Growth hormone-releasing hormone (GHRH) neurons in GHRH-enhanced green fluorescent protein transgenic mice: a ventral hypothalamic network.

The hypothalamic GHRH neurons secrete pulses of GHRH to generate episodic GH secretion, but little is known about the mechanisms involved. We have made transgenic mice expressing enhanced green fluorescent protein (eGFP) specifically targeted to the secretory vesicles in GHRH neurons. GHRH cells transported eGFP from cell bodies in the arcuate nucleus to extensively arborized varicose fiber terminals in the median eminence. Patch clamp recordings from visually identified GHRH cells in mature animals showed spontaneous action potentials, often firing in short bursts up to 10 Hz. GHRH neurons received frequent synaptic inputs, as demonstrated by the recording of abundant inward postsynaptic currents, but spikes were followed by large after-hyperpolarizations, which limited their firing rate. Because many GHRH neurons lie close to the ventral hypothalamic surface, this was examined by wide-field binocular epifluorescence stereomicroscopy. This approach revealed an extensive horizontal network of GHRH cells at low power and individual fiber projections at higher power in the intact brain. It also showed the dense terminal projections of the GHRH cell population in the intact median eminence. This model will enable us to characterize the properties of individual GHRH neurons and their structural and functional connections with other neurons and to study directly the role of the GHRH neuronal network in generating episodic secretion of GH.