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Down With Inhibition: Modulating GABAergic Neocortical Control

24 July 2008. There is evidence that local cortical circuits are disturbed in the brains of people with schizophrenia. In particular, complementary hypotheses have arisen concerning deficits in parvalbumin-containing, fast-spiking interneurons of prefrontal cortex (see SRF related news story) and abnormalities of cortical gamma band synchronization (see SRF Current Hypothesis paper by Woo and colleagues). Thus, basic science findings on control of cortical activity are of special interest to scientists investigating schizophrenia.

In a study published in the June 26 issue of Neuron, Illya Kruglikov and Bernardo Rudy of New York University report that neuromodulators, including acetylcholine, serotonin, and adenosine, regulate GABA release from fast-spiking interneurons onto neocortical excitatory cells. Specifically, they control the perisomatic release of GABA—i.e., onto the cell bodies and initial axon and dendrite segments of the pyramidal cells. The researchers were also able to show that this modulation regulates the integration of thalamocortical signaling.

Modulators of inhibition
Neuromodulators such as acetylcholine, serotonin, and norepinephrine modify cortical circuits, changing the flow of information. To do this, their effects include critical actions on neurons employing GABA, the brainís main inhibitory neurotransmitter. However, the direct effects of neuromodulators on the interneurons that release GABA are not well understood.

Kruglikov and Rudy studied the effects of activating a number of different neuromodulator receptors on fast-spiking basket cells, the primary interneurons of the mammalian neocortex. These cells provide a major source of inhibition to neocortical pyramidal cells. A subset of parvalbumin-containing, fast-spiking interneurons are of special interest to research groups that have identified deficits in these cells in people with schizophrenia (see interview with David Lewis).

The investigators initially screened a series of neuromodulators and drugs that act on neuromodulator receptors, assessing their ability to modify inhibitory post-synaptic currents (IPSCs) in cultured mouse brain slices of somatosensory cortex. They recorded from multiple pyramidal cells in the same slices, driving electrical activity of GABAergic interneurons with a nearby stimulating electrode.

Agents that blocked the inhibitory effects of the stimulation included muscarine, serotonin, adenosine, and baclofen. Muscarine stimulates the muscarinic acetylcholine receptor and baclofen stimulates the GABAB receptor. Compounds targeting metabotropic glutamate, adrenergic, histaminergic, cannabinoid, somatostatin, or cholecystokinin receptors did not modulate the IPSCs.

The researchers wanted to confirm that the IPSCs that they observed were indeed due to the fast-spiking basket cells, particularly since a variety of neuronal cell types (especially Martinotti interneurons) could have been activated by the stimulating electrode. Several pieces of evidence implicated the fast-spiking basket cells.

For one, they comprise the largest population of interneurons in layer five of somatosensory cortex, and consistent with their numbers, fast-spiking cells provided the greatest contribution to the IPSCs provoked by the stimulation. This was determined by comparing inhibitory recordings from Martinotti cell-pyramidal cell pairs versus fast-spiking cell-pyramidal cell pairs. In addition, blocking P/Q calcium channels, which are specific to the terminals of fast-spiking cells, also reduced the IPSCs. Blocking N-calcium channels, which are found in terminals of other types of interneurons, had no effect. And most importantly, according to the authors, these modulations were also observed on the IPSCs generated on a pyramidal cell when a single, synaptically connected fast-spiking interneuron was stimulated.

Kruglikov and Rudy also assessed whether the effects of the four neuromodulators on inhibitory activity were dependent on the intracellular signaling of protein kinases. Serotonin did not block the IPSCs evoked by stimulation of the slices when the protein kinase inhibitor staurosporine was present, indicating that the effects of serotonin were largely protein kinase-dependent. The inhibitory effects of the other three neuromodulators were not affected by staurosporine. Interestingly, serotonin also had a slow time course of blocking the IPSCs measured in the slices, suggesting second-messenger involvement, whereas the other three agents acted more quickly, which the authors take to indicate that these neuromodulators have effects only at the membrane, via calcium channels.

A bigger slice of cortical activity
The researchers then moved to a slice preparation that included both neocortex and its major input source, the thalamus, in order to study the interaction of neuromodulators and excitatory thalamic input. Afferents from thalamus to cortex contact both excitatory and inhibitory neurons, exciting them via glutamate release. There is a brief span of time between the excitation of the primary excitatory cells via the thalamic input and the feed-forward inhibition of the interneurons onto these cells. This "temporal integration window" is believed to be important for processing sensory inputs to neocortex.

Because muscarine was particularly effective at inhibiting GABA release from neocortical fast-spiking interneurons, and because of the powerful effect acetylcholine is known to have on cortex, Kruglikov and Rudy focused on the role that cholinergic receptors might play in reducing inhibition in the cortex. While stimulating the somatosensory region of the thalamus, they measured the effects of muscarine on both inhibitory and excitatory post-synaptic currents (EPSCs) in somatosensory pyramidal neurons. The researchers report that feed-forward IPSCs decreased, producing an increase in the window for temporal integration. Baclofen, the GABAB receptor agonist, had the same effect, but completely eliminated the feed-forward IPSCs. Nicotine, which stimulates the nicotinic subtype of the acetylcholine receptor, increased EPSCs in excitatory cells, but not in inhibitory cells.

The authors conclude that cholinergic activity increases thalamic excitation of neocortex, mainly by reducing the feed-forward inhibition produced by release of GABA from fast-spiking interneurons. This effect is mediated via muscarinic receptors and is complemented by the excitation of the excitatory neurons by acetylcholine via nicotinic receptors.

There is some sparse evidence that communication between thalamus and cortex is disturbed in schizophrenic patients (for review, see Sim et al., 2006), but the most relevant impact of this research is likely to be in the elucidation of mechanisms involved in cortical integration, suggesting neuronal circuitry of interest for researchers studying the disease. Future research may also address whether inhibitory circuits in hippocampus, another region of prime interest for schizophrenia researchers, are modulated in the same fashion.—Alisa Woods and Hakon Heimer.

Reference:
Kruglikov I, Rudy B. Perisomatic GABA release and thalamocortical integration onto neocortical excitatory cells are regulated by neuromodulators. Neuron. 2008 Jun 26;58(6):911-24. Abstract

Comments on News and Primary Papers
Comment by:  Miles Whittington
Submitted 24 July 2008
Posted 24 July 2008

This paper by Kruglikov and Rudy examines in detail the profile of neuromodulatory influences on GABA release from fast-spiking (FS), parvalbumin-containing interneurons in sensory neocortex. The work elegantly demonstrates that this interneuron subtype is exquisitely sensitive to a diverse range of neuromodulatory chemicals including those acting on muscarinic, purinergic, serotonergic, and GABAB receptors. Agonists at each of these receptors produced a strong inhibition of GABA release from electrically stimulated FS synaptic terminals and, as a result, reduced inhibitory influence both locally in cortex and on ascending thalamocortical projections. These interneurons are of particular interest currently in schizophrenia research as functional markers for them are found to be robustly reduced in postmortem brain samples from schizophrenic patients. They are also one of the key interneuron subtypes involved in the generation of certain EEG rhythms—in particular, those in the gamma (30-80 Hz) band—involved in primary sensory processing, short-term memory, and cortico-cortical communication. The pattern of generation of gamma rhythms is disrupted in people with schizophrenia.

However, it is hard to see direct implications for our understanding of processes underlying cortical dysfunction in schizophrenia from this work. This is mainly due to the fact that only a subset of parvalbumin-containing FS cells appear to be affected in schizophrenia and related animal models. It is not yet known what makes this subset of interneurons so labile in psychiatric illness, but there is much exciting work ongoing which is highly suggestive of a role for NMDA receptor-mediated excitation and changes in redox state. The paper does not subclassify the FS interneurons studied directly, but there are a couple of issues which may be of interest to the field:

1. Firstly, the importance of neuromodulation for generating gamma rhythms can clearly be seen in Figure 6. Forty Hz artificial stimulation—matching the modal frequency of activation of these neurons during network gamma rhythms—terminates pyramidal cell action potential generation in the absence of neuromodulatory influences. This suggests that inhibition-based gamma oscillations in cortex would be useless as an information coding strategy. However, the reduced inhibitory post-synaptic potential size (and thus effective duration) under muscarinic neuromodulation permits sparse, but precisely timed pyramidal cell action potentials—exactly the signature for principal cell spiking during gamma rhythms in vivo. Loss of parvalbumin from FS cells causes a large increase in GABA release, and thus increased inhibitory post-synaptic potential size and duration. It is therefore interesting to consider what the reduced parvalbumin levels in schizophrenic cortex can tell us: is it a primary cause of the observed decrease in gamma rhythm generation, owing to its enhancement of GABA release and thus perhaps termination of pyramidal cell spiking? Or is it a compensatory mechanism, as previously proposed, for an underlying deficit in FS cell excitation?

2. Secondly, it is interesting to note in this paper that cannabinoid receptor activation did not change evoked GABA release from FS cells at all. There is a growing corpus of work linking cannabis use to increase risk of psychotic episodes and exacerbate symptoms of schizophrenia. Given the proposed critical role of FS cells in cortical dysfunction in schizophrenia, it is perhaps surprising that no effect was seen. However, the study used artificial, electrical stimulation to evoke GABA release from these cells. In active networks, cannabinoid agonists can act on CCK-containing interneurons, which may indirectly change excitability of parvalbumin-containing FS interneurons, changing GABA release patterns via altered rates of action potential generation. In addition, cannabinoid receptors have been shown to directly reduce excitatory post-synaptic potential size, something that might be expected to directly reduce FS cell recruitment during network activity.

View all comments by Miles Whittington

Comments on Related News


Related News: Genetics, Expression Profiling Support GABA Deficits in Schizophrenia

Comment by:  Karoly Mirnics, SRF Advisor
Submitted 26 June 2007
Posted 26 June 2007

The evidence is becoming overwhelming that the GABA system disturbances are a critical hallmark of schizophrenia. The data indicate that these processes are present across different brain regions, albeit with some notable differences in the exact genes affected. Synthesizing the observations from the recent scientific reports strongly suggest that the observed GABA system disturbances arise as a result of complex genetic-epigenetic-environmental-adaptational events. While we currently do not understand the nature of these interactions, it is clear that this will become a major focus of translational neuroscience over the next several years, including dissecting the pathophysiology of these events using in vitro and in vivo experimental models.

View all comments by Karoly Mirnics

Related News: Genetics, Expression Profiling Support GABA Deficits in Schizophrenia

Comment by:  Schahram Akbarian
Submitted 26 June 2007
Posted 26 June 2007
  I recommend the Primary Papers

The three papers discussed in the above News article are the most recent to imply dysregulation of the cortical GABAergic system in schizophrenia and related disease. Each paper adds a new twist to the story—molecular changes in the hippocampus of schizophrenia and bipolar subjects show striking differences dependent on layer and subregion (Benes et al), and in prefrontal cortex, there is mounting evidence that changes in the "GABA-transcriptome" affect certain subtypes of inhibitory interneurons (Hashimoto et al). The polymorphisms in the GAD1/GAD67 (GABA synthesis) gene which Straub el al. identified as genetic modifiers for cognitive performance and as schizophrenia risk factors will undoubtedly spur further interest in the field; it will be interesting to find out in future studies whether these genetic variants determine the longitudinal course/outcome of the disease, treatment response etc etc.

View all comments by Schahram Akbarian