Virdee, Jasmeet K. (2008) The electrophysiological and morphological characterisation of aminergic responsive neurones within the rat hypothalamic arcuate nucleus in vitro. PhD thesis, University of Warwick.

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Abstract

1. The hypothalamic arcuate nucleus (Arc) is a key integrative centre of the central nervous
system (CNS) involved in the control and maintenance of energy balance. Whole-cell patch
clamp recording techniques were utilised, in isolated hypothalamic brain slice preparations, to
investigate the electrophysiological and morphological properties of Arc neurones. Differential
expression of subthreshold active conductances were identified and used to functionally
classify Arc neurones into 8 electrophysiological clusters. This classification was based
based upon differential expression of the following conductances: anomalous inward
rectification (Ian); hyperpolarisation-activated non-selective cation conductance (Ih); transient
outward rectification (Ia); T-type-like calcium conductance. Morphological analysis of recorded
neurones, revealed retrospectively with biocytin staining, showed four populations based upon
the orientation and number of primary dendrites. There were no obvious direct correlations
between morphology and electrophysiological properties, suggesting considerable functional
diversity of neurones and their associated circuits at the level of the Arc.
2. The physiological levels of glucose to which the brain is exposed are believed to be around
1-2.5 mM, and glucose-sensing neurones have been identified in the Arc. However, in vitro
slice studies routinely use glucose around 10 mM in aCSF. The impact of this high level of
glucose on fundamental properties and operation of hypothalamic circuits remains unclear.
Here the effect of different ambient glucose levels (10 mM, hyperglycaemic and 2 mM,
euglycaemic) on electrophysiological properties of Arc neurones was compared. Significant
differences in passive and active subthreshold membrane properties of Arc neurones were
observed, including: changes in neuronal input resistance, spontaneous activity and
magnitude of Ih and Ia. Data from this study suggests a need to re-evaluate studies previously
conducted in non-physiological levels of glucose.
3. The effects of noradrenaline (NA) on the neuronal excitability of hypothalamic Arc neurones
were studied. Application of NA induced a membrane depolarisation and increase in electrical
excitability in 51% of Arc neurones, including orexigenic NPY/AgRP neurones, a response
that persisted in the presence of TTX indicating a direct effect. NA-induced depolarisation was
mediated through α1-ARs, in particular through α1A-ARs, and associated with multiple ionic
mechanisms including: closure of a potassium conductance, activation of a non-selective
cation conductance, or a combination of the two.
4. NA also induced a membrane hyperpolarisation in a sub-population of Arc neurones (15%)
including 4/9 putative anorexigenic CART-expressing neurones, the remaining CART
neurones responded with a NA-induced excitation. NA-induced hyperpolarisation, mediated
via α2-ARs and activation of one or more potassium conductances, persisted in the presence
of TTX indicating a direct effect on Arc neurones. 7.5% of neurones responded to NA with
biphasic inhibitory/excitatory responses. Taken together, these data suggest that NA, at least
in part, excites a subpopulation of NPY/AgRP neurones and inhibits a population of CART
expressing neurones which may serve an orexigenic role at the level of the Arc.
5. Histamine induced membrane depolarisation in a population of Arc neurones (65%), most
likely through H1 receptors, via a direct effect on the postsysnaptic membrane. Histamine
induced depolarisation through multiple ionic mechanisms, including closure of a potassium
conductance or activation of an electrogenic pump.

Item Type:	Thesis (PhD)
Subjects:	R Medicine > RC Internal medicine > RC0321 Neuroscience. Biological psychiatry. Neuropsychiatry Q Science > QM Human anatomy
Library of Congress Subject Headings (LCSH):	Amines in the body, Amino acid neurotransmitters, Hypothalamus -- Research, Electrophysiology -- Research, Neurons -- Physiology
Official Date:	November 2008
Dates:	Date Event November 2008 Submitted
Institution:	University of Warwick
Theses Department:	Warwick Medical School
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Spanswick, David
Format of File:	pdf
Extent:	xv, 386 leaves : ill., charts
Language:	eng

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