proteins in mediating the potentiation of the neuronal response to NMDA by the activation of sigma receptors in the dorsal hippocampus. 2. Microiontophoretic applications as well as intravenous injections of (+)-pentazocine potentiated selectively the NMDA response in control rats as well as in PTX-pretreated animals. In contrast, the PTX pretreatment abolished the potentiation of the NMDA response by DTG, JO-1784 and NPY. Moreover, microiontophoretic applications of DTG induced a reduction of NMDA-induced neuronal activation. Neither in control nor in PTX-treated rats, did the sigma ligands and NPY have any effect on Quis-induced neuronal response. 3. In PTX-treated rats, the potentiation of the NMDA response induced by (+)-pentazocine was suppressed by haloperidol, whereas the reduction of the NMDA response by DTG was not affected by haloperidol. 4. This study provides the first in vivo functional evidence that sigma ligands and NPY modulate the NMDA response by acting on distinct receptors, differentiated by their PTX sensitivity.

Behavioral evidence for a modulating role of sigma ligands in memory processes. II. Reversion of carbon monoxide-induced amnesia

This study examined the effect of low doses of sigma ligands on amnesia induced in mice by successive carbon monoxide (CO) exposure. Mice were exposed three consecutive times to CO (10 ml/min, 30-50 s) at 38 degrees C. Spatial working memory impairment was investigated 5 days later by monitoring spontaneous alternation behavior in a Y-maze. Delayed amnesia was examined 7 days after CO exposure by using a step-down passive avoidance test. The preadministration of the sigma ligand 1,3-di-(2-tolyl)guanidine (DTG), at doses of 1 to 1000 microgram/kg, s.c., 30 min before CO exposure did not affect the resulting amnesia in either test. However, when administered 30 min before the test, i.e., 5 or 7 days after CO exposure, this agent completely reversed the CO-induced decrease in alternation performance, at doses of 10 to 100 micrograms/kg. The same effect was observed with (+)-N-allylnormetazocine ((+)-SKF 10,047), at doses of 100 to 300 micrograms/kg, but not with (-)-SKF 10,047. DTG, at the same dose range that reversed the decrease in alternation, also totally reversed the CO-induced decrease in step-down latency in the passive avoidance test. The curve for these effects was bell-shaped; the effects were not observed at the dose of 1 mg/kg. Moreover, alpha-(4-fluorophenyl-2-pyrimidinyl)-1-piperazine butanol (BMY 14802), a putative sigma antagonist (1-10 mg/kg i.p.), did not affect CO-induced amnesia, but when simultaneously administered with DTG, it completely prevented its effect in both tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective reduction of N-methyl-D-aspartate-evoked responses by 1,3-di(2-tolyl)guanidine in mouse and rat cultured hippocampal pyramidal neurones

1. The effects of 1,3-di(2-tolyl)guanidine (DTG) were examined on the responses of cultured hippocampal neurones to the excitatory amino acid analogues N-methyl-D-aspartate (NMDA), kainate, quisqualate and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). 2. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, DTG (10-100 microM) produced a concentration-dependent depression of the NMDA-evoked rises in intracellular free calcium ([Ca2+]i), an effect that was not modified by changes in the extracellular glycine concentration. DTG (at 50 and 100 microM) also attenuated, although to a lesser extent, the rises in [Ca2+]i evoked by naturally-derived quisqualate. In contrast, 50 and 100 microM DTG did not depress responses evoked by kainate, AMPA and synthetic, glutamate-free (+)-quisqualate although on occasions DTG enhanced kainate- and AMPA-evoked rises in [Ca2+]i. 3. DTG attenuated NMDA-evoked currents recorded from mouse hippocampal neurones under whole-cell voltage-clamp with an IC50 (mean +/- s.e. mean) of 37 +/- 5 microM at a holding potential of -60 mV. The DTG block of NMDA-evoked responses was not competitive in nature and was not dependent on the extracellular glycine or spermine concentration. The block did, however, exhibit both voltage-, and use-, dependency. The steady-state current evoked by naturally-derived quisqualate was also attenuated by DTG whereas those evoked by kainate and AMPA were not. 4. We conclude that DTG, applied at micromolar concentrations, is a selective NMDA antagonist in cultured hippocampal neurones, the block exhibiting both Mg(2+)- and phencyclidine-like characteristics. Given the nanomolar affinity of DTG for sigma binding sites it is unlikely that the antagonism observed here is mediated by sigma-receptors, but the data emphasize the potential danger of ascribing the functional consequences of DTG administration solely to sigma receptor-mediated events.

The sigma ligand 1,3-di-o-tolylguanidine depresses amino acid-induced excitation non-selectively in rat brain

The sigma ligand 1,3-di-o-tolylguanidine (DTG) has been applied by microiontophoresis to neurones in the rat hippocampal slice and to neurones in the neocortex and hippocampus of rats anaesthetised with urethane. DTG depressed the excitatory responses of cells to both N-methyl-D-aspartate (NMDA) and quisqualate on a majority of the units tested, in no case causing an enhancement. Haloperidol had no consistent effect of its own and did not prevent the depressant effects of DTG. It is concluded that in the preparations used, DTG did not selectively modify neuronal sensitivity to NMDA.

Characterization of specific binding sites for [3H]-1,3-di-o-tolyl-guanidine (DTG) in the rat glioma cell line C6-BU-1

The aim of the present study was to find out if a cell line of glial origin possesses sigma and/or phencyclidine (PCP) binding sites. Binding of [3H]1,3-di-o-tolyl-guanidine (DTG), a highly selective ligand for sigma binding sites, and of [3H]N-[1-(2-thienyl)cyclohexyl] piperidine ([3H]TCP), a radioligand specific for PCP receptors, to C6-BU-1 glioma cells was investigated. Binding of [3H]DTG to C6-BU-1 cell membranes was reversible, saturable (Bmax = 10.5 pmol/mg protein), and of high affinity (KD = 26 nM). C6-BU-1 cells do not possess PCP receptors as indicated by negligible specific binding of [3H]TCP to C6-BU-1 cell membranes. Specific binding of [3H]DTG was reduced in the presence of Ca2+ and to a lesser extent by Mg2+. The rank order of potency of various PCP and sigma ligands was DTG > (+)3-[(3-hydroxy-phenyl)-N-n-propyl-piperidine] [(+)3-PPP] > haloperidol > pentazocine > (-)3-PPP > PCP > metaphit > dextromethorphan > (-)butaclamol > (+)butaclamol > (-)N-allylnormetazocine [(-)SKF 10,047] > MK801 > (+)SKF 10,047 > ketamine. The drug specificity, confirmed by a reversed stereoselectivity for the benzomorphan opiate SKF 10,047, indicated that these sites correspond to a subtype of sigma binding sites, the so-called sigma 2 binding site. Thus, the C6-BU-1 cell line is the first glial cell line demonstrated to have sigma 2 binding sites.

Electrophysiological effects of phencyclidine and the sigma agonist ditolylguanidine in the cerebellum of the rat

The electrophysiological actions of phencyclidine (PCP) and the sigma agonist 1,3-di(2tolyl)guanidine (DTG) were examined in the cerebellum of urethane-anesthetized rats. The object of the study was to determine if PCP and sigma agonists shared a common mechanism of action. The cerebellar Purkinje neuron was chosen because it has sigma receptors but not N-methyl-D-aspartate receptors, where PCP has additional effects. Both DTG and PCP decreased the spontaneous discharge rate of cerebellar Purkinje neurons after parenteral administration. When the drugs were applied locally to single Purkinje neurons, using pressure ejection through multibarrel micropipettes, both compounds decreased the spontaneous activity of the neurons with equal potency. Previous studies have shown that the actions of PCP in the cerebellum are dependent upon an interaction with noradrenergic terminals from the nucleus locus coeruleus. A similar finding was made in this study for DTG. Elimination of the noradrenergic input by lesion with the neurotoxin, 6-hydroxydopamine, diminished equally the effects of PCP and DTG. Treatment of the animals with haloperidol had similar effects. It is concluded that PCP and the sigma agonist DTG both act as indirect noradrenergic agonists in the cerebellum.

The lack of utility of the rat vas deferens as a functional bioassay for sigma ligands

The present study examined the utility of the rat vas deferens preparation as a bioassay for sigma site ligands. sigma Ligands such as (+/-)-pentazocine, phencyclidine (PCP) and (+)-SK&F 10047 potentiated neurogenic twitch contractions. However, neither the order of potency nor the absolute potency of (+/-)-pentazocine and (+)-SK&F 10047 correlated with their affinity at central sigma sites. Furthermore, another potent sigma ligand, ditolyl-ortho guanidine (DTG) neither affected neurogenic twitch contractions nor inhibited twitch potentiation by PCP or (+)-SK&F 10047 at concentrations up to 30 mumol/l. These data indicate that the rat vas deferens is not a useful bioassay for the evaluation of sigma ligands. PCP, (+)-SK&F 10047 and (+/-)-pentazocine probably enhance neurogenic contractions in rat vas deferens primarily by inhibition of the neuronal uptake of noradrenaline.

Intracellular analysis of effects of phencyclidine on rat neostriatal neurons recorded in vitro

The effects of phencyclidine (PCP) on the synaptic and direct membrane properties of neostriatal neurons were assessed using intracellular recordings from 62 neurons in rat neostriatal slices. Electrophysiological measurements were obtained before, during and after bath application of PCP. At all concentrations (1-300 microM) PCP raised current thresholds for evoking synaptic potentials and reduced excitability (the firing rate of directly evoked action potentials). In a smaller number of cells PCP (50-300 microM) increased input resistance. These effects were long-lasting and neurons did not show reversal with washes as long as 75 min. Some neurons were injected with biocytin for identification. All were medium-sized spiny type I neurons typical of rodent neostriatum.

N-methyl-D-aspartate-induced neuronal activation is selectively modulated by sigma receptors

The effects of two high-affinity sigma ligands, DTG (1,3-di(2-tolyl)guanidine) and haloperidol, on the activation of dorsal hippocampus pyramidal neurons induced by microiontophoretic application of N-methyl-D-aspartate (NMDA) were assessed electrophysiologically. Low doses of DTG (0.5-3 micrograms/kg i.v.) potentiated the NMDA response. This effect of DTG was blocked by haloperidol (10 micrograms/kg i.v.), but not by spiperone, a potent dopamine antagonist with low affinity for sigma receptors. These results suggest that sigma receptors modulate the NMDA-induced neuronal activation.

Inhibition of potassium currents by the sigma receptor ligand (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine in sympathetic neurons of the mouse isolated hypogastric ganglion

The actions of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)3-PPP] on sympathetic neurons of the mouse isolated hypogastric ganglion were studied using the current clamp and single electrode voltage clamp techniques. In neurons studied under current clamp (+)3-PPP (10(-5) to 3 x 10(-4) M) evoked a concentration-dependent depolarization, which was fully reversible on washout of the drug. The depolarization was associated with an increase in membrane input resistance. At membrane potentials between -43 and -65 mV the amplitude of the depolarization was inversely related to the membrane potential. (+)3-PPP had no effect on membrane potential at potentials negative to -65 mV. The effect of (+)3-PPP on the M-current was studied in cells voltage clamped at -40 mV and stepped to -60 mV for 300-500 ms. The slow current relaxations seen during and after the voltage step are largely due to the M-current. (+)3-PPP (3 x 10(-5) to 3 x 10(-4) M) inhibited the M-current and produced an inward current in a concentration-dependent manner. (-)3-PPP (3 x 10(-5) M) had similar effects, but was less potent than (+)3-PPP. (+)3-PPP (3 x 10(-5) M) also inhibited the A-current and a calcium-dependent potassium current, but to a lesser degree than the M-current.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of sigma-receptor ligands to reduce the excitatory actions of N-methyl-DL-aspartate on rat spinal neurons in-vivo

Haloperidol and (+)-3-PPP, compounds with known affinity for the sigma-receptor, have been examined for their ability to reduce the excitatory actions of N-methyl-DL-aspartate (NMDLA), quisqualate and kainate on rat spinal neurons in-vivo. The actions of (-)-3-PPP were also tested. Haloperidol was injected intravenously whereas the 3-PPP enantiomers were administered by microelectrophoresis. Haloperidol had little effect on excitations evoked by NMDLA, quisqualate or kainate whereas both (+)- and (-)-3-PPP usually enhanced, non-selectively, responses to all three excitatory amino acid analogues. The results support suggestions that phencyclidine (PCP)-like compounds with affinity for both PCP and sigma-receptors reduce neuronal excitations mediated by the N-methyl-D-aspartate (NMDA) receptor via a selective effect at the PCP site.

Calcium-dependent displacement of haloperidol-sensitive sigma receptor binding in rat hippocampal slices following tissue depolarization

To evaluate the possible existence of an endogenous ligand for the haloperidol-sensitive sigma receptor, we developed an in vitro competition assay to measure endogenous ligand release. Depolarization of in vitro hippocampal slices by either veratridine or potassium reduced [3H]ditolylguanidine binding in a calcium-dependent and transient manner. None of the drugs or iron substitutions directly affected [3H]ditolylguanidine binding to rat brain membranes. Veratridine-induced depolarization also reduced the binding of [3H](+)3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, another sigma radioligand, in a calcium-dependent manner. Radioligand displacement was not associated with alteration in sigma receptor dissociation kinetics or receptor degradation in the hippocampal slice. In contrast, KC1 depolarization had no effect on [3H]ditolyguanidine binding to sigma receptors in liver slices. The results suggest that a calcium-dependent, depolarization-induced reduction in sigma receptor binding may have been caused by the release of an endogenous sigma ligand in rat hippocampal tissue.

An examination of the putative sigma-receptor in the mouse isolated vas deferens

1. The effects of several ligands which interact with the sigma-binding site were studied on the electrically-evoked (0.1 Hz) neurogenic twitch contractions of the mouse isolated vas deferens. 2. (+)-3-(3-Hydroxyphenyl)-N-(1-propyl)piperidine [+)-3-PPP) (10(-8) - 10(-5) M), inhibited the neurogenic twitch contractions. This inhibitory action was unaffected by naloxone (10(-6)M), idazoxan (10(-6)M), cocaine (10(-6)M) or tyramine (10(-4)-3 x 10(-4)M), but was abolished by the dopamine D2-antagonist, sulpiride (10(-6)M). Therefore, in order to study the potentiating actions of sigma ligands, sulpiride (10(-6)M) was used to prevent any inhibitory actions mediated via dopamine D2-receptors. 3. In the presence of sulpiride (10(-6)M), haloperidol (10(-6)-10(-5)M), (+)-3-PPP (10(-6)-3 x 10(-4) M) and (+)-N-allyl-N-normetazocine [+)-SKF 10,047) (10(-5)-10(-4)M) each reversibly potentiated the neurogenic twitch contractions in a concentration-dependent manner. The rank order of potency was haloperidol greater than (+)-3-PPP greater than (+)-SKF 10,047. 4. The stereoisomers of 3-PPP displayed stereoselectivity with (+)-3-PPP being more potent than (-)-3-PPP. 5. At a concentration that did not potentiate the twitch contractions, (3 x 10(-7)M), haloperidol did not antagonize the potentiating action of (+)-3-PPP (3 x 10(-5)M). 6. 1,3-Di-O-tolyguanidine (DTG) (10(-8)-10(-5)M) had no effect on the amplitude of twitch contractions and did not affect the potentiating action of (+)-3-PPP (10(-5)-3 x 10(-5)M). 7. It is concluded that a-ligands potentiate neurogenic twitch contractions of the mouse isolated vas deferens via a site that is different from the central sigma-binding site.

Comparison of opioid and GABA receptor control of excitability and membrane conductance in hippocampal CA1 pyramidal cells in rat

Opioids are thought to increase the excitability of hippocampal pyramidal cells by decreasing release of neurotransmitter from inhibitory interneurons. This study compared the actions of the opioid agonist normorphine, and the GABA receptor antagonist bicuculline, on the responses of CA1 pyramidal cells to afferent stimulation. Both normorphine and bicuculline increased the sensitivity of pyramidal cells to presynaptic stimulation, increased the number of population spikes and action potentials elicited, increased the duration of the excitatory postsynaptic potential (EPSP) and reduced the change in input conductance during the early inhibitory postsynaptic potential (IPSP). Unlike bicuculline, normorphine also decreased the change in conductance during the late inhibitory postsynaptic potential. The decreased change in the conductance of pyramidal cells caused by normorphine during both early and late inhibitory postsynaptic potentials supports the hypothesis that opioids decrease the release of GABA from inhibitory interneurons. In addition to reducing GABA-mediated changes in conductance, both normorphine and bicuculline unmasked a D-APV-sensitive conductance, measured during the early inhibitory postsynaptic potential. These results demonstrate that activation of opioid receptors enhances the excitability of CA1 pyramidal cells by decreasing GABA-mediated early and late inhibitory postsynaptic potentials and by unmasking NMDA receptors.

Differentiation of phencyclidine and sigma receptor types affecting the central inspiratory termination mechanism in cat

The effects of 1) the phencyclidine receptor ligand TCP, 2) sigma receptor ligands (+)3-PPP and DTG, and 3) N-methyl-D-aspartate receptor blockers MK-801 and dextrorphan were determined on a brainstem mechanism which controls the termination of the inspiratory phase of the breathing cycle. Inspiratory bursts were recorded from the phrenic nerve in decerebrate paralyzed cats ventilated by means of a phrenic driven servoventilator. The central mechanism which terminates inspiration was tested by withholding lung inflation, thus suppressing the contribution of the vagal feedback from the lungs to inspiratory termination. TCP increased the duration of test inspiration (tTi) by 17% at 0.03 mg/kg and by 14-fold (from 1.6 to 23 s) at 1 mg/kg. With dextrorphan, tTi was significantly increased at 3 mg/kg. In contrast, (+)3-PPP and DTG did not increase tTi at doses up to 10 mg/kg, although MK-801 (0.03 mg/kg), given after the sigma ligands, increased tTi by 59-90%. It is concluded that phencyclidine but not sigma receptor ligands block the central mechanism which terminates inspiration and that the likely site of action is the NMDA receptor complex.