In birds, prolactin secretion is stimulated by a hypothalamic prolactin releasing
hormone, vasoactive intestinal polypeptide (VIP). Active immunization against VIP
suppresses both prolactin and LH secretion. Since VIP in physiological doses, does not
stimulate LH release, it is suggested that it might act to regulate the function of LHproducing cells in a paracrine manner. The aim of this thesis is to provide biochemical,
molecular and anatomical evidence to support this hypothesis.
Vasoactive intestinal polypeptide was shown to be present in the chicken anterior
pituitary gland using high performance liquid chromatography (HPLC),
radioimmunoassay (RIA) and immunocytochemistry. The VIP antibodies used for both
RIA and immunocytochemistry showed no cross reaction with any known VIP-like
peptides, including pituitary adenylate cyclase activating polypeptide (PACAP). VIP
mRNA was also shown to be present in anterior pituitary gland, using reversetranscription polymerase chain reaction (RT-PCR) and primers designed from the chicken
VIP cDNA sequence, and by solution hybridization RNAse protection assay.
Two VIP-immunoreactive cell (VIP-ir) types were found throughout the cephalic and
caudal lobes of the anterior pituitary gland. The morphological features of the VIP-ir cell
types were similar to that of the folliculo-stellate cell type (FS-cell). One VIP-cell type
contained the Ca 2+ binding protein S-100 protein, a specific marker of FS-cells, while
the S-100 protein was not detected in the second VIP-cell type. The VIP-cells were
closely associated with gonadotrophs, lactotrophs and unidentified cell types, but VIP-ir
was not colocalised with LH or PRL. The VIP-cell characteristically enveloped several
adjacent gonadotrophs with cytoplasmic projections, which suggests that intra-pituitary
VIP may regulate the gonadotroph in a paracrine manner.
VIP receptors were localised immunocytochemically, in the anterior pituitary gland
and hypothalamus, using antibodies raised the peptide sequence of the human subtype-I
(VIP-RI) and the subtype-II VIP receptors (VIP-RII). The VIP-RI was not present in
PRL-, LH- or GH-cells, but was found exclusively in ACTH-cells. The VIP-RII was
diffusely distributed through both lobes of the chicken anterior pituitary gland, but the cell
type containing VIP-RII immunoreactivity was not identified.
The effects of VIP and PACAP on pituitary hormone secretion were determined in
vitro using cultured hemi-pituitaries. VIP and PACAP stimulated PRL secretion in a dose
dependent manner. High concentrations of VIP and PACAP stimulated GH and LH
secretion but did not affect ACTH secretion.
In conclusion, VIP is produced in the anterior pituitary gland and occurs in a FS-cell
type which is closely associated with gonadotrophs. These observations are consistent
with the view that intra-pituitary VIP may act in a paracrine manner to regulate the
function of gonadotrophs.