Olanzapine activates the rat locus coeruleus: in vivo electrophysiology and c-Fos immunoreactivity

BACKGROUND

Activation of central noradrenergic pathways by atypical antipsychotics has been hypothesized to play a role in their efficacy in treating the negative symptoms and cognitive impairment of schizophrenia. Because acute administration of the atypical antipsychotic olanzapine has been shown to increase extracellular levels of norepinephrine in the medial prefrontal cortex, we examined the effects of olanzapine on the noradrenergic cells of the locus coeruleus (LC).

METHODS

The effects of olanzapine (0.25-16 mg kg(-1), IV) on the firing rates and patterns of LC neurons were determined by extracellular, single-unit recordings in chloral hydrate-anaesthetized rats. The effects of olanzapine and clozapine on c-Fos expression in the LC, nucleus subcoeruleus part alpha (SubCA), and nucleus A5 (A5) were studied by immunohistochemistry.

RESULTS

Olanzapine increased LC cell firing rates, de-regularized firing, and induced burst firing. Induction of c-Fos expression in the LC by olanzapine and clozapine was confirmed and was also found in the SubCA, but not in A5.

CONCLUSIONS

Acute administration of olanzapine activates the noradrenergic neurons of the rat LC. This increased activity of LC neurons may play an important role in the efficacy of olanzapine and clozapine in treating both the negative symptoms and cognitive impairment observed in schizophrenic patients.

Cross infection control measures and the treatment of patients at risk of Creutzfeldt Jakob disease in UK general dental practice

AIMS

To determine the suitability of key infection control measures currently employed in UK dental practice for delivery of dental care to patients at risk of prion diseases.

MATERIALS AND METHODS

SUBJECTS

Five hundred dental surgeons currently registered with the General Dental Council of the UK.

DATA COLLECTION

Structured postal questionnaire.

ANALYSIS

Frequencies, cross-tabulations and chi-squared analysis.

RESULTS

The valid response rate to the questionnaire was 69%. 33% of practices had no policy on general disinfection and sterilisation procedures. Only 10 of the 327 responding practices (3%) possessed a vacuum autoclave. 49% of dentists reported using the BDA medical history form but less than 25% asked the specific questions recommended by the BDA to identify patients at risk of iatrogenic or familial CJD. However, 63% of practitioners would refer such patients, if identified, to a secondary care facility. Of the 107 practitioners who were prepared to provide dental treatment, 75 (70%) would do so using routine infection control procedures.

CONCLUSIONS

Most of the dental practices surveyed were not actively seeking to identify patients at risk of prion diseases. In many cases, recommended procedures for providing safe dental care for such patients were not in place.

Tyrosine depletion attenuates the behavioural stimulant effects of amphetamine and cocaine in rats

Neurochemical studies show that a tyrosine-free amino acid mixture depletes brain tyrosine and decreases dopamine synthesis and release. Here, we tested whether such a mixture would reduce the behavioural effects of amphetamine and other psychostimulants. A tyrosine-free amino acid mixture decreased the behavioural activation induced by both D-amphetamine (2 mg/kg s.c.) and cocaine (2 mg/kg s.c.). In contrast, the activation induced by the dopamine agonist, apomorphine (0.75 and 5 mg/kg s.c.), or the 5-hydroxytryptamine releasing agent, p-chloroamphetamine (2 mg/kg s.c.) was not altered. These findings provide behavioural evidence that tyrosine-free amino acid mixtures reduce presynaptic dopamine function in the brain.

Different tonic regulation of neuronal activity in the rat dorsal raphe and medial prefrontal cortex via 5-HT(1A) receptors

It has been established that 5-HT(1A) receptors are expressed both presynaptically as autoreceptors by 5-HT containing neurones, and postsynaptically by a variety of other neurones. Activation of either somatodendritic 5-HT(1A) autoreceptors or postsynaptic 5-HT(1A) receptors induces hyperpolarisation and inhibition of action potential discharge of the neurones, but it is unclear whether 5-HT(1A) receptors are under a general tonic influence by 5-HT. In the present study, using single unit recordings from both anesthetized and non-anesthetized rats, we show that the activity of neurones in the medial prefrontal cortex is not altered by systemic administration of the selective 5-HT(1A) receptor antagonist, WAY 100635. In contrast, WAY 100635 increased the firing rate of 5-HT neurones in the dorsal raphe nucleus. Our findings indicate a tonic activation of presynaptic somatodendritic but not postsynaptic cortical 5-HT(1A) receptors.

Electroconvulsive shock increases tachykinin NK(1) receptors, but not the encoding mRNA, in rat cortex

Recent studies have suggested that the substance P (tachykinin NK(1)) receptor may be a pharmacological target for the treatment of mood disorders. Here, the effects of electroconvulsive shock on tachykinin NK(1) receptor gene expression in the rat brain was investigated. Rats received either a single electroconvulsive shock or five shocks on alternate days. Quantitative autoradiography with [(125)I]Bolton Hunter-substance P, and in situ hybridisation histochemistry, were used to measure tachykinin NK(1) receptor-binding site densities and mRNA abundance, respectively. Densities of tachykinin NK(1) receptor-binding sites were significantly increased in the cerebral cortex following repeated electroconvulsive shock compared to sham treated animals. Densities remained unchanged in the hippocampus, striatum and amygdala. Neither single nor repeated electroconvulsive shock altered tachykinin NK(1) receptor mRNA in the brain regions examined. Hence, repeated electroconvulsive shock increases tachykinin NK(1) receptors in the rat brain in a regionally specific way. Upregulation of receptor-binding sites without a change in mRNA indicates that translational or post-translational mechanisms underlie this process.

Widespread distribution of binding sites for the novel Ca2+-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate, in the brain

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca(2+)-mobilizing agent in invertebrate eggs that has recently been shown to be active in certain mammalian and plant systems. Little, however, is known concerning the properties of putative NAADP receptors. Here, for the first time, we report binding sites for NAADP in brain. In contrast to sea urchin egg homogenates, [(32)P]NAADP bound reversibly to multiple sites in brain membranes. The rank order of potency of NAADP, 2',3'-cyclic NAADP and 3'-NAADP in displacing [(32)P]NAADP was, however, the same in the two systems and in agreement with their ability to mobilize Ca(2+) from homogenates. These data indicate that [(32)P]NAADP likely binds to receptors mediating Ca(2+) mobilization. Autoradiography revealed striking heterogeneity in the distribution of [(32)P]NAADP binding sites throughout the brain. Our data strongly support a role for NAADP-induced Ca(2+) signaling in the brain.

Effect of a selective 5-hydroxytryptamine reuptake inhibitor on brain extracellular noradrenaline: microdialysis studies using paroxetine

The clinical efficacy of selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors (SSRIs) is normally attributed to their ability to increase brain 5-HT function although recent preclinical findings indicate that their selectivity for 5-HT over noradrenaline may be less evident in vivo. The present study investigated the effects of the SSRI, paroxetine, on extracellular levels of noradrenaline. Microdialysis was carried out in the hippocampus of the awake rat. In rats treated twice daily for 14 days with paroxetine (5 mg/kg s.c.), dialysate levels of noradrenaline showed a maintained two-fold increase compared to saline-injected controls. Paroxetine (5 mg/kg s.c.) administered once daily for 14 days did not cause a sustained increase in noradrenaline but levels showed a moderate (+58%) increase in response to a paroxetine challenge. Acute injection of paroxetine (5 mg/kg s.c.) did not elevate noradrenaline levels. Paroxetine (5 mg/kg s.c.) elevated dialysate 5-HT after both acute and repeated (twice daily for 14 days) treatment. The paroxetine-induced increase in noradrenaline (and 5-HT) was positively correlated with plasma concentrations of the drug, which were around the therapeutic range. In comparison to paroxetine, desipramine (10 mg/kg s.c.) caused a four-fold increase in dialysate noradrenaline (but did not change 5-HT) following repeated (once daily for 14 days) treatment and a two-fold increase at for acute treatment. In summary, despite its selectivity as a 5-HT reuptake inhibitor, paroxetine increased extracellular levels of noradrenaline in rat hippocampus following repeated administration. We discuss the possibility that a facilitation of noradrenaline function might be involved in the antidepressant effect of paroxetine, and possibly other SSRIs.

Neuroanatomical targets of anxiogenic drugs in the hindbrain as revealed by Fos immunocytochemistry

It is speculated that specific hindbrain transmitter pathways centred on the periaqueductal gray and locus coeruleus are an important integrative neural substrate for the expression of anxiety and the somatic symptoms and cardiovascular changes that accompany severe anxiety states, such as in panic disorder. Here we investigated the effects of various drugs, known to induce panic in humans and to be anxiogenic in animals, on Fos expression in the periaqueductal gray, locus coeruleus and other parts of the rat hindbrain. The drugs tested were the benozodiazepine inverse agonist FG-7142, the alpha(2)-adrenoceptor antagonist yohimbine, the non-selective 5-hydroxytryptamine(2C) receptor agonist m-chlorophenyl piperazine, the adenosine antagonist caffeine and the cholecystokinin analogue BOC-CCK(4). A clear-cut finding was that administration of each anxiogenic drug caused a striking region-specific pattern of Fos expression within the hindbrain. In particular, the drugs commonly increased Fos-like immunoreactivity in the periaqueductal gray and locus coeruleus. Increased Fos expression in the periaqueductal gray was specific to the rostral dorsolateral and caudal ventrolateral regions. All the anxiogenic drugs also increased Fos-like immunoreactivity in the lateral parabrachial nucleus and nucleus of the solitary tract and all but one (BOC-CCK(4)) increased Fos in the dorsal raphe nucleus. Rats habituated to the test environment and injected with saline vehicle displayed little or no Fos-like immunoreactivity in the hindbrain areas investigated. In summary, each of the anxiogenic drugs tested (FG-7142, yohimbine, m-chlorophenyl piperazine, caffeine and BOC-CCK(4)) increased Fos expression in a restricted number of hindbrain regions, including the periaqueductal gray and locus coeruleus. Previous Fos studies have found that these same regions are activated by various fearful environmental stimuli. Therefore, a specific set of hindbrain circuits may be commonly involved in the processing of anxiety-related information evoked by pharmacological and environmental manipulation. The present findings also raise the possibility that measurement of the effect of anxiogenic drugs on Fos expression might be a useful way to model hindbrain pathways activated by anxiety and possibly panic.

Repeated administration of fluoxetine, desipramine and tranylcypromine increases dopamine D2-like but not D1-like receptor function in the rat

We tested the effect of repeated treatment (twice daily for 14 days) of rats with the antidepressant drugs fluoxetine, desipramine and tranylcypromine, on the behavioural response to the non-selective dopamine (DA) receptor agonist, apomorphine, the D1-like receptor agonists, SKF 38393 and SKF 81297 and the D2-like receptor agonists, RU 24213 and quinpirole. Agonist-induced behaviour was monitored by automated activity meters and direct observation using a checklist scoring method. Fluoxetine, desipramine and tranylcypromine enhanced (albeit to a varying degree) the behavioural responses to apomorphine (0.75 mg/kg, s.c.), quinpirole (0.25 mg/kg, s.c.) and RU 24213 (0.75 mg/kg, s.c.). In contrast, fluoxetine, desipramine and tranylcypromine did not increase the behavioural responses to SKF 38393 (7.5 mg/kg, s.c.) and SKF 81297 (0.5 mg/kg, s.c.). Finally, fluoxetine, despiramine and tranylcypromine did not modify the behavioural responses to the concomitant administration of SKF 38393 (7.5 mg/kg, s.c.) and quinpirole (0.25 mg/kg, s.c.). Our data suggest that repeated administration of fluoxetine, desipramine and tranylcypromine increases central DA D2-like but not D1-like receptor function.

beta-blocker binding to human 5-HT(1A) receptors in vivo and in vitro: implications for antidepressant therapy

A novel strategy for improving the treatment of depressive illness is augmentation of antidepressants with a 5-HT1(1A) autoreceptor antagonist. However, trials using the 5-HT1(1A)/beta-blocker pindolol are proving inconsistent. We report how positron emission tomography (PET) and in vitro autoradiography can inform trials of antidepressant augmentation. We show that in healthy volunteers, in vivo, pindolol (n = 10) and penbutolol (n = 4), but not tertatolol (n = 4) occupy the human 5-HT(1A) receptors, at clinical doses. Pindolol, as well as the beta-blockers penbutolol and tertatolol, has high affinity for human 5-HT(1A) receptors in post-mortem brain slices (n = 4). Pindolol shows preference for 5-HT(1A) autoreceptors versus the post-synaptic receptors both in vitro and in vivo. Our data reveal that pindolol doses used in antidepressant trials so far are suboptimal for significant occupancy at the 5-HT(1A) autoreceptor. Penbutolol or higher doses of pindolol are candidates for testing as antidepressant augmenting regimes in future clinical trials.

Neurochemical and electrophysiological studies on the functional significance of burst firing in serotonergic neurons

We have previously described a population of 5-hydroxytryptamine neurons which repetitively fires bursts of usually two (but occasionally three or four) action potentials, with a short (<20 ms) interspike interval within a regular low-frequency firing pattern. Here we used a paradigm of electrical stimulation comprising twin pulses (with 7- or 10-ms inter-pulse intervals) to mimic this burst firing pattern, and compared the effects of single- and twin-pulse electrical stimulations in models of pre- and postsynaptic 5-hydroxytryptamine function. Firstly, we measured the effect of direct electrical stimulation (2 Hz for 2 min) of rat brain slices on efflux of preloaded [3H]5-hydroxytryptamine. In this in vitro model, twin-pulse stimulation increased the efflux of tritium by about twice as much as did single-pulse stimulation. This effect was evident in the medial prefrontal cortex (area under the curve: 2. 59+/-0.34 vs 1.28+/-0.22% relative fractional release), as well as in the caudate-putamen (3.93+/-0.65 vs 2.17+/-0.51%) and midbrain raphe nuclei (5.42+/-1.05 vs 2.51+/-0.75%). Secondly, we used in vivo microdialysis to monitor changes in endogenous extracellular 5-hydroxytryptamine in rat medial prefrontal cortex in response to electrical stimulation (3 Hz for 10 min) of the dorsal raphe nucleus. In this model, twin-pulse stimulation of the dorsal raphe nucleus increased 5-hydroxytryptamine by approximately twice as much as did single-pulse stimulation at the same frequency (area under the curve: 50.4+/-9.0 vs 24.2+/-4.4 fmol). Finally, we used in vivo extracellular recording to follow the response of postsynaptic neurons in the rat medial prefrontal cortex to 5-hydroxytryptamine released by dorsal raphe stimulation. Electrical stimulation of the dorsal raphe nucleus (1 Hz) induced a clear-cut poststimulus inhibition in the majority of cortical neurons tested. In these experiments, the duration of poststimulus inhibition following twin-pulse stimulation was markedly longer than that induced by single-pulse stimulation (200+/-21 vs 77+/-18.5 ms). Taken together, the present in vitro and in vivo data suggest that in 5-hydroxytryptamine neurons, short bursts of action potentials will propagate along the axon to the nerve terminal and will enhance both the release of 5-hydroxytryptamine and its postsynaptic effect.

Affinity of (+/-)-pindolol, (-)-penbutolol, and (-)-tertatolol for pre- and postsynaptic serotonin 5-HT(1A) receptors in human and rat brain

There is considerable interest in the use of drugs that selectively block presynaptic (somatodendritic) serotonin 5-HT(1A) receptors for the adjunctive treatment of major depressive disorder. The 5-HT(1A)/beta-adrenoceptor ligands (+/-)-pindolol, (-)-tertatolol, and (-)-penbutolol are currently under clinical investigation, and knowledge of their affinity at different populations of central 5-HT(1A) receptors is needed. Here we have determined the affinity of these drugs for presynaptic and postsynaptic 5-HT(1A) receptors in postmortem human and rat brain using receptor autoradiography and the selective 5-HT(1A) radioligand [(3)H]WAY-100635. The binding of [(3)H]WAY-100635 was specific and saturable and showed high affinity in the rat dorsal raphe nucleus and hippocampus (K(D) = 1.5-1.7 nM). In competition studies, the three compounds had nanomolar affinity and produced monophasic displacement of [(3)H]WAY-100635 binding in all regions of both species. (-)-Penbutolol and (-)-tertatolol had similar affinity for pre-and postsynaptic 5-HT(1A) receptors in both rat and human brain. However, in the human, but not the rat, the affinity of (+/-)-pindolol in dorsal raphe nucleus (K(i) = 8.9 +/- 1. 1 nM) was slightly but significantly higher than that in hippocampus (K(i) = 14.4 +/- 1.5 nM in CA1). In summary, our data show that (+/-)-pindolol, (-)-tertatolol, and (-)-penbutolol are all high-affinity ligands at native human and rat 5-HT(1A) receptors. (-)-Penbutolol and (-)-tertatolol do not discriminate between the pre- and postsynaptic 5-HT(1A) sites tested in either species, but (+/-)-pindolol showed a slightly higher affinity for the presynaptic site in human brain. Further work is needed to establish whether the latter difference is clinically relevant.

Nicotine inhibits firing activity of dorsal raphe 5-HT neurones in vivo

It is established that the brain monoaminergic systems are among the main targets of several dependence-inducing drugs, including nicotine. In the present study extracellular electrophysiological recordings were performed to investigate the effects of nicotine on dorsal raphe 5-HT neurones. Nicotine, administered systemically (50-400 microg/kg, i.v.) in chloral hydrate-anaesthetised rats, induced a transient inhibition of the majority of 5-HT neurones recorded (38 of 45). The inhibition was rapid in onset (about 30 s) and the firing rate returned to baseline within 1-3 min. No apparent tachyphylaxis was observed to this inhibitory effect. The centrally acting nicotine antagonist mecamylamine (4 mg/kg, i.v.), but not the peripherally acting nicotine antagonist chlorisondamine (0.3 mg/kg, i.v.) antagonised the nicotine-induced inhibition of 5-HT neurones. The inhibition of 5-HT neurones was also blocked with a selective 5-HT1A receptor antagonist (WAY 100635; 0.1 mg/kg, i.v.), indicating a possible involvement of somato-dendritic 5-HT1A receptors in the effect of nicotine. Interestingly, microiontophoretic application of nicotine into the dorsal raphe failed to inhibit 5-HT neurones, suggesting an indirect effect of nicotine on 5-HT neurones, possibly involving afferent pathways.

Pindolol occupancy of 5-HT(1A) receptors measured in vivo using small animal positron emission tomography with carbon-11 labeled WAY 100635

Positron emission tomography (PET), following an intravenous injection of [carbonyl-(11)C]WAY 100635, was used to image central 5-HT(1A) receptors in rat following pretreatment with graded doses of (-)-pindolol (0.001-3 mg/kg, i.v.). The use of PET had advantages over ex vivo radioligand binding methods in that it produced parametric image volumes and reduced errors due to inter-rat variability. Time-radioactivity curves from regions of interest (ROI) acquired from individual rats enabled the estimation of specific binding of the radioligand using a compartmental model with reference tissue input. Binding potential (BP) of [(11)C]WAY 100635 was estimated for frontal cortex and hippocampus (postsynaptic), and midbrain raphe nuclei (presynaptic). In the latter ROI, pindolol dose-dependently decreased BP. The saturation curve could be fitted to a single-site model up to the lowest dose of pindolol used, giving an ED(50) (dose to cause 50% occupancy) value of 0.26 +/- 0. 05 mg/kg, and inclusion of control (nonpindolol-treated) rats did not affect the fit. In contrast, in cortex and hippocampus ROI, low doses of pindolol caused an increase in BP compared with controls. Pindolol doses greater than approximately 0.1 mg/kg, resulted in a dose-dependent decrease in BP, and ED(50) values in cortex and hippocampus were estimated as 0.44 +/- 0.13 and 0.48 +/- 0.12 mg/kg, respectively. The increase in [(11)C]WAY 100635 binding at low pindolol doses is feasibly related to a decrease in basal receptor occupancy following reduced release of endogenous 5-HT. Considering the apparently greater potency of pindolol at the midbrain raphe ROI, this effect could be mediated via agonist activity at the autoreceptor.

Expression of 5-HT receptors and the 5-HT transporter in rat brain after electroconvulsive shock

Modulation of central 5-HT receptor sensitivity is implicated in the therapeutic response to electroconvulsive shock (ECS). Altered 5-HT receptor expression may play a role in this process. We have measured the mRNAs encoding 5-HT1A, 5-HT2A, 5-HT2C and 5-HT7 receptors, and the 5-HT transporter, in rat brain after single ECS, repeated ECS, and 3 weeks after repeated ECS. Hippocampal 5-HT1A receptor mRNA was decreased in CA4 and increased in dentate gyrus by single or repeated ECS, with parallel alterations in [3H]8-OH-DPAT binding site densities. Repeated ECS increased cortical [3H]ketanserin binding and 5-HT2A receptor mRNA. The other mRNAs were unchanged. The results show that ECS has subtype specific, anatomically discrete, and temporally selective effects on 5-HT receptor expression.

Comparison of the effects of alpha-methyl-p-tyrosine and a tyrosine-free amino acid load on extracellular noradrenaline in the rat hippocampus in vivo

Peripheral administration of an amino acid load lacking tyrosine and its precursor, phenylalanine, causes a lowering of central tyrosine levels. The aim of the present study was to examine the effects of tyrosine depletion on extracellular noradrenaline using microdialysis. Extracellular noradrenaline was measured in hippocampus of the anaesthetized rat under both baseline conditions (with reuptake inhibitor, desipramine, in the perfusion medium) and following administration of the alpha2-adrenoreceptor antagonist, idazoxan. The tyrosine free amino acid load did not alter either baseline noradrenaline or the twofold rise in noradrenaline evoked by idazoxan compared with saline controls. In contrast, the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, caused a marked reduction in baseline extracellular noradrenaline and abolished the rise induced by idazoxan. In conclusion, the present data indicate that under the conditions used, a tyrosine-free amino acid mixture may not be an effective means to interfere with central noradrenaline function. This contrasts with recent findings demonstrating that the tyrosine-depletion approach can be used to decrease presynaptic dopamine function.

Effect of 5-HT(1A) receptor ligands on Fos-like immunoreactivity in rat brain: evidence for activation of noradrenergic transmission

This study investigated the effects of 8-OH-DPAT and various other 5-HT(1A) receptor agonists on brain noradrenergic transmission using Fos-like immunoreactivity (Fos-LI) as a marker of neural activation. Administration of 8-OH-DPAT (0.1 and 1 mg/kg) induced a marked and dose-related increase in the number of cells positive for Fos-LI in the locus coeruleus (LC), the main source of noradrenergic projections to the forebrain. This effect was also induced by the non-selective, partial 5-HT(1A) receptor agonist buspirone (10 mg/kg). The effect of both 8-OH-DPAT (0.1 mg/kg) and buspirone (10 mg/kg) on Fos-LI in the LC was blocked by pretreatment with the selective 5-HT(1A) receptor antagonist WAY 100635 (1 mg/kg). The active S(-)-enantiomer of the partial 5-HT(1A) receptor agonist (+/-)-MDL 75005EF (1 mg/kg) also induced the expression of Fos-LI in the LC, whereas the inactive R(+)-enantiomer of (+/-)-MDL 73005EF at the same dose did not. In addition to the LC, 8-OH-DPAT (0.1 mg/kg) also induced a marked increase in Fos-LI in various forebrain areas including the medial prefrontal cortex (infralimbic and cingulate cortical areas). More detailed analysis of the Fos response to 8-OH-DPAT in the medial prefrontal cortex revealed that the effect was attenuated by pretreatment with a combination of the beta(1)- and beta(2)-adrenoceptor antagonists ICI 118551 (4 mg/kg) and metoprolol (4 mg/kg), but not the alpha(1)-adrenoceptor antagonist prazosin (5 mg/kg). Taken together, the present findings provide immunocytochemical evidence that 5-HT(1A) receptor agonists activate noradrenergic neurones in the LC and that this leads to increased noradrenergic transmission at postsynaptic sites in the forebrain (specifically medial prefrontal cortex).

Attenuation of some subjective effects of amphetamine following tyrosine depletion

Fifteen healthy volunteers received d-amphetamine (20 mg orally) 2 h after ingesting either a nutritionally balanced amino acid mixture or one lacking the catecholamine precursors, tyrosine and phenylalanine (TYR-free). Plasma tyrosine levels were significantly lowered in subjects who received the TYR-free mixture but mean plasma amphetamine levels were higher. Despite this, the TYR-free mixture appeared to decrease the subjective psychostimulant effects of amphetamine, as determined by visual analogue scales. In contrast, the TYR-free mixture failed to lower the subjective anorectic effect of amphetamine. These findings are consistent with animal experimental studies indicating that tyrosine depletion attenuates the release of dopamine produced by amphetamine but not the release of noradrenaline.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Risk factors for predicting surgical salvage of sternal wound-healing complications

A retrospective study was performed to determine whether clinical factors can predict which complicated poststernotomy wounds can be managed successfully by debridement and reclosure, and which wounds require a muscle flap for healing. Between January 1990 and December 1996, 3,435 median sternotomies were performed at Indiana University Medical Center and affiliated hospitals. A total of 91 patients (2.6%) were reoperated for sternal wound-healing complications. Seventy-six patients (83.5%) underwent debridement and rewiring, and 15 patients (16.5%) underwent primary flap coverage. Of the 76 patients who underwent initial rewiring, 45 (59%) healed and 31 (41%) required additional operative procedures. Of the 31 rewiring failures, 26 patients (84%) were healed with muscle flaps and 4 patients (13%) were managed with a second successful rewiring. The following clinical factors were correlated with operative procedure and outcome: history of smoking, chronic obstructive pulmonary disease, steroid use, previous sternotomy, age, diabetes, harvest of the left or right internal mammary artery (IMA), emergency operation, operation time, pump time, cross-clamp time, ischemic time, coronary artery bypass grafting alone versus combined with a valve replacement, positive wound cultures, positive blood cultures, elevated white blood cell count, and fever. When comparing patients with successful rewiring with those who had a failed rewiring, positive wound and blood cultures were significant risk factors (p < 0.05) on univariate analysis. Presence of a positive wound culture was significant on multivariate analysis. When comparing risk factors in patients who were rewired successfully versus all patients who had muscle flap coverage, the presence of a positive blood culture was significant on both univariate and multivariate analyses. We conclude patients most likely to fail rewiring and to require muscle flap closure are those with infected wounds, positive blood cultures, and possibly left IMA bypasses.

Handling complaints in a constructive way

This article offers advice on dealing with specific complaints and looks at the nurse's role and responsibilities in handling and resolving them.

Effects of (-)-tertatolol, (-)-penbutolol and (+/-)-pindolol in combination with paroxetine on presynaptic 5-HT function: an in vivo microdialysis and electrophysiological study

The antidepressant efficacy of selective serotonin reuptake inhibitors (SSRIs) might be enhanced by co-administration of 5-HT1A receptor antagonists. Thus, we have recently shown that the selective 5-HT1A receptor antagonist, WAY 100635, blocks the inhibitory effect of an SSRI on 5-HT cell firing, and enhances its ability to elevate extracellular 5-HT in the forebrain. Here we determined whether the beta-adrenoceptor/5-HT1A receptor ligands (+/-)-pindolol, (-)-tertatolol and (-)-penbutolol, interact with paroxetine in a similar manner. Both (-)-tertatolol (2.4 mg kg(-1) i.v.) and (-)-penbutolol (2.4 mg kg(-1) i.v.) enhanced the effect of paroxetine (0.8 mg kg(-1) i.v.) on extracellular 5-HT in the frontal cortex, whilst (+/-)-pindolol (4 mg kg(-1) i.v.) did not. (-)-Tertatolol (2.4 mg kg(-1) i.v.) alone caused a slight increase in 5-HT however, (-)-penbutolol (2.4 mg kg(-1) i.v.) alone had no effect. In electrophysiological studies (-)-tertatolol (2.4 mg kg(-1) i.v.) alone had no effect on 5-HT cell firing but blocked the inhibitory effect of paroxetine. In contrast, (-)-penbutolol (0.1-0.8 mg kg(-1) i.v.) itself inhibited 5-HT cell firing, and this effect was reversed by WAY 100635 (0.1 mg kg(-1) i.v.). We have recently shown that (+/-)-pindolol inhibits 5-HT cell firing via a WAY 100635-sensitive mechanism. Our data suggest that (-)-tertatolol enhances the effect of paroxetine on forebrain 5-HT via blockade of 5-HT1A autoreceptors which mediate paroxetine-induced inhibition of 5-HT cell firing. In comparison, the mechanisms by which (-)-penbutolol enhances the effect of paroxetine on extracellular 5-HT is unclear, since (-)-penbutolol itself appears to have agonist properties at the 5-HT1A autoreceptor. Indeed, the agonist action of (+/-)-pindolol at 5-HT1A autoreceptors probably explains its inability to enhance the effect of paroxetine on 5-HT in the frontal cortex. Overall, our data suggest that both (-)-tertatolol and (-)-penbutolol are superior to (+/-)-pindolol in terms of enhancing the effect of an SSRI on extracellular 5-HT. Both (-)-tertatolol and (-)-penbutolol are worthy of investigation for use as adjuncts to SSRIs in the treatment of major depression.

Role of the medial prefrontal cortex in 5-HT1A receptor-induced inhibition of 5-HT neuronal activity in the rat

1. We examined the involvement of the frontal cortex in the 5-HT2A receptor-induced inhibition of 5-HT neurones in the dorsal raphe nucleus (DRN) of the anaesthetized rat using single-unit recordings complemented by Fos-immunocytochemistry. 2. Both transection of the frontal cortex as well as ablation of the medial region of the prefrontal cortex (mPFC) significantly attenuated the inhibition of 5-HT neurones induced by systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.5-16 microg kg(-1), i.v.). In comparison, the response to 8-OH-DPAT was not altered by ablation of the parietal cortex. The inhibitory effect of 8-OH-DPAT was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.) in all neurones tested. 3. In contrast, cortical transection did not alter the sensitivity of 5-HT neurones to iontophoretic application of 8-OH-DPAT into the DRN. Similarly, cortical transection did not alter the sensitivity of 5-HT neurones to systemic administration of the selective 5-HT reuptake inhibitor, paroxetine (0.1-0.8 mg kg(-1) , i.v.). 4. 8-OH-DPAT evoked excitation of mPFC neurones at doses (0.5-32 microg kg(-1), i.v.) in the range of those which inhibited 5-HT cell firing. At higher doses (32-512 microg kg(-1), i.v.) 8-OH-DPAT inhibited mPFC neurones. 8-OH-DPAT (0.1 mg kg(-1), s.c.) also induced Fos expression in the mPFC. The neuronal excitation and inhibition, as well as the Fos expression, were antagonized by WAY 100635. 5. These data add further support to the view that the inhibitory effect of 5-HT1A receptor agonists on the firing activity of DRN 5-HT neurones involves, in part, activation of a 5-HT1A receptor-mediated postsynaptic feedback loop centred on the mPFC.

Influence of 5-HT1A receptors on central noradrenergic activity: microdialysis studies using (+/-)-MDL 73005EF and its enantiomers

Recent studies indicate that 5-HT1A receptor agonists stimulate noradrenaline release in the brain. Here we investigate the mechanism underlying the increase in extracellular noradrenaline induced by (+/-)-MDL 73005EF, a weak 5-HT1A receptor agonist. Extracellular noradrenaline was measured in the hippocampus of the awake rat using microdialysis. (+/-)-MDL 73005EF (0.1, 1 and 5 mg/kg s.c.) caused a dose-related increase in noradrenaline. The active S(-)- enantiomer of MDL 73005EF (1 mg/kg s.c.) also increased noradrenaline whereas the inactive R(+)- enantiomer (1 mg/kg s.c.) did not. Measurements of extracellular 5-HT in hippocampus of anaesthetised rats confirmed that the 5-HT1A receptor agonist action of (+/-)-MDL 73005EF resides in the S(-)- enantiomer. Thus, S(-)-MDL 73005EF (0.3 and 1 mg/kg s.c.) markedly decreased 5-HT, whereas R(+)-MDL 73005EF (1 mg/kg s.c.) did not. The noradrenaline response to (+/-)-MDL 73005EF (1 mg/kg s.c.) was significantly blocked by the selective 5-T1A receptor antagonist, WAY 100635 (1 but not 0.3 mg/kg s.c). The noradrenaline response to (+/-)-MDL 73005EF (1 mg/kg s.c.) was not modified by pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine. Intra-hippocampal application of (+/-)-MDL 73005EF (10 microM in perfusion medium) did not increase noradrenaline. Although (+/-)-MDL 73005EF has moderate affinity for dopamine D2 binding sites, the dopamine D2 receptor antagonist, remoxipride (1 mg/kg s.c.) did not increase noradrenaline. In conclusion, our data suggest that (+/-)-MDL 73005EF increases noradrenaline release in rat hippocampus through activation of 5HT1A receptors that appear to be located postsynaptically. These data are discussed in relation to the antidepressant/anxiolytic effects of 5-HT1A agonists.

Effect of a tyrosine-free amino acid mixture on regional brain catecholamine synthesis and release

We report the effects of a tyrosine (and phenylalanine)-free amino acid mixture on tyrosine levels, ex vivo catecholamine synthesis and in vivo catecholamine release in brain regions of the rat. Administration of a tyrosine-free amino acid load reduced tissue levels of tyrosine (-50% after 2 h) in all brain regions examined (frontal cortex, hippocampus, striatum). The tyrosine-free amino acid mixture also reduced DOPA accumulation: this effect was most marked in striatum (-44%) and nucleus accumbens (-34%), areas with a predominantly dopaminergic innervation. Smaller decreases (-20-24%) were detected in other areas (cortex, hippocampus and hypothalamus). The effect on DOPA accumulation was prevented by supplementing the mixture with tyrosine/phenylalanine. The tyrosine-free amino acid mixture did not alter 5-HTP accumulation in any region. In microdialysis experiments, the tyrosine-free amino acid mixture did not consistently alter striatal extracellular dopamine under basal conditions but markedly, and dose-dependently, reduced the release of dopamine induced by amphetamine. In contrast, the tyrosine-free amino acid mixture did not alter either basal or amphetamine-evoked release of noradrenaline in hippocampus. Overall, these studies indicate that administration of a tyrosine-free amino acid mixture to rats depletes brain tyrosine to cause a decrease in regional brain catecholamine synthesis and release. Dopaminergic neurones appear to be more vulnerable to tyrosine depletion than noradrenergic neurones.