Secretory activity of neurons and related electrical activity

The neuroendocrine and endocrine systems in insect - Historical perspective and overview

A complex cascade of events leads to the initiation and maintenance of a behavioral act in response to both internally and externally derived stimuli. These events are part of a transition of the animal into a new behavioral state, coordinated by chemicals that bias tissues and organs towards a new functional state of the animal. This form of integration is defined by the neuroendocrine (or neurosecretory) system and the endocrine system that release neurohormones or hormones, respectively. Here we describe the classical neuroendocrine and endocrine systems in insects to provide an historic perspective and overview of how neurohormones and hormones support plasticity in behavioral expression. Additionally, we describe peripheral tissues such as the midgut, epitracheal glands, and ovaries, which, whilst not necessarily being endocrine glands in the pure sense of the term, do produce and release hormones, thereby providing even more flexibility for inter-organ communication and regulation.

Bag cell control of egg laying in freely behaving aplysia

Neuroendocrine (bag cell) control of egg laying was studied in freely behaving Aplysia. Surgical lesions showed that bag cells are not necessary for egg laying, although they play a crucial role in its control, and that the pleurovisceral connectives are the afferent pathway to the bag cells. Recording in vivo showed that synchronous bag cell spikes progressively invade the network, leading to prolonged repetitive firing that initiates natural egg laying.

Peptidergic modulation of insect voltage-gated Ca(2+) currents: role of resting Ca(2+) current and protein kinases A and C

The modulation of voltage-gated Ca(2+) currents in isolated dorsal unpaired median (DUM) neurons of cockroach was investigated using whole cell patch clamp. The neuropeptide neurohormone D (NHD), a member of the adipokinetic hormone family, affected Ca(2+) currents at pico- to nanomolar concentrations. It strongly enhanced currents activating at lower depolarizations, whereas those activating at strong depolarizations were slightly attenuated. The first effect results from upregulation of a previously characterized omega-conotoxin MVIIC- and omega-agatoxin IVA-sensitive "mid/low voltage-activated" (M-LVA) Ca(2+) current. The cAMP-analogue 8-bromo-cAMP, forskolin, and the catalytic subunit of protein kinase A (PKA) mimicked the stimulating action of NHD. In addition, preincubation of neurons with the PKA inhibitor KT 5720 abolished the action of NHD. Thus NHD seems to upregulate the M-LVA current via channel phosphorylation by PKA. Activation of protein kinase C by oleoylacetylglycerol (OAG) mimicked the effect of NHD, and subsequent NHD application only enhanced the current to a moderate extent. On the other hand, inhibition of protein kinase C (PKC) by Gö 6976 abolished the NHD effect. These results indicate that also PKC, too, may play a role in the peptidergic modulation of the M-LVA Ca(2+) current. The reduction of Ca(2+) currents in the high-voltage-range is caused by the NHD-induced upregulation of a voltage-independent Ca(2+) resting current, I(Ca,R), which most probably leads to enhanced Ca(2+)-dependent inactivation of voltage-gated Ca(2+) currents. To assess the major consequences of the Ca(2+) current changes, current-clamp investigations were performed. Experiments with iberiotoxin, a specific blocker of BK-type Ca(2+)-dependent K(+) currents, and the M-LVA current-blocking omega-toxins suggested that NHD causes-via increasing Ca(2+)-dependent K(+) currents-a larger hyperpolarization of action potentials. The lowering in the action potential threshold produced by NHD, however, seems to be a direct consequence of the hyperpolarizing shift of the activation curve of total Ca(2+) current resulting from NHD-induced upregulation of the M-LVA current component.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Peptidergic modulation of an insect Na(+) current: role of protein kinase A and protein kinase C

The modulation of voltage-gated Na(+) currents in isolated somata of dorsal unpaired median (DUM) neurons of the cockroach Periplaneta americana was investigated using the patch-clamp technique. The neuropeptide Neurohormone D (NHD), which belongs to the family of adipokinetic hormones, reversibly reduced the Na(+) current in concentration-dependent manner (1 pM to 10 nM). At 10 nM, NHD caused an attenuation of the maximum of current-voltage (I-V) relation for peak currents by 23 +/- 6%. An analysis of NHD action on current kinetics in terms of the Hodgkin-Huxley formalism revealed that NHD reduces the time constant of inactivation, whereas steady-state activation and inactivation as well as the time constant of activation were not affected. In addition, NHD prolonged the recovery from inactivation. The cAMP analogue 8-bromo-cAMP, forskolin, and the catalytic subunit of protein kinase A mimicked the action of NHD. Furthermore, preincubation of cells with the protein kinase A inhibitor KT 5720 abolished the action of NHD. Thus NHD seems to modify the Na(+) current via channel phosphorylation by protein kinase A. Activation of protein kinase C by oleoylacetylglycerol (OAG) also reduced the Na(+) current, but it did not occlude the action of NHD. On the other hand, inhibition of protein kinase C by chelerythrine or Gö 6976 did not essentially impair the NHD effects.

Dynamic neuronal-glial interactions in hypothalamus and pituitary: implications for control of hormone synthesis and release

Various lines of evidence have suggested that astrocytes play a dynamic role in control of hormone synthesis and release from the CNS. The model system most studied has been the rat hypothalamo-neurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal terminals. Neurons of this system manufacture and secrete oxytocin and vasopressin. Electron microscopic studies have shown that certain physiological conditions (e.g., dehydration, lactation) produce increases in direct apposition among these neurosecretory cells, an effect due to withdrawal of glial processes from between the neurons. Neurohypophysial astrocytes (pituicytes) show dynamic interactions with the neurons at the level of the terminals, by engulfing them and interposing processes between the terminals and the basement membrane when hormone demand is low. Pituicyte processes retract from both areas when hormone demand is high, allowing the neuronal terminals direct access to the perivascular space. Recently, osmotic manipulations (in the physiological range) have shown that these changes can be produced in vitro in neurohypophysial explants without stimulated hormone release. Experiments on cultured adult rat pituicytes have revealed similar morphological changes in response to noradrenaline. These changes were reversed or blocked by propranolol. The increase in direct soma-somatic apposition (7-9 nm separation) of magnocellular neurons could produce a tonic rise in (K+)o which would increase protein synthesis and contribute to the raised excitability of these neurons. Also, the removal of interposed glia could allow the formation of gap junctions and specialised synapses which are known to occur between these neurons. These in turn may participate in producing the coordinated firing that maximizes hormone release. The interactions of pituicytes with the terminals in the neurohypophysis suggests that these astrocytes are also a part of the mechanism of control of hormone release.

Retinal illumination produces synaptic inhibition of a neurosecretory organ in the crayfish, Pacifastacus leniusculus (Dana)

We have identified a cluster of neurosecretory cells in the crayfish eyestalk that possess dendrites in the second optic neuropil (Medulla) and project axons to the first optic neuropil (Lamina). Illumination of the ipsilateral retina produces a synaptic inhibition of these cells that is mimicked by iontophoresis of gamma-aminobutyric acid within the medullary neuropil. The neurosecretory nature of the cells, the efferent projection of their axons, and the strong inhibition of their spiking activity upon retinal illumination suggest that they may be involved in the feedback control of dark adaptation and/or circadian changes in visual sensitivity.

Serial section reconstruction of the neural poles of hair cells in the human organ of Corti. I. Inner hair cells

Study of the anatomy of the cochlea, and in particular the morphology of synaptic relationships between hair cells and cochlear neurons, is essential for elucidation of the mechanisms of transduction of mechanical acoustic signals into electrical neural events. Because considerable gaps remain in our understanding of the microscopic anatomy of these synapses, particularly in the human, a reconstruction of neural pole of inner hair cells of the human organ of Corti was performed. The data are based on 526 serial sections from the basal turn (10 mm region) and 356 serial sections from the middle turn (26 mm region). This provided complete data on 3 and partial data on 5 inner hair cells. Afferent terminals on inner hair cells were variable in size, ranging 1 to 20 micrometers in diameter. Branching of large fibers to produce multiple terminals innervating from 1 to 3 inner hair cells was common. Each inner hair cell received approximately 6 to 8 different nerve terminals. In addition, each terminal possessed a variable number of synaptic contacts. Junctional membrane specialization consisted of synapses, desmosomes, coated vesicles and arrays of microtubules and membrane cisternae. Specialization at synapses consisted of asymmetrical membrane thickening. At inner hair cells the postsynaptic membrane was thicker than the presynaptic membrane. Eighty-three percent of synapses had presynaptic bodies. Vesiculated efferent terminals synapsed on afferent fibers at the base of inner hair cells, but never directly on the inner hair cell. These anatomical data demonstrate distinct differences between the human and animal inner ear, which are important in the interpretation of neurophysiological data in animals and the formulation of hypotheses that involve assumptions crossing species.

Ultrastructural identification of neurohaemal sites in a tick: evidence that the dorsal complex may be a true endocrine gland

A fine structural study has been made of the 'paraganglionic plates' associated with the perineurium and of the 'retrocerebral organ' associated with the periganglionic sheath of the tick Boophilus microplus; these structures have been postulated, from descriptions by light microscopy, to be tick neurohaemal organs. Neurosecretory terminals are observed frequently in the neural lamella/perineurial sheath, particularly in a dorso-lateral area which may correspond to the 'paraganglionic plates'. No evidence was found of a discrete peripheral neurohaemal organ such as the corpus cardiacum of insects. The 'retrocerebral organ' is comprised of periganglionic sheath cells, which appear to be glandular rather than neurohaemal, and peripheral ganglionic cells.

Na+- and Ca2+-dependent components in action potentials of the ovulation hormone producing caudo-dorsal cells in Lymnaea stagnalis (Gastropoda)

Action potentials in the afterdischarge of the ovulation hormone producing caudo-dorsal cells (CDC) of Lymnaea stagnalis are strikingly different from electrically evoked spikes in the silent resting and inhibited states of these cells. Spikes evoked in the silent states consist of one fast peak (80-100 mV; 10-15 ms). The overshoot in Na+- and Ca2+-dependent. Spikes are blocked in Na+-free saline and by TTX. Repolarization is retarded by TEA. Co2+ increases the overshoot. Active state action potentials (60-80 mV) last up to 125 ms, due to activation of a slow component following the TTX-sensitive spike. The slow component is Na+- and Ca2+-dependent. In normal saline it is blocked by Co2+ and La3+. In Ca2+-free saline the remaining part of the slow component is blocked by La3+ only. The slow component is voltage-dependent in a graded fashion. Activation is bound to the active state in which the CDC are depolarized by 20 mV. TEA and Ca2+-free saline greatly increase spike duration in the active state. This suggests that, in addition to the classical TEA-sensitive channel, a Ca2+-dependent K+ channel is involved in repolarization of active state action potentials. The underlying membrane properties and the functional significance are discussed in relation to the pacemaking mechanism of the CDC.

Presence of T-bars, intramembranous particle arrays and exocytotic profiles in neuroendocrine terminals of an insect

Definitive evidence is presented to show that arthropod neurohaemal terminals contain electron-dense T-bar structures with clustered microvesicles similar to those present in neuropilar and neuromuscular terminals. In terminal membranes of the locus corpus cardiacum, studied by freeze-fracture, intramembranous particle arrays, considered to correlate with the dense bars, are seen. However, there does not appear to be a spatial association between the arrays and the exocytotic profiles seen following exposure to stimulants for hormone release. The presence of the densities in both neuroendocrine and conventional terminals is discussed in the light of current theories for mechanisms of release of neurotransmitters and neurohormones in arthropods and vertebrates.

Synaptic inputs and action potentials of magnocellular neuropeptidergic cells: intracellular recording and staining in slices of rat hypothalamus

Excitatory postsynaptic potentials (EPSPs) and action potentials of magnocellular neuropeptidergic cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) were studied with intracellular recording in coronal slices of rat hypothalamus. The fluorescent dye Lucifer Yellow (LY) was injected intracellularly and the cells were subsequently identified as magnocellular (somata greater than 15 x 15 micrometer). These cells generally had a large cytoplasm-to-nucleus ratio. In PVN it was frequently possible to trace filled dendrites to the ependyma of the third ventricle, and occasionally dendritic spines could be seen. Electrical stimuli in areas dorsolateral and ventrolateral to the fornix column evoked EPSPs in some anatomically identified MNCs of PVN, which indicates that presynaptic fibers innervating MNCs approach PVN from this region. Short-latency (less than 1 msec) spikes could be evoked in many MNCs of PVN by stimulation near SON, which is consistent with the known projection to the neurohypophysis of many MNCs. Action potentials in MNCs of PVN and SON had significantly longer durations at one-third spike height (mean +/- S.D. = 2.06 +/- 0.6 msec) than hippocampal CA1 pyramidal cells (1.17 +/- 0.29 msec). This suggests that neuroendocrine cells in mammals and some lower vertebrates and invertebrates are similar in this regard.

A light and electron microscopic investigation of the neurosecretory bag cells of Aplysia

The two bilateral clusters of neurosecretory bag cells of Aplysia were studied with both light and electron microscopy. Autoradiography revealed that the bag cells rapidly accumulate 3H-labelled amino acids and that after 1-2 h, heavy concentrations of silver grains appear over Golgi complexes and in the proximal axons. Intrasomatic injections of CoCl2 or lucifer yellow showed clear branch points and numerous varicosities along individual axons. Many of the bag cells are multipolar. Electron-microscopic observations confirmed that individual fibres branch and showed that the varicosities are packed with dense-cored vesicles similar in size (180 nm diameter) and electron density to those found in the somata. The axons of several cells are usually associated into bundles that travel (within the connective tissue sheath) either rostrally up the pleurovisceral connective or toward the contralateral bag cell cluster. Bundled in groups of tens to hundreds, a total of many thousands of axons fill the sheath around each cell cluster and around the proximal 2-5 mm of the pleurovisceral connective; the number of axon bundles in the sheath decreases rapidly with distance from the cluster. Individual axons reaching the outer edges of bundles from neurosecretory endings near blood sinuses in the sheath, creating an extensive neurohemal release area. Dense-cored vesicles are packed into the endings, often in very close apposition to the plasma membrane. Possible release profiles (omega-shaped) and smaller clear vesicles (85 nm diameter) were observed in the axon endings. A number of axons also enter and travel among the conventional (non-neurosecretory) axons in the core of the pleurovisceral connective nerve. These 'core' bag cell axons project for several millimetres beyond the terminations of the bundled axons of the sheath. The findings support the hypothesis proposed in physiological studies that the distribution and branching of the axonal tree are the basis for the extracellularly recorded wave forms and of the potentiation of electrical signals during bag-cell activity. Additional evidence indicates that exocytosis is the means by which bag-cell hormone is released during afterdischarges.

Intracellular recordings from the paraventricular nucleus in slices of rat hypothalamus

1. The electrical activity of thirty-five neurones in the lateral area of the paraventricular nucleus (p.v.n.) was recorded intracellularly in vitro from slices of rat hypothalamus. 2. Spontaneously occurring action potentials were observed in twenty-four of the neurones. The temporal pattern of action potentials was generally slow and irregular; occasionally some cells fired bursts of action potentials. 3. Depolarizations with a fast rising phase and slow decay occurred spontaneously in most cells. These depolarizations exhibited a wide range of amplitudes in each cell (up to 33 mV), showed temporal summation, and could serve as pre-potentials for spontaneously occurring action potentials. Presumably, these depolarizations were excitatory post-synaptic potentials (e.p.s.p.s.). 4. Depolarizing current injection could evoke action potentials. Extracellular stimuli dorsolateral to the fornix column occasionally elicited action potentials which had a short and invariant latency and which could respond to stimulation rates of 100 Hz. In some cases, extracellular stimuli in the same area evoked depolarizations which had long and variable latency and were similar to those occurring spontaneously. These two types of responses probably represent antidromic and orthodromic activation respectively. 5. Intracellular injections of horseradish peroxidase suggest that these recordings were obtained primarily, but not exclusively, from magnocellular neuroendocrine cells. This is consistent with previous anatomical studies on the location of magnocellular elements in p.v.n.