Periodic Reporting for period 2 - TOGETHER (E Pluribus Unum: Principles and Plasticity of Electrical Coupling in a Neuronal Network)
Reporting period: 2023-04-01 to 2024-09-30
The fundamental purpose of the brain is communication and information transfer; from neuron to neuron, and from neurons to the other cells of the body. This communication takes place in specialized contacts between cells. The best known (and, arguably, best studied) such contact is the chemical synapse, where an electrical nerve impulse triggers the release of transmitters from the endings of one neuron to act on receptors at a receiving element in the next neuron. However, many neurons in the central nervous system also communicate via so-called electrical synapses, formed by direct channels that connect the interior of one neuron with the interior of another. The electrical synapse, or gap junctions as is the broader term for such contacts between the body’s cells, is less well studied, but can have a powerful impact on the operations of neuronal networks by their intriguing properties: they allow for the immediate and bidirectional flow of a number of ions and substances, which can change both electrical excitability and the molecular state of cells, and can coordinate the activity of large ensembles of neurons, allowing them to act as a collective. Due to their wide-spread distribution, they are likely to be involved in virtually all brain functions, and have been linked to brain disorders such as epilepsy. A better understanding of how electrical synapses are structurally organized and operate, and their impact on the myriad operations that the brain performs each day, has the potential to aid the development of new therapeutic strategies for the various disorders of brain function.
In this project, we study the organization, functional properties and role of electrical synapse. We take advantage of a discovery we made some years ago in an ERC-supported project that one of the key brain systems for controlling hormone secretion, the so-called tuberoinfundibular dopamine (TIDA) neurons in the hypothalamus (an evolutionarily ancient part of the brain that controls much of our survival behaviours, autonomic functions and hormone release) are powerfully linked by electrical synapses in the rat, perhaps more than any other system discovered to date. We further found that the very same neurons in the mouse completely lack electrical synapses, and that this difference explains why male mice express parental behaviours and care for their offspring while male rats largely ignore their young. This serendipitous discovery (unique in the literature) provided us with a powerful “experiment of nature” to explore gap junctions, which have been notoriously difficult to investigate by conventional means due to shortcomings of available pharmacological and genetic tools. In this project (“TOGETHER”) we aim to provide fundamental new insight into electrical synapses by 1) leveraging genetic techniques to convert rat TIDA neurons to their mouse counterpart and vice versa by disrupting and introducing the expression of gap junction proteins, respectively; 2) determining if dynamic coupling and uncoupling of electrical synapses explain how a female rodent prepares for motherhood; 3) explore if electrical coupling is regulated on a second-millisecond scale to change the output of a neuronal network depending on state and 4) investigate how the presence of electrical coupling alters the operation of neurons and networks and impacts on the cellular identity of neurons.
In this project, we study the organization, functional properties and role of electrical synapse. We take advantage of a discovery we made some years ago in an ERC-supported project that one of the key brain systems for controlling hormone secretion, the so-called tuberoinfundibular dopamine (TIDA) neurons in the hypothalamus (an evolutionarily ancient part of the brain that controls much of our survival behaviours, autonomic functions and hormone release) are powerfully linked by electrical synapses in the rat, perhaps more than any other system discovered to date. We further found that the very same neurons in the mouse completely lack electrical synapses, and that this difference explains why male mice express parental behaviours and care for their offspring while male rats largely ignore their young. This serendipitous discovery (unique in the literature) provided us with a powerful “experiment of nature” to explore gap junctions, which have been notoriously difficult to investigate by conventional means due to shortcomings of available pharmacological and genetic tools. In this project (“TOGETHER”) we aim to provide fundamental new insight into electrical synapses by 1) leveraging genetic techniques to convert rat TIDA neurons to their mouse counterpart and vice versa by disrupting and introducing the expression of gap junction proteins, respectively; 2) determining if dynamic coupling and uncoupling of electrical synapses explain how a female rodent prepares for motherhood; 3) explore if electrical coupling is regulated on a second-millisecond scale to change the output of a neuronal network depending on state and 4) investigate how the presence of electrical coupling alters the operation of neurons and networks and impacts on the cellular identity of neurons.
In order to implement the objectives of this project, we have first recruited new highly qualified members to the group following international advertising, purchased several state-of-the-art pieces of equipment and established a series of new techniques in the laboratory. These investments, made possible by the generous support of the ERC, have made it possible for us to make progress related to all the objectives. – In one project, we are performing a “species conversion” by introducing electrical synapses to mouse TIDA neurons (which normally lack such connections) and, in parallel, dissolving these intercellular channels between rat TIDA neurons, a system where electrical synapses normally are abundant. The latter (rat) part is still in initial stages but for the mouse project, we were able to accomplish some level of connectivity which support our hypotheses of electrical synapses and their role in network operation in a novel way. At the same time, the fact that a full functional “species conversion” was not achieved suggests the existence of (as yet unknown) additional components required for the formation of electrical connectivity, which is being followed up by a new series of experiments. Secondly, we have discovered that in female rats, the normally strongly coupled TIDA system dissolves its electrical synapses during pregnancy in what we interpret as an adaptation to motherhood. This is a novel finding that suggests, among other things, that gap junctions are highly dynamic constructs and that they can be added or removed as the life conditions of an animal changes. Thirdly, since rat TIDA neurons undergo a highly regular oscillation with a period of ca 3 secs, we are examining if and how electrical synapses alter their strength across the oscillation cycle. Indeed, even at this brief time scale, neurons alter their coupling strength such that they are maximally connected when the period of electrical discharge commences. Fourth, we are exploring how the passage of current can be asymmetrical, i.e. stronger from one cell to another than in the opposite direction of the same pair. Our experiments suggest that this may be caused by the expression of multiple types of gap junction proteins within the pair, including proteins that previously were believed to be absent from neurons. In addition, we have also discovered how TIDA neurons are regulated by estrogen, that could explain why this sex hormone is such a powerful release-promoting agent for the hormone, prolactin, which plays a key role in maternal behaviour and reproduction, and how TIDA neurons are regulated by the brain’s main excitatory transmitter, glutamate, through a number of parallel paths.
We are cautious at this point of the grant duration, when most projects are still in the midst of implementation to make claims about going beyond state of the art, but one finding that we ourselves are very excited about is the finding that electrical synapses seem to disappear from TIDA neurons as a female rat undergoes pregnancy. We believe this could be a key step for the adaptation to motherhood, a role for gap junctions that has not previously been proposed. In general, we believe our strategy of using species differences as a platform to explore the importance of a protein/form of neuronal information transfer is a novel approach to unlock biological mysteries.