It is thought that CA3 pyramidal neurons communicate mainly through bursts of spikes rather than so-called trains of regular firing action potentials. Reports of both burst firing and nonburst firing CA3 cells suggest that they may fire with more than one output pattern. With the use of whole-cell recording methods we studied the firing properties of rat hippocampal pyramidal neurons in vitro within the CA3b subregion and found three distinct types of firing patterns. Approximately 37% of cells were regular firing where spikes generated by minimal current injection (rheobase) were elicited with a short latency and with stronger current intensities trains of spikes exhibited spike frequency adaptation (SFA). Another 46% of neurons exhibited a delayed onset at rheobase with a weakly-adapting firing pattern upon stronger stimulation. The remaining 17% of cells showed a burst-firing pattern, though only elicited in response to strong current injection and spontaneous bursts were never observed. Control experiments indicated that the distinct firing patterns were not due to our particular slicing methods or recording techniques. Finally, computer modeling was used to identify how relative differences in K+ conductances, specifically K(C), K(M), and K(D), between cells contribute to the different characteristics of the three types of firing patterns observed experimentally.

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Pyramidal neurons in the CA3 region of the hippocampal formation integrate synaptic information arriving in the dendrites within discrete laminar regions. At potentials near or below the resting potential integration of synaptic signals is most affected by the passive properties of the cell and hyperpolarization-activated currents (Ih). Here we focused specifically on a subset of neurons within the CA3b subregion in order to better understand their membrane response within subthreshold voltage ranges. Using a combined experimental and computational approach we found that the passive properties of these neurons varied up to 5-fold between cells. Likewise, there was a large variance in the expression of Ih channels. However, the contribution of Ih was minimal at resting potentials endowing the membrane with an apparent linear response to somatic current injection within ±10 mV. Unlike in CA1 pyramidal neurons, however, Ih activation was not potentiated in an activity dependent-manner. Computer modeling, based on a combination of voltage- and current-clamp data, suggested that an increasing density of these channels with distance from the soma, compared with a uniform distribution, would have no significant effect on the general properties of the cell because of their relatively lower expression. Nonetheless, temporal summation of excitatory inputs was affected by the presence of Ih in the dendrites in a frequency- and distance-dependent fashion.

The impact of the A-type γ-aminobutyric acid (GABA-A) receptor in gonadotropin releasing hormone (GnRH) neurons is controversial. In adult GnRH neurons, the GABA-A receptor conductance has been reported to either hyperpolarize or depolarize GnRH neurons. Regardless of whether GABA is inhibitory or excitatory in GnRH neurons, GABAergic input would be integrated with post-synaptic potentials generated by other synaptic inputs. We used dynamic current clamping and compartmental computer modeling to examine the integration of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamatergic input and GABA-mediated input in both the hyperpolarizing (inhibitory) and depolarizing (excitatory) modes. In both living and model neurons, action potentials were most likely a few ms after a maximum in AMPA conductance coincided with a minimum in inhibitory GABA. Excitatory GABA interacted differently with AMPA, with spikes most likely, in both dynamic clamping of living neurons and in model neurons, when a maximum in AMPA coincided with the decay from peak of a maximum in GABA. Distributing synapses along the dendrite maximized the temporal relationship between AMPA and GABA conductances and therefore, the potential for spiking. Thus, these two dominant neurotransmitters could interact in multiple frames to generate action potentials in GnRH neurons. KeywordsGnRH; synaptic integration; compartmental model; glutamate; GABA; hypothalamus Hypothalamic GnRH neurons form the final pathway for integration of signals regulating reproduction. The control of GnRH neurons has been proposed to reflect both their intrinsic electrical properties and the input they receive from presynaptic neurons (Kusano et al., 1995; reviewed by Lopez et al., 1998). Despite the relatively precise requirements in the timing of GnRH pulses for proper reproductive function (Pohl et al., 1983), GnRH neurons have heterogeneous electrophysiological properties (Sim et al., 2001;Spergel et al., 1999). Moreover, emerging evidence suggests that GnRH neurons may differ in their responses to at Corresponding author: Kelly Suter, University of Texas at San Antonio, 1 UTSA Circle, San Antonio TX, 78249. kelly.suter@utsa.edu. Requested Section Editor: Menahem Segal, Department of Neurobiology, Weizmann Institute of Science (Rehovot, Israel) Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptNeuroscience. Author manuscript; available in PMC 2009 July 17. Published in final edited form as:Neuroscience. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manus...

Adult GnRH neurons exhibit a stereotypic morphology with a small soma, single axon, and single dendrite arising from the soma with little branching. The adult morphology of GnRH neurons in mice reflects an anatomical consolidation of dendrites over postnatal development. We examined this issue in rat GnRH neurons with biocytin filling in live hypothalamic slices from infant males, as adult littermates and in gonad-intact males, castrated males, and in males with one of three levels of testosterone (T) treatment. Somatic area and total dendritic length were significantly greater in infant males than in adults. Moreover, total numbers of dendrite branches were greater in infant males as compared with adults. The number of higher order branches and the lengths of higher order branches were also greater in infant males than in adults. Most interestingly, in adults a single dendrite arose from the somata, consistently at 180° from the axon. In contrast, prepubertal animals had an average of 2.2 ± 0.2 primary dendrites arising from somata (range, one to seven primary dendrites). Angles relative to the axon at which dendrites in prepubertal males emanated from GnRH somata were highly variable. Castration at 25 d of age and castration at 25 d of age with one of three levels of T treatment did not influence morphological parameters when GnRH neurons were examined between 40 d and 48 d of age. Thus, a spatially selective remodeling of primary dendrites and consolidation of distal GnRH dendritic arbors occurs during postnatal development and is largely independent of T.