Presence of functional vasopressin V1 receptors in rat vagal afferent neurones

Infrared light elicits endothelium-dependent vasodilation in isolated occipital arteries of the rat via soluble guanylyl cyclase-dependent mechanisms

The left and right occipital arteries provide blood supply to afferent cell bodies in the ipsilateral nodose and petrosal ganglia. This supply is free of an effective blood-ganglion barrier, so changes in occipital artery blood flow directly affect the access of circulating factors to the afferent cell bodies. The application of infrared (IR) light to modulate neural and other cell processes has yielded information about basic biological processes within tissues and is gaining traction as a potential therapy for a variety of disease processes. To address whether IR can directly modulate vascular function, we performed wire myography studies to determine the actions of IR on occipital arteries isolated from male Sprague-Dawley rats. Based on our previous research that functionally-important differences exist between occipital artery segments close to their origin at the external carotid artery (ECA) and those closer to the nodose ganglion, the occipital arteries were dissected into two segments, one closer to the ECA and the other closer to the nodose ganglion. Segments were constricted with 5-hydroxytryptamine to a level equal to 50% of the maximal response generated by the application of a high (80 mM) concentration of K+ ions. The direct application of pulsed IR (1,460 nm) for 5 s produced a rapid vasodilation in occipital arteries that was significantly more pronounced in segments closest to the ECA, although the ECA itself was minimally responsive. The vasodilation remained for a substantial time (at least 120 s) after cessation of IR application. The vasodilation during and following cessation of the IR application was markedly diminished in occipital arteries denuded of the endothelium. In addition, the vasodilation elicited by IR in endothelium-intact occipital arteries was substantially reduced in the presence of a selective inhibitor of the nitric oxide-sensitive guanylate cyclase, 1H-[1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ). It appears that IR causes endothelium-dependent, nitric-oxide-mediated vasodilation in the occipital arteries of the rat. The ability of IR to generate rapid and sustained vasodilation may provide new therapeutic approaches for restoring or improving blood flow to targeted tissues.

The vagus nerve mediates the suppressing effects of peripherally administered oxytocin on methamphetamine self-administration and seeking in rats

The neuropeptide oxytocin has emerged as a promising pharmacotherapy for methamphetamine (METH) addiction, and clinical trials of intranasal oxytocin are underway. However, there is debate as to how peripherally administered oxytocin alters brain signalling to modulate addiction processes. Interestingly, there is evidence for functional interactions between peripheral oxytocin administration and the vagus nerve. Therefore, this study investigated whether the effects of peripherally administered oxytocin require vagal signalling to reduce METH self-administration and reinstatement of METH-seeking behaviours. Male and female Sprague-Dawley rats underwent surgery for jugular catheterisation and either subdiaphragmatic vagotomy (SDV) or a sham operation. Rats were trained to self-administer METH, and the effect of peripherally administered oxytocin on METH intake was assessed. Rats then underwent extinction, and effects of oxytocin were assessed on cue- and METH-induced reinstatement of METH-seeking. Oxytocin treatment robustly attenuated METH intake in both sexes, and SDV entirely prevented the suppressant effect of oxytocin (0.3 mg/kg) on METH intake, and partially prevented the effects of 1 mg/kg oxytocin in both sexes. After extinction, SDV decreased the suppressing effects of oxytocin on cue- and METH-primed reinstatement in males, but not females. SDV was functionally confirmed by measuring food intake following administration of the vagal dependent peptide, cholecyostokin-8. Our data suggest that vagus nerve signalling is required for the inhibitory effects of peripherally administered oxytocin on METH self-administration and reinstatement, and that this vagal dependency is partially mediated by sex and drug withdrawal. This study has implications for the use of oxytocin as a therapy for METH use disorder for both sexes.

Occipital Artery Function during the Development of 2-Kidney, 1-Clip Hypertension in Rats

This study compared the contractile responses elicited by angiotensin II (AII), arginine vasopressin (AVP), and 5-hydroxytryptamine (5-HT) in isolated occipital arteries (OAs) from sham-operated (SHAM) and 2-kidney, 1-clip (2K-1C) hypertensive rats. OAs were isolated and bisected into proximal segments (closer to the common carotid artery) and distal segments (closer to the nodose ganglion) and mounted separately on myographs. On day 9, 2K-1C rats had higher mean arterial blood pressures, heart rates, and plasma renin concentrations than SHAM rats. The contractile responses to AII were markedly diminished in both proximal and distal segments of OAs from 2K-1C rats as compared to those from SHAM rats. The responses elicited by AVP were substantially greater in distal than in proximal segments of OAs from SHAM rats and that AVP elicited similar responses in OA segments from 2K-1C rats. The responses elicited by 5-HT were similar in proximal and distal segments from SHAM and 2K-1C rats. These results demonstrate that continued exposure to circulating AII and AVP in 2K-1C rats reduces the contractile efficacy of AII but not AVP or 5-HT. The diminished responsiveness to AII may alter the physiological status of OAs in vivo.

Ca++ influx is essential for the hypotensive response to arginine vasopressin-induced neuron activation of the area postrema in the rat

We attempted to relate the signal pathway to the hypotension induced by arginine vasopressin (AVP) injection into the area postrema (AP) in urethane-anesthetized and ventilated rats with vagotomy. A femoral artery and vein were catheterized to measure the blood pressure (BP) and administer drugs, respectively. The rat was placed on a stereotaxic apparatus to expose the calamus sriptorius (CS) by craniostomy and maintained at normocapnia in hyperoxia. In protocol 1, hypotension evoked by AVP (3.0 x 10(-5) IU) microinjected into the AP 0.2 mm rostral to the CS of the midline was abolished by V(1A) antagonist, U73122 (phospholipase C blocker), and BAPTA-AM (Ca(++) chelator), suggesting that an increasing intracellular Ca(++) is essential for AVP-induced hypotension. In protocol 2, AVP-induced hypotension was abolished by EGTA (extracellular Ca(++) chelator) and Ca(++) blockers such as nifedipine, nimodipine (L-types), and omega-conotoxin MVIIC (P/Q-type), but not by omega-conotoxin GVIA (N-type). In protocol 3, AVP-induced hypotension was blocked by calphostin C (protein kinase C inhibitor) and mimicked by an increase in intracellular K(+) ions that was reversed by EGTA. Vehicle injections produced no changes in BP. In protocol 4, glutamate-induced hypotension was reversed by BAPTA-AM but not by EGTA or V(1A) antagonist. Our data suggest that AVP-induced hypotension depends on Ca(++) influx through a signal pathway from phospholipase C to protein kinase C which inactivates K(+) channels that may depolarize AP neurons to activate L- and P/Q-type Ca(++) channels. This may provide new insights into establishing a relationship between the signal pathway and physiological functions.

5-HT activates vagal afferent cell bodies in vivo: Role of 5-HT2 and 5-HT3 receptors

Occipital artery (OA) injections of 5-HT elicit pronounced reductions in heart rate and mean arterial blood pressure (MAP) in urethane-anesthetized rats by activation of vagal afferent cell bodies in the ipsilateral nodose ganglion. In contrast, internal carotid artery (ICA) and i.v. injections elicit similar cardiovascular responses by activation of peripheral vagal afferent terminals. The aim of this study was to examine the roles of 5-HT3 and 5-HT2 receptors in the 5-HT-induced activation of vagal afferent cell bodies and peripheral afferent terminals in urethane-anesthetized rats. OA, ICA and i.v. injections of 5-HT elicited dose-dependent reductions in heart rate and MAP that were virtually abolished after i.v. administration of the 5-HT3 receptor antagonists, MDL 7222 or ICS 205-930. The responses elicited by the OA injections of 5-HT were markedly diminished after i.v. injection of the 5-HT2 receptor antagonists, xylamidine or ketanserin, whereas the responses elicited by i.v. or ICA injections of 5-HT were not affected. The present findings suggest that (1) 5-HT3 and 5-HT2 receptor antagonists gain ready access to nodose ganglion cells upon i.v. administration, and (2) functional 5-HT3 and 5-HT2 receptors exist on the cell bodies of vagal afferent neurons mediating the cardiovascular responses elicited by OA injections of 5-HT. These findings also support a wealth of evidence that 5-HT3 receptors exist on the peripheral terminals of vagal afferents, and although they do not discount the possibility that 5-HT2 receptors exist on peripheral vagal afferent terminals, it appears that activation of these receptors does not have pronounced effects on 5-HT3 receptor activity on terminals that mediate the hemodynamic responses to 5-HT.

Occipital artery injections of 5-HT may directly activate the cell bodies of vagal and glossopharyngeal afferent cell bodies in the rat

The primary objective of this study was to determine whether circulating factors gain direct access to and affect the activity of vagal afferent cell bodies in the nodose ganglia and glossopharyngeal afferents cell bodies in the petrosal ganglia, of the rat. We found that the occipital and internal carotid arteries provided the sole blood supply to the nodose ganglia, and that i.v. injections of the tracer, Basic Blue 9, elicited strong cytoplasmic staining in vagal and glossopharyngeal afferent cell bodies that was prevented by prior ligation of the occipital but not the internal carotid arteries. We also found that occipital artery injections of 5-HT elicited pronounced dose-dependent reductions in heart rate and diastolic arterial blood pressure that were (1) virtually abolished after application of the local anesthetic, procaine, to the ipsilateral nodose and petrosal ganglia, (2) markedly attenuated after transection of the ipsilateral vagus between the nodose ganglion and brain and virtually abolished after subsequent transection of the ipsilateral glossopharyngeal nerve between the petrosal ganglion and the brain, (3) augmented after ipsilateral transection of the aortic depressor and carotid sinus nerves, and (4) augmented after transection of all ipsilateral glossopharyngeal and vagal afferent nerves except for vagal cardiopulmonary afferents. These findings suggest that blood-borne 5-HT in the occipital artery gains direct access to and activates the cell bodies of vagal cardiopulmonary afferents of the rat and glossopharyngeal afferents of undetermined modalities.

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- 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 and oxytocin action in the brain: cellular neurophysiological studies

During the last two decades it has become apparent that vasopressin (VP) and oxytocin (OT), in addition to playing a role as peptide hormones, also act as neurotransmitters. Morphological studies and electrophysiological recordings have shown a close anatomical correlation between the presence of these receptors and the neuronal responsiveness to VP or OT. These compounds have been found to affect membrane excitability in neurons located in the hippocampus, hypothalamus, lateral septum, brainstem, spinal cord and superior cervical ganglion. Sharp electrode intracellular and whole-cell recordings, done in brainstem motoneurons, have revealed that VP and OT can directly affect neuronal excitability by opening non-specific cationic channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas in some hypothalamic neurons OT appears to mobilize intracellular calcium, as revealed by calcium imaging techniques, in the brainstem the action of this neuropeptide is mediated by a second messenger which is distinct from the second messenger activated in peripheral target cells. Future studies should be aimed at elucidating the properties of the cationic channels responsible for the neuronal action of VP and OT, at identifying the brain-specific second messengers activated by these neuropeptides and at determining whether endogenous VP and OT can exert neuronal effects similar to those elicited by exogenous neuropeptides.

Autoradiographic visualisation of axonal transport of adenosine A1 receptors along the rat vagus nerve and characterisation of adenosine A1 receptor binding in the dorsal vagal complex of hypertensive and normotensive rats

The present study had employed in vitro receptor autoradiography with [3H]DPCPX to visualise the presence of adenosine A1 receptors on the rat nodose ganglion, which contains the perikarya of vagal afferent neurons projecting the the nucleus tractus solitarius (NTS). In addition, unilateral vagal ligation resulted in an accumulation of [3H]DPCPX binding adjacent to the ligatures, indication that adenosine A1 receptors are subject to axoplasmic flow along the rat vagus nerve. Radioligand binding assays were utilised to characterise the properties of adenosine A1 receptors in the dorsal vagal complex (NTS, area postrema and dorsal motor nucleus of the vagus) of pup and adult normotensive (Wistar Kyoto, WKY) and hypertensive (spontaneously hypertensive, SHR) rats. Saturation binding indicated that the affinity (KD) of [3H]DPCPX, and the binding site density (Bmax) were not different between the adult WKY and SHR, although the pup SHR had a lower KD value than the pup WKY rat. Competition binding assays revealed complex differences between the two rat strains; however, with respect to hypertension, the affinity of the selective adenosine A1 agonist, cyclohexyladenosine (CHA), was markedly reduced in the membranes from SHR (Ki approximately 93 nM) compared to WKY (approximately 6 nM). Such an observation is consistent with the attenuated responses of SHRs to intra-NTS injections of adenosine.

Bradykinin B2 receptors in nodose ganglia of rat and human

The present study has employed in vitro electrophysiology to characterise the ability of bradykinin to depolarise the rat isolated nodose ganglion preparation, containing the perikarya of vagal afferent neurons. Both bradykinin and kallidin elicited a concentration-dependent (1-100 nM) depolarisation when applied to the superfusate bathing the nodose ganglia, whereas the bradykinin B1 receptor agonist, des-Arg9-bradykinin, was only effective in the micromolar range. Furthermore, the electrophysiological response to bradykinin was antagonised by the bradykinin B2 receptor antagonist, D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-t hienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl+ ++-L-(2alpha,3beta,7abeta)-octahydro-1H-indole-2-carbonyl-L- arginine (Hoe 140), in a concentration-related manner. To determine the anatomical location of functional bradykinin B2 receptors, in vitro autoradiography with [125I]para-iodophenyl Hoe 140 was performed on sections of rat and human inferior vagal (nodose) ganglia and confirmed the presence of binding over vagal perikarya. Collectively, these data provide evidence for functionally relevant bradykinin B2 receptors on vagal afferent neurons, which are apparently also present on human vagal perikarya.

Functional GABAA receptors on rat vagal afferent neurones

1. In the present study, in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess gamma-aminobutyric acid (GABA)A receptors. 2. GABA (1-100 microM) and isoguvacine (3-100 microM) caused a concentration-dependent depolarization of the rat isolated nodose ganglion preparation at room temperature. When applied to the tissue 20 min before the agonist, SR95531 (3 microM) and bicuculline (3 microM) caused a parallel shift to the right of the GABA and isoguvacine concentration-response curves, yielding shifts of 81 fold and 117 fold for SR95531 and 4 fold and 12 fold for bicuculline, respectively. 3. Baclofen (10 nM-100 microM) was unable to elicit a depolarization of the rat isolated nodose ganglion preparation at either room temperature or at 36 degrees C, whilst 5-aminovaleric acid (10 microM), a GABAB receptor antagonist, was unable to antagonize significantly the GABA-induced depolarization at either room temperature or at 36 degrees C. 4. [3H]-SR95531 (7.2 nM), a GABAA receptor-selective antagonist, bound topographically to sections of rat brainstem. Specific binding was highest in the medial nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMVN). Binding was also observed in certain medullary reticular nuclei, in particular the parvocellular reticular nucleus. 5. Unilateral nodose ganglionectomy caused a reduction in GABAA binding site density in the medial NTS from 93 +/- 7 to 68 +/- 6 d.p.m./mm2. This procedure also caused a reduction in GABAA binding site density in the side of the NTS contralateral to the lesion, from 151 +/- 12 to 93 +/- 7 d.p.m./mm2. Sham surgery had no effect on the binding of [3H]-SR95531 in rat brainstem. 6. The present data provide evidence for the presence of GABAA receptors located on the soma and central terminals of rat vagal afferent neurones. Additionally, a population of GABAA receptors is evidenced postsynaptically in the rat NTS with respect to vagal afferent terminals. These data are discussed in relation to the functional pharmacology of GABA in this region of the NTS.

Radioligand binding and autoradiographic visualization of adenosine transport sites in human inferior vagal ganglia and their axonal transport along rat vagal afferent neurons

The present study has employed membrane-binding studies and in vitro autoradiography to demonstrate the presence of adenosine transport sites in human inferior vagal ganglia using [3H]nitrobenzylthioinosine ([3H]NBMPR), a potent inhibitor of adenosine transport. In addition, [3H]NBMPR was used to determine whether adenosine transport sites are subject to axonal transport along the rat vagus nerve. Binding of [3H]NBMPR to human inferior vagal ganglia membranes was saturable and reversible. Saturation experiments revealed a single class of high affinity-binding sites with a Kd of 93.73 +/- 23.13 pM and Bmax of 413.50 +/- 50.40 fmol/mg protein. In displacement experiments, the adenosine transport inhibitor dipyridamole was the most potent displacer of [3H]NBMPR binding (Ki = 42.7 +/- 28.0 nM). Adenosine itself was able to fully displace [3H]NBMPR binding with a Ki of 115.0 +/- 34.0 microM. The A1/A2a adenosine receptor agonist 5'-(N-ethylcarboxamido)-adenosine (NECA) was able to fully displace [3H]NBMPR binding in only one experiment at a concentration of 100 microM, yielding an affinity 1000-fold higher than its affinity for adenosine receptors. All competition curves obtained from displacement experiments displayed monophasic profiles, indicating the presence of a single class of [3H]NBMPR binding sites. Incubation of human inferior vagal ganglia sections with [3H]NBMPR (0.7 nM) revealed dense binding which appeared to be consistent with the distribution of neuronal cell bodies in this tissue. Following unilateral ligation of the vagus nerve in the rat, accumulation of [3H]NBMPR binding sites occurred both proximal and distal to the vagal ligatures. These results suggest that [3H]NBMPR binds with high affinity to a single class of adenosine transport sites, and that these sites are present on vagal afferent neurons in the human and undergo bidirectional axonal transport along the rat vagus nerve.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Ontogeny of arginine vasopressin-like immunoreactivity in the Brazilian opossum brain

The neuropeptide arginine vasopressin is involved in many centrally mediated functions and brain development. In this study, we have examined the ontogeny of arginine vasopressin-like immunoreactivity (AVP-IR) in the Brazilian opossum (Monodelphis domestica) brain to further understand the involvement of AVP in the forming central nervous system. Monodelphis is a small pouchless marsupial and its pups are born in an extremely immature state before neurogenesis is completed. In the adult brain, cell bodies containing AVP-IR were found in several nuclear groups and areas, and immunoreactive fibers were found to be widely distributed throughout the brain. The distribution of AVP-IR in the adult opossum brain generally resembled that reported for other species including the rat, however, some differences in localization of immunoreactive cells were observed. In the developing opossum brain, AVP-IR was first seen in the mesencephalon and diencephalon between embryonic days 12 and 13. Subsequently, a distinct group of AVP immunoreactive cells was present in the forming supraoptic nucleus on day 1 of postnatal life (1 PN) and at 3 PN in the paraventricular nucleus. Between 1 and 3 PN, a few cells transiently expressed AVP-IR in the forming thalamus and tegmental area. At these ages a few immunoreactive fibers were also detected in the forming cerebellum. These fibers were not seen at later ages in these areas. By 5 PN, an increased expression of AVP-IR was seen in the forming supraoptic and paraventricular hypothalamic nuclei, median eminence, and posterior pituitary. At 7 PN, immunoreactive cells and fibers were seen in several forebrain areas. The distribution pattern of AVP-IR became adult-like by 60 PN. A sex difference in the amount of AVP-IR in the lateral septum was also observed in the opossum brain at 60 PN. This difference persisted in the adult brain. Due to the early presence of AVP-IR in the Monodelphis brain before neurogenesis and morphogenesis is completed, we suggest that AVP may be involved in morphogenesis of the central nervous system. In addition, AVP may have a significant physiological function in regard to homeostasis before the forebrain contributes to these control mechanisms. Further studies, including physiological and developmental manipulations, will define the significance of the early presence of AVP during the differentiation and maturation of the central nervous system in Monodelphis.

Characterization of vasopressin receptors in cultured cells derived from the region of rat brain circumventricular organs

The aim of the present study was to characterize vasopressin receptors within the two circumventricular organs located in the lamina terminalis of the rat brain, namely the organum vasculosum of the lamina terminalis and the subfornical organ. Cells derived from both structures were isolated, cultured and intracellular Ca2+ concentrations were measured in single fura-2 loaded neurons and astrocytes after application of vasopressin and various vasopressin analogues. Subsequent to Ca2+ measurements, the identification of neurons and astrocytes was verified using immunocytochemistry with cell type-specific antibodies. High proportions of subfornical organ (34%) and organum vasculosum laminae terminalis (28%) neurons exhibited increased intracellular Ca2+ concentration after exposure to 1-1000 nM vasopressin. Within single cells, the response was dose-dependent. Similar results were obtained in subfornical organ (62%) and organum vasculosum laminae terminalis (38%) astrocytes with minor differences in the transient amplitude and pattern distribution when compared with neurons. Since omission of extracellular Ca2+ preserved vasopressin responsiveness, it is likely that intracellular stores were the main source of mobilized Ca2+. The preincubation of neurons and astrocytes with the V1 receptor-specific antagonist d(CH2)5[Tyr(Me)2]8-arginine vasopressin (10-100 nM) selectively and reversibly blocked the vasopressin-mediated response. Oxytocin-induced Ca2+ transients (0.32-1000 nM), which were observed in 32% (63%) or organum vasculosum laminae terminalis and in 54% (42%) of subfornical organ neurons (astrocytes), were not affected by the V1-specific antagonist. These data indicate the presence of a V1-like vasopressin receptor and an oxytocin receptor in cultured neurons and astrocytes from both circumventricular organ structures. In addition, the exposure to the highly selective V2 receptor agonist, 1-desamino,8-D-arginine vasopressin, evoked Ca2+ transients almost exclusively in organum vasculosum laminae terminalis neurons (eight of 18 tested). Only 1 (n = 14) subfornical organ neuron and none of the astrocytes tested (n = 26) responded to 1-desamino,8-D-arginine vasopressin. Since 1-desamino,8-D-arginine vasopressin acting via "classical" V2 receptors is not expected to affect the intracellular Ca2+ concentration, these data indicate the tissue and cell type-specific expression of a 1-desamino,8-D-arginine vasopressin-sensitive vasopressin receptor in neurons of the organum vasculosum laminae terminalis. In summary, the results indicate a heterogeneity of neurohypophyseal peptide receptor subtypes in the primary cell culture of both circumventricular structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional dopamine D2 receptors on rat vagal afferent neurones

1. In the present study in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess dopamine D2 receptors. 2. Dopamine (10-300 microM) elicited a temperature- and concentration-dependent depolarization of the rat isolated nodose ganglion preparation. When applied to the tissue 15 min prior to agonist, raclopride (10 microM), clozapine (10 microM) or a mixture of raclopride and clozapine (10 microM each) all produced a threefold parallel shift to the right of the dopamine concentration-response curve. In contrast, SCH 23390 (100 nM), phentolamine and propranolol (1 microM each) failed to antagonize the dopamine-mediated depolarization. 3. [125I]-NCQ 298 (0.5 nM), a D2 selective radioligand, bound topographically to sections of rat brainstem. Densitometric quantification of autoradiograms revealed 93.8 +/- 0.5% specific binding of this salicylamide radioligand, as determined by raclopride (10 microM, n = 10 animals). Binding was highest in the nucleus tractus solitarius (NTS), particularly the medial and gelatinous subnuclei. In addition, specific binding was also observed in the interpolar spinal trigeminal nucleus and the inferior olive. 4. Unilateral nodose ganglionectomy caused a 36.6 +/- 3.0% reduction in specific binding in the denervated NTS compared to the contralateral NTS. Furthermore, the loss of binding was confined to the dorsal aspect of the medial subnucleus of the NTS. Sham surgery had no effect on the binding of [125I]-NCQ 298 in rat brainstem. 5. The present data provide evidence for the presence of functionally relevant dopamine D2 receptors on both the soma and central terminals of rat vagal afferent neurones. In addition, the majority of D2 receptors in the rat NTS appear to be located postsynaptically with respect to vagal terminals, and are presumably located either on ascending glossopharyngeal terminals, descending terminals from higher brain regions or on neuronal cell bodies within the NTS.

[Arg8]vasotocin excites neurones in the dorsal vagal complex in vitro: evidence for an action through novel class(es) of CNS receptors

Using extracellular recordings from brainstem slices in vitro, it was demonstrated that a high proportion (38/56) of neurones in the dorsal vagal complex of dioestrus, virgin female rats exhibit an excitatory response to [Arg8]-vasotocin (AVT). Pharmacological characterization suggests that these responses cannot be entirely explained by interaction with either of the currently known classes of central receptors for oxytocin (OT) and vasopressin (V1a). Comparison of the responses with those to the OT receptor-specific agonist [Thr4,Gly7]-OT (TGOT), showed that not all neurones that responded to TGOT also responded to AVT (3/27). Furthermore, while the effects of 10(-7) M TGOT could be blocked either by the broad-spectrum antagonist d(CH2)5[d-Tyr(OEt)2,Val4,Cit8]-vasopressin or by the selective OT receptor antagonist d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin, these peptides did not completely block the responses to AVT, indicating that AVT is unlikely to act through the central OT receptor. The responses to AVT and [Arg8]-vasopressin (AVP) indicated the presence of at least 2 classes of receptor with which these agonists could act. Of 42 neurones tested with both AVP and AVT, none responded to AVP in the absence of a response to AVT, while 7/42 responded to AVT without a response to AVP. This might be explained by AVP acting through only the V1 receptor, while AVT acts through both the V1 and its own novel class of receptor. This was substantiated by the fact that two OT/V1 receptor antagonists, d(CH2)5[d-Tyr(OEt)2,Val4,Cit8]-VP and d(CH2)5[Tyr(Me)2,Tyr-NH2(9)]-AVP, were unable to block completely all the responses to AVT at a dose which suppressed responses to both AVP and TGOT.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic adenosine A2a receptors on soma and central terminals of rat vagal afferent neurons

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A1 antagonist PACPX. Autoradiography using [3H]NECA (4 nM) with suppression of A1 binding enabled the first visualisation of high affinity adenosine A2 receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.

Vasopressin anti-idiotypic antibody staining in the rat brain: colocalization with [35S] [pGlu4, Cyt6]AVP(4-9) binding sites

Vasopressin and its fragment peptides such as [pGlu4, Cyt6]AVP(4-9) (AVP(4-9) represent putative neuromodulators within central nervous homeostatic, memory and behavioural circuits. To localize their central receptor systems, the previously characterized monoclonal anti-idiotypic antibody mAb 237 was employed in immunocytological investigations of rat brain tissue sections. This antibody was raised to the monoclonal idiotypic anti-AVP antibody mAb 113 which preferentially binds to the acyclic C-terminal portion of the AVP molecule and is therefore also capable of binding the naturally occurring AVP(4-9) fragment. Immunoreactive magnocellular neurones were detected in the AVP-synthesizing supraoptic but not paraventricular nuclei. Dense staining was observed within circumventricular organs lacking a blood-brain barrier (BBB). These structures include the subfornical organ, the organum vasculosum laminae terminalis, the internal layer of the median eminence, the body of the pineal gland, the choroid plexus and the area postrema, where immunoreactivity was found on capillaries, neurones and fibres. Further staining was found in the nucleus of the solitari tract and the arcuate nucleus, endowed with a leaky BBB. Distinct cell patches in the ependymal lining of the third ventricle as well as dendritic processes of juxtaependymal neurones were labelled by the anti-idiotypic antibody mAb 237. The observed staining pattern did not parallel that obtained in autoradiographic studies performed using either radiolabelled AVP or a V1-receptor antagonist, but that found with the [35S]-labelled AVP(4-9) fragment. Using [35S]-labelled AVP(4-9) fragment, specific high density binding sites could be localized autoradiographically in structures within and outside the BBB, in complete agreement with the anti-idiotypic immunoreactivity. Since the anti-idiotypic methodology is based on transfer of complementary structures, and the epitope recognized by the corresponding idiotypic antibody resembles the sequence of AVP(4-9), the anti-idiotypic antibodies might recognize the AVP(4-9) receptor with high affinity.