Changes in dendritic architecture: not your "usual suspect" in control of the onset of puberty in male rats.

Until the recent past, the search for the underlying drive for the pubertal increase in gonadotropin-releasing hormone (GnRH) hormone from the GnRH-containing neurons in the hypothalamus was largely focused on extrinsic factors. The most recent evidence however indicates changes in the structure of GnRH neurons themselves may contribute to this fundamental event in development. Based on our studies in males, dendritic architecture is not static from birth until adulthood. Instead, dendrites undergo a dramatic remodeling during the postnatal period which is independent of testosterone and occurs before the pubertal increase in GnRH release. First, the number of dendrites emanating from somata is reduced between infancy and adulthood. Moreover, a dendrite of adult GnRH neurons invariability arises at angle of 180°from the axon as opposed to the extraordinary variability in location during infancy. In fact, in some neurons from infants, no dendrite even resides in the adult location. Thus, there is a spatially selective remodeling of primary dendrites. Secondly, dendrites of GnRH neurons from infants were highly branched prior to assuming the compact morphology of adults. Finally, other morphological aspects of GnRH neurons such as total dendritic length, the numbers of dendrite branches and the lengths of higher order branches were significantly greater in infants than adults, indicating a consolidation of dendritic arbors. Activity in multi-compartment models of GnRH neurons, suggest the impact of structure on neuronal activity is exerted with both active and passive dendrites. Thus, passive properties make a defining contribution to function. Accordingly, changes in morphology alone are likely to have functional consequences for the pattern of activity in GnRH neurons. Our findings suggest structural remodeling of dendrites during the postnatal period likely facilitates repetitive action potentials and thus, GnRH release at the time of puberty.

Simulated GABA synaptic input and L-type calcium channels form functional microdomains in hypothalamic gonadotropin-releasing hormone neurons.

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons integrate the multiple internal and external cues that regulate sexual reproduction. In contrast to other neurons that exhibit extensive dendritic arbors, GnRH neurons usually have a single dendrite with relatively little branching. This largely precludes the integration strategy in which a single dendritic branch serves as a unit of integration. In the present study, we identify a gradient in L-type calcium channels in dendrites of mouse GnRH neurons and its interaction with GABAergic and glutamatergic inputs. Higher levels of L-type calcium channels are in somata/proximal dendrites (i.e., 0-26 µm) and distal dendrites (∼130 µm dendrite length), but intervening midlengths of dendrite (∼27-130 µm) have reduced L-type calcium channels. Using uncaging of GABA, there is a decreasing GABAergic influence along the dendrite and the impact of GABA(A) receptors is dependent on activation of L-type calcium channels. This results in amplification of proximal GABAergic signals and attenuation of distal dendritic signals. Most interestingly, the intervening dendritic regions create a filter through which only relatively high-amplitude, low-frequency GABAergic signaling to dendrites elicits action potentials. The findings of the present study suggest that GnRH dendrites adopt an integration strategy whereby segments of single nonbranching GnRH dendrites create functional microdomains and thus serve as units of integration.

Hormone secretion in transgenic rats and electrophysiological activity in their gonadotropin releasing-hormone neurons.

Expression of GFP in GnRH neurons has allowed for studies of individual GnRH neurons. We have demonstrated previously the preservation of physiological function in male GnRH-GFP mice. In the present study, we confirm using biocytin-filled GFP-positive neurons in the hypothalamic slice preparation that GFP-expressing somata, axons, and dendrites in hypothalamic slices from GnRH-GFP rats are GnRH1 peptide positive. Second, we used repetitive sampling to study hormone secretion from GnRH-GFP transgenic rats in the homozygous, heterozygous, and wild-type state and between transgenic and Wistar males after ~4 yr of backcrossing. Parameters of hormone secretion were not different between the three genetic groups or between transgenic males and Wistar controls. Finally, we performed long-term recording in as many GFP-identified GnRH neurons as possible in hypothalamic slices to determine their patterns of discharge. In some cases, we obtained GnRH neuronal recordings from individual males in which blood samples had been collected the previous day. Activity in individual GnRH neurons was expressed as total quiescence, a continuous pattern of firing of either low or relatively high frequencies or an intermittent pattern of firing. In males with both intensive blood sampling (at 6-min intervals) and recordings from their GnRH neurons, we analyzed the activity of GnRH neurons with intermittent activity above 2 Hz using cluster analysis on both data sets. The average number of pulses was 3.9 ± 0.6/h. The average number of episodes of firing was 4.0 ± 0.6/h. Therefore, the GnRH pulse generator may be maintained in the sagittal hypothalamic slice preparation.