Accessory Neurons Research Articles

Diversity of central oxytocinergic projections.

Localization and distribution of hypothalamic neurons expressing the nonapeptide oxytocin has been extensively studied. Their projections to the neurohypophyseal system release oxytocin into the systemic circulation thus controlling endocrine events associated with reproduction in males and females. Oxytocinergic neurons seem to be confined to the ventral hypothalamus in all mammals. Groups of such cells located outside the supraoptic and the paraventricular nuclei are summarized as "accessory neurons." Although evolutionary probably associated with the classical magocellular nuclei, accessory oxytocin neurons seem to consist of rather heterogenous groups: Periventricular oxytocin neurons may gain contact to the third ventricle to secrete the peptide into the cerebrospinal fluid. Perivascular neurons may be involved in control of cerebral blood flow. They may also gain access to the portal circulation of the anterior pituitary lobe. Central projections of oxytocinergic neurons extend to portions of the limbic system, to the mesencephalon and to the brain stem. Such projections have been associated with control of behaviors, central stress response as well as motor and vegetative functions. Activity of the different oxytocinergic systems seems to be malleable to functional status, strongly influenced by systemic levels of steroid hormones.

Voltage-gated Ca2+ channels in accessory lobe neurons of the chick

Birds have ten pairs of protrusions, "accessory lobes", on the lateral sides of the lumbosacral spinal cord. It has been proposed that accessory lobes act as a sensory organ of equilibrium and neurons in accessory lobes transmit sensory information to the motor center. We have reported that cells in chick accessory lobes express functional voltage-gated Na(+) and K(+) channels and generate action potentials. In this study, we examined properties of voltage-gated Ca(2+) channels (VGCCs). The amplitude of voltage-gated Ca(2+) channel currents carried by Ca(2+) and Ba(2+) increased gradually during 10 min rather than showing the usual run-down. The current-voltage relationship of Ba(2+) currents was consistent with that of the high-voltage-activated Ca(2+) channel. The proportion of Ba(2+) currents inhibited by ω-conotoxin GVIA was larger than 80%, indicating that the major subtype is N type. Amplitudes of tail currents of Ca(2+) currents evoked by repetitive pulses at 50 Hz are stable for 1 s. If the major subtype of VGCCs at synaptic terminals is also N type, this property may contribute to the establishment of stable synaptic connections between accessory lobe neurons, which are reported to fire at frequencies higher than 15 Hz, and postsynaptic neurons in the spinal cord.

Glutamate evokes firing through activation of kainate receptors in chick accessory lobe neurons

Ten pairs of protrusions, called accessory lobes (ALs), exist at the lateral sides of avian lumbosacral spinal cords. Histological and behavioral evidence suggests that neurons are present in ALs and the AL acts as a sensory organ of equilibrium during walking. Neurons in the outer layer of the AL consistently show glutamate-like immunoreactivity and neurons in the central region of the AL show glutamate receptor-like immunoreactivity. However, it is unknown how glutamate acts on the functional activity of AL neurons. In this study, we examined the effects of glutamate on the electrical activities of AL neurons using the patch clamp technique. There are two types of neurons among isolated AL neurons: spontaneously firing and silent neurons. Among silent neurons, 42% of neurons responded to glutamate and generated repetitive firing. Kainate and glutamate in combination with the NMDA receptor antagonist, MK-801, also induced firing and evoked an inward current. On the other hand, the application of AMPA, NMDA or glutamate in combination with the non-NMDA receptor antagonist, CNQX, did not. These results indicate that chick AL neurons express functional kainate receptors to respond to glutamate and suggest that the glutamatergic transmission plays a role in excitatory regulation of AL neurons of the chick.

Segmental and regional differences in neuronal expression of the leech Hox genes Lox1 and Lox2 during embryogenesis.

Using double immunofluorescence experiments, we described the expression of the leech Hox genes, Lox1 and Lox2 by central neurons that stained for either serotonin or the leech-specific neuronal marker, Laz1-1. The goal is to determine whether the segmental boundaries of Lox1 and Lox2 expression in identified neurons coincide with segmental and regional differences in the differentiation of these cells. A number of neurons described here have been previously identified. The anteromedial serotonergic neurons are restricted to rostral ganglion 1 (R1) to midbody ganglion 3 (M3), but only express Lox1 in M2 and M3. The posteromedial serotonergic neurons which are situated in all segments as bilateral pairs early in development, but later become unpaired starting at M3, expressed Lox1 only in M2 and M3, and Lox2 in M8 to M21, in all paired and unpaired stages. The Retzius neurons, which stain for serotonin, express Lox2 in M7 to M21 where they exhibit different morphologies from their segmental homologs of the sex ganglia in M5 and M6. The Laz1-1 immunoreactive (Laz1-1+) heart accessory-like neurons express Lox1 in M4 and Lox2 in M7 to M17, but not in their segmental homologs of the heart accessory (HA) neurons located exclusively in M5 and M6. Also, Laz1-1+ neurons, which we named Lz3 expressed Lox1 in M4 to M8 where they are unpaired, but express Lox2 in M9 to M16 where they are bilaterally paired. Other Laz1-1 cells show more restricted and isolated Lox1 and Lox2 expression patterns. These results suggest a role of Lox1 and/or Lox2 in defining the anteroposterior boundaries of segmentally iterated neurons.

Analysis of GABA-induced inhibition of spontaneous firing in chick accessory lobe neurons

It has been hypothesized that chick accessory lobes (ALs) contain functional neurons and act as a sensory organ of equilibrium. It was reported that neurons located in an outer layer of ALs showed γ-aminobutyric acid (GABA)- and glutamic acid decarboxylase (GAD)-like immunoreactivity more strongly than centrally located neurons, which were surrounded by the GAD-immunoreactive terminals. We investigated effects of GABA on the electrical activity of AL neurons. About 50% of embryonic AL neurons exhibited spontaneous firing. In the on-cell recording, GABA, muscimol, and GABA in combination with CGP35348 inhibited this firing. In whole-cell voltage clamp recordings, GABA and muscimol evoked a transient current. The mean reversal potential of GABA-evoked currents was close to the theoretical reversal potential of Cl⁻. These results indicate that GABA exerts the inhibitory effect on the firing through the activation of GABA(A) receptors. In addition, the intracellular concentration of Cl⁻ was estimated to be about 16 mM in measurements with the gramicidin-perforated configuration, indicating the physiological reversal potential of the GABA current was about -60 mV. In conclusion, AL neurons have an intrinsic mechanism to evoke the spontaneous firing, which can be arrested by the inhibitory mechanism through the activation of the GABA(A) receptors.

Zolpidem, a selective GABA A receptor α 1 subunit agonist, induces comparable Fos expression in oxytocinergic neurons of the hypothalamic paraventricular and accessory but not supraoptic nuclei in the rat

Functional activation of oxytocinergic (OXY) cells in the hypothalamic paraventricular (PVN), supraoptic (SON), and accessory (ACC) nuclei was investigated in response to acute treatment with Zolpidem (a GABA A receptor agonist with selectivity for α 1 subunits) utilizing dual Fos/OXY immunohistochemistry. Zolpidem was administered intraperitoneally in dose 10 mg/kg of BW and 60 min later the animals were sacrificed by transcardial perfusion with fixative. The Fos/OXY co-labelings were analyzed on 40 μm thick serial coronal sections using computerized light microscopy. Zolpidem elicited a concordant Fos/OXY staining in all four PVN sub-areas investigated, including the anterior (15.71 ± 2.35%), middle (14.52 ± 2.53%), dorsal (13.34 ± 2.61%), and periventricular (18.21 ± 4.75%) ones, however, had no significant stimulatory effect on OXY cells in the SON. In response to Zolpidem, statistically significant activations were also seen in certain groups of accessory structures including the circular nucleus (13.99 ± 3.43%), small clusters of accessory neurons (10.55 ± 1.94%), and the lateral hypothalamic perivascular nucleus (9.42 ± 2.74%). Between the naive and vehicle controls, the dual Fos/OXY labelings did not elicit any significant differences. Our data provide insight into the topographic patterns of brain activity within the clusters of magnocellular OXY cells in the hypothalamus associated with stimulation of GABA A benzodiazepine receptors and for the first time illustrate the triggering contemporaneousness within the cells of the principal and accessory magnocellular nuclei in response to Zolpidem treatment. The present study provides a comparative background that may help in the further understanding of a possible extend of Zolpidem effect on the brain.

Peripheral targets of centrally located putative accessory neurons of MRO in the isopod Ligia exotica.

The three centrally located putative accessory neurons of the muscle receptor organ (MRO) of the isopod Ligia exotica were identified to the third segmental nerve (N3) of the thoracic ganglion by backfilling with Lucifer Yellow. These neurons were then studied intracellularly and extracellularly to determine whether they suppressed the stretch-activated responses of thoracic stretch receptors. Intracellular injection of depolarizing currents into these three putative accessory neurons revealed that only neuron #3 had an inhibitory effect, suggesting that it is an inhibitory accessory neuron related to thoracic stretch receptors. We searched for the peripheral targets of neurons #1 and #2 by intracellular filling with Lucifer Yellow or by recording of junctional potentials in extensor muscles, and show that they are motor neurons that innervate the deep extensor and superficial extensor muscles, respectively.

Architectonics of crayfish ganglia.

The central nervous system of crayfish consists of a chain of segmental ganglia that are linked by cables of intersegmental axons. Each ganglion contains a highly-ordered core of longitudinal tracts, vertical tracts, commissures, and synaptic neuropils. We review from a technical perspective the history of the description of these ganglia, and recognize four episodes of progress. Each major innovation in anatomical methods has led to new insight into the structure and function of this nervous system, and new awareness of the structural patterns that are common to the CNS of all arthropods. Ganglia in different segments of the body differ in size, and appear to differ in anatomy. From a comparison of the structures of the cores of abdominal, thoracic, and subesophageal ganglia, we argue that this apparent difference is illusory. Rather, each of these ganglia is organized on the same plan, a plan also found in insect segmental ganglia. The apparent differences follow from longitudinal compression during development and from allometric growth of particular neuropils associated with innervation of the walking legs. Different authors have described the internal organization of ganglia in different segments, so we provide a cross-reference to the nomenclatures they have introduced. We compare the locations of cell bodies of motor neurons and accessory neurons that innervate different peripheral structures, and demonstrate double-labeling of certain GABAergic peripheral inhibitory neurons. Finally, we describe the construction of digital movies of serial sections of these ganglia, and discuss their utility.

Segmental differentiation in the leech central nervous system: Proposed segmental homologs of the heart accessory neurons

As part of an on-going study of segmental differentiation in the central nervous system (CNS) of the leech Hirudo medicinalis, a search was made for putative segmental homologs of the heart accessory (HA) neurons, which exist exclusively as a bilateral pair in the ganglia of the fifth and sixth body segments. As it is not yet feasible to obtain adequate cell lineage information in H. medicinalis, potential homologs of the HA neurons were determined using morphological, immunohistochemical, and electrophysiological criteria. Among cells in other body ganglia with somata in the same locations as HA neurons, a pair was found having extensive morphological and physiological similarities to HA neurons. These we have called HA-like (HAL) neurons. Adult HA and HAL neurons have closely related patterns of primary branching, in terms of shape, intraganglionic pathways taken, and extraganglionic projections. The number, location, and relative thickness of branches are also similar among these cells. In embryos 10 to 11 days old, HA and HAL neurons have virtually identical branching patterns, with primary and secondary branches of nearly uniform caliber. Differences in branch thickness develop gradually; by embryonic day 20, they resemble those found in adult neurons. Two features found to differ between HA and HAL neurons were the cell body diameter (larger for the HA cells) and the expression of antigens recognized by the monoclonal antibody Laz1-1 (absent at a detectable level in the HA neurons). At a physiological level, the HA and HAL neurons showed action potentials of similar size and shape, as well as inhibitory synaptic inputs from a common source, the heart interneurons (HN). The observations presented here suggest that there is a common developmental origin for the HA and HAL neurons, and hence that their fates are positionally determined by as yet unknown factors.

Peptide immunoreactivity of the ciliary ganglion and its accessory cells in the rat

By means of immunohistochemistry, calcitonin gene-related peptide, Leu-enkephalin and neuropeptide Y localize to rat ciliary and accessory ganglion cells. The proportion of ciliary and accessory neurons immunoreactive to each peptide is provided and compared to previous data for vasoactive intestinal peptide. These findings indicate considerable neurochemical complexity for a parasympathetic ganglion with a small cell population.

Extension and retraction of axonal projections by some developing neurons in the leech depends upon the existence of neighboring homologues. II. The AP and AE neurons.

To assess the generality of our previous finding (Gao and Macagno, 1987) that segmental homologues play a role in the establishment of the pattern of axonal projections of the heart accessory HA neurons, we have extended our studies to two other identified leech neurons: the anterior pagoda (AP) neurons and the annulus erector (AE) motor neurons. Bilateral pairs of AP neurons are found in the first through the twentieth segmental ganglia (SG1 through SG20) of the leech ventral nerve cord. All AP neurons initially extend axonal projections to the contralateral periphery as well as longitudinal projections along the contralateral interganglionic connective nerves toward anterior and posterior neighboring ganglia. Although the peripheral projections are maintained by all AP neurons throughout the life of the animal, the longitudinal projections disappear in all but two segments: the AP neurons in SG1 maintain their anterior projections and extend them into the head ganglion, and those in SG20 maintain their posterior projections and extend them into SG21 and the tail ganglion. When single AP neurons are deleted anywhere along the nerve cord before processes begin to atrophy, however, the longitudinal projections are retained by their ipsilateral homologues in adjacent ganglia. The rescued processes appear to take over the projections of the deleted neurons. In cases where two or more AP neurons on the same side of the nerve cord are deleted from adjacent ganglia, a contralateral homologue sometimes extends projections to the periphery ipsilaterally or on both sides. We obtained similar results when we deleted single AE neurons from midbody ganglia. Thus, our experiments with three different identified neurons consistently show that the initial pattern of projections is the same in all ganglia, but that the existence of homologues in adjacent ganglia leads to the pruning of some of the initial projections. A consequence of this homologue-dependent process retraction is that neurons normally lacking neighboring homologues will have patterns of projections different from those neurons that do have such neighbors. Process loss by the HA, AP, and AE neurons may be the result either of competition for targets, inputs, or growth factors or of direct interactions among homologous cells.

Extension and retraction of axonal projections by some developing neurons in the leech depends upon the existence of neighboring homologues. I. The HA cells.

The role of homologues in the establishment of the pattern of axonal projections of identified segmentally homologous neurons was investigated by means of selective cell ablation and dye injection. The cells studied were the bilateral pairs of heart accessory (HA) neurons found in the fifth and sixth segmental ganglia of the leech ventral nerve cord. Homologues start their morphological differentiation with identical axonal projections, and segmental differences are manifested later, when specific branches stop growing and disappear. The deletion of single HA cells at early stages, however, permits these branches to survive in their ipsilateral homologues and to grow and take over the projections of the deleted neurons. In addition, if both HA homologues on the same side of the nerve cord, or three of the four HA cells, are deleted in an animal, the remaining HA cells often extend novel projections. These observations suggest that either competition for targets, inputs or growth factors, or direct interactions among homologous cells may play a role in the differentiation of segment specific patterns of axonal projections.

Cellular and ultrastructural features of the adult and the embryonic eye in the marine gastropod, Ilyanassa obsoleta.

Light and electron microscopic techniques show that the eye of the marine prosobranch gastropod, Ilyanassa obsoleta, is composed of an optic cavity, lens, cornea, retina, and neuropile, and is surrounded by a connective tissue capsule. The adult retina is a columnar epithelium containing three morphologically distinct cell types: photoreceptor, pigmented, and ciliated cells. The retina is continuous anteriorly with a cuboidal corneal epithelium. The neuropile, located immediately behind the retina, is composed of photoreceptor cell axons, accessory neurons, and their neurites. The embryonic eye is formed from surface ectoderm, which sinks inward as a pigmented cellular mass. At this time, the eye primordium already contains presumptive photoreceptor cells, pigmented retinal cells, and corneal cells. Several days later, just before hatching, the embryonic eye remains in intimate contact with the cerebral ganglion. It has no ciliated retinal cells, neuropile, optic nerve, or connective tissue capsule and its photoreceptor cells lack the electron-lucent vesicles and multivesicular bodies of adult photoreceptor cells. As the eye and the cerebral ganglion grow apart, the optic nerve, neuropile, and connective tissue capsule develop.

Neural control of the hearts in the leech,Hirudo medicinalis

1. Bilateral pairs of heart accessory neurons (HA cells) have been identified on the ventral surface of the fifth and sixth segmental ganglia ofHirudo medicinalis. These neurons are rhythmically active and fire in a metastable phase relation to the heartbeat central pattern generator and heart activity. 2. The HA neurons project into the contralateral roots of their own ganglion and innervate the contralateral heart. The HA neurons of the fifth ganglion (HA (5) cells) send axons into the contralateral anterior connectives and branch in several more anterior ganglia to form axons that project into the contralateral roots. The HA(6) neurons are similar but their intersegmental projection is directed posteriorly. 3. The HA neurons receive rhythmic inhibitory synaptic input directly from the neurons of the heartbeat central pattern generator (HN interneurons). The HA cells also receive excitatory synaptic input, which can be spontaneous or driven by discharge in N mechanosensory cells. 4. The HA neurons can induce myogenic activity in quiescent hearts and accelerate the rhythm of hearts that are already myogenic. The HA neurons can also increase the beat tension in hearts that are entrained by phasic excitatory drive from heart motor neurons (i.e. the activity of the heartbeat central pattern generator). These influences, are mediated by tonic activity in the HA neurons. 5. The HA neurons appear to modulate the myogenic properties and contractile strength of the hearts.

A simulation study of a neuron in a simple muscle control system

The mechanism underlying signal transmission through the accessory neuron of the muscle receptor organ in the crayfish abdomen was examined experimentally and by computer simulation. The mathematical model of the accessory neuron and its synaptic inputs was based, in part, on previous neuronal models studied by Stein (1965) and Rall (1964). The model predicted with considerable accuracy the behaviour of the experimental system in a number of situations. It was possible to determine in some detail the mechanisms underlying the frequency response characterized by a threshold and an approximately linearly increasing response for stimulus intensities above threshold. Furthermore, we were able to explain the observation that the accessory cell transforms a regular input train from a stretch receptor into an irregular output train. The highly branched structure of the dendritic tree, combined with a firing threshold in the dendrites which prevents action potentials from invading large parts of the dendritic tree, seems essential in determining these properties.

Summation of excitation and inhibition in the slowly adapting stretch receptor neuron of the crayfish

The slowly adapting stretch receptor of the crayfish is inhibited via the large accessory neuron both by reflex activation of this inhibitory interneuron from the stretch receptor itself (autogenic inhibition) and by activation of the interneuron from stretch receptors in other abdominal segments (neighbourinhibition). Neighbour-inhibition increases proportionally with the increase in impulse frequency in the large accessory neuron produced by activity in neighbouring receptors and largely independently of the level of excitation in the stretch receptor itself. A simple model based on intracellular recordings from the receptor neuron predicts this behaviour fairly accurately. In this model each receptor impulse is followed by an IPSP after a delay proportional to the uninhibited interspike interval of the receptor (autogenic inhibition). The other IPSP's arrive randomly distributed in time (neighbour-inhibition). An alternative model in which all IPSP's arrive randomly produces similar results. This latter model can be modified to fit other neuronal systems.

Analysis of postural motoneuron activity in crayfish abdomen. I. Coordination by premotoneuron connections

The activity of the crayfish abdominal postural motoneurons and their associated neurons (the accessory neuron(s) and the MRO(1)) were examined with the aid of techniques for the analysis of simultaneously recorded spike trains. A means of reliably identifying the spikes of the individual motoneurons based on their relative axon conduction velocities is presented. The analyses show that: 1) the large, phasically active synergist motoneurons innervating muscles producing the same movement show a marked similarity in their average responses, which is independent of the input source; 2) the small, tonically active and the middle-sized, tonicphasic synergist motoneurons innervating the same muscle and similar synergist motoneurons innervating antagonistic muscles are coordinated entirely by premotoneuron connections; 3) the accessory neuron is coordinated in its activity with the phasically active flexor excitor motoneurons and the extensor inhibitor motoneuron and thereby functions as a flexor synergist; and 4) the simultaneous presentation of flexion-producing and extension-producing inputs to the postural system results in a reciprocal oscillation in flexor-extensor motoneuron output. The functional significance of these results with respect to the operation of the postural system are discussed.

Analysis of postural motoneuron activity in crayfish abdomen. II. Coordination by excitatory and inhibitory connections between motoneurons

The identified spike trains of the crayfish abdominal postural efferent neurons were recorded simultaneously from one or more segments. The efferent activity was analyzed using cross-correlation histograms, peristimulus time scatter diagrams, and specialized antidromic techniques. The analyses show that the larger, phasically active motoneurons are coordinated in their activity by cross connections made at the motoneuron level. The cross connections are both excitatory and inhibitor in nature and result in significant alterations in spike output. Further, the accessory neuron receives an inhibitory cross connection from a middle-sized extensor excitor motoneuron or motoneurons. In each case, the cross connections appear to be appropriate to the function of the postural system.