Effects of rabbit anti-α-melanocyte-stimulating hormone (α-MSH) immunoglobulins on α-MSH signaling related to food intake control.

Anti-α-melanocyte-stimulating hormone (α-MSH) polyclonal antibodies have been used for α-MSH neutralization in functional studies, but the results are sometime inconsistent with the antibody expected blocking properties. The present study aimed to determine if rabbit (Rb) anti-α-MSH immunoglobulins (Ig) may inhibit or enhance α-MSH signaling on melanocortin receptor type 4 (MC4R) and α-MSH-induced anorexigenic effect if presented as immune complexes with α-MSH. Polyclonal Rb anti-α-MSH IgG were commercially available and their ability to bind α-MSH has been confirmed by the immunohistochemical detection of α-MSH neurons in the rat hypothalamus. In vitro assay of the cyclic-adenosine mono-phosphate (cAMP) secreted by cells transfected with MC4R was performed to analyze effect of Rb IgG on α-MSH-induced cAMP production. We found that adding Rb IgG to α-MSH resulted in stimulation of cAMP detected at lower peptide concentrations as compared to α-MSH alone. To determine effects of Rb IgG on food intake, rats were injected into the arcuate hypothalamic nucleus with either α-MSH, Rb IgG alone or Rb IgG preincubated with α-MSH. During 2 days after injections, food intake was increased in both groups of rats receiving Rb IgG. However, during following 4 days when food was restricted to 1h/day, only the Rb IgG group displayed higher food intake. Furthermore, after the refeeding, 24h food intake was lower in rats receiving Rb IgG - α-MSH immune complexes. This group of rats was also characterized by higher number of immunopositive neurons in the arcuate nucleus expressing α-MSH and agouti-related protein but not tyrosine hydroxylase. Taken together, these results show that Rb anti-α-MSH antisera, although efficient for immunohistochemical detection of α-MSH, does not always display α-MSH blocking properties but, in contrast, may enhance α-MSH binding to MC4R and increase α-MSH anorexigenic effects when presented as immune complexes with the peptide.