|Dr Roger A.L. Dampney||Reader (in-charge)||University||1977-|
|Dr Yoshitaka Hirooka||Visiting Fellow||Kyushu Univ.||1992-|
|Jaimie W. Polson||Research Assistant II||NHMRC||1987-|
|Matthew J. Coleman||PhD student (0.5)||University||1990-|
|Patrick Potts||PhD student||1994-|
|Ayesha Hakim||BMedSc(Hons) student|
(co-supervisor: M. Bennett)
|Emma Dickens||BMedSc(Hons) student||1995|
|Marcha Tuyau||BMedSc(Hons) student|
Current effective full-time personnel (with those co-supervised taken as 0.5) = 6.0
The Laboratory investigates the connections and functions of neurons in the brain stem that play an important role in the regulation of arterial blood pressure. A variety of techniques are used, including electrophysiology, neural pathway tracing, immediate early gene expression and immunohistochemistry.
Use of immediate early gene expression to identify central cardiovascular pathways
In previous work, the Lab has used the method of c-fos functional mapping to identify central neurons that are involved in the regulation of blood pressure. The principle of this technique is that when a neuron is activated for a prolonged period, it expresses a proto-oncogene called c-fos that in turn causes the production of a protein called Fos. The presence of Fos can be detected in a neuron by using immunocytochemistry. Thus, if a stimulus is applied that is known to activate blood-pressure regulating neurons in the brain, the Fos immunocytochemical procedure can be used to label the activated neurons. Furthermore, because it is possible to combine the Fos technique with other immunohistochemical and anatomical tracing techniques, it has been possible to identify, to some extent, the neurotransmitter content and connections of central neurons involved in blood pressure regulation.
In 1994, Yoshi Hirooka and Jaimie Polson used this technique to identify the central pathways in the lower brainstem that mediate the cardiovascular reflex response triggered by a hypoxic stimulus (lack of oxygen). This reflex is characterized by a sympathetically-mediated constriction of blood vessels, which has the effect of conserving oxygen and thus helping to protect the body under hypoxic conditions. The results of this study indicated that hypoxia causes stimulation of neurons within several discrete brain regions. Furthermore, Yoshi Hirooka and Jaimie Polson obtained strong evidence that a key component of the central neuronal mechanisms subserving the cardiovascular response to hypoxia is a direct pathway from the nucleus of the solitary tract (a major sensory nucleus in the medulla oblongata which in turn receives inputs from peripheral oxygen sensors) and cardiovascular neurons within the rostral ventrolateral medulla (VLM), which in turn regulate the sympathetic out-flow to the heart and blood vessels. In addition, there is also a direct pathway from the parabrachial complex in the pons to the rostral VLM that is also activated by the hypoxic stimulus.
The Lab has shown previously that induced changes in blood pressure result in activation of neurons in a number of discrete regions throughout the brain. In 1994, Patrick Potts showed for the first time that the activation of neurons within the medulla oblongata in response to perturbations in blood pressure is entirely dependent upon signals originating from arterial baroreceptors. The significance of this observation is that it will allow examination of the specific central pathways that subserve the cardiovascular reflex responses that follow changes in blood volume.
Role of central angiotensin in blood pressure regulation
The Lab has previously carried out a number of studies that have demonstrated the importance of central angiotensin in modulating the activity of blood-pressure regulating neurons in the medulla oblongata. In particular, together with Fred Mendelsohn and Andrew Allen, it was shown that there is a high density of angiotensin receptors within the VLM, in the subregions known to contain cardiovascular neurons. Furthermore, it was shown previously that angiotensin can excite cardiovascular neurons in the VLM. During 1994, two separate studies were carried out to examine further the role of central angiotensin in blood pressure regulation. First, Yoshi Hirooka and Patrick Potts, in collaboration with Geoff Head, Baker Institute, Melbourne, used the Fos technique to identify for the first time the population of neurons in the medulla and pons that are activated by the peptide angiotensin. This study showed that medullary neurons activated by angiotensin are confined to very discrete regions within the VLM and nucleus of the solitary tract. Of particular interest was the observation that the large majority of VLM neurons activated by angiotensin were catecholamine- synthesizing neurons. Thus, the results suggest that angiotensin acts specifically on a chemically distinct subset of neurons within the VLM. Secondly, Yoshi Hirooka carried out an electrophysiological study in which he showed that blockade of angiotensin receptors within the rostral VLM causes a significant inhibition of synaptic transmission to rostral VLM pressor neurons, indicating that endogenous angiotensin within this region has a tonic facilitatory effect on synaptic transmission.
Chart recording showing the effect on arterial pressure and renal sympathetic activity (rSNA) of injection of L-NAME, an inhibitor of nitric oxide synthesis, into the lower thoracic spinal cord. Note the marked and prolonged fall in rSNA following injection of L-NAME, which indicates that nitric oxide is continuously produced in the spinal cord and helps to maintain the resting level of sympathetic vasomotor activity.
Role of nitric oxide in the tonic excitation of renal sympathetic preganglionic neurones
Nitric oxide (NO) is believed to be a neuronal messenger in many central and peripheral synapses. It has been shown previously that the enzymes for NO synthesis are present in sympathetic preganglionic neurons, but there is little information on its functional role in these neurons. In 1994, Ayesha Hakim, Yoshi Hirooka and Matthew Coleman made the exciting discovery that injection of the NO precursor, L-arginine, directly into the lower thoracic spinal cord, which contains the large majority of sympathetic preganglionic neurons controlling the kidney, causes a large increase in the firing rate of these neurons. Even more interestingly, injection of L-NAME, a drug that blocks the synthesis of NO, causes a large and prolonged decrease in the activity of these neurons. This suggests that NO is tonically released in the spinal cord, and that it helps to maintain the resting activity of sympathetic preganglionic neurons.
Role of the caudal medullary raphe in cardiovascular regulation
Matthew Coleman in this Laboratory has previously shown that neurons in the caudal medullary raphe are capable of producing large decreases in sympathetic activity and blood pressure. Roger Dampney, during six months study leave in 1994 at Dr Bridget Lumb's Laboratory at the University of Bristol, participated in an electrophysiological project aimed at determining the inputs that normally drive the cells in this region. The results of these experiments demonstrated that neurons within this region are excited by visceral noxious stimulation, which suggests the possibility that they mediate the reflex decrease in blood pressure that can be elicited by visceral noxious stimulation. Moreover, these experiments also showed that many of these neurons also receive inputs from a subregion within the periaqueductal gray in the midbrain, which is well known from the work of others to play an important role in mediating the cardiovascular and other physiological responses to noxious stimuli.
In 1995 the Laboratory plans to continue its studies on the physiological, anatomical and pharmacological properties of cells in the brain that control blood pressure. In particular, the organization of brainstem pathways involved in the control of blood volume will be studied. Studies will also continue on the role of nitric oxide in modulating synaptic transmission to sympathetic neurons in the spinal cord, and will be extended to study the role of nitric oxide in modulating the activity of vasomotor neurons in the medulla. Finally, studies will be continued on the role of depressor neurons in the caudal midline medulla, with particular emphasis on the pathways involved and the functional relationship between these neurons and other central neurons that subserve the baroreceptor reflex.
The Laboratory is located in room 353, while laboratory personnel have their desks in room 275 of the Anderson Stuart building. Dr Dampney's office is room 276. The laboratory is equipped with a large range of cardiovascular and electrophysiological instrumentation. It also has histological, histochemical and immunohisto-chemical facilities, including two fluorescence microscopes. In addition, there are microcomputers for data analysis which are interfaced with microscopes to enable mapping of labelled structures in microscope sections.
MOST RECENT TOTAL ANNUAL CITATIONS (for 1993): 170
|NHMRC|| Functions and connections of |
medullary cardiovascular nuclei
|NHF||Functional mapping of central|
Total for 1994: $114,723
Total for 1995: $121,238
Lectures: 10, on cardiovascular physiology.
|HLS 2||Dent 2||Hons||Total|
|Practical classes (no.)||20||(4)||-||-||20|
Total formal contact teaching time = 49 h
In addition, much time was spent speaking to and advising students as well as exam and essay marking and course design and development.
(see also OTHER RESEARCH ACTIVITIES in 1994)