Corticosterone and serotonin similarly influence GABAergic and purinergic pathways to affect cortical inhibitory networks

Both serotonin (5‐HT) and stress exert changes in cortical inhibitory tone to shape the activity of cortical networks. Because astrocytes are also known to affect inhibition through established purinergic pathways, we assessed the role of GABA and purinergic pathways with respect to the effects of rapid corticosterone (CORT) and 5‐HT on cortical inhibition. We used a paired‐pulse paradigm (P1 and P2) in acutely isolated mouse brain slices to evaluate changes in cortical evoked inhibition. Normally, 5‐HT decreases the amplitude of the first pulse P1, whereas it increases the amplitude of P2 (increasing frequency transmission). Interestingly, it was observed that CORT application decreased P1 and increased P2 similar to that of 5‐HT application. Given that CORT and 5‐HT are known to modulate inhibition, we applied the GABA_A antagonist bicuculline in the presence of both and found that the increase in P2 and the P2/P1 was lost, providing evidence for a common mechanism involving GABA_A receptor signalling. Additional occlusion experiments (ie, 5‐HT in presence of CORT and CORT in presence of 5‐HT) provide further support for a common mechanism. Because both 5‐HT and CORT blocked the increase in P2 and P2/P1 with respect to the other, we suggest 5‐HT/CORT already utilise the shared mechanism to affect cortical inhibition. Using low concentrations of the GAPDH inhibitor iodoacetate, as commonly used to selectively disrupt astrocyte metabolism, we found that the increase in P2 and P2/P1 was similarly blocked in response to both CORT and 5‐HT. Because astrocyte signalling depends in large part on purinergic pathways, the purinergic contribution was assessed using Ab129 (P2Y antagonist) and SCH 58261 (A2A antagonist). Once again, P2Y and A2A receptor blockade similarly disrupted 5‐HT‐ or CORT‐mediated increases in P2 and P2/P1. Taken together, these results support the common involvement of GABAergic and purinergic pathways in the effects of CORT and 5‐HT that may also involve astrocytes.     

Antiabsence drugs and inhibitory pathways

Conditioning stimuli to the coronal gyrus or periventricular gray matter inhibit the activity of spinal trigeminal neurons. Valproate decreased the corticofugal inhibition of the spinal trigeminal nucleus, as did ethosuximide, trimethadione, and imipramine. Valproate and ethosuximide also decreased the periventricular inhibition of the spinal trigeminal nucleus, indicating that antiabsence drug depress subcortical inhibitory pathways as well as pathways of cortical origin. These results support the hypothesis that ability to depress inhibitory pathways is an important characteristic of antiabsence drugs. The effect of valproate and ethosuximide on periventricular inhibition also suggests that these anticonvulsants may act by preventing the spread of seizure activity through subcortical pathways.     

Depression of cortical activity in humans by mild hypercapnia

The effects of neural activity on cerebral hemodynamics underlie human brain imaging with functional magnetic resonance imaging and positron emission tomography. However, the threshold and characteristics of the converse effects, wherein the cerebral hemodynamic and metabolic milieu influence neural activity, remain unclear. We tested whether mild hypercapnia (5% CO2 ) decreases the magnetoencephalogram response to auditory pattern recognition and visual semantic tasks. Hypercapnia induced statistically significant decreases in event-related fields without affecting behavioral performance. Decreases were observed in early sensory components in both auditory and visual modalities as well as later cognitive components related to memory and language. Effects were distributed across cortical regions. Decreases were comparable in evoked versus spontaneous spectral power. Hypercapnia is commonly used with hemodynamic models to calibrate the blood oxygenation level-dependent response. Modifying model assumptions to incorporate the current findings produce a modest but measurable decrease in the estimated cerebral metabolic rate for oxygen change with activation. Because under normal conditions, low cerebral pH would arise when bloodflow is unable to keep pace with neuronal activity, the cortical depression observed here may reflect a homeostatic mechanism by which neuronal activity is adjusted to a level that can be sustained by available bloodflow. Animal studies suggest that these effects may be mediated by pH-modulating presynaptic adenosine receptors. Although the data is not clear, comparable changes in cortical pH to those induced here may occur during sleep apnea, sleep, and exercise. If so, these results suggest that such activities may in turn have generalized depressive effects on cortical activity.     

Handedness and the excitability of cortical inhibitory circuits

Inhibitory processes play a significant role in the control of goal-directed actions. To increase insights into these mechanisms as a function of handedness, we measured the transient inhibition of volitional motor activity induced by single pulse transcranial magnetic stimulation during bimanual isometric contractions with symmetrical and asymmetrical force demands. Here, we assess the cortical silent period (cSP), which associates with intrahemispheric inhibition, and the ipsilateral silent period (iSP), which provides an estimation of interhemispheric inhibition. The data showed that inhibitory processes support the functional regulation of bimanual motor output. Furthermore, right-handers demonstrated asymmetries in intra- and interhemispheric inhibition due to asymmetrical force requirements and hand dominance, whereas left-handers did not show marked differences. In particular, right-handers demonstrated increased inhibitory processing that favoured control of the dominant (left) hemisphere whereas both motor cortices exhibited equal capabilities in left-handers. These observations were specific to the bimanual nature of the task. The present results underline distinct organisational mechanisms of coordinated behaviour in right- and left-handers.     

Hierarchical cortical control of neuroimmunomodulatory pathways

The central nervous system and the immune system are both stimulus response systems with sophisticated memories mediating defence and adaptation to external and internal threats. There is mounting evidence that these two systems share their information in a bidirectional flow of cytokines, steroids, and neuropeptides. This review examines the influence of higher cognitive centres on immunity and highlights the central role played by the hypothalamus in enabling these two systems to function as an integrated unit to maintain homeostasis. The search for novel compounds that are capable of enhancing immunity by regulating these brain–immune feedback loops offers one of the most exciting areas for future neurobiological research.     

Cortico-spinal pathways and inhibitory mechanisms in hepatic encephalopathy

Transcranial magnetic stimulation of the cerebral cortex was used to study motor system function in 31 cirrhotics (29 post-necrotic and 2 cryptogenic) with and without hepatic encephalopathy (HE). The results were compared with those of 14 healthy subjects matched for age. A significant increase of central motor conduction time, a significant raising of the motor evoked potential (MEP) threshold at rest and a significant reduction of the MEP/muscle action potential (MAP) amplitude ratio were found only in patients with chronic stable (12 patients) and recurrent (9 patients) HE. Vice versa, a significant shortening of the central silent period was observed in all 31 cirrhotic patients. The peripheral silent period was normal in all instances. These results indicate that the damage to the cortico-spinal pathways is related to the progression of cirrhosis to HE, and that cirrhotic patients present a dysfunction of the inhibitory motor mechanisms before HE is clinically manifest.     

Differential effect of baclofen on cortical and spinal inhibitory circuits

ObjectiveThe cutaneous silent period (SP) is a spinal inhibitory reflex, which suppresses activity in spinal motor nuclei. Transcranial magnetic stimulation (TMS) elicits a cortical SP, which represents GABA_B receptor-mediated inhibition of cortical excitability. Baclofen as a strong GABA_B agonist effectively reduces muscle hypertonia, however, it is not known whether intrathecal baclofen (ITB) may modulate spinal inhibitory circuits.MethodsWe evaluated clinical and neurophysiological effects of ITB in ten patients with severe spasticity due to spinal cord injury (n = 9) and chronic progressive multiple sclerosis (n = 1). Neurophysiological assessment included H reflex and cutaneous and cortical SPs, before and 15, 30, 60, 90, 120, and 180 min after ITB bolus administration.ResultsITB suppressed soleus H reflex as early as 15 min after lumbar bolus injection; MAS scores declined after 1 h. Cortical SP end latency and duration increased progressively with a significant maximum 3 h following ITB bolus, whereas cutaneous SP latency and duration did not change significantly.ConclusionThe present findings suggest that baclofen does not affect the cutaneous SP, but prolongs the cortical SP.SignificanceThe spinal inhibitory circuitry of the cutaneous SP is not modulated by GABA_B receptor-mediated activity, in contrast to the cortical inhibitory circuitry of the cortical SP, which is subject to powerful GABA_B control.     

Maturation of inhibitory and excitatory motor cortex pathways in children

Objective

To study intracortical inhibition and facilitation with paired-pulse transcranial magnetic stimulation in children, adolescents and adults. Methods: Paired-pulse transcranial magnetic stimulation (interstimulus intervals (ISI): 1, 3, 5, 10 and 20 ms) was applied over the primary motor cortex (M1) in 30 healthy subjects (range 6–30 years, median age 15 years and 8 months, SD 7,9) divided in three groups: adults (? 18 years), adolescents (> 10 and < 18 years) and children (? 10 years). Results: We observed significantly less intracortical inhibition (SICI) in children’s M1 compared to that of adults. Adolescents showed significantly less SICI at the 5 ms interval than did adults. No significant differences were apparent in intracortical facilitation (ICF). Conclusion: We postulate that, as in adults, the maturing M1 possesses horizontal glutamatergic cross-links that represent the neuronal substrate of excitatory intracortical pathways. GABAergic interneurons, the neuronal substrate of inhibitory intracortical pathways, mature between childhood and adulthood. Reduced GABAergic inhibition may facilitate neuronal plasticity and motor learning in children.     

Mouse cortical inhibitory neuron type that coexpresses somatostatin and calretinin

Mammalian cortex contains a diversity of inhibitory neuron types, each with distinct morphological, immunochemical, and/or physiological properties. In rat cortex, chemical markers distinguish at least four distinct and nonoverlapping neuron classes based on expression of parvalbumin (PV), somatostatin (SST), calretinin (CR), and cholecystokinin (CCK). It has generally been assumed that these classifications should also apply to other rodent species. In mouse cortex, however, we found significant colocalization of SST and CR in inhibitory neurons; about 30% of SST-positive cells contained CR, and about 33% of CR-positive cells contained SST across frontal, somatosensory (S1), and visual cortex (V1). The SST and CR colocalized cells were concentrated in layer 2/3. We further characterized morphological and physiological properties of the mouse cortical inhibitory neuron types that express SST by using "GIN" transgenic mice, in which GFP is expressed in a subset of SST inhibitory neurons (see Oliva et al. [2000] J Neurosci 20:3354-3368). Generally, both SST/CR+ cells and SST/CR- cells exhibited morphological features of Martinotti cells as described in rat cortex, and they also had similar accommodating spike-firing patterns. However, they differed significantly in quantitative comparisons of morphology and spike shapes. SST/CR+ cells had more horizontally extended dendritic fields and more primary process than did SST/CR- cells; and SST/CR- cells had narrower action potential widths and faster afterhyperpolarization than did SST/CR+ cells. Thus, our data show an important species difference in the chemical distinction of inhibitory neuron subtypes, and indicate that colocalization of CR in SST cells correlates with different morphological and physiological features.     

Abnormalities of cortical inhibitory neurons in amyotrophic lateral sclerosis

We have used peristimulus time histograms to study how paired, transcranial magnetic stimulation alters the firing of single motor units and the magnitude of unitary excitatory postsynaptic potentials (EPSPs) recorded from the extensor digitorum communis muscle. With stimulus intensity at threshold and an interstimulus interval of 30 ms, normal subjects (n = 20) demonstrated marked inhibition with a mean test/conditioning EPSP ratio of 13.8% (range 0–51%) and in 7 subjects the ratio was 0 (100% inhibition). In amyotrophic lateral sclerosis (ALS) the ratio was 133% (range 64–267%), P < 0.001. Fifty percent of patients had a test/conditioning EPSP ratio greater than 100% (0 inhibition). The abnormalities were independent of disease severity, bulbar versus spinal ALS, more prominent upper versus lower motor neuron findings, and disease duration. Normal inhibition occurred in 3 individuals, 1 each with multiple sclerosis, Kennedy's syndrome, and monomelic amyotrophy. We speculate that the marked loss of inhibition seen in all patients with ALS, which may be unique to this disorder, reflects loss of inhibitory modulation of the corticomotoneuron and could result in their chronic excitatory drive and eventual demise. © 1997 John Wiley & Sons, Inc.     

Dendritic calcium mechanisms and long-term potentiation in cortical inhibitory interneurons

Calcium (Ca(2+) ) is a major second messenger in the regulation of different forms of synaptic and intrinsic plasticity. Tightly organized in space and time, postsynaptic Ca(2+) transients trigger the activation of many distinct Ca(2+) signaling cascades, providing a means for a highly specific signal transduction and plasticity induction. High-resolution two-photon microscopy combined with highly sensitive synthetic Ca(2+) indicators in brain slices allowed for the quantification and analysis of postsynaptic Ca(2+) dynamics in great detail. Much of our current knowledge about postsynaptic Ca(2+) mechanisms is derived from studying Ca(2+) transients in the dendrites and spines of pyramidal neurons. However, postsynaptic Ca(2+) dynamics differ considerably among different cell types. In particular, distinct rules of postsynaptic Ca(2+) signaling and, accordingly, of Ca(2+) -dependent plasticity operate in GABAergic interneurons. Here, I review recent progress in understanding the complex organization of postsynaptic Ca(2+) signaling and its relevance to several forms of long-term potentiation at excitatory synapses in cortical GABAergic interneurons.     

Subtle hemispheric asymmetry of motor cortical inhibitory tone.

OBJECTIVE: To test whether a novel paired transcranial magnetic stimulation (TMS) protocol (J Physiol 545.1 (2002) 153) detects hemispheric differences in motor cortical inhibition. METHODS: Nine right-handers and 8 left-handers participated. Focal paired TMS was applied to the hand area of the dominant (M1-D) or non-dominant motor cortex (M1-ND). Motor evoked potentials (MEP) were recorded from the relaxed contralateral abductor digiti minimi. The first (S1) and second pulse (S2) were separated by 1.5 or 2.1 ms. Nine stimulus intensities of S1 and S2 (i.e. 9x9 intensity conditions) ranging from 60 to 140% of resting motor threshold (RMT) were tested. The interaction between S1 and S2 was expressed by MEP(S1+S2)/(MEP(S1)+MEP(S2))*100%. Values below and above 100% indicate short-interval intracortical inhibition (SICI) and facilitation (SICF), respectively. RESULTS: In right-handers, RMT was lower, SICI was present with fewer intensity conditions and the magnitude of SICI was less in M1-D than M1-ND. No hemispheric asymmetry was found for SICF. Left-handers showed no hemispheric difference for any of these measures. CONCLUSIONS: Findings suggest that, in right-handers, M1-D is controlled by less inhibitory tone than M1-ND. This may put the M1-D to an advantage for processes that are associated with a reduction of SICI, such as voluntary activation and use-dependent plasticity.     

Reduced inhibitory effect of imipramine on radiolabeled serotonin uptake into platelets in geriatric depression

Tritiated imipramine binding, uptake of radiolabeled serotonin, and inhibition of uptake by imipramine in vitro were studied in platelets obtained from four groups of subjects: (1) normal controls 50 years of age or younger, (2) patients with major depression 50 years of age or younger, (3) normal controls 60 years of age or older, and (4) patients with major depression 60 years of age or older. Depression in both age groups was associated with a substantial decrease in the number of [3H]imipramine binding sites; the elderly depressed patients exhibited a small but significant (p less than 0.05) reduction in platelet [3H]serotonin uptake. However, the inhibition of serotonin uptake into platelets by imipramine was markedly reduced only in the elderly depressed patients. This reduced sensitivity to imipramine may explain the reduced responsiveness of patients with geriatric depression to the therapeutic effects of imipramine and other tricyclic antidepressants.     

In vitro metabolism of chlorpromazine by cytochromes P450 4F4 and 4F5 and the inhibitory effect of imipramine

The metabolism of chlorpromazine by expressed recombinant cytochromes P450 4F4 and 4F5 cloned from rat brain was analyzed to characterize the individual activities of the isoforms. Both isoforms metabolized chlorpromazine to both the N-demethylated and the S-oxide products. When isoforms were incubated with chloropromazine in the presence of increasing concentrations of imipramine, imipramine significantly inhibited both N-demethylation and S-oxidation of chlorpromazine. A dilution of the serum fraction of anti-4F antibody was also found to significantly inhibit both S-oxidation and N-demethylation of chlorpromazine by both 4F4 and 4F5.     

Immunochemical characterization of inhibitory mouse cortical neurons: Three chemically distinct classes of inhibitory cells

The cerebral cortex has diverse types of inhibitory neurons. In rat cortex, past research has shown that parvalbumin (PV), somatostatin (SOM), calretinin (CR), and cholecystokinin (CCK) label four distinct chemical classes of GABAergic interneurons. However, in contrast to rat cortex, previous studies indicate that there is significant colocalization of SOM and CR in mouse cortical inhibitory neurons. In the present study we further characterized immunochemical distinctions among mouse inhibitory cortical neurons by double immunochemical labeling with chemical markers. We found that, PV, SOM, and vasointenstinal peptide (VIP) reliably identify three nonoverlapping distinct subpopulations, as there was no overlap of immunoreactivity between PV and all the other chemical markers tested, and SOM and VIP did not show any overlap in labeled neurons in all the cortical areas. In comparison, there was significant overlap in combinations of other chemical markers. With some laminar and regional variations, the average overlap of SOM/CR (percentage of SOM+ cells expressing CR) and SOM/neuropeptide tyrosine (NPY) across all examined layers and cortical regions was 21.6% and 7.1%, respectively. The average overlap of VIP/CR, VIP/NPY, and CR/NPY was 34.2%, 9.5%, and 10%, respectively. We quantified and assessed the percentages of marker‐positive GABAergic cells, and showed that the nonoverlapping subpopulations (i.e., PV+, SOM+ and VIP+ cells) accounted for about 60% of the GABAergic cell population. Taken together, our data reveal important chemical distinctions between mouse inhibitory cortical neurons and indicate that PV, SOM, and VIP can differentially label a majority of mouse inhibitory cortical neurons. J. Comp. Neurol. 518:389–404, 2010. © 2009 Wiley‐Liss, Inc.     

Parvalbumin-producing cortical interneurons receive inhibitory inputs on proximal portions and cortical excitatory inputs on distal dendrites

To examine inputs to parvalbumin (PV)-producing interneurons, we generated transgenic mice expressing somatodendritic membrane-targeted green fluorescent protein specifically in the interneurons, and completely visualized their dendrites and somata. Using immunolabeling for vesicular glutamate transporter (VGluT)1, VGluT2, and vesicular GABA transporter, we found that VGluT1-positive terminals made contacts 4- and 3.1-fold more frequently with PV-producing interneurons than VGluT2-positive and GABAergic terminals, respectively, in the primary somatosensory cortex. Even in layer 4, where VGluT2-positive terminals were most densely distributed, VGluT1-positive inputs to PV-producing interneurons were 2.4-fold more frequent than VGluT2-positive inputs. Furthermore, although GABAergic inputs to PV-producing interneurons were as numerous as VGluT2-positive inputs in most cortical layers, GABAergic inputs clearly preferred the proximal dendrites and somata of the interneurons, indicating that the sites of GABAergic inputs were more optimized than those of VGluT2-positive inputs. Simulation analysis with a PV-producing interneuron model compatible with the present morphological data revealed a plausible reason for this observation, by showing that GABAergic and glutamatergic postsynaptic potentials evoked by inputs to distal dendrites were attenuated to 60 and 87%, respectively, of those evoked by somatic inputs. As VGluT1-positive and VGluT2-positive axon terminals were presumed to be cortical and thalamic glutamatergic inputs, respectively, cortical excitatory inputs to PV-producing interneurons outnumbered the thalamic excitatory and intrinsic inhibitory inputs more than two-fold in any cortical layer. Although thalamic inputs are known to evoke about two-fold larger unitary excitatory postsynaptic potentials than cortical ones, the present results suggest that cortical inputs control PV-producing interneurons at least as strongly as thalamic inputs.     

Different modulation of inhibitory and stimulatory pathways mediated by adenosine after chronic in vivo agonist exposure

After 6 days of in vivo treatment with two selective adenosine receptor agonists, 5'-N-ethylcarboxamido adenosine (NECA) and R-N6-phenylisopropiladenosine (R-PIA), we investigated their effects on adenosine receptors/adenylyl cyclase system in synaptic plasma membranes isolated from rat brain. NECA treatment caused a significant loss of NECA-stimulated adenylyl cyclase activity, suggesting a desensitization of the adenosine A2 receptors-mediated pathway. No significant differences in total adenosine A2 receptors were observed, but Gs protein levels were decreased, suggesting Gs down-regulation as a mechanism for desensitization. On the other hand, NECA treatment caused a significant decrease in high-affinity adenosine A1 receptors population; however, no changes in CHA-inhibited adenylyl cyclase activity or Gi protein level were observed. Finally, when we studied the effects of R-PIA, a selective adenosine A1 receptor agonist, on stimulatory pathway of adenosine, low-affinity adenosine A2 binding sites were decreased without affecting the functionality of the pathway. These results show that adenosine A1 and A2 receptors are modulated in a different way after chronic agonist exposure and suggest the existence of cross-talk mechanisms between both stimulatory an inhibitory pathways mediated by adenosine.