Vasopressin-induced currents in rat neonatal spinal lateral horn neurons are G-protein mediated and involve two conductances

Activation of V1a vasopressin receptors excite subicular pyramidal neurons by activating TRPV1 and depressing GIRK channels

Arginine vasopressin (AVP) is a nonapeptide that serves as a neuromodulator in the brain and a hormone in the periphery that regulates water homeostasis and vasoconstriction. The subiculum is the major output region of the hippocampus and an integral component in the networks that processes sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whereas the subiculum expresses high densities of AVP-binding sites and AVP has been shown to increase the synaptic excitability of subicular pyramidal neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of V_1a receptors increased the excitability of subicular pyramidal neurons via activation of TRPV1 channels and depression of the GIRK channels. V_1a receptor-induced excitation of subicular pyramidal neurons required the function of phospholipase Cβ, but was independent of intracellular Ca²⁺ release. Protein kinase C was responsible for AVP-mediated depression of GIRK channels, whereas degradation of phosphatidylinositol 4,5-bisphosphate was involved in V_1a receptor-elicited activation of TRPV1 channels. Our results may provide one of the cellular and molecular mechanisms to explain the physiological functions of AVP in the brain.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

Expression of arginine vasotocin receptors in the developing zebrafish CNS

Vasotocin/vasopressin is a neuropeptide that regulates social and reproductive behaviors in a variety of animals including fish. Arginine vasotocin (AVT) is expressed by cells in the ventral hypothalamic and preoptic areas in the diencephalon during embryogenesis in zebrafish suggesting that vasotocin might mediate other functions within the CNS prior to the development of social and reproductive behaviors. In order to examine potential early roles for vasotocin we cloned two zebrafish vasotocin receptors homologous to AVPR1a. The receptors are expressed primarily in the CNS in similar but generally non-overlapping patterns. Both receptors are expressed in the forebrain, midbrain and hindbrain by larval stage. Of note, AVTR1a-expressing neurons in the hindbrain appear to be contacted by the axons of preoptic neurons in the forebrain that include avt+ neurons and sensory axons in the lateral longitudinal fasciculus (LLF). Furthermore, AVTR1a-expressing hindbrain neurons extend axons into the medial longitudinal fasciculus (MLF) that contains axons of many neurons thought to be involved in locomotor responses to sensory stimulation. One hypothesis consistent with this anatomy is that AVT signaling mediates or gates sensory input to motor circuits in the hindbrain and spinal cord.

Neuromodulation by oxytocin and vasopressin

Oxytocin (OT) and vasopressin (VP) are two closely related neuropeptides, widely known for their peripheral hormonal effects. Specific receptors have also been found in the brain, where their neuromodulatory actions have meanwhile been described in a large number of regions. Recently, it has become possible to study their endogenous neuropeptide release with the help of OT/VP promoter-driven expression of fluorescent proteins and light-activated ion channels. In this review, I summarize the neuromodulatory effects of OT and VP in different brain regions by grouping these into different behavioral systems, highlighting their concerted, and at times opposite, effects on different aspects of behavior.

Cardiac sympatho-excitatory action of PVN-spinal oxytocin neurones

A significant proportion of the spinally projecting neurones in the paraventricular nucleus are immunoreactive for oxytocin. Some of these oxytocin neurones terminate on sympathetic preganglionic neurones in the upper thoracic spinal cord, a region from which cardiac sympathetic neurones originate. No studies have so far identified a cardiac action of the supraspinal oxytocin neurones. The present study was designed to test the hypothesis that these oxytocin neurones excite spinal cardiac sympathetic neurones. This was done by measuring heart rate changes in response to intrathecal oxytocin and a selective agonist, and to stimulation of paraventricular neurones before and during blockade of spinal sites with selective antagonists. Rats were anaesthetised with chloralose and urethane (50 mg and 650 mg/kg) and recordings were made of heart rate and blood pressure. Drugs in a volume of 10 microl were applied to the upper thoracic spinal cord via a catheter placed intrathecally with its tip at T2. The paraventricular nucleus was explored with a glass micropipette, placed stereotaxically, and filled with d,l-homocysteic acid (DLH, 200 mM) for exciting neurones and pontamine sky blue for marking the position. Oxytocin (0.002 mM) applied to the spinal cord elicited increases in heart rate (26+/-5 beats per minute). This was mimicked by a highly selective oxytocin agonist. These heart rate increases were blocked selectively by two different oxytocin antagonists but unaffected by a V(1a) vasopressin antagonist. Excitation of sites in dorsal and medial parvocellular sub-nuclei of the paraventricular nucleus elicited increases in heart rate (36+/-3 bpm) which were significantly reduced by oxytocin antagonists but not affected by V(1a) antagonist. Also these induced increases in heart rate were unaffected by vagotomy or i.v. atropine but were abolished by i.v. esmolol. It is concluded that there is a population of paraventricular-spinal oxytocin neurones that excite cardiac sympathetic preganglionic neurones controlling heart rate.

Overview of cellular electrophysiological actions of vasopressin

The nonapeptide vasopressin acts both as a hormone and as a neurotransmitter/neuromodulator. As a hormone, its target organs include kidney, blood vessels, liver, platelets and anterior pituitary. As a neurotransmitter/neuromodulator, vasopressin plays a role in autonomic functions, such as cardiovascular regulation and temperature regulation and is involved in complex behavioral and cognitive functions, such as sexual behavior, pair-bond formation and social recognition. At the neuronal level, vasopressin acts by enhancing membrane excitability and by modulating synaptic transmission. The present review will focus on the electrophysiological effects of vasopressin at the cellular level. A large proportion of the experiments summarized here have been performed in in vitro systems, especially in brain and spinal cord slices of the rat. Vasopressin exerts a powerful excitatory action on motoneurons of young rats and mice. It acts by generating a cationic inward current and/or by reducing a potassium conductance. In addition, vasopressin enhances the inhibitory synaptic input to motoneurons. By virtue of these actions, vasopressin may regulate the functioning of neuronal networks involved in motor control. In the amygdala, vasopressin can directly excite a subpopulation of neurons, whereas oxytocin, a related neuropeptide, can indirectly inhibit these same neurons. In the lateral septum, vasopressin exerts a similar dual action: it excites directly a neuronal subpopulation, but causes indirect inhibition of virtually all lateral septal neurons. The actions of vasopressin in the amygdala and lateral septum may represent at least part of the neuronal substrate by which vasopressin influences fear and anxiety-related behavior and social recognition, respectively. Central vasopressin can modulate cardiovascular parameters by causing excitation of spinal sympathetic preganglionic neurons, by increasing the inhibitory input to cardiac parasympathetic neurons in the nucleus ambiguus, by depressing the excitatory input to parabrachial neurons, or by inhibiting glutamate release at solitary tract axon terminals. By acting in or near the hypothalamic supraoptic nucleus, vasopressin can influence magnocellular neuron activity, suggesting that the peptide may exert some control on its own release at neurohypophyseal axon terminals. The central actions of vasopressin are mainly mediated by receptors of the V(1A) type, although recent studies have also reported the presence of vasopressin V(1B) receptors in the brain. Major unsolved problems are: (i) what is the transduction pathway activated following stimulation of central vasopressin V(1A) receptors? (ii) What is the precise nature of the cation channels and/or potassium channels operated by vasopressin? (iii) Does vasopressin, by virtue of its second messenger(s), interfere with other neurotransmitter/neuromodulator systems? In recent years, information concerning the mechanism of action of vasopressin at the neuronal level and its possible role and function at the whole-animal level has been accumulating. Translation of peptide actions at the cellular level into autonomic, behavioral and cognitive effects requires an intermediate level of integration, i.e. the level of neuronal circuitry. Here, detailed information is lacking. Further progress will probably require the introduction of new techniques, such as targeted in vivo whole-cell recording, large-scale recordings from neuronal ensembles or in vivo imaging in small animals.

Cholecystokinin B-type receptors mediate a G-protein-dependent depolarizing action of sulphated cholecystokinin ocatapeptide (CCK-8s) on rodent neonatal spinal ventral horn neurons

Reports of cholecystokinin (CCK) binding and expression of CCK receptors in neonatal rodent spinal cord suggest that CCK may influence neuronal excitability. In patch-clamp recordings from 19/21 ventral horn motoneurons in neonatal (PN 5-12 days) rat spinal cord slices, we noted a slowly rising and prolonged membrane depolarization induced by bath-applied sulfated CCK octapeptide (CCK-8s; 1 microM), blockable by the CCK B receptor antagonist L-365,260 (1 microM). Responses to nonsulfated CCK-8 or CCK-4 were significantly weaker. Under voltage clamp (V H -65 mV), 22/24 motoneurons displayed a CCK-8s-induced tetrodotoxin-resistant inward current [peak: -136 +/- 28 pA] with a similar time course, mediated via reduction in a potassium conductance. In 29/31 unidentified neurons, CCK-8s induced a significantly smaller inward current (peak: -42.8 +/- 5.6 pA), and I-V plots revealed either membrane conductance decrease with net inward current reversal at 101.3 +/- 4.4 mV (n = 16), membrane conductance increase with net current reversing at 36.1 +/- 3.8 mV (n = 4), or parallel shift (n = 9). Intracellular GTP-gamma-S significantly prolonged the effect of CCK-8s (n = 6), whereas GDP-beta-S significantly reduced the CCK-8s response (n = 6). Peak inward currents were significantly reduced after 5-min perfusion with N-ethylmaleimide. In isolated neonatal mouse spinal cord preparations, CCK-8s (30-300 nM) increased the amplitude and discharge of spontaneous depolarizations recorded from lumbosacral ventral roots. These observations imply functional postsynaptic G-protein-coupled CCK B receptors are prevalent in neonatal rodent spinal cord.

Identified motoneurons involved in sexual and eliminative functions in the rat are powerfully excited by vasopressin and tachykinins

The pudendal motor system is constituted by striated muscles of the pelvic floor and the spinal motoneurons that innervate them. It plays a role in eliminative functions of the bladder and intestine and in sexual function. Pudendal motoneurons are located in the ventral horn of the caudal lumbar spinal cord and send their axon into the pudendal nerve. In the rat, binding sites for vasopressin and tachykinin are present in the dorsomedial and dorsolateral pudendal nuclei, suggesting that these neuropeptides may affect pudendal motoneurons. The aim of the present study was to investigate possible effects of vasopressin and tachykinins on these motoneurons. Recordings were performed in spinal cord slices of young male rats using the whole-cell patch-clamp technique. Before recording, motoneurons were identified by 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine, 4-chlorobenzenesulfonate retrograde labeling. The identification was confirmed, a posteriori, by choline acetyltransferase immunocytochemistry. Vasopressin and tachykinins caused a powerful excitation of pudendal motoneurons. The peptide-evoked depolarization, or the peptide-evoked inward current, persisted in the presence of tetrodotoxin, indicating that these effects were mainly postsynaptic. By using selective receptor agonists and antagonist, we determined that vasopressin acted via vasopressin 1a (V1a), but not V1b, V2, or oxytocin receptors, whereas tachykinins acted via neurokinin 1 (NK1), but not NK2 or NK3, receptors. Vasopressin acted by enhancing a nonselective cationic conductance; in some motoneurons, it also probably suppressed a resting K+ conductance. Our data show that vasopressin and tachykinins can excite pudendal motoneurons and thus influence the force of striated perineal muscles involved in eliminative and sexual functions.

Vasopressin inhibits glutamate release via two distinct modes in the brainstem

The hypothalamus coordinates autonomic responses in part through arginine vasopressin (AVP) released in medial nucleus tractus solitarius (NTS). However, the mechanisms and sites of AVP action within NTS pathways are uncertain. In brainstem slices, we activated solitary tract (ST) primary afferents to release glutamate and tested whether AVP modulated synaptic transmission to second-order neurons. NTS neurons were classified as second order by ST synaptic characteristics or the presence of anterograde tracers from peripheral baroreceptor afferents. Stimulus recruitment curves indicated ST-EPSCs on individual neurons were evoked by stimulation of single ST axons. Variance-mean (V-M) analysis of ST-EPSCs in individual neurons revealed uniformly high release probability (p approximately 0.9) from an average of 19 release sites (N) and a quantal size (q) of 34.0 +/- 4.7 pA. In 26 of 49 neurons, AVP inhibited afferent synaptic transmission. In most neurons, AVP reduced ST-EPSC amplitudes (n = 20) by decreasing p to 0.65, whereas q, N, and conduction times were unaffected. The V1a antagonist SR49059 alone decreased ST-EPSC V and increased M, suggesting tonic AVP actions, and blocked exogenous AVP action (n = 4). In other neurons with identical ST release properties, AVP induced synaptic failures and increased conduction time without altering the V-M relationship of successful ST-EPSCs (n = 6). Interestingly, frequency-depressed ST-EPSCs were not affected by AVP. AVP failed to alter holding or voltage-dependent potassium currents. Thus, AVP regulates NTS neurons by two distinct novel and state-dependent mechanisms: one, an analog, graded presynaptic inhibition of terminal glutamate release and the other, a binary, extraterminal block of conducted excitation.

Vasopressin induces depolarization and state-dependent firing patterns in rat thalamic paraventricular nucleus neurons in vitro

The thalamic midline paraventricular nucleus (PVT) is prominently innervated by vasopressin-immunoreactive neurons from the suprachiasmatic nucleus (SCN), site of the brain's biological clock. Using patch-clamp recordings in slice preparations taken from Wistar rats during the subjective day, we examined 90 PVT neurons for responses to bath-applied AVP (0.5–2 μM; 1–3 min). In current clamp at resting membrane potentials (−65 ± 1 mV), PVT neurons displayed low-threshold spikes (LTSs) and burst firing patterns. In 50% of cells tested, AVP induced a slowly rising, prolonged membrane depolarization and tonic firing, returning to burst firing upon recovery. AVP modulated hyperpolarization-activated LTSs by decreasing the time to the initial sodium spike at the onset of LTS, also increasing the duration of the afterdepolarization. Responses were blockable with a V1a receptor antagonist (Manning compound). Under voltage clamp, AVP induced a TTX-resistant, slowly rising, and prolonged (∼15 min) inward current (<40 pA). Current-voltage relationship ( I-V) analyses of the AVP responses revealed a decrease in membrane conductance to 73.1 ± 6.2% of control, with net AVP current reversing at −106 ± 4 mV, and decreased inward rectification at negative potentials. These observations are consistent with an AVP-induced closure of an inwardly rectifying potassium conductance. On the basis of these in vitro observations, we suggest that the SCN vasopressinergic innervation of PVT is excitatory in nature, possibly releasing AVP with circadian rhythmicity and contributing to state-dependent firing patterns in PVT neurons over the sleep-wake cycle.

Presynaptic Angiotensin II AT1 Receptors Enhance Inhibitory and Excitatory Synaptic Neurotransmission to Motoneurons and Other Ventral Horn Neurons in Neonatal Rat Spinal Cord

In neonatal spinal cord, we previously reported that exogenous angiotensin II (ANG II) acts at postsynaptic AT1 receptors to depolarize neonatal rat spinal ventral horn neurons in vitro. This study evaluated an associated increase in synaptic activity. Patch clamp recordings revealed that 38/81 thoracolumbar (T7–L5) motoneurons responded to bath applied ANG II (0.3–1 μM; 30 s) with a prolonged (5–10 min) and reversible increase in spontaneous postsynaptic activity, selectively blockable with Losartan ( n = 5) but not PD123319 ( n = 5). ANG-II-induced events included both spontaneous inhibitory (IPSCs; n = 6) and excitatory postsynaptic currents (EPSCs; n = 5). While most ANG induced events were tetrodotoxin-sensitive, ANG induced a significant tetrodotoxin-resistant increase in frequency but not amplitude of miniature IPSCs ( n = 7/13 cells) and EPSCs ( n = 2/7 cells). In 35/77 unidentified neurons, ANG II also induced a tetrodotoxin-sensitive and prolonged increase in their spontaneous synaptic activity that featured both IPSCs ( n = 5) and EPSCs ( n = 4) when tested in the presence of selective amino acid receptor antagonists. When tested in the presence of tetrodotoxin, ANG II was noted to induce a significant increase in the frequency but not the amplitude of mIPSCs ( n = 9) and mEPSCs ( n = 8). ANG also increased spontaneous motor activity from isolated mouse lumbar ventral rootlets. Collectively, these observations support the existence of a wide pre- and postsynaptic distribution of ANG II AT1 receptors in neonatal ventral spinal cord that are capable of influencing both inhibitory and excitatory neurotransmission.

Peptidergic activation of locomotor pattern generators in the neonatal spinal cord

The development of motor networks in the spinal cord is partly activity-dependent. We have observed receptor-mediated excitatory effects of two peptides, arginine vasopressin (AVP) and oxytocin (OXT), on motor network activity in the neonate. With the use of an en bloc in vitro preparation of mouse spinal cord (2-3 d old), which either was isolated completely or had muscles of the hindlimb left intact, we show that the bath application of AVP or OXT can evoke an increase in population bursting of motoneurons recorded from the lumbar ventral roots. By using antagonists for AVP and OXT, we found that these peptides were binding primarily to V1a and OXT receptors, respectively. Western blot analysis revealed a 48 kDa V1a and a 55 kDa OXT receptor immunoreactive band that was expressed in tissue obtained from L1-L6 sections of spinal cord. AVP, but not OXT, could, on occasion, evoke sustained periods of locomotor-like activity. In addition, when we applied AVP or OXT in combination with a 5-HT2 agonist, bouts of locomotor-like activity could be observed in a majority of preparations. Collectively, these data point to a novel role for AVP and OXT in the activation of spinal motor networks.

Developmental aspects of spinal locomotor function: insights from using the in vitro mouse spinal cord preparation

Over the last five years, rapid advances have been made in our understanding of the location, function, and recently, organization of the central pattern generator (CPG) for locomotion. In the mammal, the use of the neonatal rat has largely contributed to these advances. Additionally, the use of the in vitro mouse spinal cord preparation is becoming more common, catalysed in part by the potential for the use of genetic approaches to study locomotor function. Although tempting, it is necessary to resist the a priori assumption that the organization of the spinal CPG is identical in the rat and mouse. This review will describe the development of locomotor-like behaviour in the mouse from embryonic day 12 to postnatal day 14. While there are still many gaps in our knowledge, compared with the rat, the in vitro mouse appears to follow a qualitatively similar course of locomotor development. The emphasis in this review is the use or potential use of the mouse as a complement to existing data using the neonatal rat preparation.

Angiotensin AT(1)-receptors depolarize neonatal spinal motoneurons and other ventral horn neurons via two different conductances

Angiotensin receptors are highly expressed in neonatal spinal cord. To identify their influence on neuronal excitability, we used patch-clamp recordings in spinal cord slices to assess responses of neonatal rat (5-12 days) ventral horn neurons to bath-applied angiotensin II (ANG II; 1 microM). In 14/34 identified motoneurons tested under current clamp, ANG II induced a slowly rising and prolonged membrane depolarization, blockable with Losartan (n = 5) and (Sar(1), Val(5), Ala(8))-ANG II (Saralasin, n = 4) but not PD123319 (1 microM each; n = 4). Under voltage clamp (V(H) -65 mV), 7/22 motoneurons displayed an ANG-II-induced tetrodotoxin-resistant inward current (-128 +/- 31 pA) with a similar time course, an associated reduction in membrane conductance and net current reversal at -98.8 +/- 3.9 mV. Losartan-sensitive ANG II responses were also evoked in 27/78 tested ventral horn "interneurons." By contrast with motoneurons, their ANG-II-induced inward current was smaller (-39.9 +/- 5.2 pA) and analysis of their I-V plots revealed three patterns. In eight cells, membrane conductance decreased with net inward current reversing at -103.8 +/- 4.1 mV. In seven cells, membrane conductance increased with net current reversing at -37.9 +/- 3.6 mV. In 12 cells, I-V lines remained parallel with no reversal within the current range tested. Intracellular dialysis with GTP-gamma-S significantly prolonged the ANG II effect in seven responsive interneurons and GDP-beta-S significantly reduced the ANG II response in four other cells. Peak inward currents were significantly reduced in all 13 responding neurons recorded in slices incubated in pertussis toxin (5 microgram/ml) for 12-18 h or in 12 neurons perfused with N-ethylmaleimide. Of 29 interneurons sensitive to pertussis toxin or N-ethylmaleimide treatment, 9 cells displayed a decrease in membrane conductance that reversed at -101.3 +/- 3.8 mV. In eight cells, membrane conductance increased and reversed at -38.7 +/- 3.4 mV. In 12 cells, the I-V lines remained parallel with no reversal within the current range tested, suggesting that both conductances are modulated by pertussis toxin-sensitive G proteins. These observations reveal a direct, G-protein-mediated depolarizing action of ANG II on neonatal rat ventral horn neurons. They also imply involvement of two distinct conductances that are differentially distributed among different cell types.

Effects of vasopressin on isolated rat adrenal chromaffin cells

It has been demonstrated that arginine vasopressin (AVP) is synthesized not only in specific hypothalamic nuclei, but also in the adrenal medulla where it is thought to regulate adrenal functions by autocrine and paracrine mechanisms. In order to further characterise the effects of AVP on rat adrenal chromaffin cells, we examined: (a) the mRNA expression for V(1a) and V(1b) AVP receptors in these cells; (b) the effects of AVP on the membrane potential and membrane currents measured with the whole-cell patch-clamp technique; and (c) effect of AVP on catecholamine release from single adrenal chromaffin cells measured with carbon fibre microelectrodes. Reverse transcription-polymerase chain reaction (RT-PCR) on tissue punch samples obtained from the adrenal medulla demonstrated message for both the V(1a) and V(1b) receptors, while material obtained from the adrenal cortex showed expression of the V(1a) receptor only. Single-cell RT-PCR conducted on acutely isolated chromaffin cells showed message for the V(1a) receptor in 84% of cells, while 38% of cells also contained message for the V(1b) receptor (n=45). Under current-clamp recording, responses to AVP application (4-40 microM) were variable; 22/34 (65%) tested cells were depolarised, 29% hyperpolarised, and the remaining cells showed a biphasic response. Changes in membrane potential of either direction were dose-dependent and accompanied by a decrease in cell membrane resistance. Under voltage-clamp (V(hold)=-60 mV), AVP evoked inward current in 27/52 (52%) and outward current in 16/52 (31%) chromaffin cells. Both types of AVP-evoked responses were blocked by co-application of a nonselective V(1a)/V(1b) antagonist. Application of AVP evoked prolonged bursts of amperometric currents (indicative of catecholamine release) in 4/9 tested cells, but reduced the currents evoked by ACh application in all tested cells (n=7). These findings demonstrate a complex action of AVP on adrenal chromaffin cells, with individual adrenal chromaffin cells responding with either excitation or inhibition. This response pattern may be related to the expression of V(1) receptor subtypes.

Dark-field microscopy visualization of unstained axonal pathways using oil of wintergreen

Despite enormous progress in the development of new morphological techniques, there is still not a simple technique for visualization of the fiber architecture in the mammalian brain. To develop such a technique, thick (400-600 microm) sections of the rat, mice, calf or postmortal human brain were fixed in paraformaldehyde, dehydrated in a series of ethanol and finally immersed in methyl salicylate. The major principle of this newly developed method was to make the neural tissue transparent, and then utilize the ability of neuronal fibers to deflect and deviate light directed from the side to render them visible. Dark-field illumination was used to create illuminating rays of light arriving at an angle exceeding the collecting angle of the objective lens, thus causing only the axonal pathways to be visible as a bright silver silhouette against a dark background. As a result, a three-dimensional structure of the whole white matter of the brain slice became clearly viewable. This technique worked equally well for mammalian brain frontal, sagittal and horizontal sections, as well as for the spinal cord sections. The method was appropriate for verification of axonal fiber courses in brain slice preparations used in electrophysiological experiments, including special applications, such as visualization of axonal bundles within neural transplants. Due to its simplicity, the technique can be successfully used even in an amateur laboratory having basic microscopy equipment and reagents.

Electrophysiological evidence for vasopressin V(1) receptors on neonatal motoneurons, premotor and other ventral horn neurons

Prominent arginine-vasopressin (AVP) binding and AVP V(1) type receptors are expressed early in the developing rat spinal cord. We sought to characterize their influence on neural excitability by using patch-clamp techniques to record AVP-induced responses from a population of motoneurons and interneurons in neonatal (5-18 days) rat spinal cord slices. Data were obtained from 58 thoracolumbar (T(7)-L(5)) motoneurons and 166 local interneurons. A majority (>90%) of neurons responded to bath applied AVP (10 nM to 3 microM) and (Phe(2), Orn(8))-vasotocin, a V(1) receptor agonist, but not V(2) or oxytocin receptor agonists. In voltage-clamp, postsynaptic responses in motoneurons were characterized by slowly rising, prolonged (7-10 min) and tetrodotoxin-resistant inward currents associated with a 25% reduction in a membrane potassium conductance that reversed near -100 mV. In interneurons, net AVP-induced inward currents displayed three patterns: decreasing membrane conductance with reversal near -100 mV, i.e., similar to that in motoneurons (24 cells); increasing conductance with reversal near -40 mV (21 cells); small reduction in conductance with no reversal within the current range tested (41 cells). A presynaptic component recorded in most neurons was evident as an increase in the frequency but not amplitude (in motoneurons) of inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs), in large part due to AVP-induced firing in inhibitory (mainly glycinergic) and excitatory (glutamatergic) neurons synapsing on the recorded cells. An increase in frequency but not amplitude of miniature IPSCs and EPSCs also indicated an AVP enhancement of neurotransmitter release from axon terminals of inhibitory and excitatory interneurons. These observations provide support for a broad presynaptic and postsynaptic distribution of AVP V(1) type receptors and indicate that their activation can enhance the excitability of a majority of neurons in neonatal ventral spinal cord.

Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems

During the last two decades, it has become apparent that vasopressin and oxytocin, in addition to playing a role as peptide hormones, also act as neurotransmitters/neuromodulators. A number of arguments support this notion: (i) vasopressin and oxytocin are synthesized not only in hypothalamo-neurohypophysial cells, but also in other hypothalamic and extrahypothalamic cell bodies, whose axon projects to the limbic system, the brainstem and the spinal cord. (ii) Vasopressin and oxytocin can be shed from central axons as are classical neurotransmitters. (iii) Specific binding sites, i.e. membrane receptors having high affinity for vasopressin and oxytocin are present in the central nervous system. (iv) Vasopressin and oxytocin can alter the firing rate of selected neuronal populations. (v) In-situ injection of vasopressin and oxytocin receptor agonists and antagonists can interfere with behavior or physiological regulations. Morphological studies and electrophysiological recordings have evidenced a close anatomical correlation between the presence of vasopressin and oxytocin receptors in the brain and the neuronal responsiveness to vasopressin or oxytocin. These compounds have been found to affect membrane excitability in neurons located in the limbic system, hypothalamus, circumventricular organs, brainstem, and spinal cord. Sharp electrode intracellular recordings and whole-cell recordings, done in brainstem motoneurons or in spinal cord neurons, have revealed that vasopressin and oxytocin can directly affect neuronal excitability by opening non-specific cationic channels or by closing K(+) channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas, in cultured neurons, vasopressin and oxytocin appear to mobilize intracellular Ca(++), in brainstem slices, the action of oxytocin is mediated by a second messenger that is distinct from the second messenger activated in peripheral target cells. In this review, we will summarize studies carried out at the cellular level, i.e. we will concentrate on in-vitro approaches. Vasopressin and oxytocin will be treated together. Though acting via distinct receptors in distinct brain areas, these two neuropeptides appear to exert similar effects upon neuronal excitability.

Vasopressin increases GABAergic inhibition of rat hypothalamic paraventricular nucleus neurons in vitro

This investigation used an in vitro hypothalamic brain slice preparation and whole cell and perforated-patch recording to examine the response of magnocellular neurons in hypothalamic paraventricular nucleus (PVN) to bath applications of vasopressin (VP; 100-500 nM). In 22/38 cells, responses were characterized by an increase in the frequency of bicuculline-sensitive inhibitory postsynaptic potentials or currents with no detectable influence on excitatory postsynaptic events. Perforated-patch recordings confirmed that VP did not have an effect on intrinsic membrane properties of magnocellular PVN neurons (n = 17). Analysis of intrinsic membrane properties obtained with perforated-patch recording (n = 23) demonstrated that all of nine VP-sensitive neurons showed a rebound depolarization after transient membrane hyperpolarization from rest. By contrast, 12/14 nonresponding neurons displayed a delayed return to resting membrane potentials. Recordings of reversed inhibitory postsynaptic currents with chloride-loaded electrodes showed that responses to VP persisted in media containing glutamate receptor antagonists but were abolished in the presence of tetrodotoxin. In addition, responses were mimicked by vasotocin [Phe(2), Orn(8)], a selective V(1a) receptor agonist, and blocked by [beta-Mercapto-beta, beta-cyclopentamethylenepropionyl(1),O-Me-Tyr(2), Arg(8)]-VP (Manning compound), a V(1a)/OT receptor antagonist. Neither [deamino-Cys(1),Val(4),D-Arg(8)]-VP, a selective V(2) receptor agonist, nor oxytocin were effective. Collectively, the results imply that VP acts at V(1a) receptors to excite GABAergic neurons that are presynaptic to a population of magnocellular PVN neurons the identity of which features a unique rebound depolarization. Endogenous sources of VP may be VP-synthesizing neurons in suprachiasmatic nucleus, known to project toward the perinuclear regions of PVN, and/or the magnocellular neurons within PVN.