This morphology raises important questions about the ways in which electrical information is processed by GnRH neurons. Thus, GnRH neurons are simple yet extraordinarily long unipolar or bipolar processing units. More remarkably, these studies have revealed that a GnRH neuron’s dendrite extends for considerable distances, often over 1000 μm, only occasionally branches, and remains spiny until its endpoint ( 5, 6, 7). However, very recent dye-filling experiments have challenged this view by revealing that the GnRH neurons in situ exhibit numerous spines on both their cell somata and dendrites ( 5, 6, 7). Controversy has surrounded the role that the GnRH neurons themselves may play in the processing of this information because early electron microscopic studies failed to identify more than a few synapses on individual GnRH neurons ( 3). The mechanisms underlying this integration are unclear. The regulation of fertility requires the precise integration of a wide variety of internal homeostatic and external environmental cues ( 4). Once located in the hypothalamus, the GnRH neurons exhibit simple unipolar or bipolar morphologies and extend axons from their somata to the median eminence from where they secrete GnRH into the hypophyseal portal system to control pituitary gonadotropin secretion ( 3). These neurons are unique in that they are born in the nasal placode and migrate into the brain during embryogenesis ( 1, 2). THE GnRH NEURONS represent the final output neurons of the neuronal network controlling fertility in all mammals. Together, these studies demonstrate a novel pattern of spike generation in mammalian neurons and indicate that afferent inputs within distal dendritic microdomains directly initiate action potentials. Compartmental modeling indicates that sites of spike initiation depend upon location of excitatory input and dendrite geometry. Simultaneous recordings from distal dendrites and somata of individual GnRH neurons indicate distal dendrites are the primary site of spike initiation in these cells. We used imaging, electrophysiological, and modeling studies to understand how they integrate and process information along dendrites. These neurons control fertility and must integrate a variety of internal homeostatic and external environmental cues. GnRH neurons provide an interesting neuronal phenotype with simple, relatively unbranched, unipolar or bipolar dendrites of extensive lengths (>1000 μm) covered in spines. However, dendrites often exhibit conductances necessary for spike generation and represent functionally independent processing compartments within neurons. It is dogma that action potentials are initiated at the soma/axon hillock of neurons.
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