Negative-feedback control of neocortical activity by the neuromodulator adenosine

The neocortex is instantly recognisable as the folded, sheet-like tissue that makes up much of the visible surface of the human brain. It is central to all the high-level brain functions of mammals, receiving information from our senses, combining this with short-term and long-term memory, performing cognitive processes (or "thought", in other words) and planning our movements. At the cellular level, these processes have been linked to activity across networks of neurons, comprising periods where the networks are quiet and periods where the networks are active with neurons communicating via fast synapses and firing at rates around 10 times per second. These collective activity states are thought to underlie many normal physiological processes and have been seen localised in small regions of neocortex (less than one cubic millimetre), propagating as waves of activity or in massive synchronous events spread wide across centimetres of neocortex as observed during sleep.

Models of neurons that include synaptic connections capture many aspects of neocortical activity but a full, detailed understanding of these processes is still required. In particular, the role of neuromodulators - chemicals that are released and diffuse far throughout tissue with powerful effects on neural properties - have not been incorporated into the picture. An important modulator that suppresses neuronal activity is adenosine, which belongs to the chemical family of purines (caffeine is another well-known purine). Adenosine binds to receptors on the surfaces of neurons and effectively slows activity down. Adenosine has a role in a wide range of healthy processes such as sleep, breathing and control of movement, and plays an important protective role during pathological states like epilepsy and stroke. The University of Warwick has developed novel biosensors that can measure the dynamics of concentrations of adenosine in neural tissue. This unique technology has allowed for significant advances in the understanding of adenosine in other brain regions such as the hippocampus and cerebellum.

Using biosensor technology, we have recently performed experiments on neocortical tissue that, for the first time, have demonstrated a link between neocortical activity and adenosine release. We have shown that activity is suppressed by naturally present adenosine and also that activity causes adenosine to be released. Interestingly, it appears that adenosine is rapidly broken-down in tissue, giving this signalling mechanism a short range despite the fact that adenosine diffuses. Our results provide evidence for an important negative feedback mechanism for adenosine in the neocortex, giving it a potentially key role in initiating and terminating activity.

To fully understand adenosine signalling, we will use a unique combination of state-of-the-art experimental and theoretical methods to precisely map and correlate the activity within neocortical networks with areas of adenosine release. By combining this with detailed information on the synaptic connections that are targeted by adenosine we can produce a model that fully defines the actions of adenosine in the neocortex during activity. This will increase the fundamental understanding of neocortical processes and has the potential to eventually inform the design of new therapeutic interventions for treating diseases such as epilepsy, which can be the result of over-excitation in the cortex.

We will provide the first quantitative description of a key physiological feedback loop between neocortical network activity and the diffusive neuromodulator adenosine. The system is complex and requires a systems-biology approach in which experiment and modeling are fully integrated. The theory and experimental PIs are leaders in the fields of network modeling and adenosine neurobiology, and have an established collaboration, and thus constitute a capable and well-suited research team for the analysis of this fundamental system.

In our preliminary data we provided clear evidence that:

(i) Adenosine is released during neocortical activity and plays a negative-feedback role in the termination of this activity.

(ii) A basic model of adenosine release and diffusion from a store captures the measured adenosine release dynamics during patterns of neocortical activity.

(ii) Despite being a diffusive neuromodulator adenosine is rapidly removed from the extracellular space potentially giving it a highly local action.

(iii) Adenosine suppresses excitatory layer-5 pyramidal cells and their synapses, and alters the short-term synaptic dynamics, with implications for neuronal coding.

We now propose to extend this preliminary study to develop an experimentally verified model of the negative-feedback loop between network activity and neuromodulator action. The activity-dependent release and potent range of adenosine will be quantified using a novel combination of experimental technologies: wide-field calcium imaging and purine biosensors. The effects of adenosine on synaptic and cellular properties will be quantified using multiple patch-clamp recording and state-of-the-art neuron modeling. These two isolated components will then be brought together to provide the first verified model of the adenosine-activity negative-feedback loop and will constitute a key benchmark for future studies on the interaction between activity and neuromodulators.

The impacts of this research are: (i) the understanding of a fundamental physiological process, with ultimate follow-on benefits to the health sector; (ii) the development and refinement of instrumentation - our experiments will, for the first time, combine wide-field calcium imaging with unique biosensor technology with potential transfer to a wide range of related projects in academic and industrial research.

Principal beneficiaries are academics, the medical community and our industrial partners. The first two are treated in the Academic Beneficiaries section, with the long-term benefit to the health of the nation arising from a quantitative understanding of the role of adenosine in pathologies (epilepsy and ischemia), and in therapies (deep brain stimulation).

Benefits for UK industry: this project uses unique biosensor technology (including the latest 7um-size carbon-fibre sensors) developed by our partner Sarissa Biomedical Ltd. The extensive use of biosensors in this project, coupled with detailed instrument modelling, has the potential for significant technology refinement. Our collaborative links with the industrial partner will ensure rapid exploitation of any technology improvements that arise. Warwick has in place specialist business engagement staff in Warwick Ventures and Development Office, and also with Advantage West Midlands (our regional development agency) to ensure relevant results are exploited. Because the project combines calcium recordings and biosensors for the first time, any suggestions relating to compatibility improvements will also be communicated to manufacturers of both technologies.

Communication
The PI and co-PI have a demonstrated record of academic communication through publications, invited conferences and seminars. They organise a number of wider communication activities, such as workshops at the University and hospitals that bring together academics and health professionals. They regularly participate in outreach activities to the general public, including panel discussions, internet video of popularised research, and regular experimental demonstrations at primary and secondary schools.

Training
The University of Warwick has a dedicated staff training section with Warwick Systems Biology Doctoral Training Centre additionally providing an excellent transferable skills programme. The RAs will be encouraged to follow any relevant courses, particularly on Science Communication and team building. General transferable skills of the project include computer and web-based skills, team-working, presentation and high-level writing skills. The specific skills to be learned during the project are also of wide use in the bio-sector and pharmaceutical industry (electrophysiology), and any quantitative sector such as high-finance (mathematical modelling).