Effects of the metabotropic glutamate receptor agonist, ACPD, on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of the rat during the normal process of aging

The dopamine D1 receptor positive allosteric modulator, DETQ, improves cognition and social interaction in aged mice and enhances cortical and hippocampal acetylcholine efflux

Dopamine (DA) D1 and D2 receptors (Rs) are critical for cognitive functioning. D1 positive allosteric modulators (D1PAMs) activate D1Rs without desensitization or an inverted U-shaped dose response curve. DETQ, [2-(2,6-dichlorophenyl)-1-((1S,3R)-3-(hydroxymethyl)-5-(2-hydroxypropan-2-yl)-1-methyl-3,4-dihydroisoquinolin-2(1H)-yl)ethan-1-one] is highly selective for the human D1Rs as shown in humanized D1R knock-in (hD1Ki) mice. Here, we have ascertained the efficacy of DETQ in aged [13-23-month-old (mo)] hD1Ki mice and their corresponding age-matched wild-type (WT; C57BL/6NTac) controls. We found that in aged mice, DETQ, given acutely, subchronically, and chronically, rescued both novel object recognition memory and social behaviors, using novel object recognition (NOR) and social interaction (SI) tasks, respectively without any adverse effect on body weight or mortality. We have also shown, using in vivo microdialysis, a significant decrease in basal DA and norepinephrine, increase in glutamate (Glu) and gamma-amino butyric acid (GABA) efflux with no significant changes in acetylcholine (ACh) levels in aged vs young mice. In young and aged hD1Ki mice, DETQ, acutely and subchronically increased ACh in the medial prefrontal cortex and hippocampal regions in aged hD1Ki mice without affecting Glu. These results suggest that the D1PAM mechanism is of interest as potential treatment for cognitive and social behavioral deficits in neuropsychiatric disorders including but not restricted to neurodegenerative disorders, such as Parkinson's disease.

Altered glutamate/GABA equilibrium in aged mice cortex influences cortical plasticity

Age-related molecular changes in the synapse can cause plasticity decline. We found an impairment of experience-dependent cortical plasticity is induced by short lasting sensory conditioning in aged mice. However, extending the training procedure from 3 to 7 days triggered plasticity in the aged cortex of the same range as in young mice. Additionally, GABAergic markers (GABA, GAD67, VGAT) in young and aged groups that showed the plastic changes were upregulated. This effect was absent in the aged group with impaired plasticity, while the expression of Vglut1 increased in all trained groups. This may reflect the inefficiency of inhibitory mechanisms in the aging brain used to control increased excitation after training and to shape proper signal to noise ratio, which is essential for appropriate stimuli processing. HPLC analysis showed that the glutamate/GABA ratio was significantly reduced in aged animals due to a significant decrease in glutamate level. We also observed a decreased expression of several presynaptic markers involved in excitatory (vesicular glutamate transporter-vglut2) and inhibitory (glutamic acid decarboxylase-GAD67, vesicular GABA transporter VGAT) transmission in the aged barrel cortex. These changes may weaken the plasticity potential of neurons and impede the experience-dependent reorganization of cortical connections. We suggest that the imbalance toward inhibition resulting from a decrease of glutamate content in the aging cerebral cortex, together with GABAergic system ineffectiveness in upregulating GABA level after sensory training, contributes to the impairment of learning-dependent cortical plasticity.

Modulation of pyramidal cell output in the medial prefrontal cortex by mGluR5 interacting with CB1

The medial prefrontal cortex (mPFC) serves executive cognitive functions such as decision-making that are impaired in neuropsychiatric disorders and pain. We showed previously that amygdala-driven abnormal inhibition and decreased output of mPFC pyramidal cells contribute to pain-related impaired decision-making (Ji et al., 2010). Therefore, modulating pyramidal output is desirable therapeutic goal. Targeting metabotropic glutamate receptor subtype mGluR5 has emerged as a cognitive-enhancing strategy in neuropsychiatric disorders, but synaptic and cellular actions of mGluR5 in the mPFC remain to be determined. The present study determined synaptic and cellular actions of mGluR5 to test the hypothesis that increasing mGluR5 function can enhance pyramidal cell output. Whole-cell voltage- and current-clamp recordings were made from visually identified pyramidal neurons in layer V of the mPFC in rat brain slices. Both the prototypical mGluR5 agonist CHPG and a positive allosteric modulator (PAM) for mGluR5 (VU0360172) increased synaptically evoked spiking (E-S coupling) in mPFC pyramidal cells. The facilitatory effects of CHPG and VU0360172 were inhibited by an mGluR5 antagonist (MTEP). CHPG, but not VU0360172, increased neuronal excitability (frequency-current [F-I] function). VU0360172, but not CHPG, increased evoked excitatory synaptic currents (EPSCs) and amplitude, but not frequency, of miniature EPSCs, indicating a postsynaptic action. VU0360172, but not CHPG, decreased evoked inhibitory synaptic currents (IPSCs) through an action that involved cannabinoid receptor CB1, because a CB1 receptor antagonist (AM281) blocked the inhibitory effect of VU0360172 on synaptic inhibition. VU0360172 also increased and prolonged CB1-mediated depolarization-induced suppression of synaptic inhibition (DSI). Activation of CB1 with ACEA decreased inhibitory transmission through a presynaptic mechanism. The results show that increasing mGluR5 function enhances mPFC output. This effect can be accomplished by increasing excitability with an orthosteric agonist (CHPG) or by increasing excitatory synaptic drive and CB1-mediated presynaptic suppression of synaptic inhibition ("dis-inhibition") with a PAM (VU0360172). Therefore, mGluR5 may be a useful target in conditions of impaired mPFC output. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Stress, neurotransmitters, corticosterone and body-brain integration

Stress can be defined as a brain-body reaction towards stimuli arising from the environment or from internal cues that are interpreted as a disruption of homeostasis. The organization of the response to a stressful situation involves not only the activity of different types of neurotransmitter systems in several areas of the limbic system, but also the response of neurons in these areas to several other chemicals and hormones, chiefly glucocorticoids, released from peripheral organs and glands. Thus, stress is probably the process through which body-brain integration plays a major role. Here we review first the responses to an acute stress in terms of neurotransmitters such as dopamine, acetylcholine, glutamate and GABA in areas of the brain involved in the regulation of stress responses. These areas include the prefrontal cortex, amygdala, hippocampus and nucleus accumbens and the interaction among those areas. Then, we consider the role of glucocorticoids and review some recent data about the interaction of these steroids with several neurotransmitters in those same areas of the brain. Also the actions of other substances (neuromodulators) released from peripheral organs such as the pancreas, liver or gonads (insulin, IGF-1, estrogens) are reviewed. The role of an environmental enrichment on these same responses is also discussed. Finally a section is devoted to put into perspective all these environmental-brain-body-brain interactions during stress and their consequences on aging. It is concluded that the integrative perspective framed in this review is relevant for better understanding of how the organism responds to stressful challenges and how this can be modified through different environmental conditions during the process of aging. This article is part of a Special Issue entitled: Brain Integration.

Pain-related deactivation of medial prefrontal cortical neurons involves mGluR1 and GABA(A) receptors

Pain-related hyperactivity in the amygdala leads to deactivation of the medial prefrontal cortex (mPFC) and decision-making deficits. The mechanisms of pain-related inhibition of the mPFC are not yet known. Here, we used extracellular single-unit recordings of prelimbic mPFC neurons to determine the role of GABA(A) receptors and metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5, in pain-related activity changes of mPFC neurons. Background and evoked activity of mPFC neurons decreased after arthritis induction. To determine pain-related changes, the same neuron was recorded continuously before and after induction of arthritis in one knee joint by intra-articular injection of kaolin/carrageenan. Stereotaxic administration of a GABA(A) receptor antagonist {[R-(R*,S*)]-5-(6,8-dihydro-8-oxofuro[3,4-e]-1,3-benzodioxol-6-yl)-5,6,7,8-tetrahydro-6,6-dimethyl-1,3-dioxolo[4,5-g]isoquinolinium iodide (bicuculline)} into the mPFC by microdialysis reversed pain-related inhibition, whereas offsite injections into the adjacent anterior cingulate cortex had no or opposite effects on prelimbic mPFC neurons. A selective mGluR1/5 agonist [(S)-3,5-dihydroxyphenylglycine (DHPG)] inhibited background and evoked activity under normal conditions through a GABAergic mechanism, because the inhibitory effect was blocked with bicuculline. In the arthritis pain state, DHPG, alone or in the presence of bicuculline, had no effect. Consistent with the involvement of mGluR1 in pain-related inhibition of the mPFC, a selective mGluR1 antagonist [(S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid] reversed the pain-related decrease of background and evoked activity of mPFC neurons in arthritis, whereas a selective mGluR5 antagonist [2-methyl-6-(phenylethynyl)pyridine hydrochloride] had no effect. The mGluR antagonists had no effect under normal conditions. We interpret our data to suggest that pain-related inhibition of mPFC neurons in the arthritis model depends on mGluR1-mediated endogenous activation of GABA(A) receptors. Exogenous activation of mGluR1/5 produces GABAergic inhibition under normal conditions. Restoring normal activity in the mPFC may be a therapeutic strategy to improve cognitive deficits associated with persistent pain.

mGluR1, but not mGluR5, activates feed-forward inhibition in the medial prefrontal cortex to impair decision making

Cognitive flexibility depends on the integrity of the prefrontal cortex (PFC). We showed previously that impaired decision making in pain results from amygdala-driven inhibition of medial PFC neurons, but the underlying mechanisms remain to be determined. Using whole cell patch clamp in rat brain slices and a cognitive behavioral task, we tested the hypothesis that group I metabotropic glutamate receptors (mGluRs) activate feed-forward inhibition to decrease excitability and output function of PFC pyramidal cells, thus impairing decision making. Polysynaptic inhibitory postsynaptic currents (IPSCs) and monosynaptic excitatory postsynaptic currents (EPSCs) were evoked in layer V pyramidal cells by stimulating presumed amygdala afferents. An mGluR1/5 agonist [(S)-3,5-dihydroxyphenylglycine, DHPG] increased synaptic inhibition more strongly than excitatory transmission. The facilitatory effects were blocked by an mGluR1 [(S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid, LY367385], but not mGluR5, antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine. IPSCs were blocked by bicuculline and decreased by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium salt (NBQX). Facilitation of synaptic inhibition by DHPG was glutamate driven because it was blocked by NBQX. DHPG increased frequency but not amplitude of spontaneous IPSCs; consistent with action potential-dependent synaptic inhibition, tetrodotoxin (TTX) prevented the facilitatory effects. DHPG decreased synaptically evoked spikes (E-S coupling) and depolarization-induced spiking [frequency-current (f-I) relationship]. This effect was indirect, resulting from glutamate-driven synaptic inhibition, because it persisted when a G protein blocker was included in the pipette but was blocked by GABA(A) receptor antagonists and NBQX. In contrast, DHPG increased E-S coupling and f-I relationships in mPFC interneurons through a presynaptic action, further supporting the concept of feed-forward inhibition. DHPG also impaired the ability of the animals to switch strategies in a decision-making task; bicuculline restored normal decision making, whereas a GABA(A) receptor agonist (muscimol) mimicked the decision-making deficit. The results show that mGluR1 activates feed-forward inhibition of PFC pyramidal cells to impair cognitive functions.

Forced, moderate-intensity treadmill exercise suppresses apoptosis by increasing the level of NGF and stimulating phosphatidylinositol 3-kinase signaling in the hippocampus of induced aging rats

While nerve growth factor (NGF) activates various signaling cascades, the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway plays a pivotal role in controlling the survival of neurons, although this activity declines during the aging process. We investigated the effect of forced moderate-intensity treadmill exercise on the level of NGF and the PI3-K/Akt signaling pathway in the hippocampus of induced aging rats. Forty-five male Sprague-Dawley rats were divided into the following three groups: (1) control group, in which aging was not induced (CON: n=15), (2) aging-control group, in which aging was induced but the rats were not subjected to exercise (ACON: n=15), and (3) the aging-exercise group, in which aging was induced and the rats were subjected to treadmill exercise (AEX: n=15). d-Galactose (50mg/kg) was injected into the abdominal cavity for 8 weeks to induce aging. Rats were subjected to treadmill exercise 5 days a week for 8 weeks, and the speed of the treadmill was gradually increased. The protein levels of NGF, P-PI3-K, and P-Akt were significantly high in the AEX group (p<0.01, p<0.01, and p<0.001, respectively). Tyrosine kinase A (Trk A) receptor level was significantly higher in the CON and AEX groups than in the ACON group (p<0.01). TUNEL assay showed a significant reduction in apoptosis in the AEX group (p<0.001). Caspase-3 activation was significantly decreased in the AEX and CON groups (p<0.05). These results show that forced moderate-intensity treadmill exercise increases the level of NGF and activates P-PI3-K to induce P-Akt in order to suppress apoptotic cell death in the hippocampus of induced aging rats.

Glutamate-dopamine-GABA interactions in the aging basal ganglia

The study of neurotransmitter interactions gives a better understanding of the physiology of specific circuits in the brain. In this review we focus mostly on our own results on the interaction of the neurotransmitters glutamate, dopamine and GABA in the basal ganglia during the normal process of aging. We review first the studies on the action of endogenous glutamate on the extracellular concentrations of dopamine and GABA in the neostriatum and nucleus accumbens during aging. It was found that there exists an age-related change in the interaction of glutamate, dopamine and GABA and that these effects of aging exhibit a dorsal-to-ventral pattern of effects with no changes in the dorsal parts (dorsal striatum) and changes in the most ventral parts (nucleus accumbens). Second we reviewed the data on the effects of different ionotropic and metabotropic glutamate receptor agonists on the extracellular concentrations of dopamine and GABA in the nucleus accumbens. The results obtained clearly show the different contribution of each glutamate receptor subtype in the age-related changes produced on the interaction of glutamate, dopamine and GABA in this area of the brain. Third the effects of an enriched environment on the action of AMPA and NMDA-receptor agonists in the nucleus accumbens of rats during aging are also evaluated. Finally, and since the nucleus accumbens has been suggested to play a role in emotion and motivation and also motor behaviour, we speculated on the possibility of a specific contribution for the different glutamatergic pathways terminating in the nucleus accumbens and their interaction with a decreased dopamine playing a relevant role in motor behaviour during aging.

Aging, plasticity and environmental enrichment: structural changes and neurotransmitter dynamics in several areas of the brain

Cajal was probably the first neurobiologist to suggest that plasticity of nerve cells almost completely disappeared during aging. However, we know today that neural plasticity is still present in the brain during aging. In this review we suggest that aging is a physiological process that occurs asynchronously in different areas of the brain and that the rate of that process is modulated by environmental factors and related to the neuronal-synaptic-molecular substrates of each area. We review here some of the most recent results on aging of the brain in relation to the plastic changes that occur in young and aged animals as a result of living in an enriched environment. We highlight the results from our own laboratory on the dynamics of neurotransmitters in different areas of the brain. Specifically we review first the effects of aging on neurons, dendrites, synapses, and also on molecular and functional plasticity. Second, the effects of environmental enrichment on the brain of young and aged animals. And third the effects of an enriched environment on the age-related changes in neurogenesis and in the extracellular concentrations of glutamate and GABA in hippocampus, and on dopamine, acetylcholine, glutamate and GABA under a situation of acute mild stress in the prefrontal cortex.

Applicability of reverse microdialysis in pharmacological and toxicological studies

A recent application of microdialysis is the introduction of a substance into the extracellular space via the microdialysis probe. The inclusion of a higher amount of a drug in the perfusate allows the drug to diffuse through the microdialysis membrane to the tissue. This technique, actually called as reverse microdialysis, not only allows the local administration of a substance but also permits the simultaneous sampling of the extracellular levels of endogenous compounds. Local effects of exogenous compounds have been studied in the central nervous system, hepatic tissue, dermis, heart and corpora luteae of experimental animals by means of reverse microdialysis. In central nervous studies, reverse microdialysis has been extensively used for the study of the effects on neurotransmission at different central nuclei of diverse pharmacological and toxicological agents, such as antidepressants, antipsychotics, antiparkinsonians, hallucinogens, drugs of abuse and experimental drugs. In the clinical setting, reverse microdialysis has been used for the study of local effects of drugs in the adipose tissue, skeletal muscle and dermis. The aim of this review is to describe the principles of the reverse microdialysis, to compare the technique with other available methods and finally to describe the applicability of reverse microdialysis in the study of drugs properties both in basic and clinical research.