Whisker-related circuitry in the trigeminal nucleus principalis: Topographic precision

Abstract

Single whiskers are topographically represented in the trigeminal (V) nucleus principalis (PrV) by a set of cylindrical aggregates of primary afferent terminals and somata (barrelettes). This isomorphic pattern is transmitted to the thalamus and barrel cortex. However, it is not known if terminals in PrV from neighboring whiskers interdigitate so as to violate rules of spatial parcellation predicted by barrelette borders; nor is it known the extent to which higher order inputs are topographic. The existence of inter-whisker arbor overlap or diffuse higher order inputs would demand additional theoretical principles to account for single whisker dominance in PrV cell responses. In adult rats, first, primary afferent pairs responding to the same or neighboring whiskers and injected with Neurobiotin or horseradish peroxidase were rendered brown or black to color-code their terminal boutons. When collaterals from both fibers appeared in the same topographic plane through PrV, the percentage of the summed area of the two arbor envelopes that overlapped was computed. For same-whisker pairs, overlap was 5?±?6% (mean?±?SD). For within-row neighbors, overlap was 2?±?5%. For between-row neighbors, overlap was 1?±?4%. Second, the areas of whisker primary afferent arbors and their corresponding barrelettes in the PrV were compared. In the transverse plane, arbor envelopes significantly exceeded the areas of cytochrome oxidase-stained barrelettes; arbors often extended into neighboring barrelettes. Third, bulk tracing of the projections from the spinal V subnucleus interpolaris (SpVi) to the PrV revealed strict topography such that they connect same-whisker barrelettes in the SpVi and PrV. Thus, whisker primary afferents do not exclusively project to their corresponding PrV barrelette, whereas higher order SpVi inputs to the PrV are precisely topographic.     

Integration of Sensory Quanta in Cuneate Nucleus Neurons In Vivo

Discriminative touch relies on afferent information carried to the central nervous system by action potentials (spikes) in ensembles of primary afferents bundled in peripheral nerves. These sensory quanta are first processed by the cuneate nucleus before the afferent information is transmitted to brain networks serving specific perceptual and sensorimotor functions. Here we report data on the integration of primary afferent synaptic inputs obtained with in vivo whole cell patch clamp recordings from the neurons of this nucleus. We find that the synaptic integration in individual cuneate neurons is dominated by 4–8 primary afferent inputs with large synaptic weights. In a simulation we show that the arrangement with a low number of primary afferent inputs can maximize transfer over the cuneate nucleus of information encoded in the spatiotemporal patterns of spikes generated when a human fingertip contact objects. Hence, the observed distributions of synaptic weights support high fidelity transfer of signals from ensembles of tactile afferents. Various anatomical estimates suggest that a cuneate neuron may receive hundreds of primary afferents rather than 4–8. Therefore, we discuss the possibility that adaptation of synaptic weight distribution, possibly involving silent synapses, may function to maximize information transfer in somatosensory pathways.     

大鼠口面部躯体和上消化道内脏伤害性传入汇聚于三叉神经脊束间质核含CB神经元

三叉神经脊束间质核(interstitial nucleus of the spinal trigeminal tract,INV)为位于三叉神经脊束内的一些灰质团块,经三叉神经和舌咽及迷走神经接受口面部的三叉神经躯体传入与上消化道的内脏伤害性传入。INV内含有大量含calbinding D-28k(CB)神经元,但尚不清楚支配口面部的三叉神经躯体传入与支配上消化道的内脏伤害性传入是否汇聚于INV内含CB的神经元。本文应用跨节追踪法并结合CB和Fos免疫组织化学的激光共聚焦显微镜和电镜技术,研究了下牙槽神经(interior alveolarnerve．IAN)的初级传入和上消化道伤害性信息向INV内含CB神经元的汇聚。结果如下：(1)将生物素化的葡聚糖胺(biotinylated dextran amine,BDA)和甲醛分别注入IAN和上消化道后,BDA跨节标记的浓密初级传入纤维和末梢分布于同侧INV内,在其膨大部较为集中;大量的CB和Fos免疫反应阳性神经元分布于双侧INV内,并与BDA注射侧的BDA标记末梢区相重叠：共聚焦显微镜观察显示,约半数CB免疫反应阳性的神经元同时呈Fos阳性的双标记神经元(74／153),其中部分双标神经元与IAN末梢形成紧密接触状。(2)辣根过氧化物酶(horseradish peroxidase,HRP)注射到IAN后,HRP跨节标记的纤维和末梢的分布与BDA标记的分布相似;电镜下观察到,INV内有大量CB免疫反应阳性神经元的树突和少量胞体,以及HRP标记的传入末梢,其中一些HRP标记的轴突终末分别与CB免疫反应阳性树突和胞体形成非对称型轴-树或轴-体突触。结果提示口面部躯体初级传入信息和内脏伤害性信息汇聚于INV内含CB的神经元上,可能在躯体传入信息对内脏伤害性信息的调制和内脏心血管活动中发挥重要作用。     

Presynaptic selection of afferent inflow in the spinal cord.

The synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates can be centrally controlled by means of specific sets of GABAergic interneurons that make axo-axonic synapses with the terminal arborizations of the afferent fibers. In the steady state, the intracellular concentration of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Following the release of GABA by spinal interneurons and activation of GABA(A) receptors in the afferent terminals, there is an outwardly directed efflux of chloride ions that produces primary afferent depolarization (PAD) and reduces transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of single afferent terminals (which is reduced during PAD), have further indicated that muscle and cutaneous afferents have distinctive, but modifiable PAD patterns in response to segmental and descending stimuli. This has suggested that PAD and presynaptic inhibition in the various types of afferents is mediated by separate sets of last-order GABAergic interneurons. Direct activation, by means of intraspinal microstimulation, of single or small groups of last-order PAD-mediating interneurons shows that the monosynaptic PAD elicited in Ia and Ib afferents can remain confined to some sets of the intraspinal collaterals and not spread to nearby collaterals. The local character of PAD allows cutaneous and descending inputs to selectively inhibit the PAD of segmental and ascending intraspinal collaterals of individual muscle spindle afferents. It thus seems that the intraspinal branches of the sensory fibers are not hard wired routes that diverge excitation to spinal neurons, but are instead dynamic pathways that can be centrally controlled to address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.     

大鼠三叉神经尾侧亚核内SPR阳性神经元与初级传入和下行投射纤维之间的突触联系

用追踪和免疫电镜技术研究三叉神经尾侧亚核（Ｖｃ）内Ｐ物质受体（ＳＰＲ）阳性神经元与初级传入和下行投射之间的突触联系。光镜观察发现,在Ｖｃ浅层,ＳＰＲ阳性神经元的分布与ＲＭｇ下行投射终末的分布有重叠。电镜观察发现,三叉初级传入终末和ＳＰＲ阳性神经元树突形成非对称性轴树突触;ＲＭｇ下行投射终末与ＳＰＲ阳性神经元树突也形成非对称性轴树突触,提示ＲＭｇ下行投射纤维可能通过直接作用于丘脑投射神经元对三叉初级传入的伤害性信息进行调控。     

Fine structural morphology of identified X- and Y-cells in the cat's lateral geniculate nucleus

Four physiologically identified neurons in the A laminae of the cat's dorsal lateral geniculate nucleus were filled with horseradish peroxidase and studied using the electron microscope. Two were X-cells and two were Y-cells. Each had electrophysiological properties appropriate for its X- Or Y-cell class, and each also had an axon that projected into the optic radiation, indicative of a geniculocortical relay cell. Representative samples from about 10% of each neuron's entire dendritic arbor (proximal and distal) were taken to obtain an estimate of the types and distributions of synapses contacting these arbors. One X-cell had a cytoplasmic laminar body, but there were no other significant cytological differences seen among the neurons. Common to each of the neurons were the following synaptic features: (i) retinal terminals (r.l.p.) were mostly or entirely restricted to proximal dendrites or dendritic appendages (less than 100 microns from the soma). These terminals constituted about 15-25% of the synapses on the proximal dendrites. (ii) Terminals with flattened or pleomorphic synaptic vesicles (f. terminals) were predominant on the proximal dendrites (30-55% of the total synapses for that region) and were mainly located near the retinal terminals. A smaller percentage (10-20%) were also distributed onto the distal dendrites. (iii) Small terminals with round synaptic vesicles (r.s.d.), many presumably having a cortical origin, predominated (60-80%) on distal dendrites (greater than 100 microns), but also formed a large proportion (40-70%) of the synapses on the intermediate (50-150 microns) dendrites. Total synaptic contacts for one X-cell and one Y-cell were estimated at about 4000 and 5000, respectively. The major fine structural differences observed between X- and Y-cells were almost entirely related to the retinal afferents. First, the retinal synapses for X-cells were mostly made on to dendritic appendages (spines, etc.), whereas Y-cells had most of their retinal synapses onto the shafts of primary and proximal secondary dendrites (that is, near branch points. Second, the retinal terminals that contacted X-cell dendrites nearly always formed triadic arrangements that included nearby f. terminals, but those on Y-cells rarely did so. Finally, the main type of f. terminals associated with X-cells were morphologically different from most of those associated with the Y-cells, and this also related directly to the triadic arrangements; that is, f. terminals in the triadic arrangements were morphologically distinguishable from f. terminals that did not participate in triadic arrangements.(ABSTRACT TRUNCATED AT 400 WORDS)     

The synaptic relationships between the primary afferent terminals and the cuneo-thalamic relay neurons in the rat cuneate nucleus

In order to establish the synaptic relationship between the primary afferent terminals and the cuneothalamic relay neurons in the cuneate nucleus, the combined retrograde transport of horseradish peroxidase (HRP) and experimental degeneration have been applied in the young adult albino rats. 10 to 30% HRP was injected contralaterally (0.5 microliter) in the ventrobasal thalamic nucleus and multiple dorsal rhizotomies (C5 to T1) in the cervicothoracic dorsal roots were performed on the side ipsilateral to the cuneate nucleus. The results showed that: The cuneo-thalamic relay (CTN) neurons were the major neuronal type of the nucleus. More than 55% of neurons have been labelled. These neurons were 18-30 micron X 15-25 micron in sizes. They distributed in the whole rostrocaudal extent of the nucleus, particularly dense in the middle portion. The cells varied from round, oval, spindle to multipolar in shapes. They were rich in cytoplasmic organelles and had well-developed roughed endoplasmic reticulum. Their nucleus was either centrally or eccentrically located and was rather regular. The HRP-positive granules were randomly distribute in the perikaryon, dendrites and initial segment of the axons; At least three types of the experimental degeneration of the primary afferent terminals (PAT) were observed in the cuneate nucleus two to three days after dorsal rhizotomy, namely, electron-dense, granular and neurofilamentous. These PAT were mostly large and contained round vesicles. They were commonly found within synaptic complex, in which they were presynaptic to dendrites of various sizes, and were themselves postsynaptic to smaller axon terminals containing flattened vesicles. Degenerating PAT forming isolated synapses were less commonly seen; The PAT in the synaptic complex were directly presynaptic to the dendrites originating from the CTN neurons. The dendrites forming PAT-CTN synases were of large and medium-sized. The PAT did not form direct axo-somatic synapses with the somata of CTN or of any other cell types in the cuneate nucleus.     

Glutamic Acid Decarboxylase-Containing Neurons in the Dorsal Column Nuclei of the Cat

The retrograde transport of horseradish peroxidase (HRP) and immunocytochemistry for glutamic acid decarboxylase (GAD) have been employed to examine whether local circuit neurons (LCNs) exist in the dorsal column nuclei (DCN) and whether these neurons may be GABA-ergic. Observations focused on the dorsal part of the middle cuneate nucleus (MCd), since this region has been previously shown to contain projecting neurons whose axons terminate almost exclusively in the contralateral thalamus. After large injections of HRP in the nucleus ventralis posterolateralis and surrounding structures of the feline thalamus, the majority of neurons in MCd are labeled. These represent about 89% of the neurons in MCd as counted in 40-μm frozen sections, and about 69% as counted in plastic-embedded, 2.5-μm-thick section. Unlabeled by the same injections are some medium to largeneurons at the dorsal rim of MCd, and many characteristically small (X ±250/μm²) neurons at the periphery of the cell clusters formed by thalamic-projecting neurons. These small neurons represent 10-12% of the neuronal population of MCd, as counted in 40-μm-thick frozen sections, and about 30%, as counted in plastic-embedded, 2.5-μm-thick sections. Neurons in this size range are also unlabeled after injection of retrograde tracer in the pretectal area, inferior and superior colliculi, inferior olivary complex, and/or spinal cord. These injections, however, result in the labeling of neurons along the dorsal rim of MCd and/or in other regions of the cuneate nucleus.

In adult, colchicine-treated cats, the use of anti-GAD serum reveals a population of labeled neurons uniformly distributed throughout the DCN. In MCd, these are small (X =±235 μm²) neurons mainly intercalated between cell clusters, and represent about 25% of the neuronal population of this nuclear subdivision as counted in plastic-embedded, 2.5-μm-thick sections. Labeled processes densely infiltrate the cell clusters, and labeled varicosities appear to cover the soma and dendrites of unlabeled neurons. At the electron-microscopic level, most labeled profiles contain vesicles and correpond to F boutons usually involved in “axoaxonic” contacts with terminals of dorsal root afferent and presynaptic to dendrites. Other vesicle-containing, GAD-positive endings seem to correspond to the P boutons described by Ellis and Rustioni (1981) and are believed to be, at least in part, of dendritic origin. It is suggested that GAD-positive neurons are GABA-ergic LCNs and that these can mediate both pre- and postsynaptic inhibition. Their integrative role is likely to be more complex than postulated by previous electrophysiological studies.     

Synaptic organization of substance P, glutamate and GABA-immunoreactive boutons on functionally identified neurons in cat spinal deeper dorsal horn

In order to determine how nociceptive input conveyed by the C-fibers terminating in superficial lam-inae of the spinal cord reaches the wide dynamic range (WDR) cells in deeper dorsal horn, which functions as ascend-ing projection pathway, the morphological features of some WDR cells in the deeper dorsal horn of the cat lumbar spinal cord were studied by intracellular injection of horseradish peroxidase and physiological characterization. One of the fully stained neurons with somata in lamina V and dendrites that entered lamina Ⅱ were examined by electron mi-croscopy. Immunogold staining of ultrathin sections through the labeled proximal dendrites in lamina Ⅱ revealed that these dendrites received numerous synapses from substance P and glutamate immunoreactive (IR) axons, which were considered originating from C-fibers. In addition, many GABA-IR terminals were found presynaptic to the labeled dendrites. The results, therefore, suggest that the information carried by primary afferent can be sent from t     

Extrastriate feedback to primary visual cortex in primates: a quantitative analysis of connectivity.

Knowledge-based or top-down influences on primary visual cortex (area V1) are believed to originate from information conveyed by extrastriate feedback axon connections. Understanding how this information is communicated to area V1 neurons relies in part on elucidating the quantitative as well as the qualitative nature of extrastriate pathway connectivity. A quantitative analysis of the connectivity based on anatomical data regarding the feedback pathway from extrastriate area V2 to area V1 in macaque monkey suggests (i) a total of around ten million or more area V2 axons project to area V1; (ii) the mean number of synaptic inputs from area V2 per upper-layer pyramidal cell in area V1 is less than 6% of all excitatory inputs; and (iii) the mean degree of convergence of area V2 afferents may be high, perhaps more than 100 afferent axons per cell. These results are consistent with empirical observations of the density of radial myelinated axons present in the upper layers in macaque area V1 and the proportion of excitatory extrastriate feedback synaptic inputs onto upper-layer neurons in rat visual cortex. Thus, in primate area V1, extrastriate feedback synapses onto upper-layer cells may, like geniculocortical afferent synapses onto layer IVC neurons, form only a small percentage of the total excitatory synaptic input.     

Nonclassical neuronal communications

Examples from classical neuronal communications are discussed in the light of biochemical and anatomical data. These are the nonsynaptic axo-axonic interactions of the enkephalinergic neurons on nerve terminals of peptidergic primary sensory afferents and dopaminergic nigrostriatal fibers. Examples of dendrites as presynaptic sites are discussed in three very different situations, namely, the dopaminergic dendrites of the substantia nigra neurons, the gamma-aminobutyric acid--ergic dendrites involved in reciprocal dendro-dendritic synapses in the olfactory bulb, and the peripheral branches of the substance P-containing primary sensory neurons.     

GABA-immunoreactive neurons and terminals in the cat periaqueductal gray matter: A light and electron microscopic study

Immunocytochemical and electron microscopic methods were used to study the GABAergic innervation in adult cat periaqueductal gray matter (PAG). A mouse monoclonal antibody against γ -aminobutyric acid (GABA) was used to visualize the inhibitory neuronal system of PAG. At light microscopy, GABA-immunopositive (GABA_IP) neurons formed two longitudinally oriented columns in the dorsolateral and ventrolateral PAG that accounted for 36% of the neuronal population of both PAG columns; their perikaryal cross-sectional area was smaller than that of unlabeled (UNL) neurons found in the same PAG subdivisions. At electron microscopic level, patches of GABA immunoreactivity were readily detected in neuronal cell bodies, proximal and distal dendrites, axons and axon terminals. Approximately 35–36% of all terminals were GABA_IP; they established symmetric synapses with dendrites (84.72% of the sample in the dorsolateral PAG and 86.09% of the sample in the ventrolateral PAG) or with cell bodies (7–10% of the sample). Moreover, 49.15% of GABA_IP axon terminals in the dorsolateral and 52.16% in the ventrolateral PAG established symmetric synapses with GABA_IP dendrites. Immunopositive axon terminals and unlabeled terminals were also involved in the formation of a complex synaptic arrangment, i.e. clusters of synaptic terminals in close contact between them that were often observed in the PAG neuropil. Moreover, a fair number of axo-axonic synapses between GABA_IP and/or UNL axon terminals were present in both PAG subdivisions. Several dendro-dendritic synapses between labeled and unlabeled dendrites were also observed in both PAG subdivisions. These results suggest that in the cat PAG there exist at least two classes of GABArgic neurons. The first class could exert a tonic control on PAG projecting neurons, the second could act on those GABAergic neurons that in turn keep PAG projecting neurons under tonic inhibition. The functional implications of this type of GABAergic synapse organization are discussed in relation to the dishinibitory processes that take place in the PAG.     

A synaptic strategy for consolidation of convergent visuotopic maps

The mechanisms by which experience guides refinement of converging afferent pathways are poorly understood. We describe a vision-driven refinement of corticocollicular inputs that determines the consolidation of retinal and visual cortical (VC) synapses on individual neurons in the superficial superior colliculus (sSC). Highly refined corticocollicular terminals form 1-2 days after eye-opening (EO), accompanied by VC-dependent filopodia sprouting on proximal dendrites, and PSD-95 and VC-dependent quadrupling of functional synapses. Delayed EO eliminates synapses, corticocollicular terminals, and spines on VC-recipient dendrites. Awake recordings after EO show that VC and retina cooperate to activate sSC neurons, and VC light responses precede sSC responses within intervals promoting potentiation. Eyelid closure is associated with more protracted cortical visual responses, causing the majority of VC spikes to follow those of the colliculus. These data implicate spike-timing plasticity as a mechanism for cortical input survival, and support a cooperative strategy for retinal and cortical coinnervation of the sSC.     

NMDA channels control meal size via central vagal afferent terminals

The N-methyl-D-aspartate (NMDA) ion channel blocker MK-801 administered systemically or as a nanoliter injection into the nucleus of the solitary tract (NTS), increases meal size. Furthermore, we have observed that ablation of the NTS abolishes increased meal size following systemic injection of dizocilpine (MK-801) and that MK-801-induced increases in intake are attenuated in rats pretreated with capsaicin to destroy small, unmyelinated, primary afferent neurons. These findings led us to hypothesize that NMDA receptors on central vagal afferent terminals or on higher-order NTS neurons innervated by these vagal afferents might mediate increased food intake. To evaluate this hypothesis, we examined 15% sucrose intake after 50-nl MK-801 injections ipsilateral or contralateral to unilateral nodose ganglion removal (ganglionectomy). On the side contralateral to ganglionectomy, vagal afferent terminals would be intact and functional, whereas ipsilateral to ganglionectomy vagal afferent terminals would be absent. Three additional control preparations also were included: 1) sham ganglionectomy and 2) subnodose vagotomy either contralateral or ipsilateral to NTS cannula placement. We found that rats with subnodose vagotomies increased their sucrose intake after injections of MK-801 compared with saline, regardless of whether injections were made contralateral (12.6 +/- 0.2 vs. 9.6 +/- 0.3 ml) or ipsilateral (14.2 +/- 0.6 vs. 9.7 +/- 0.4 ml) to vagotomy. Rats with NTS cannula placements contralateral to nodose ganglionectomy also increased their intake after MK-801 (12.2 +/- 0.9 and 9.2 +/- 1.1 ml for MK-801 and saline, respectively). However, rats with placements ipsilateral to ganglionectomy did not respond to MK-801 (8.0 +/- 0.5 ml) compared with saline (8.3 +/- 0.4 ml). We conclude that central vagal afferent terminals are necessary for increased food intake in response to NMDA ion channel blockade. The function of central vagal afferent processes or the activity of higher-order NTS neurons driven by vagal afferents may be modulated by NMDA receptors to control meal size.     

Synaptic relationship between somatostatin- and neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats

J. Neurochem. (2012) 122, 923-933. ABSTRACT: The pre-B?tzinger complex (pre-B?tC) in the ventrolateral medulla oblongata is critical for the generation of respiratory rhythm in mammals. Somatostatin (SST) and neurokinin 1 receptor (NK1R) immunoreactivity have been used as markers of the pre-B?tC. SST immunoreactivity almost completely overlaps with small fusiform NK1R-immunoreactive (ir) neurons, the presumed rhythmogenic neurons, but not with large multipolar NK1R-ir neurons. Understanding the neurochemical characteristics, especially the synaptic relationship of SST/NK1R-ir neurons within the pre-B?tC network is essential in providing cellular and structural bases for understanding their physiological significance. This work has not been documented so far. We found that SST immunoreactivity was highly expressed in terminals, somas, and primary dendrites in the pre-B?tC. Besides the small fusiform neurons, a small population of medium-sized NK1R-ir neurons also colocalized with SST. Large NK1R-ir neurons were not SST-ir, but received somatostatinergic inputs. SST-ir terminals were glutamatergic or GABAergic, and synapsed with NK1R-ir neurons. Most of synapses between them were of the symmetric type, indicating their inhibitory nature. Asymmetric synapses were evident between SST-ir terminals and NK1R-ir dendrites, strongly suggesting an excitatory innervation from the presumed rhythmogenic neurons as these neurons are glutamatergic. We speculate that SST-mediated excitatory and inhibitory synaptic transmission onto NK1R-ir rhythmogenic and follower neurons synchronizes their activity to contribute to respiratory rhythmogenesis and control.     

Synaptic relations of neurotensinergic neurons in the dorsal raphe nucleus

The ultrastructure and synaptic relations of neurotensinergic neurons in the rat dorsal raphe nucleus (DRN) were examined. The neurotensin-like immunoreactive (NT-LI) neurons in the DRN were fusiform or spherical. The NT-LI perikarya could only be detected in colchicine-treated animals whereas the immunoreactive axon terminals could only be found in the anirnals not treated with colchicine. Although many NT-LI dendrites received synapses from nonimmunoreactive axon terminals, the NT-LI perikarya received few synapses. NT-LI axon terminals also made synapses on nonimmunoreactive dendrites. Occasionally, synapses were found between the NT-LI axon terminals and NT-LI dendrites in the cases in which the animals were not treated with colchicine.     

Balanced GABAergic and glutamatergic synapse development in hippocampal neurons

Coordinated development of excitatory and inhibitory synapses is crucial for normal function of neuronal circuits. Using homo- and heterochronic cultures of hippocampal neurons, we compared the formation of glutamatergic and GABAergic synapses at different stages and asked whether the age of dendrites affects their ability to accept new glutamatergic and GABAergic synapses. Neurons were transfected with either CFP-actin as a dendritic marker or GFP-synaptophysin as a presynaptic marker. We found that GFP-synaptophysin clusters formed on CFP-actin-labeled dendrites at similar density regardless of pre- and postsynaptic cell type or the age of dendrites (0-2 weeks) upon co-culturing. Therefore, the age of mature dendrites does not affect their ability to accept new synapses. Because GABAergic transmission switches from depolarizing to hyperpolarizing during 1-2 weeks in these cultures, our observations also suggest that this developmental switch does not alter the formation of GABAergic synapses.     

Effects of neonatal axoplasmic transport attenuation on the response properties of vibrissae-sensitive neurons in the trigeminal principal sensory nucleus of the rat

We have previously shown that attenuation of axoplasmic transport by application of vinblastine to the developing infraorbital nerve (ION) results in a loss of central vibrissae-related patterns that is not accompanied by changes in the receptive field sizes for the V primary afferents innervating the whisker follicles. The present study examines the relationship between the loss of central vibrissae-related patterns and alterations in the response properties of neurons in the V principal sensory nucleus (PrV) of adult rats that sustained application of vinblastine to the ION at birth. Absence of histochemically demonstrable vibrissae-related patterns in PrV resulted in only modest changes in the receptive fields and response properties of vibrissae-sensitive neurons in this nucleus that projected to the contralateral thalamus. Response latencies to electrical activation of the V ganglion were similar in treated and untreated animals. The mean receptive field size was significantly increased from 1.3 +/- 0.7 vibrissae in controls to 1.7 +/- 0.9 vibrissae in vinblastine-treated animals, and the percentage of cells yielding a tonic response to vibrissae deflection was markedly reduced (p < 0.01 for both measures). Phasically responding cells recorded in vinblastine-treated animals showed a significant reduction in the mean number of spikes per stimulus following deflection of the vibrissae in either the preferred or non-preferred direction relative to cells recorded in normal animals (p < 0.05). The present results indicate that disruption of the normal vibrissae-related aggregates of neurons in PrV by application of vinblastine to the ION has limited effects on the functional representation of the vibrissae in this nucleus.     

Electron microscopy examination of galanin-like peptide (GALP)-containing neurons in the rat hypothalamus

Galanin-like peptide (GALP) is a novel peptide which is isolated from the porcine hypothalamus. GALP-containing neurons are present in the arcuate nucleus (ARC), being particularly densely concentrated in medial posterior regions. To observe the ultrastructure and synaptic relationships of GALP-containing neurons in the ARC, light and immunoelectron microscopy techniques were used. At the light microscope level, GALP-containing neurons were observed distributed rostrocaudally throughout the ARC, with the majority present in the posterior, periventricular zones. At the electron microscope level, many immunopositive dense-cored vesicles were evident in the perikarya, dendrites and axon terminals of the GALP-containing neurons. Furthermore, these neurons received synapses from immunonegative axon terminals that were symmetric in the case of synapses made on perikarya, and both asymmetric and symmetric for synapses made on dendrites. Axon terminals of GALP-containing neurons often made synapses on immunonegative dendrites. Such synapses were all symmetric. Synapses were also found between axon terminals and perikarya as well as dendrites of GALP-containing neurons. These findings suggest that the physiological role of the GALP-containing neurons in the ARC is based on complex synaptic relationships between GALP-containing neurons and either GALP-immunopositive or -immunonegative neurons.     

Emerging methodologies for the study of hypothalamic gonadotropin-releasing-hormone (GnRH) neurons

Gonadotropin-releasing-hormone (GnRH) neurons form part of a central neural oscillator that controls sexual reproduction through intermittent release of the GnRH peptide. Activity of GnRH neurons, and by extension release of GnRH, has been proposed to reflect intrinsic properties and synaptic input of GnRH neurons. To study GnRH neurons, we used traditional electrophysiology and computational methods. These emerging methodologies enhance the elucidation of processing in GnRH neurons. We used dynamic current-clamping to understand how living GnRH somata process input from glutamate and GABA, two key neurotransmitters in the neuroendocrine hypothalamus. In order to study the impact of synaptic integration in dendrites and neuronal morphology, we have developed full-morphology models of GnRH neurons. Using dynamic clamping, we have demonstrated that small-amplitude glutamatergic currents can drive repetitive firing in GnRH neurons. Furthermore, application of simulated GABAergic synapses with a depolarized reversal potential have revealed two functional subpopulations of GnRH neurons: one population in which GABA chronically depolarizes membrane potential (without inducing action potentials) and a second population in which GABAergic excitation results in slow spiking. Finally, when AMPA-type and GABA-type simulated inputs are applied together, action potentials occur when the AMPA-type conductance occurs during the descending phase of GABAergic excitation and at the nadir of GABAergic inhibition. Compartmental computer models have shown that excitatory synapses at >300 microns from somtata are unable to drive spiking with purely passive dendrites. In models with active dendrites, distal synapses are more efficient at driving spiking than somatic inputs. We then used our models to extend the results from dynamic current clamping at GnRH somata to distribute synaptic inputs along the dendrite. We show that propagation delays for dendritic synapses alter synaptic integration in GnRH neurons by widening the temporal window of interaction for the generation of action potentials. Finally, we have shown that changes in dendrite morphology can modulate the output of GnRH neurons by altering the efficacy of action potential generation in response to after-depolarization potentials (ADPs). Taken together, the methodologies of dynamic current clamping and multi-compartmental modeling can make major contributions to the study of synaptic integration and structure-function relationships in hypothalamic GnRH neurons. Use of these methodological approaches will continue to provide keen insights leading to conceptual advances in our understanding of reproductive hormone secretion in normal and pathological physiology and open the door to understanding whether the mechanisms of pulsatile GnRH release are conserved across species.