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Abstract:
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The present study demonstrate the functional alterations of the GABAA and GABAB
receptors and the gene expression during the regeneration of pancreas following
partial pancreatectomy. The role of these receptors in insulin secretion and
pancreatic DNA synthesis using the specific agonists and antagonists also are studied
in vitro. The alterations of GABAA and GABAR receptor function and gene
expression in the brain stem, crebellum and hypothalamus play an important role in
the sympathetic regulation of insulin secretion during pancreatic regeneration.
Previous studies have given much information linking functional interaction between
GABA and the peripheral nervous system. The involvement of specific receptor
subtypes functional regulation during pancreatic regeneration has not given emphasis
and research in this area seems to be scarce. We have observed a decreased GABA
content, down regulation of GABAA receptors and an up regulation of GABAB
receptors in the cerebral cortex, brain stem and hypothalamus. Real Time-PCR
analysis confirmed the receptor data in the brain regions. These alterations in the
GABAA and GABAB receptors of the brain are suggested to govern the regenerative
response and growth regulation of the pancreas through sympathetic innervation. In
addition, receptor binding studies and Real Time-PCR analysis revealed that during
pancreatic regeneration GABAA receptors were down regulated and GABAB
receptors were up regulated in pancreatic islets. This suggests an inhibitory role for
GABAA receptors in islet cell proliferation i.e., the down regulation of this receptor
facilitates proliferation. Insulin secretion study during 1 hour showed GABA has
inhibited the insulin secretion in a dose dependent manner in normal and
hyperglycaemic conditions. Bicuculline did not antagonize this effect. GABAA
agonist, muscimol inhibited glucose stimulated insulin secretion from pancreatic
islets except in the lowest concentration of 1O-9M in presence of 4mM glucose.Musclmol enhanced insulin secretion at 10-7 and 10-4M muscimol in presence of
20mM glucose- 4mM glucose represents normal and 20mM represent
hyperglycaemic conditions. GABAB agonist, baclofen also inhibited glucose induced
insulin secretion and enhanced at the concentration of 1O-5M at 4mM glucose and at
10-9M baclofen in presence of 20mM glucose. This shows a differential control of
the GABAA and GABAB receptors over insulin release from the pancreatic islets.
During 24 hours in vitro insulin secretion study it showed that low concentration of
GABA has inhibited glucose stimulated insulin secretion from pancreatic islets.
Muscimol, the GABAA agonist, inhibited the insulin secretion but, gave an enhanced
secretion of insulin in presence of 4mM glucose at 10-7
, 10-5 and 1O-4M muscimol.
But in presence of 20mM glucose muscimol significantly inhibited the insulin
secretion. GABAB agonist, baclofen also inhibited glucose induced insulin secretion
in presence of both 4mM and 20mM glucose. This shows the inhibitory role of
GABA and its specific receptor subtypes over insulin synthesis from pancreatic bete-islets.
In vitro DNA synthesis studies showed that activation of GABAA receptor by
adding muscimol, a specific agonist, inhibited islet DNA synthesis. Also, the
addition of baclofen, a specific agonist of GABAB receptor resulted in the stimulation
of DNA synthesis.Thus the brain and pancreatic GABAA and GABAB receptor
gene expression differentially regulates pancreatic insulin secretion and islet cell
proliferation during pancreatic regeneration. This will have immense clinical
significance in therapeutic applications in the management of Diabetes mellitus. |