The neurotransmitter candidature of sulphur-containing excitatory amino acids in the mammalian central nervous system.

While L-glutamate (L-Glu) is considered to be the predominant excitatory amino acid transmitter in the mammalian CNS, other amino acids have come under scrutiny as possible rivals for such a role. These include four sulphur-containing analogues of L-Glu and L-aspartate known as the SAAs. The L-Glu analogues are L-homocysteic acid and L-homocysteine sulphinic acid, while the L-aspartate analogues are L-cysteic acid and L-cysteine sulphinic acid. They are mixed agonists of excitatory amino acid receptors on a variety of neurones and are reported to be present in and released from mammalian CNS tissue. This review serves to summarize the current state of research into the possibility that one or more of these compounds is indeed a transmitter within the mammalian CNS.

Further evidence that behavioral tests and neuropeptide mRNA and tissue level alterations can differentiate between typical and atypical antipsychotic drugs.

This study was designed to compare some behavioral and biochemical effects of chronic treatment with a range of antipsychotic drugs. Gene expression of enkephalin, chromogranin A, chromogranin B, and secretogranin II and their respective peptide products were studied with in situ hybridization and radioimmunoassays after daily oral administration of haloperidol, clozapine, risperidone, or zotepine for 21 days. In behavioral tests, significant catalepsy was induced by haloperidol only. All four antipsychotic drugs increased hind paw retraction time but only haloperidol also increased forelimb retraction time. In the caudate putamen, haloperidol increased both enkephalin mRNA expression and enkephalin tissue levels. Neither of these parameters was altered by the other three drugs. In the prefrontal cortex, antipsychotic drugs generated a distinct pattern of gene expression in two regards. First, the dopamine D(2) receptor antagonist, haloperidol, did not significantly alter synaptic protein levels or their encoding mRNAs. Secondly, there was a differential change in tissue levels and mRNA expression since secretogranin II was not affected by any tested antipsychotic drug. This study shows that different types of antipsychotic drug induce distinct behavioural effects as well as differential changes in the biosynthesis of synaptic proteins and their encoding mRNAs. The data reinforce the notion that haloperidol can be classed as a typical antipsychotic drug whilst clozapine, zotepine, and risperidone reflect their atypical classification.

An investigation into signal transduction mechanisms involved in DHPG-induced LTD in the CA1 region of the hippocampus.

Previously, we have found that activation of mGlu receptors using a group I-specific mGlu receptor agonist, (RS)-3,5-DHPG, can induce long-term depression (LTD) in the CA1 region of the hippocampus and that, once established, this synaptic depression can be reversed by application of the mGlu receptor antagonist, (S)-MCPG [Palmer et al., 1997. Neuropharmacology 36, 1517-1532]. We have started to investigate the signal transduction mechanisms involved in these effects. Group I mGlu receptors couple to phospholipase C and therefore can activate protein kinase C and mobilise Ca2+ from intracellular stores. However, neither protein kinase C inhibitors (chelerythrine or Ro 31-8220) nor agents which deplete intracellular Ca2+ stores (thapsigargin or cyclopiazonic acid) were able to prevent DHPG-induced LTD. Furthermore, the ability of MCPG to reverse DHPG-induced LTD was not prevented by these compounds. These results suggest that it is unlikely that DHPG-induced LTD, or its reversal by MCPG, is produced via activation of either protein kinase C or by release of Ca2+ from intracellular stores.

The actions of a range of excitatory amino acids at (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-depolarizing receptors on neonatal rat motoneurones.

Depolarizations induced by a range of amino acids including some sulphur-containing excitatory transmitter candidates were evoked from motoneurones in the neonatal rat spinal cord under conditions that precluded activation of known ionotropic glutamate receptors. The responses could be partially and differentially depressed by continuous application of several metabotropic glutamate receptor (mGluR) antagonists or by receptor desensitization with the mGluR agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]. In most cases [the exceptions being (1S,3R)-ACPD and to a lesser extent, quisqualate], the major component of these depolarizations was resistant to antagonism by phenylglycine-derived mGluR antagonists or desensitization of (1S,3R)-ACPD-sensitive receptors. Of the excitatory responses observed with the tested agonists, those evoked by L-glutamate itself were generally the least affected by blockade of known glutamate receptors.

A CaMKII inhibitor, KN-62, facilitates DHPG-induced LTD in the CA1 region of the hippocampus.

We have shown previously that activation of mGlu receptors using a group I specific mGlu receptor agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG), can induce long-term depression (LTD) in the CA1 region of the hippocampus (Palmer et al., 1997). We now report that DHPG-induced LTD is facilitated by treatment with KN-62, an inhibitor of certain Ca2+/calmodulin-dependent protein kinases (CaMKs), including CaMKII.

Elevation of extracellular cortical noradrenaline may contribute to the antidepressant activity of zotepine: an in vivo microdialysis study in freely moving rats.

The antipsychotic, zotepine, as well as possessing affinity for dopamine D1- and D2-1ike receptors, has high affinity for the noradrenaline (NA) transporter and inhibits [3H]NA uptake by rat frontal cortex synaptosomes, in vitro. The present studies investigated the effects of zotepine on extracellular NA in the frontal cortex of freely moving rats using in vivo microdialysis. Removal of calcium from the perfusate reduced extracellular NA by 70.5% and prevented the 50 mM KCl-stimulated increase in NA levels. Zotepine (0.5-1.5 mg kg(-1) i.p.), evoked biphasic, dose-dependent rises in extracellular NA with maximal increases observed at 60 min (+ 171.0%) and 240 min (+ 211.5%) post-treatment. The increases in NA levels were sustained for up to 100 min post-dosing. Clozapine (10.0 mg kg(-1) i.p.), resulted in a smaller, transient increase in NA levels (+ 72.0%) which lasted for 20 min post-treatment. Neither ziprasidone (3.0 mg kg(-1) i.p.) nor olanzapine (1.0 mg kg(-1) i.p.) influenced extracellular NA. Systemic treatment with the antidepressant desipramine (0.3 mg kg(-1) i.p.) resulted in a prolonged elevation of NA levels over 240 min (maximal increase of + 354.3%), whilst local infusion of nisoxetine (1-100 microM) through the dialysis probe increased NA levels in a concentration-dependent manner (up to 587.8% of control values). These data suggest that the inhibition of NA uptake by zotepine and its subsequent prolonged elevation of extracellular cortical NA may underlie the reported antidepressant properties of zotepine in schizophrenic patients.

Differential actions of 3-(4-chlorophenyl) glutamic acid stereoisomers and L-trans-pyrrolidine-2,4-dicarboxylic acid upon L-homocysteic acid- and L-glutamic acid-induced responses from rat spinal motoneurones.

The four recently synthesized stereoisomers of 3-(4-chlorophenyl) glutamic acid (chlorpheg) were individually examined for their abilities to potentiate depolarizations of neonatal rat motoneurones evoked by L-homocysteic acid (L-HCA, 10 microM). This property had previously been observed using the racemate and is believed to be mediated by uptake inhibition. Both the (2S,3S)- and (2S,3R)- isomers were selective potentiators of L-HCA- (vs L-Glu) induced depolarizations although the (2S,3S)- isomer was more effective. The (2R,3S)- isomer had a slight but significant depressant action which could be attributed to N-methyl-D-aspartate (NMDA) receptor antagonism. Comparison of the potentiating properties of (2S,3S)- and (2S,3R)-chlorpheg with those of L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC, a L-Glu uptake inhibitor) upon L-HCA- and L-Glu-evoked responses revealed that both chlorpheg isomers (500 microM each) selectively potentiated responses evoked by L-HCA (10 microM) but had no significant effect upon those evoked by L-Glu (50 microM). On the other hand, use of tPDC at the same concentration significantly enhanced the depolarizations evoked by both amino acids, although its action on L-Glu-evoked responses was greater. It is concluded that (i) the (2S,3S)- isomer and to a lesser extent, the (2S,3R)- isomer of chlorpheg are responsible for the potentiating actions seen with the chlorpheg racemate used in previous studies and (ii) (2R,3S)-chlorpheg is a weak NMDA antagonist. The apparently selective action of (2S,3S)- and (2S,3R)-chlorpheg upon L-HCA-relative to L-Glu-induced depolarizations supports the existence of multiple excitatory amino acid uptake sites, some of which may yet be unidentified.

Diastereoselective synthesis of all four isomers of 3-(4-chlorophenyl) glutamic acid: identification of the isomers responsible for the potentiation of L-homocysteic acid-evoked depolarizations in neonatal rat motoneurons.

All four isomers of 3-(4-chlorophenyl)glutamic acid (5-8) were prepared by diastereoselective synthesis. Addition of (6S)-(+)-bis-lactim ether 15 to cis-4-chlorocinnamate 12 gave a mixture comprising mainly the (2R,3S)- and (2R,3R)-isomers 5 and 6, respectively (in a ratio of 56:40), while addition of (6R)-(-)-bis-lactim ether 16 to 4-chlorocinnamate 12 gave a mixture comprising mainly the (2S,3R)- and (2S,3S)-isomers 8 and 7, respectively (in a ratio of 56:42). The four stereoisomers (5-8) were therefore conveniently prepared by addition of either 3-lithio-(6S)- or -(6R)-bis-lactim ether (15 or 16, respectively) to 4-chlorocinnamate 12 and separation of the resultant mixtures of diastereoisomers (23-26) by flash silica gel chromatography. The absolute configurations of 6 and 7 were confirmed by X-ray crystallography. Both the (2S,3S)- and (2S,3R)-isomers (7 and 8, respectively) at a concentration of 100 microM significantly potentiated depolarizations induced by 10 microM L-homocysteic acid (L-HCA) (% control +/- sem: 130.4 +/- 3.6, n = 20 and 114.5 +/- 2.4, n = 11, respectively) while the (2R,3S)-isomer 5 significantly reduced L-HCA response amplitude (94.2 +/- 1.4, n = 9) and the (2R,3R)-isomer 6 was inactive. Experiments designed to compare the agonist-potentiating actions of 7 and 8 in the neonatal rat spinal cord with L-trans-pyrrolidine-2,4-dicarboxylic acid, the well-known L-Glu uptake inhibitor, provided additional evidence for the selective enhancement of depolarizations due to L-HCA and not those due to L-Glu. This selective action supports the existence of multiple excitatory amino acid uptake sites.

Thalamic control of dopaminergic functions in the caudate-putamen of the rat--I. The influence of electrical stimulation of the parafascicular nucleus on dopamine utilization.

A neurochemical response of four dopamine-rich brain regions to unilateral electrical stimulation of the parafascicular thalamic nucleus was examined in the halothane-anaesthetized rat. Tissue concentrations of dopamine and its two major metabolites, 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid, were assayed by a high performance liquid chromatographic technique in samples of caudate-putamen complex, nucleus accumbens, prefrontal cortex and substantia nigra. The ratios of metabolite to parent amine concentrations were taken as indices of dopamine utilization. Halothane anaesthesia alone evoked significant bilateral increases of dopamine utilization in every brain region studied. Electrical stimulation of one parafascicular nucleus produced further bilateral elevations of dopamine utilization in the caudate-putamen complex without altering these parameters in the substantia nigra. In the prefrontal cortex, however, thalamic stimulation resulted in significant bilateral decreases of dopamine utilization. Electrical stimulation of cortical or other thalamic areas did not evoke this regional pattern of dopamine utilization. It is argued that these indices of dopamine utilization together serve as reliable indicators of synaptic dopamine release and it is concluded that the parafascicular thalamus is capable of facilitating dopaminergic neurotransmission in the caudate-putamen by a mechanism that is probably independent of changes in dopamine cell firing rate. An anatomical analysis suggests that a thalamo-cortical-striatal route is most likely to mediate this function.

Interactions between dopamine and gamma-aminobutyrate in the substantia nigra: implications for the striatonigral output hypothesis.

Experiments employing a rodent circling model were conducted to test the predictive capacity of the theory which states that striatonigral gamma-aminobutyrate neurones transmit striatal information influencing the animal's locomotion and orientation. In agreement with this proposal, blocking nerve conduction in one substantia nigra with procaine, or nigral gamma-aminobutyrate receptors with bicuculline administered stereotaxically, frequently forced rats to move ipsiversively to systemic apomorphine, as though the treatment had impaired striatonigral transmission on that side of the brain. Attempts to reverse the direction of apomorphine circling by stimulating gamma-aminobutyrate receptors with muscimol, by facilitating the amino acid's action with flurazepam, or by increasing its synaptic concentration either with a breakdown inhibitor (ethanolamine O-sulphate or 4-amino-hex-5-enoic acid) or an uptake blocker (cis-1,3-aminocyclohexane carboxylic acid) in one nigra, proved unsuccessful. In fact, ethanolamine O-sulphate, flurazepam and muscimol all gave the appearance of hindering rather than enhancing the passage of striatal-derived motor information through the nigra. Broadly speaking, these drugs gave predictable behavioral responses from the ventromedial thalamus, suggesting they were acting in accordance with known mechanisms. The anomalous behaviour with ethanolamine O-sulphate may be attributed to its elevating gamma-aminobutyrate levels in other brain areas, since similar ipsiversive rotations occurred if gamma-aminobutyrate catabolism was prevented at a wide variety of extranigral sites. A simple explanation for the paradoxical ipsiversive behaviours produced by intranigral flurazepam or muscimol in combination with systemic or intracerebral injection of dopamine agonists, is that they act via presynaptic receptors to inhibit the release of endogenous gamma-aminobutyrate and thereby impede striatonigral outflow ipsilaterally.

Behavioural role of dopamine D1 receptors in the reserpine-treated mouse.

The effects of 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393) (D1 agonist) on the motor behaviour of mice rendered hypokinetic with reserpine, were studied in the absence and presence of additional treatment with N-n-propyl-N-phenylethyl-p(3-hydroxyphenyl)ethylamine hydrochloride (RU 24213), lisuride (D2 agonists) or apomorphine (mixed D1/D2 agonist). Three hours after reserpine (5 mg/kg) stimulating dopamine D2 receptors evoked slow, ponderous walking and head-down sniffing. SKF 38393 (1.5-15 mg/kg) had no direct effect of its own, but greatly amplified the D2 response, giving more fluent locomotion, rearing and grooming. The facilitatory action of SKF 38393 was inhibited by the D1 antagonist (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin l -7-ol (SCH 23390) (0.05 mg/kg), whereas D2-mediated responses were sensitive both to SCH 23390 and the D2 antagonist metoclopramide (0.5 mg/kg). Mice treated with reserpine for 24 h became more sensitive to the motor stimulant actions of all four agonists. SKF 38393 now promoted rapid locomotion, rearing and grooming directly. The effects of D2 stimulation were weak by comparison and often antagonistic (not synergistic) with those of the D1 agonist. Both sets of agonists were now attenuated only by their respective antagonists. Reserpine caused pronounced falls in the concentrations of dopamine, 5-hydroxytryptamine and noradrenaline in the striatum, olfactory tubercle and cerebral cortex, with correspondingly elevated metabolite levels. These results indicate that D1 and D2 agonists at doses that are relatively ineffective at stimulating behaviour when given in isolation 3 h after reserpine, interact when given together to partially restore locomotion, rearing and grooming. This interaction is not apparent 24 h post-reserpine, a time at which D1 and D2 agonists produce significant effects of their own.

Evidence for the participation of nigrotectal gamma-aminobutyrate-containing neurones in striatal and nigral-derived circling in the rat.

The gamma-aminobutyrate-containing nature of nigrotectal neurones and the possible involvement of the tectum in circling behaviour were investigated in the rat. Electrolytic or kainic acid lesions of the substantia nigra reduced gamma-aminobutyrate levels on average by 19-29% in intermediate and deep, but not superficial superior colliculus. Placement of lesions or injection of muscimol (40 ng) into these gamma-aminobutyrate-innervated layers of superior colliculus gave only weak ipsilateral posturing or circling that was intensified by apomorphine, but which strongly antagonized contraversive apomorphine-induced circling in 6-hydroxydopamine pretreated rats(lateral greater than medial sites). Contraversive circling to unilateral muscimol (40 ng) was significantly attenuated by lesions or muscimol injections placed in the ipsi- or contralateral superior colliculus. Picrotoxin (40 ng) and tetanus toxin (30 mouse LD50 doses) evoked explosive motor behaviour from medial colliculus and vigorous contraversive circling when injected into the lateral colliculus. The latter offset ipsiversive asymmetries to kainate (0,8 micrograms) in the corresponding substantia nigra. Bilateral intratectal picrotoxin produced hyperactivity that reversed haloperidol catalepsy. Similar bilateral administration of muscimol did not produce catalepsy but a state of frozen immobility. Kainic acid introduced into the superior colliculus gave mixed excitatory-inhibitory responses initially followed by ipsiversive circling only and loss of tectal perikarya. None of these drug effects occurred from the overlying cerebral cortex or subjacent tegmentum. We propose that separate medial "non-postural" and lateral "postural" tectal locomotor regions may exist in the superior colliculus that are situated within a striato-nigrotectal outflow system capable of influencing the animal's motor activity and posture.

Thalamic control of dopaminergic functions in the caudate-putamen of the rat--III. The effects of lesions in the parafascicular-intralaminar nuclei on D2 dopamine receptors and high affinity dopamine uptake.

Dopamine receptor binding in the caudate-putamen was studied following bilateral lesions of the thalamostriatal pathway. Receptor binding was assayed using [3H]spiperone and defined with both (+)-butaclamol and S(-)-sulpiride. Radiofrequency lesions resulted in an increase in the Bmax of [3H]spiperone binding defined with both (+)-butaclamol and S(-)-sulpiride between 7 and 14 days following surgery. At longer survival times a fluctuating response was seen in which a decrease in receptor binding was observed at 28 days following lesion and a further rise again at 70 days. At no time point was significant change in Kd recorded. Further experiments were carried out to control for the possible effects of damage to fibres of passage and for inadvertent damage to habenula, as well as to define the receptor subtype involved. Ibotenic acid lesions resulted in similar effects to those reported with the radiofrequency method. Thus, 7 days following lesion, Bmax for (+)-butaclamol-defined [3H]spiperone binding increased by approximately 14-20% over that recorded in sham-lesioned animals. Using S(-)-sulpiride to define binding, Bmax was found to increase 13-17% in the same membrane preparations. Similar results were obtained in experiments at 14 days following ibotenic acid induced lesions. Again, no change in Kd was recorded. When radiofrequency lesions were made, which were largely restricted to habenula and associated fibres of passage, only small [(+)-butaclamol defined] or insignificant [S(-)-sulpiride defined] changes in Bmax were recorded. Combined radiofrequency lesions of habenula and ibotenic acid lesions of the thalamus resulted in a larger increase in Bmax for (+)-butaclamol defined binding than with S(-)-sulpiride defined binding. Our interpretation of these findings, in the light of the histology of the lesions, is that the predominant effect of removing thalamic input to the caudate-putamen is an increase in the number of D2 receptors, but without any change of affinity. A small component of the change in Bmax defined with (+)-butaclamol found with radiofrequency lesions may be due to a response at non-dopamine sites (possibly a 5-hydroxytryptamine receptor subtype) following damage to other caudate-putamen afferents which pass near the habenula or fasciculus retroflexus. Following unilateral ibotenic acid lesions of the thalamus, the number of high affinity uptake sites for dopamine was increased at long survival times.(ABSTRACT TRUNCATED AT 400 WORDS)

Thalamic control of dopaminergic functions in the caudate-putamen of the rat--II. Studies using ibotenic acid injection of the parafascicular-intralaminar nuclei.

We have investigated the role of the parafascicular-intralaminar thalamus in the regulation of dopaminergic function in the caudate-putamen by making unilateral injections of the excitotoxin, ibotenic acid, into the thalamus of the halothane-anaesthetized rat. Dopamine utilization was measured at 4 h, 18 h, and 7 days after operation in microdissected tissue from caudate-putamen, substantia nigra, and nucleus accumbens. High performance liquid chromatography with electrochemical detection was used to simultaneously determine dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid. Dopamine utilization was recorded as a ratio of metabolite to its parent amine. At 4 h following injection large bilateral increases in dopamine utilization were recorded in both medial and lateral sectors of caudate-putamen. The percentage increases found for homovanillic acid-based ratios were larger than those found for 3,4-dihydroxyphenylacetic acid-based ratios. These results probably reflect increased dopamine release resulting from the acute effects of ibotenic acid. These changes were independent of dopamine utilization ratios recorded in the substantia nigra, which showed no change either ipsilateral or contralateral to the injection. In contrast to these findings, at 7 days following injection, dopamine utilization ratios were reduced both ipsilateral and contralateral to the injection, although only the ipsilateral reductions were significant. Again, no change was found for this survival time in the substantia nigra. At 18 h survival an intermediate pattern between the 4 h and 7 day result was found. In the nucleus accumbens, ibotenic acid injection produced similar results to those found in caudate-putamen, i.e. a bilateral increase in dopamine utilization at early time intervals and a unilateral ipsilateral decrease at long intervals following injection. These results show that dopamine release in the caudate-putamen is sensitive to experimentally induced changes in neural activity and lesions of its thalamic input. Since the effect of presumed stimulation is markedly greater than lesion, it would appear that, under the conditions employed in these experiments, the thalamus is relatively silent; a suggestion consistent with other evidence. Furthermore, since the changes found occurred in the absence of changes in utilization ratios in the substantia nigra, the mechanisms whereby thalamus regulates dopamine release may be exerted via a local circuit and/or a presynaptic mechanism in the region of dopamine terminals. The anatomical routes responsible for these effects are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevention by (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin of both catalepsy and the rises in rat striatal dopamine metabolism caused by haloperidol.

1. The influence of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on haloperidol-induced increases in the dopamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA), was measured in three microdissected brain regions of the rat following a quantitative assessment of catalepsy. 2. Haloperidol alone (2.66 mumol kg-1, i.p.) caused a robust cataleptic response. Given 30 min after haloperidol, 8-OH-DPAT (76 or 760 nmol kg-1, s.c.) prevented catalepsy in 30% and 100% of rats, respectively. 3. Haloperidol significantly increased the DOPAC (by 2 to 4 fold) and HVA (by 3 to 7 fold) contents of the caudate-putamen, nucleus accumbens and medial prefrontal cortex. Given alone, only the lower dose of 8-OH-DPAT caused a significant biochemical change, a doubling of cortical DOPAC. 4. In the cases where catalepsy was prevented by either dose of 8-OH-DPAT, the haloperidol-induced increases in DOPAC and HVA were consistently lower in the caudate-putamen. This pattern was true for the rise in cortical HVA but only in response to the lower dose of 8-OH-DPAT. In contrast, neither dose of 8-OH-DPAT was able to influence the haloperidol-induced rises in cortical DOPAC. In the nucleus accumbens, 8-OH-DPAT did not affect the haloperidol-induced increases in the dopamine metabolites, irrespective of the dose employed or the resulting behaviour. When catalepsy was not prevented, 8-OH-DPAT did not alter the neurochemical responses to haloperidol in any region. 5. These results suggest that part of the mechanism by which 8-OH-DPAT prevents haloperidol-induced catalepsy is reflected by a reversal of the compensatory increase in meso-striatal and/or meso-cortical dopamine neuronal activity that normally accompanies postsynaptic dopamine receptor blockade with haloperidol.

Protein phosphatase inhibitors facilitate DHPG-induced LTD in the CA1 region of the hippocampus.

We have shown earlier that activation of metabotropic glutamate (mGlu) receptors using a group I-specific mGlu receptor agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG), can induce long-term depression (LTD) in the CA1 region of the hippocampus. In an attempt to determine the signal transduction mechanisms involved in this form of synaptic plasticity, we have tested the effects of a range of inhibitors on DHPG-induced LTD. In vitro grease-gap electrophysiological recordings were performed in the rat hippocampal CA1 region. We have found that DHPG-induced LTD is resistant to the two potent protein kinase C (PKC) inhibitors, Gö 6976 (10 microM) and Gö 6983 (10 microM), the potent and selective protein kinase A (PKA) inhibitor, KT 5720 (10 microM), and the potent broad spectrum kinase inhibitor, staurosporine (10 microM). In contrast, non-selective inhibitors of protein phosphatases (PP1 and PP2A), okadaic acid (1 microM) or calyculin A (1 microM), facilitated DHPG-induced LTD. However, an inhibitor of protein phosphatase 2B, FK 506 (1 microM), did not influence this process. The PP1/PP2A protein phosphatase inhibitors, but none of the other agents tested, also inhibited (S)-alpha-methyl-4-carboxyphenylglycine (MCPG)-induced reversal of DHPG-induced LTD. These data suggest that activation of neither PKC nor PKA is involved in DHPG-induced LTD. They do, however, suggest that the process is under regulation by protein phosphorylation and dephosphorylation.

Influence of acute and chronic haloperidol treatment on dopamine metabolism in the rat caudate-putamen, prefrontal cortex and amygdala.

The present study investigated the actions of single and repeated injections of the classical antipsychotic drug, haloperidol (1 mg.kg-1 IP), on dopamine (DA) metabolism in three distinct rat brain regions, namely the prefrontal cortex, amygdala and caudate-putamen (CP), using a high-performance liquid chromatographic assay. Acute administration of the drug caused significant elevations in concentrations of two major DA metabolites in all three areas studied. Less marked acute increases were seen in the CP following 10 days of repeated haloperidol treatment. However, in both the prefrontal cortex and the amygdala, the development of such "tolerance" was somewhat delayed in comparison, occurring only after a 22-day treatment schedule. The amygdala displayed the greatest degree of neurochemical tolerance, returning to control values by day 22 of chronic treatment. When allowance was made for the withdrawal effects of antipsychotic drug administration, a genuine tolerance phenomenon was observed in all three areas examined. These data suggest that if neurochemical tolerance is a prerequisite for functional DA receptor blockade and hence therapeutic efficacy, then both the prefrontal cortex and amygdala should be considered as potential therapeutic targets of haloperidol and perhaps antipsychotic drugs in general.

The influence of assay conditions on measurement of excitatory dibasic sulphinic and sulphonic alpha-amino acids in nervous tissue.

Major improvements to the HPLC separation of fluorescent derivatives of excitatory sulphur-containing amino acids have been made. Quisqualate was used as the internal standard since no endogenous derivatives coeluted with it. The artefactual generation of sulphinic and sulphonic amino acids from the oxidation of cysteine (56 microM) and homocysteine (1.2 microM) has been investigated using deionised water, an acidic phosphate/methanol mixture, perchloric acid and trichloroacetic acid (TCA) as extraction media. Of the four extraction media examined, TCA in combination with ether extraction was shown to be the most potent oxidative treatment and resulted in 23% oxidation of original cysteine or homocysteine to sulphinic and sulphonic acids. This oxidation was partially resistant to the presence of physiological concentrations of glutathione (1.5 mM) such that in the case of cysteine, 6% oxidation was observed. A 10% (v/v) mixture of methanol in 75 mM phosphate solution (pH 4.6) was found to be the most artefact-free extraction method and in spinal cord tissue processed with this medium, cysteine sulphinic acid was the only excitatory sulphur-containing amino acid consistently detectable (0.24 +/- 0.01 pmol/mg wet weight, n = 6).

Thalamic control of subcortical dopamine function in the rat and the effects of lesions applied to the medial prefrontal cortex.

Dopamine (DA) utilisation has been assessed in medial and lateral segments of the caudate-putamen complex (CPM and CPL, respectively) in response to unilateral manipulations aimed at the thalamic mediodorsal nucleus, lateral division (MDL). The ratios of 3,4-dihydroxyphenylacetic acid (DOPAC):DA and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid, HVA):DA are used as indices of DA utilisation and, in the case of HVA:DA, may also reflect DA release. Neither electrical stimulation nor ibotenate (IBO) treatment followed by long recovery periods (2 days or 1 week) had any significant effect on DA utilisation in CPM or CPL. Cell-specific activation of neurones produced by short-term (1 h recovery) infusions of IBO aimed unilaterally at MDL (right side) resulted in bilateral increases of DA utilisation in both CP sectors. These changes tended to be slightly more marked in the hemisphere ipsilateral to the side of IBO infusion. Unilateral infusions of IBO were then aimed at MDL of either (1) the left or right hemisphere of animals which had already received a 1-week-old unilateral (right side) prefrontal cortex (FCx) lesion or (2) the right hemisphere of animals which had previously received a 1 week-old bilateral FCx lesion. The pattern of changes, when expressed relative to the 'sham-operated' animals which received the FCx lesion alone, were similar to those described above following intra-MDL infusions of IBO into animals with an intact cortex. The FCx lesions themselves were shown to have no significant effect on DA utilisation in any CP sector. In view of the known neuroanatomical connections, it is likely that the effects observed in CP are not due to activation of MDL neurones themselves but are more likely the result of activation of neurones in the intralaminar nuclei which border MDL. Nevertheless, these findings support the concept that activation of thalamic nuclei will enhance DA function in a variety of forebrain areas in the rat.

Evidence that thalamic efferent neurones are non-cholinergic: a study in the rat with special reference to the thalamostriatal pathway.

Controversy surrounds the question as to whether some fibres of the thalamostriatal projection, are cholinergic. The present experiments show that lesions of the parafascicular-intralaminar thalamus produced no reductions in choline acetyltransferase (ChAT) activities in any area of microdissected rat caudate-putamen complex or dorsolateral frontal cortex. We conclude that thalamostriatal projections are entirely non-cholinergic. Furthermore, lesions of the mediodorsal or periventricular thalamus resulted in no change in ChAT activities in their terminal projection areas (medio-/orbitofrontal cortices and nucleus accumbens, respectively). The probability that all thalamic outputs are non-cholinergic is discussed.