Gonadotropin-inhibiting hormone (GnIH) stimulates food intake and inhibits reproduction. Neuropeptide Y (NPY) is also involved in appetite regulation and reproduction. Here we investigated whether the effects of GnIH could be mediated via actions on NPY neurons. Mice were used so that we could take advantage of genetically modified animals in which the NPY neurons expressed GFP, enabling their in vitro identification.
Coronal brain slices 250 mm thick and including the arcuate nucleus, were cut and mounted on an upright microscope for patch-clamp electrophysiology recordings. Slices were continuously superfused with artificial cerebrospinal fluid (aCSF) at 32°C and viewed with infrared DIC and epifluorescence optics. GnIH, 10 nM, 100 nM and 1 mM, was applied locally for 30s.
First, most NPY cells were inhibited by GnIH at all concentrations. This was manifest by a reversible reduction in firing rate of spontaneous action potentials together with hyperpolarization of the membrane. Current-voltage relationships revealed that the effects of GnIH generally involved inhibition of a mixed cation conductance and activation of a K+ conductance, which explain the hyperpolarization and inhibition. Second, GnIH inhibited secretion of NPY in incubated hypothalamic blocks, consistent with the electrophysiology. Third, tetrodotoxin (TTX), to inhibit action potential activity, had effects that were strikingly similar to those of GnIH, indicating the possibility of an interneuron in the circuitry. Fourth, neither GniH nor TTX had significant effects on spontaneous excitatory postsynaptic current frequency or amplitude, thus excluding presynaptic effects. Fifth, immunohistochemistry revealed that only 20% of NPY cells had close GnIH contacts.
Thus, GnIH has several actions on NPY neurons in mouse arcuate nucleus, with inhibition being prominent. These effects may reflect subpopulations of NPY neurons. The main effect of GnIH may be to inhibit an excitatory input to NPY neurons, thereby inhibiting the activity of NPY neurons in an indirect manner.