Division of Neuroscience

Movement and Memory Laboratory | Blood Vessel Laboratory | Developmental Neurobiology Laboratory | Cerebral Cortex Laboratory | Learning and Emotions Laboratory | Neuronal Signalling Laboratory | Synapse and Hearing Laboratory | Synaptic Biochemistry

Research in the Division of Neuroscience is aimed at understanding the fundamental cellular and molecular mechanisms of the nervous system. It is only through such knowledge that disorders of the nervous system will be properly understood. The nervous system must be investigated at all levels, from the most basic molecular interactions occurring inside nerve cells, to the complex behaviour of vast networks of thousands or millions of interconnected neurons.

Bruce Walmsley

Groups in the Division are studying the neural mechanisms underlying vision, hearing, movement, memory, emotion, higher order cortical processing, development and cardiovascular control.

The past year has seen the establishment in the Division of a two-photon/confocal microscope facility for the study of living nerve cells. This facility allows individual nerve cells to be visualized and sub-cellular processes to be observed by injecting special fluorescent dyes into neurons. Interesting results have already been obtained which reveal processes within synaptic terminals too small (in the order of several microns or millionths of a meter long) to be studied by other means. This year has also seen a further move towards the use of molecular and transgenic techniques, to identify important receptors and investigate the role of particular proteins in neuronal processes.

A major highlight of the year was the IUPS Symposium on Synaptic Transmission which was organized by members of the Division and held on Heron Island in September. The meeting was attended by many top international neuroscientists, including Nobel Prize Laureates Professors B Sakmann and E Neher, and provided Australian neuroscientists with an excellent opportunity for interaction.

Dr Bruce Walmsley, Head of Division

Movement and Memory Laboratory

Serial Correlation in Transmitter Release at a Central Synapse
At the synaptic contacts formed between a pair of neurons, the amount of transmitter released each time the synaptic terminal is activated by a nerve impulse varies from trial to trial. During low frequency activation, this variation has always been assumed to be the result of a random process. At synapses between layer V neurones in somatosensory cortex of young rats, we have found that the variability is not following a random process. Failures to release transmitter follow similar failures more frequently than would occur by chance. Similarly, when release occurs, the size of the post-synaptic response varies less for short sequences of impulses than would occur by chance. These patterns of release show up as a positive serial correlation in the sequential amplitudes of the evoked synaptic responses. These patterns can be disrupted by preventing the uptake of calcium into an intracellular store in the nerve terminal. Calcium imaging of these nerve terminals has revealed the presence of brief periods of high levels of internal calcium, followed by longer periods of recovery to basal levels of calcium. These calcium 'sparks/puffs' can be prevented by disrupting intracellular calcium stores. Our conjecture is that during the calcium 'spark/puff', the probability of transmitter release is elevated, and a sequence of multi-quantal responses can be evoked. During the recovery phase, calcium approaches baseline levels, and a period of low release probability ensues, leading to a sequence of successive failures.

Christian Stricker, Anna Cowan and Stephen Redman

The Role of Different Calcium Sources in the induction of Long-Term Potentiation at CAI Hippocampus Synapses
Long-term potentiation (LTP) is the maintained enhancement of synaptic strength following activation of the synapses combined with depolarization of the post-synaptic neurone. The essential trigger for this mechanism is an elevation of post-synaptic calcium. This calcium can be derived from a number of sources. These are voltage-dependent calcium channels (VDCCs), ryanodine-sensitive stores, inositol triphosphate (IP3)-activated calcium release, and NMDA receptors. With the exception of the pivotal role for calcium via NMDA receptors, the relative importance of calcium derived from each of these stores during LTP induction is unknown. We have shown that conditioning stimulation that induces a weak LTP is sensitive to the release of calcium from ryanodine sensitive stores, but insensitive to supply from VDCCs and IP3 sensitive stores. Stronger conditioning that results in more pronounced and maintained LTP is insensitive to calcium derived from ryanodine sensitive stores and VDCCs, but is sensitive to calcium release from IP3 sensitive stores. Maximal LTP, induced by strong conditioning stimuli, requires calcium entry via VDCCs, but does not require calcium to be released from either store.

There are two possible interpretations of these results. The first is that each level of LTP is achieved by a different signalling pathway, and that the critical calcium source for each level of LTP is specifically coupled to that pathway. The alternative explanation is that each level of LTP requires a different calcium concentration to be achieved. Calcium elevation from IP3 sensitive stores swamps the calcium derived from ryanodine sensitive stores in reaching the required threshold for the intermediate level of LTP. Similarly, the calcium entering via VDCCs swamps the calcium derived from both types of calcium stores, and reaches the threshold required for the strongest LTP. Direct imaging of calcium concentration changes in spines and dendrites during LTP induction will provide further insight into these phenomena.

Clarke Raymond and Stephen Redman

 

Blood Vessel Laboratory

Structural and Functional Changes in Arteries during Hypertension

Cardiovascular disease, including heart attack and stroke, remains the most significant killer disease in Australia today and hypertension, or high blood pressure, is an important risk factor for all of its different forms. Blood pressure is determined in part by the pumping of the heart muscle and in part by the resistance imposed by the blood vessels which serve to distribute the oxygen and nutrients in the blood to the organs of the body. The blood vessels, which carry the blood from the pumping heart, are like pipes of continuously decreasing diameter, composed of an inner layer of endothelial cells surrounded by one to multiple layers of muscle cells, arranged transversely around the vessel. Blood flow is reduced and blood pressure is increased when the muscle cells contract and vessel diameter is reduced. Conversely, blood flow is increased and blood pressure decreased when the muscle cells relax and vessel diameter is increased. In vascular disease, such as hypertension, the ability of blood vessels to relax is decreased. This occurs through both decreases in the production and release of relaxing factors and increases in constricting factors and in certain blood vessels, spontaneous, rhythmical contractions appear or become more common.

A number of gross structural alterations are found in the walls of blood vessels from hypertensive animals. We have been interested in whether other changes occur at a more detailed level and whether these changes in the structure of blood vessels during hypertension may underlie changes in function. Of particular interest are structures called gap junctions. These are channels which form between adjacent cells in the walls of blood vessels. These gap junctions enable the transfer of both electrical and chemical information between the vascular cells, and are therefore important for the coordination of contractions in blood vessels. Gap junctions, which are composed of protein subunits called connexins, are found not only between adjacent cells within the two cell layers of the blood vessel wall, but also between the two cellular layers.

In blood vessels taken from animals with hypertension, we have found that the number of gap junctions between adjacent endothelial cells is decreased and their connexin makeup is altered. The endothelial cells themselves are also smaller. By studying animals in the prehypertensive phase, we have concluded that these structural changes coincide with the increases in blood pressure of these animals. In the same hypertensive vessels, we have also found alterations in the incidence of the gap junctions connecting the two cellular layers and in the number of muscle cells in the layers surrounding the endothelium. These structural changes have the potential to significantly alter vascular function.

Cerebral blood vessels which supply the brain, normally undergo spontaneous, rhythmical contractions like those which are upregulated during hypertension. Our studies have investigated the mechanisms underlying these events by correlating voltage changes and changes in intracellular calcium with these contractions. Through the use of various inhibitors, we have shown that these contractions result from a complex series of interactions involving multiple sources of intracellular and extracellular calcium and voltage dependent ion channels in the cell membrane. If similar mechanisms exist in hypertensive vessels, new therapeutic targets to control these contractions may be identified and their role in hypertension identified.

Caryl Hill

 

Developmental Neurobiology Laboratory

One of the major problems for the survival of nerve cells is their extreme length, sometimes over a meter from the cell body along the axon to the terminal. Nerve cells are dependent on information they receive from the postsynaptic cell in order to survive and mature. Survival of neurons during development of the nervous system is dependent upon both soluble and insoluble trophic factors signalling from the nerve terminal to the cell body. These factors act on receptors at the nerve terminal and are then faced with the problem of transducing their signal along the axon to the cell body.

Understanding the processes that control the flow of information from the nerve terminal to the cell body is essential if we are to understand neuronal development and apply this understanding to enhance nerve regeneration.

The family of neurotrophic factors include nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and neurotrophin 4 (NT-4). These factors promote the survival of sympathetic and sensory neuronal populations by binding to receptors present both in the cell body and on the nerve terminal. There are two classes of receptors, one is a low affinity neurotrophin receptor named p75NTR, which binds all the neurotrophins. The other class consists of the high affinity tyrosine kinase receptors: TrkA that binds NGF, TrkB that binds NT-4 and BDNF and TrkC that binds NT-3. At the nerve terminal the neurotrophins bind their receptors, and the neurotrophin-receptor complex is then internalised and transported to the cell body. We have shown that the different neurotrophins require different sets of receptors for their efficient transport. NGF uses both TrkA and p75 for its internalisation and transport. NT-3 and NT-4 on the other hand are totally dependant on p75 for their transport and their retrograde transport is prevented in knockout animals lacking the receptor for p75. Inhibition of the kinase activity of TrkA blocks the transport of NGF but has no effect on the transport of NT-3 and NT-4. This transport occurs in multivesicular bodies in which the machinery required for the signal transduction is physically moved from the terminal to the cell body where it can then interact with the signal cascade leading to nuclear responses. Molecules that are not themselves retrogradely transported may generate stable second messenger molecules at the nerve terminal, which are stable enough to survive the long time for transport to the cell body. We have examined the downstream signalling pathways looking at molecules that are associated with retrogradely transported NGF and found that molecules that could be activated at the nerve terminal will move along the axon together with NGF and that blockage of their activation will prevent this association. The recruitment of associated second messengers to the signalling organelle requires their activation at the nerve terminal and not at the cell body. Thus activation of other signalling pathways may regulate the mix of proteins retrogradely transported to the cell body and thus alter the final signal delivered. Our current research aims to examine this targeting to try to find a way to increase the availability of neurotrophins during regeneration.

Other molecules that are not transported but generate second messengers in the nerve terminal may influence neurotrophin endocytosis or transport. We have been investigating the mechanisms of control of the retrograde axonal transport of ¹²⁵I - bNGF, ¹²⁵I - NT - 3, ¹²⁵I - NT - 4/5, ¹²⁵I - BDNF to determine the additional factors that may regulate this pathway of signal transduction and thus be able to act in synergy with the neurotrophins. These molecular mechanisms have been investigated by identifying potential signalling molecules using specific pharmacological inhibitors. The activators of protein kinase C (PKC) when applied at the nerve terminal block retrograde axonal transport of the neurotrophins and the inhibitors of the calcium independent isoforms of PKC reverse this blockade. This inhibition of retrograde axonal transport by activation of PKC may be due to either or both of two mechanisms. Firstly, it has been shown that PKC can activate metaloproteases that can then cleave the receptors from the cell surface and thus prevent binding, uptake and internalisation of the neurotrophins. Secondly, it has been shown that activation of PKC can lead to changes that disrupt the actin cytoskeleton and we have shown that this can also lead to an inhibition of retrograde axonal transport. Our results using inhibitors of the metaloproteases have indicated that the effects of activation of PKC are predominantly due to the disruption in the actin cytoskeleton rather than activation of the metaloproteases. The results of this study will suggest alternate ways to enhance neuronal regeneration by perturbing the second messenger cascades promoting axonal transport.

In collaboration with the Gene Targeting Group, we have made mice deficient in the GTP-binding protein, G_za. The group has been studying the phenotype of these animals in order to discover the role of this protein in neuronal signalling mechanisms. The animals fail to thrive over the first 3 weeks of age and G_za deficient mice develop tolerance to morphine more rapidly and to a greater extent than control mice. The adrenal gland of the knockout animals is smaller at 3 weeks of age and is enlarged in adult animals due to hypertrophy of the adrenal cortex. We have investigated the hormonal state of these mice and have shown that in the young animals the levels of ACTH are low in the knockout animals and the corticosterone is not altered while in the adult animals ACTH is unchanged and corticosterone is elevated in the knockouts. These results suggest that the knockout animals are more responsive to ACTH. The differences in the steroid levels may reflect an altered stress level for the mice and may account for the alteration in morphine tolerance and dependence.

Ian Hendry

 

Cerebral Cortex Laboratory

Research in the Synaptic Biophysics Group aims to answer basic questions about the mechanisms of synaptic transmission and synaptic integration in the mammalian central nervous system, in particular the cerebral cortex. Our main experimental approach is to make electrical recordings from single, identified neurons in tissue obtained from rodents. We study neurons in dissociated cell cultures, in brain slices, and 'in vivo' in intact brains. By studying the brain at all these different levels of reductionism, we hope to contribute to an understanding of how the healthy brain carries out its computations, and what goes wrong in disease states such as epilepsy.

During 2001, members of the Group have worked on four main projects:

Voltage-gated potassium currents in large Layer 5 neocortical pyramidal neurons.
Large Layer 5 neocortical pyramids are the main output neurons of the cerebral cortex, making them an important cell type in which to study synaptic integration, which is the process by which neurons summate their synaptic inputs to generate an output of action potentials. Synaptic integration is known to involve voltage-gated potassium (Kv) currents. Earlier work done in the Group identified two functionally distinct classes of Kv currents in these neurons: the A-current and the delayed rectifier current. During 2001 we identified and characterized a third class of Kv current, the D-current. Interestingly, all three types of Kv current were found to have distinct properties, dendritic distributions, and effects on neuronal excitability. We speculated that each class of Kv current is tuned to a specific role in synaptic integration. This hypothesis that will be tested in future work.

Mechanisms of neurotransmitter release at hippocampal synapses in cell culture.
Neurotransmitter release is known to involve an array of specialised presynaptic proteins, but the exact function of each these proteins remains unclear. We are addressing this issue by using molecular techniques to substitute a non-native form of one of these proteins, Syntaxin, at hippocampal synapses in culture. We will then use electrophysiological methods to test our hypotheses about the effect of this substitution on synaptic transmission. So far we have worked on optimising a single-cell electroporation method for efficiently transfecting our neuronal cultures with a Syntaxin-EGFP construct. Our next step is to proceed to the electrophysiology. This project is in collaboration with Dr Catherine Morgans (Synaptic Biochemistry Group). In a separate, but related, project we are studying the turnover of presynaptic vesicles using a novel electrophysiological assay. This technique involves loading the vesicles of single, isolated glutamatergic neurons with GABA by stimulating them in the presence of external GABA. When the external GABA is removed, further stimulation reveals a GABAergic IPSC, the amplitude of which declines with number of stimulations as the supply of GABA-loaded vesicles is exhausted. By studying the rate of this decline, we can estimate the size of the cycling pool of presynaptic vesicles, this being a fundamental determinant of synaptic efficacy.

Neuronal excitability in the presence of in vivo synaptic noise.
Neurons studied in acute slices (as in Project 1 above) typically lack the very large number of active synaptic inputs present in vivo, because many of the afferents are cut off during the slicing procedure. For this reason, many neurons in slices are, electrically, much quieter than their counterparts in the intact brain. In this project our general aim is to examine the extent to which findings about synaptic integration, typically made in the quiet environment of the slice, generalise to the noisy environment of the real brain. To do this, we have established an in vivo patch clamp technique for making stable recordings from single neurons in the cortex of anaesthetised rats. As a first step, we are comparing the basic electrophysiological properties of identified cell types in vivo and in vitro. In future work we plan to study in vivo the coupling between weak synaptic input and action potential output. Theoretical studies indicate that this coupling is critically dependent on the level of synaptic noise; this idea will be tested.

Developmental changes in the trigeminal nucleus of the rat.
This project is being undertaken by Dr. Stephen Ho, who recently joined the Group as a Visiting Fellow. The trigeminal nucleus, which is a brainstem relay between the whiskers and the somatosensory cortex, undergoes a striking change during early postnatal development, organising into a somatotopic map of whisker inputs. We hypothesise that this process depends critically on synaptic plasticity, similar to that which occurs in the hippocampus during learning and in other regions of the brain during development. We shall test this idea by looking for so-called silent synapses and correlating their appearance with developmental indicators. We shall also explore other possible mechanisms underlying map formation, such as the changeover from excitation to inhibition of GABAergic inputs.

John Bekkers

 

Learning and Emotions Laboratory

The amygdala is a part of the limbic system that is involved in assigning emotional significance to cognitive events. In particular, it is involved in the processing of fear producing stimuli. One simple form of learning in which the amygdala is involved, is fear conditioning.

Fear conditioning is the process during which a normally innocuous stimulus such as a flashing light becomes associated with a fear producing stimulus (like an electric shock) so that the innocuous stimulus itself subsequently produces a fear response. It represents a form of learning and involves the storage of 'emotional' memories. Fear conditioning has been shown to occur in every species that has been examined from flies to humans and its expression shows a remarkably conserved pattern of symptoms which include increases in heart rate and blood pressure, reduction in salivation and freezing of ongoing movement. Associated with the autonomic symptoms of the fear response there are, in humans, cognitive effects such as feelings of dread and despair. Disorders of the storage or expression of fear responses are thought to underlie such mental disorders as panic attacks, anxiety and post traumatic stress disorder.

A converging body of literature over the last forty years indicates that the amygdala is critically involved in assigning emotional significance or value to events through associative learning. Stimulation of the amygdala can elicit the same constellation of symptoms as fear, and lesions of the amygdala reduce the acquisition and expression of fear. An understanding of the function of this structure is thus essential in the development of rational therapies for a range of related anxiety disorders. The anatomical organisation of the amygdala is now fairly well understood. However, its physiology is just beginning to be elucidated. The main focus of the cellular neurophysiology group is to elucidate the basic electrophysiological properties of cells within the amygdala and study its synaptic connectivity.

It is known that the major inputs to the amygdala use glutamate as the principal transmitter. Glutamatergic synapses can undergo a type of plasticity which has been implicated in the storage of memories. In the amygdala, these synapses are likely to be involved in the acquisition of fear conditioning. The amygdala is broadly divided into three main subnuclei: the lateral, basal and central. Sensory and cortical inputs enter the amygdala at the level of the lateral and basal nuclei. The different subnuclei are extensively interconnected and finally project to the central nucleus. Cells within the central nucleus project to brainstem and hypothalamic nuclei responsible for evoking the physiological responses associated with fear.

One project in our group is involved with examining the properties of cells in the input side of the amygdala. We have shown that cells within the lateral and basal nuclei can be divided into two broad categories: pyramidal cells and interneurones. Pyramidal cells form the major type of cell (93%) and are similar to excitatory cells found throughout the cortex. The remaining cells (7%) are interneurons which are inhibitory and form extensive connections with the excitatory cells in the amygdala. Surprisingly we found that the properties of synaptic inputs onto interneurons were quite different from those onto pyramidal cells. These findings indicate that the modulation of inhibitory pathways may be an important control mechanism within the amygdala.

Another project is studying the output side of the amygdala - the central nucleus. This structure is divided into two main parts, the medial and lateral. It has recently been shown that cells in the lateral division are inhibitory and make local circuits while cells in the medial division project out of the amygdala. We have been examining the effects of a class of drugs called benzodiazepines (eg diazepam or valium). These drugs are widely used as anxiolytics and their role in the amygdala is of great interest. These drugs are thought to work by potentiating the actions of the major inhibitory transmitter in the brain, gamma amino butyric acid (GABA). We have found that the central nucleus also contains a second type of GABA receptor which is inhibited by benzodiazepines. This finding may have therapeutic implications as a potential target for new classes of drugs.

Pankaj Sah

 

Neuronal Signalling Laboratory

The brain is made up of billions of neurons connected in complex networks. The working hypothesis is that when we think, move and feel what is really happening is that particular sets of neuron networks are activated. Perhaps the best evidence that this is indeed the case comes from the experiments of Wilder Penfield, who showed in the 1950s that electrical stimulation of different areas of the human brain could elicit movement, the sensations of sound and light, and even provoke the recall of long lost memories.

The main focus of my group is to understand how individual neurons process synaptic inputs from other neurons. We do this by making electrical recordings from single neurons in cortical brain slices from rats. As dendrites are the main site where synaptic inputs from other neurons are made, much of our research is focused on understanding how dendrites influence the way single neurons process information. In the past year we (S Williams and G Stuart) investigated whether the size of excitatory postsynaptic potentials (EPSPs) depends on their location in the dendritic tree. In particular, we investigated whether EPSPs made at sites further from the soma are larger than to those made more proximally. This would make some sense, as EPSPs attenuate as they spread from the distal dendrites to the soma. Hence, more distal EPSPs would need to be larger to overcome this attenuation, and thereby have an equal vote at the soma with respect to influencing action potential initiation. It turns out that the average synaptic conductance at different dendritic locations is similar. However, because more distal dendrites tend to be smaller, the same synaptic conductance generates a larger local EPSP at distal dendritic locations. If large enough, this local dendritic EPSP could trigger a dendritic spike, which could travel to the soma and trigger action potential initiation in the axon. These results indicate that despite a uniform synaptic conductance at different dendritic locations, distal EPSPs can nevertheless have a powerful influence on action potential initiation.

We have also been investigating the role of GABAergic inhibitory postsynaptic potentials (IPSPs) in synaptic integration. Interestingly, we (A Gulledge and G Stuart) found that dendritic GABAergic inputs could increase, rather than decrease, the likelihood of action potential generation, that is they could be excitatory. GABA released synaptically onto dendrites could have the same excitatory effect. This occurred as while the reversal potential for GABA is below threshold, it is positive to the resting membrane potential. Hence, GABA elicits a depolarising response from resting membrane potentials. When combined with a somatic EPSP at the right time the depolarisation due to activation of dendritic GABAergic inputs can sum with the somatic EPSP and lead to action potential generation. This result will have important implications for understanding the role of GABA in both the physiology and pathology of the cortex (eg during epilepsy).

Finally, we have investigated how the repetitive activation of action potentials together with EPSPs can lead to long term changes in synaptic strength. Previous work indicates that changes in synaptic strength may underlie learning and memory. Furthermore, activation of NMDA receptors is thought to be involved. We (B Kampa and G Stuart) have found direct evidence that repetitive activation of action potentials together with EPSPs can enhance the activation of synaptic NMDA receptors, which would be expected to increase the chance of inducing synaptic plasticity. This research will increase our understanding of how our brains make memories, and should in the long run help in the development of therapies to treat conditions associated with memory loss (eg. Alzheimer's disease).

Greg Stuart

 

Synapse and Hearing Laboratory

Hearing and the Brain
The auditory system processes sound information extremely rapidly. As a consequence, synaptic connections between nerve cells in those parts of the brain which process auditory signals contain some of the fastest and most powerful synaptic connections in the brain. In the Synaptic Structure and Function group, we study the fundamental mechanisms underlying the generation of synaptic signals and the processing of these signals in neurons of the auditory brainstem. Our experiments involve studies of the structure of synapses using electron-microscopy and fluorescent labelling of receptors in the target cell, and functional studies using electrical recording from individual neurons in living slices of the auditory brainstem. In addition, due to their large size, we are able to visualize individual synaptic terminals and obtain direct recordings from them. This allows us to perform molecular studies of the fundamental (quantal) nature of neurotransmitter release from these terminals. Over the past year, we have completed a study on synaptic transmission in the central pathways of congenitally deaf mice. The deaf mouse serves as a valuable model of some forms of human congenital deafness, in which the hair cells in the cochlear are not functional although the central connections of the auditory system remain intact.

We were interested in whether or not these synaptic connections were normal, since they developed in the absence of auditory input from the cochlear. We found that surprisingly, the excitatory synaptic inputs from auditory nerves were much stronger in the deaf mice. Our results showed that the increase in synaptic strength was entirely due to increases in the amount of transmitter released from the auditory nerve terminals. We also found that the calcium handling capacity of the auditory nerve terminals was impaired in deaf mice, and that this resulted in a much greater desynchronized release of transmitter.

In the absence of nerve impulses, synaptic terminals spontaneously release neurotransmitter, and it has been assumed that this represents the release of a quantum of transmitter contained in small vesicles in the terminal. However, it has recently been proposed that some spontaneous transmitter release is the result of the spontaneous release of 'sparks' of calcium in the presynaptic terminal, which then triggers the release of multiple vesicles. We have investigated this at inhibitory glycinergic synapses on auditory neurons, where the synaptic contacts are made with the cell bodies, thus avoiding the complications of synaptic contacts on dendrites. Our results reveal that calcium released from intracellular stores contributes to the occurrence of very large spontaneous synaptic currents, which may be the result of synchronous release from adjacent release sites or the simultaneous release of multiple quanta at the same site. Ongoing experiments are aimed at determining if nerve impulses are capable of triggering calcium release from intra-terminal stores and increasing the amount of neurotransmitter released from the terminal. We have also examined individual auditory synapses under the electron-microscope. Our results show that there are great differences between synapses arising from different auditory nerves. The size of the synaptic connection and the number of synaptic vesicles docked at each synapse varies greatly. These results may explain the variability in transmitter release which we have previously shown in electrophysiological experiments. Our immediate goal is to understand the role of presynaptic nerve activity and development in the regulation of synaptic strength. The auditory system provides an excellent model system in which to study these synaptic mechanisms.

Bruce Walmsley

Synaptic Biochemistry

Congenital Night Blindness Human beings are visual animals. Genetic defects affecting our vision tend to have a drastic impact on our way of life. Many congenital visual diseases are caused by mutations in genes encoding proteins required for the normal detection of light by the retina and the transmission of visual information from the retina to the brain. Research in the Synaptic Biochemistry Group is on the proteins involved in the transmission of visual information within the retina, and for the past two years the subject of our research has been a recently discovered retinal calcium channel (called alpha-1F) encoded by a gene, mutations in which have been found to cause incomplete congenital night blindness (CSNB2).

People with CSNB2 carry mutations in the CACNA1F gene that abolish function of the channel causing them to have near normal day vision but drastically reduced night-time vision.

Visual processing begins with the detection of light entering the eye by specialized, photon-absorbing neurons in the retina - the photoreceptors. The light signal then undergoes two stages of information processing within the retina before being transmitted to the brain. This processing occurs at points of contact (called synapses) between retinal neurons. The retina is thus organized into two synaptic layers interspersed between layers of neuronal cell bodies. The first synaptic layer occurs between photoreceptors and bipolar cells, the second-order neurons in the retina. The second synaptic layer occurs between the bipolar cells and the ganglion cells, which finally transmit light signals to the brain via the optic nerve.

Neurons transmit signals from one to another at the synapse, where a chemical transmitter is secreted by the pre-synaptic neuron and detected by the post-synaptic neuron. Neurotransmitter release is triggered by the influx of calcium ions into the presynaptic neuron. Calcium enters the neuron through calcium channels: electrically sensitive, pore-forming proteins embedded in the cell membrane. The calcium channels controlling transmitter release in the retina have unique physiological characteristics that appear optimized for visual function.

In the Synaptic Biochemistry Group, we have shown recently that the calcium channel mediating transmitter release from photoreceptors and bipolar cells is none other than the product of the CACNA1F gene. We have cloned the rat CACNA1F gene and shown that the alpha-1F channel is localized exactly at the sites of neurotransmitter release in the photoreceptor and bipolar cell pre-synaptic terminals. Thus, the symptoms of CSNB2 can be explained as an impairment in the release of neurotransmitter from photoreceptors and bipolar cells.

Catherine Morgans