The pharmacology and function of central VPAC/PAC receptors

• Main Author: Dickson, Louise
• Published: University of Edinburgh 2007
• Subjects: 615.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.649541

Standardised [cAMP]_iand [Ca²⁺]_iassays (96-well plate) were used to directly compare hVPAC/PAC receptor pharmacology in CHO-K1 cells stably expressing the receptors, with additional studies evaluating cell lines with endogenous receptor expression. A range of peptide agonists were utilised in these studies, including non-selective (e.g. PACAP-27) and classical ligands (e.g. VIP), as well as recently described (e.g. R3P65) and highly selective compounds (e.g. maxadilan). The agonist rank order of potencies were identical between assays for each receptor in all of the cell lines examined, with EC₅₀ values consistently ~ 100 fold lower in the [Ca²⁺]_iassay. The pharmacology of the reportedly selective peptide antagonists PG7-269 (VPAC₁), PG99-465 (VPAC₂) and M65 (PAC₁) was also examined, with these studies identifying complexities in the pharmacology of PG99-465. The standardised assay conditions and the excellent correlation between the [cAMP]_iand [Ca²⁺]_idata, underpinned the establishment of an HT [Ca²⁺]_iassay (384-well plate) which was used to identify hVPAC receptor selective ligands from the compound libraries of the Fujisawa Pharmaceutical Company. Approximately 100,000 compounds were tested, with several non-peptides identified as potential ligands. The potency and selectivity of these compounds were subsequently fully characterised using the 96- and 384-well [Ca²⁺]_iassay formats, although none were as potent as the peptides examined in chapter one. In the final section of this thesis, putative roles for VPAC/PAC receptors in the brain were investigated. Firstly, as VIP and PACAP are thought to stimulate the release of cytokines from astrocytes in the brain, VPAC/PAC receptor pharmacology was examined in primary cultures of rat cortical astrocytes (RCA). Receptor characterisation studies ([cAMP]_iassays) established that the PAC₁ was the predominant VPAC/PAC receptor subtype in RCA, with preliminary studies also showing that VIP and PACAP-27 stimulated the release of the cytokines IL-1β and TNF-α from these cells. In addition, behavioural studies using mice further examined the role of the VPAC₂ receptor in the maintenance of circadian activity rhythms.