Michael W. Conway

Role of 5-HT1A and 5-HT1B Receptors in the Mediation of Behavior in the Forced Swim Test in Mice

The purpose of this study was to further examine the effect of activation of 5-HT1A and 5-HT1B receptors in the forced swim test in mice and to determine if activation of these receptors played a role in the mediation of the effects of the tricyclic antidepressant imipramine. The 5-HT1A agonist 8-OH-DPAT decreased immobility in the forced swim test in mice as previously described. Both the selective 5-HT1B agonist anpirtoline (1.25–5 mg/kg) and mixed 5-HT1A/B agonist RU24969 (0.6–2.5 mg/kg) significantly increased time spent swimming in the FST.

Diverse and often opposite behavioural effects of NMDA receptor antagonists in rats: implications for “NMDA antagonist modelling” of schizophrenia

RationaleLittle attention has been paid to the relative equivalence of behavioural effects of NMDA receptor antagonists in rodents, with different compounds often used interchangeably to “model” aspects of schizophrenia in preclinical studies.ObjectivesTo further resolve such conjecture, the present study systematically compared eight different NMDA receptor antagonists: MK-801, PCP, ketamine, memantine, SDZ 220,581, Ro 25-6981, CP 101-606 and NVP-AAM077, in a series of variable interval (VI) schedules of reinforcement. Aspects of motivation as indexed in these tasks may well be impaired in schizophrenia and undoubtedly impact on the capacity to perform more complex, explicit tasks of cognition.Methods and resultsAn initial locomotor activity assessment demonstrated that all antagonists tested, except the NR2A-subunit preferring antagonist NVP-AAM077, induced hyperactivity, albeit of greatly differing magnitudes, qualities and temporal profiles. Three distinct patterns of antagonist effect were evident from the VI assays used: a uniform decrease in responding produced by (S)-(+)-ketamine, memantine and NVP-AAM077, a uniform increase in responding caused by the NR2B-subunit preferring antagonists Ro 25-6981 and CP 101-606, and variable bidirectional effects of PCP, SDZ 220,581 and MK-801.ConclusionDespite nominally common mechanisms of action and often presumed biological equivalence, the NMDA antagonists tested produced very diverse effects on the expression of instrumental action. Other aspects of responding were left intact, including switching and matching behaviours, and the ability to respond to conditional stimuli. The implications of such findings with regard to animal modelling of schizophrenic psychotic symptoms are manifold.

Characterisation of carbon paste electrodes for real-time amperometric monitoring of brain tissue oxygen.

Tissue O₂ can be monitored using a variety of electrochemical techniques and electrodes. In vitro and in vivo characterisation studies for O₂ reduction at carbon paste electrodes (CPEs) using constant potential amperometry (CPA) are presented. Cyclic voltammetry indicated that an applied potential of -650 mV is required for O₂ reduction at CPEs. High sensitivity (-1.49 ± 0.01 nA/μM), low detection limit (ca. 0.1 μM) and good linear response characteristics (R² > 0.99) were observed in calibration experiments performed at this potential. There was also no effect of pH, temperature, and ion changes, and no dependence upon flow/fluid convection (stirring). Several compounds (e.g. dopamine and its metabolites) present in brain extracellular fluid were tested at physiological concentrations and shown not to interfere with the CPA O₂ signal. In vivo experiments confirmed a sub-second response time observed in vitro and demonstrated long-term stability extending over twelve weeks, with minimal O₂ consumption (ca. 1 nmol/h). These results indicate that CPEs operating amperometrically at a constant potential of -650 mV (vs. SCE) can be used reliably to continuously monitor brain extracellular tissue O₂.

Group II metabotropic glutamate receptor antagonists LY341495 and LY366457 increase locomotor activity in mice.

The group II metabotropic glutamate receptor (mGluR) antagonists LY341495 and LY366457 were profiled for their effects on locomotor activity in mice. Both compounds significantly increased locomotor activity. Observational studies showed that rearing was also selectively increased. LY366457-induced hyperactivity was significantly attenuated by the selective D1 dopamine receptor antagonist SCH23390 and also by the D2 dopamine receptor antagonist haloperidol but only at doses that significantly suppressed spontaneous locomotion. The selective 5-HT(2A) antagonist MDL100907 had no effect on LY366457-induced hyperactivity, while the less selective 5-HT(2A-C) antagonist ritanserin had only a modest effect. In all cases, the doses of antagonists that reduced the locomotor response to LY366457 were greater than those previously shown to reduce the locomotor response to the psychostimulants amphetamine and cocaine and MK-801. Pretreatment with reserpine also significantly attenuated the response to LY366457, possibly implicating a monoaminergic substrate in the mediation of this effect. The phenomenonology and pharmacology of the locomotor activation induced by the mGluR antagonists differs markedly from that induced by locomotor stimulants such as amphetamine, cocaine or MK-801. These results suggest that group II mGluRs may be involved in the tonic suppression of locomotor and exploratory activity, and this suppression can be disinhibited in the presence of a group II mGluR antagonist.

Selective imidazoline I2 ligands do not show antidepressant-like activity in the forced swim test in mice

Clonidine is an adrenergic agonist with high affinity for [.alpha]2 adrenoceptors that also has affinity for imidazoline receptors. Clonidine has previously been shown to reduce immobility in the forced swim test (FST) in mice. In the present study, this effect was blocked by idazoxan (0.06 mg/kg s.c.) and by yohimbine (1.0 mg/kg s.c.) suggesting that clonidines effects in this test are mediated via its action at [.alpha]2 sites. Imidazoline I2 site ligands have been shown to inhibit monoamine oxidase and thus may also have antidepressant activity. Three compounds with selective affinity for I2 receptors (BU224, BU239, BDF 8082) were also tested in the FST. These compounds showed no activity either alone or in combination with a subthreshold dose of imipramine in the FST. These results suggest that I2 receptor ligands do not show antidepressant-like activity in the FST in mice. Furthermore the activity of the mixed [.alpha]2/I1 agonist clonidine is most likely to be due to its action at [.alpha]2 sites.

Dissociable Effects of Antipsychotics on Ketamine-Induced Changes in Regional Oxygenation and Inter-Regional Coherence of Low Frequency Oxygen Fluctuations in the Rat

Typical and atypical antipsychotics have been shown to alleviate N-methyl-D-aspartate (NMDA) receptor antagonist-induced BOLD signals in healthy humans and animals to differing degrees; factors that might relate to their different molecular mechanisms and clinical profiles. Recent studies have also extended these investigations to the analysis of resting state functional connectivity measures of BOLD signals in different brain regions. Using constant potential amperometry, we examined the effects of the NMDA receptor antagonist S-(+)-ketamine on tissue oxygen levels in medial prefrontal cortex (mPFC) and medial ventral striatum (mVS), and temporal coherence of low-frequency oxygen fluctuations between these regions in freely moving rats. Furthermore, we assessed the extent to which the atypical antipsychotic clozapine and the typical antipsychotic haloperidol could modulate the effects of S-(+)-ketamine on these measures. Acute S-(+)-ketamine (5–25 mg/kg) produced dose-dependent increases in both tissue O2 levels and coherence. Although effects of clozapine and haloperidol alone were relatively minor, their effects on ketamine-induced signals were markedly more distinct. Clozapine dose-dependently attenuated the absolute S-(+)-ketamine (25 mg/kg) O2 signal in both regions, and also attenuated ketamine-induced increases in regional coherence. Haloperidol had no effect on the absolute ketamine O2 signal yet potentiated increases in regional coherence. The dissociable effects of haloperidol and clozapine on ketamine-induced hyperoxygenation and mPFC–mVS coherence elucidate potentially important mechanistic differences between these classes of pharmacology. This study demonstrates for the first time that in vivo amperometry can measure both regional brain tissue O2 levels and inter-regional coherence, advancing BOLD-like measurements of functional connectivity into awake, unconstrained animals.

Differential Contributions of Infralimbic Prefrontal Cortex and Nucleus Accumbens during Reward-Based Learning and Extinction

Using environmental cues for the prediction of future events is essential for survival. Such cue-outcome associations are thought to depend on mesolimbic circuitry involving the nucleus accumbens (NAc) and prefrontal cortex (PFC). Several studies have identified roles for both NAc and PFC in the expression of stable goal-directed behaviors, but much remains unknown about their roles during learning of such behaviors. To further address this question, we used in vivo oxygen amperometry, a proxy for blood oxygen level-dependent (BOLD) signal measurement in human functional magnetic resonance imaging, in rats performing a cued lever-pressing task requiring discrimination between a rewarded and nonrewarded cue. Simultaneous oxygen recordings were obtained from infralimbic PFC (IFC) and NAc throughout both acquisition and extinction of this task. Activation of NAc was specifically observed following rewarded cue onset during the entire acquisition phase and also during the first days of extinction. In contrast, IFC activated only during the earliest periods of acquisition and extinction, more specifically to the nonrewarded cue. Thus, in vivo oxygen amperometry permits a novel, stable form of longitudinal analysis of brain activity in behaving animals, allowing dissociation of the roles of different brain regions over time during learning of reward-driven instrumental action. The present results offer a unique temporal perspective on how NAc may promote actions directed toward anticipated positive outcome throughout learning, while IFC might suppress actions that no longer result in reward, but only during critical periods of learning.

Hemodynamic responses in amygdala and hippocampus distinguish between aversive and neutral cues during Pavlovian fear conditioning in behaving rats.

Lesion and electrophysiological studies in rodents have identified the amygdala and hippocampus (HPC) as key structures for Pavlovian fear conditioning, but human functional neuroimaging studies have not consistently found activation of these structures. This could be because hemodynamic responses cannot detect the sparse neuronal activity proposed to underlie conditioned fear. Alternatively, differences in experimental design or fear levels could account for the discrepant findings between rodents and humans. To help distinguish between these alternatives, we used tissue oxygen amperometry to record hemodynamic responses from the basolateral amygdala (BLA), dorsal HPC (dHPC) and ventral HPC (vHPC) in freely‐moving rats during the acquisition and extinction of conditioned fear. To enable specific comparison with human studies we used a discriminative paradigm, with one auditory cue [conditioned stimulus (CS)+] that was always followed by footshock, and another auditory cue (CS−) that was never followed by footshock. BLA tissue oxygen signals were significantly higher during CS+ than CS− trials during training and early extinction. In contrast, they were lower during CS+ than CS− trials by the end of extinction. dHPC and vHPC tissue oxygen signals were significantly lower during CS+ than CS− trials throughout extinction. Thus, hemodynamic signals in the amygdala and HPC can detect the different patterns of neuronal activity evoked by threatening vs. neutral stimuli during fear conditioning. Discrepant neuroimaging findings may be due to differences in experimental design and/or fear levels evoked in participants. Our methodology offers a way to improve translation between rodent models and human neuroimaging.

Coordinated acetylcholine release in prefrontal cortex and hippocampus is associated with arousal and reward on distinct timescales

Summary Cholinergic neurotransmission throughout the neocortex and hippocampus regulates arousal, learning, and attention. However, owing to the poorly characterized timing and location of acetylcholine release, its detailed behavioral functions remain unclear. Using electrochemical biosensors chronically implanted in mice, we made continuous measurements of the spatiotemporal dynamics of acetylcholine release across multiple behavioral states. We found that tonic levels of acetylcholine release were coordinated between the prefrontal cortex and hippocampus and maximal during training on a rewarded working memory task. Tonic release also increased during REM sleep but was contingent on subsequent wakefulness. In contrast, coordinated phasic acetylcholine release occurred only during the memory task and was strongly localized to reward delivery areas without being contingent on trial outcome. These results show that coordinated acetylcholine release between the prefrontal cortex and hippocampus is associated with reward and arousal on distinct timescales, providing dual mechanisms to support learned behavior acquisition during cognitive task performance.