下丘脑室旁核内GABA在中枢高渗刺激诱发的心血管反应中的作用

目的：探讨下丘脑室旁核（PVN）内的γ-氨基丁酸（GABA）在中枢高渗刺激诱发的应激性心血管反应中的作用及其机制。方法：在清醒自由活动大鼠,用脑部微量透析法和高效液相色谱法观察中枢高渗刺激对PVN区域GABA含量的影响,并同时记录血压和心率的变化;用GABAA受体阻断剂Bicuculline或GABAB受体阻断剂Saelofen直接灌流PVN区并给予中枢高渗刺激,进一步探讨PVN区GABA在中枢高渗刺激诱发的应激性心血管反应中的作用。结果：①PVN局部灌流0．6mol／L盐水时,血压和心率都显著增加（均为P〈0．01）,同时,PVN区细胞外液中GABA水平也明显增加到刺激前的561．96％±173．96％（P〈0．05）;②PVN局部灌流Bicuculline或Saclofen的同时,给予0．6mol／L盐水的刺激,可使高渗刺激引起的血压增加幅度明显降低（均为P〈0．01）,而心率的增加幅度未受明显影响（均为P〈0．05）。结论：中枢高渗刺激可引起PVN内GABA的分泌,而后者可通过GABAA和GABAB受体产生血压的升高反应。     

Plasticity of pre- and postsynaptic GABAB receptor function in the paraventricular nucleus in spontaneously hypertensive rats

GABA(B) receptor function is upregulated in the paraventricular nucleus (PVN) of the hypothalamus in spontaneously hypertensive rats (SHR), but it is unclear whether this upregulation occurs pre- or postsynaptically. We therefore determined pre- and postsynaptic GABA(B) receptor function in retrogradely labeled spinally projecting PVN neurons using whole cell patch-clamp recording in brain slices in SHR and Wistar-Kyoto (WKY) rats. Bath application of the GABA(B) receptor agonist baclofen significantly decreased the spontaneous firing activity of labeled PVN neurons in both SHR and WKY rats. However, the magnitude of reduction in the firing rate was significantly greater in SHR than in WKY rats. Furthermore, baclofen produced larger membrane hyperpolarization and outward currents in labeled PVN neurons in SHR than in WKY rats. The baclofen-induced current was abolished by either including G protein inhibitor GDPbetaS in the pipette solution or bath application of the GABA(B) receptor antagonist in both SHR and WKY rats. Blocking N-methyl-d-aspartic acid receptors had no significant effect on baclofen-elicited outward currents in SHR. In addition, baclofen caused significantly greater inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) in labeled PVN neurons in brain slices from SHR than WKY rats. By contrast, baclofen produced significantly less inhibition of GABAergic inhibitory postsynaptic currents (IPSCs) in labeled PVN neurons in SHR than in WKY rats. Although microinjection of the GABA(B) antagonist into the PVN increases sympathetic vasomotor tone in SHR, the GABA(B) antagonist did not affect EPSCs and IPSCs of the PVN neurons in vitro. These findings suggest that postsynaptic GABA(B) receptor function is upregulated in PVN presympathetic neurons in SHR. Whereas presynaptic GABA(B) receptor control of glutamatergic synaptic inputs is enhanced, presynaptic GABA(B) receptor control of GABAergic inputs in the PVN is attenuated in SHR. Changes in both pre- and postsynaptic GABA(B) receptors in the PVN may contribute to the control of sympathetic outflow in hypertension.     

AT(1) and glutamatergic receptors in paraventricular nucleus support blood pressure during water deprivation

Water deprivation activates sympathoexcitatory neurons in the paraventricular nucleus (PVN); however, the neurotransmitters that mediate this activation are unknown. To test the hypothesis that ANG II and glutamate are involved, effects on blood pressure (BP) of bilateral PVN microinjections of ANG II type 1 receptor (AT1R) antagonists, candesartan and valsartan, or the ionotropic glutamate receptor antagonist, kynurenate, were determined in urethane-anesthetized water-deprived and water-replete male rats. Because PVN may activate sympathetic neurons via the rostral ventrolateral medulla (RVLM) and because PVN disinhibition increases sympathetic activity in part via increased drive of AT1R in the RVLM, candesartan was also bilaterally microinjected into the RVLM. Total blockade of the PVN with bilateral microinjections of muscimol, a GABA(A) agonist, decreased BP more (P < 0.05) in water-deprived (-29 +/- 8 mmHg) than in water-replete (-7 +/- 2 mmHg) rats, verifying that the PVN is required for BP maintenance during water deprivation. PVN candesartan slowly lowered BP by 7 +/- 1 mmHg (P < 0.05). In water-replete rats, however, candesartan did not alter BP (1 +/- 1 mmHg). Valsartan also produced a slowly developing decrease in arterial pressure (-6 +/- 1 mmHg; P < 0.05) in water-deprived but not in water-replete (-1 +/- 1 mmHg) rats. In water-deprived rats, PVN kynurenate rapidly decreased BP (-19 +/- 3 mmHg), and the response was greater (P < 0.05) than in water-replete rats (-4 +/- 1 mmHg). Finally, as in PVN, candesartan in RVLM slowly decreased BP in water-deprived (-8 +/- 1 mmHg; P < 0.05) but not in water-replete (-3 +/- 1 mmHg) rats. These data suggest that activation of AT(1) and glutamate receptors in PVN, as well as of AT1R in RVLM, contributes to BP maintenance during water deprivation.     

Interaction between glutamate and GABA systems in the integration of sympathetic outflow by the paraventricular nucleus of the hypothalamus

The paraventricular nucleus (PVN) of the hypothalamus is a central site known to modulate sympathetic outflow. Excitatory and inhibitory neurotransmitters within the PVN dictate final outflow. The goal of the present study was to examine the role of the interaction between the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter GABA in the regulation of sympathetic activity. In alpha-chloralose- and urethane-anesthetized rats, microinjection of glutamate and N-methyl-D-aspartate (NMDA; 50, 100, and 200 pmol) into the PVN produced dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate. These responses were blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). Microinjection of bicuculline, a GABA(A) receptor antagonist, into the PVN (50, 100, and 200 pmol) also produced significant, dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate; AP-5 also blocked these responses. Using microdialysis and HPLC/electrochemical detection techniques, we observed that bicuculline infusion into the PVN increased glutamate release. Using an in vitro hypothalamic slice preparation, we found that bicuculline increased the frequency of glutamate-mediated excitatory postsynaptic currents in PVN-rostral ventrolateral medullary projecting neurons, supporting a GABA(A)-mediated tonic inhibition of this excitatory input into these neurons. Together, these data indicate that 1) glutamate, via NMDA receptors, excites the presympathetic neurons within the PVN and increases sympathetic outflow and 2) this glutamate excitatory input is tonically inhibited by a GABA(A)-mediated mechanism.     

Activation of the serotonergic 5-HT1A receptor in the paraventricular nucleus of the hypothalamus inhibits water intake and increases urinary excretion in water-deprived rats

The paraventricular nucleus (PVN) may be considered as a dynamic mosaic of chemically-specified subgroups of neurons. 5-HT(1A) is one of the prime receptors identified and there is expressed throughout all magnocellular regions of the PVN. Several reports have demonstrated that a subpopulation of the magnocellular neurons expressing 5-HT(1A) receptors are oxytocin (OT) neurons and activation of 5-HT(1A) receptors in the PVN increases the plasma OT. Increasing evidence shows that OT inhibits water intake and increases urinary excretion in rats. The aim of this study was to investigate the role of serotonergic 5-HT(1A) receptors in the lateral-medial posterior magnocellular region of the PVN in the water intake and diuresis induced by 24 h of water deprivation. Cannulae were implanted in the PVN of rats. 5-HT injections in the PVN reduced water intake and increased urinary excretion. 8-OH-DPAT (a 5-HT(1A) agonist) injections blocked the water intake and increased urinary output in all the periods of the observation. pMPPF (a 5-HT(1A) antagonist) injected bilaterally before the 8-OH-DPAT blocked its inhibitory effect on water intake and its diuretic effect. We suggest that antidipsogenic and diuretic responses seem to be mediated via 5-HT(1A) receptors of the lateral-medial posterior magnocellular region of the PVN in water-deprived rats.     

GABAergic mediation of stress-induced secretion of corticosterone and oxytocin, but not prolactin, by the hypothalamic paraventricular nucleus

The hypothalamus-pituitary-adrenal axis (HPA) participates in mediating the response to stressful stimuli. Within the HPA, neurons in the medial parvocellular region of paraventricular nucleus (PVN) of the hypothalamus integrate excitatory and inhibitory signals triggering secretion of corticotropin-releasing hormone (CRH), the main secretagogue of adrenocorticotropic hormone (ACTH). Stressful situations alter CRH secretion as well as other hormones, including prolactin and oxytocin. Most inputs to the PVN are of local origin, half of which are GABAergic neurons, and both GABA-A and GABA-B receptors are present in the PVN. The objective of the present study was to investigate the role of GABA-A and GABA-B receptors in the PVN's control of stress-induced corticosterone, oxytocin and prolactin secretion. Rats were microinjected with saline or different doses (0.5, 5 and 50 pmol) of GABA-A (bicuculine) or GABA-B (phaclofen) antagonists in the PVN. Ten minutes later, they were subjected to a stressor (ether inhalation) and blood samples were collected 30 min before and 10, 30, 60, 90 and 120 min after the stressful stimulus to measure hormone levels by radioimmunoassay. Our results indicate that GABA acts in the PVN to inhibit stress-induced corticosterone secretion via both its receptor subtypes, especially GABA-B. In contrast, GABA in the PVN stimulates oxytocin secretion through GABA-B receptors and does not alter prolactin secretion.     

Plasticity of GABAergic control of hypothalamic presympathetic neurons in hypertension

Increased sympathetic outflow contributes to the pathogenesis of hypertension. However, the mechanisms of increased sympathetic drive in hypertension remain unclear. We examined the tonic GABAergic inhibition in control of the excitability of paraventricular (PVN) presympathetic neurons in spontaneously hypertensive rats (SHR) and normotensive controls, including Sprague-Dawley (SD) and Wistar-Kyoto (WKY) rats. Whole cell patch-clamp recordings were performed on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla (RVLM) in brain slices. The basal firing rate of PVN neurons was significantly decreased in 13-wk-old SD and WKY rats but increased in 13-wk-old SHR, compared with their respective 6-wk-old controls. The GABA(A) antagonist bicuculline consistently increased the firing of PVN neurons in normotensive controls. Surprisingly, bicuculline either decreased the firing or had no effect in 59.3% of labeled cells in 13-wk-old SHR. In contrast, the GABA(B) antagonist CGP-55845 had no effect on the firing of PVN neurons in normotensive controls but significantly increased the firing of 75% of cells studied in 13-wk-old SHR. Furthermore, the evoked GABA(A) current decreased significantly in labeled PVN neurons of 13-wk-old SHR compared with that in normotensive controls. Both the frequency and amplitude of GABAergic spontaneously inhibitory postsynaptic currents were also reduced in 13-wk-old SHR. This study demonstrates an unexpected functional change in GABA(A) and GABA(B) receptors in regulation of the firing activity of PVN-RVLM neurons in SHR. This change in GABA(A) receptor function and GABAergic inputs to PVN output neurons may contribute to increased sympathetic outflow in hypertension.     

Paraventricular oxytocin neurons are involved in neural modulation of breathing.

In this study, we determined the projections of oxytocin-containing neurons of the paraventricular nucleus (PVN) to phrenic nuclei and to the rostral ventrolateral medullary (RVLM) region, which is known to be involved in respiratory rhythm generation. Studies were also designed to determine oxytocin-receptor expression within the RVLM and the physiological effects of their activation on respiratory drive and arterial blood pressure. Oxytocin immunohistochemistry combined with cholera toxin B, a retrograde tracer, showed that a subpopulation of oxytocin-containing parvocellular neurons in the dorsal and medial ventral regions of the PVN projects to phrenic nuclei. Similarly, a subpopulation of pseudorabies virus-labeled neurons in the PVN coexpressed oxytocin after injection of pseudorabies virus, a transynaptic retrograde marker, into the costal region of the diaphragm. A subpopulation of oxytocin expressing neurons was also found to project to the RVLM. Activation of this site by microinjection of oxytocin into the RVLM (0.2 nmol/200 nl) significantly increased diaphragm electromyographic activity and frequency discharge (P < 0.05). In addition, oxytocin increased blood pressure and heart rate (P < 0.05). These data indicate that oxytocin participates in the regulation of respiratory and cardiovascular activity, partly via projections to the RVLM and phrenic nuclei.     

Reduced endogenous GABA-mediated inhibition in the PVN on renal nerve discharge in rats with heart failure

One characteristic of heart failure (HF) is increased sympathetic activation. The paraventricular nucleus (PVN) of the hypothalamus (involved in control of sympathetic outflow) has been shown to have increased neuronal activation during HF. This study examined the influence of endogenous GABA input (inhibitory in nature) into the PVN on renal sympathetic nerve discharge (RSND), arterial blood pressure (BP), and heart rate (HR) in rats with HF induced by coronary artery ligation. In alpha-chloralose- and urethane-anesthetized rats, microinjection of bicuculline (a GABA antagonist) into the PVN produced a dose-dependent increase in RSND, BP, and HR in both sham-operated control and HF rats. Bicuculline attenuated the increase in RSND and BP in HF rats compared with control rats. Alternatively, microinjection of the GABA agonist muscimol produced a dose-dependent decrease in RSND, BP, and HR in both control and HF rats. Muscimol was also less effective in decreasing RSND, BP, and HR in HF rats than in control rats. These results suggest that endogenous GABA-mediated input into the PVN of rats with HF is less effective in suppressing RSND and BP compared with control rats. This is partly due to the post-release actions of GABA, possibly caused by altered function of post-synaptic GABA receptors in the PVN of rats with HF. Reduced GABA-mediated inhibition in the PVN may contribute to increased sympathetic outflow, which is commonly observed during HF.     

Role of the hypothalamic PVN in the reflex reduction in mesenteric blood flow elicited by hyperthermia

The hypothalamic paraventricular nucleus (PVN) is an important integrative center in the brain. In the present study, we investigated whether the PVN is a key region in the mesenteric vasoconstriction that normally accompanies an increase in core body temperature. Anesthetized rats were monitored for blood pressure, heart rate, mesenteric blood flow, and vascular conductance. In control rats, elevation of core body temperature to 41 degrees C had no significant effect on blood pressure, increased heart rate, and reduced mesenteric blood flow by 21%. In a separate group of rats, muscimol was microinjected bilaterally (1 nmol/side) into the PVN. Compared with the control group, there was no significant difference in the blood pressure and heart rate responses elicited by the increase in core body temperature. In contrast to control animals, however, mesenteric blood flow did not fall in the muscimol-treated rats in response to the elevation in core body temperature. In a separate group, in which muscimol was microinjected into regions outside the PVN, elevating core body temperature elicited the normal reduction in mesenteric blood flow. The results suggest that the PVN may play a key role in the reflex decrease in mesenteric blood flow elicited by hyperthermia.     

Aldosterone-induced brain MAPK signaling and sympathetic excitation are angiotensin II type-1 receptor dependent

Angiotensin II (ANG II)-induced mitogen-activated protein kinase (MAPK) signaling upregulates angiotensin II type-1 receptors (AT(1)R) in hypothalamic paraventricular nucleus (PVN) and contributes to AT(1)R-mediated sympathetic excitation in heart failure. Aldosterone has similar effects to increase AT(1)R expression in the PVN and sympathetic drive. The present study was undertaken to determine whether aldosterone also activates the sympathetic nervous system via MAPK signaling and, if so, whether its effect is independent of ANG II and AT(1)R. In anesthetized rats, a 4-h intravenous infusion of aldosterone induced increases (P < 0.05) in phosphorylated (p-) p44/42 MAPK in PVN, PVN neuronal excitation, renal sympathetic nerve activity (RSNA), mean blood pressure (MBP), and heart rate (HR). Intracerebroventricular or bilateral PVN microinjection of the p44/42 MAPK inhibitor PD-98059 reduced the aldosterone-induced RSNA, HR, and MBP responses. Intracerebroventricular pretreatment (5 days earlier) with pooled small interfering RNAs targeting p44/42 MAPK reduced total and p-p44/42 MAPK, aldosterone-induced c-Fos expression in the PVN, and the aldosterone-induced increases in RSNA, HR, and MBP. Intracerebroventricular infusion of either the mineralocorticoid receptor antagonist RU-28318 or the AT(1)R antagonist losartan blocked aldosterone-induced phosphorylation of p44/42 MAPK and prevented the increases in RSNA, HR, and MBP. These data suggest that aldosterone-induced sympathetic excitation depends upon that AT(1)R-induced MAPK signaling in the brain. The short time course of this interaction suggests a nongenomic mechanism, perhaps via an aldosterone-induced transactivation of the AT(1)R as described in peripheral tissues.     

GABA in the paraventricular nucleus tonically suppresses baroreflex function: alterations during pregnancy

It is well established that GABAergic inputs to the paraventricular nucleus of the hypothalamus (PVN) tonically suppress heart rate and the activity of several sympathetic nerves. However, whether GABA similarly inhibits PVN control of baroreflex function has not been previously investigated. To test this hypothesis, it was determined whether microinjection of the GABA(A) antagonist, bicuculline, into the PVN enhances the baroreflex in anesthetized female virgin rats. In addition, because GABAergic inhibition of PVN preautonomic neurons is decreased during pregnancy, it was also determined whether the effects of PVN bicuculline administration on baroreflex function were less in pregnant animals. In virgin rats, PVN microinjection of bicuculline increased (P < 0.05) baroreflex gain and maximum levels of heart rate (gain, from 1.6 ± 0.6 to 3.8 ± 1.3 bpm/mmHg; maximum, from 406 ± 18 to 475 ± 14 bpm) and of lumbar sympathetic nerve activity (gain from 2.6 ± 0.7 to 4.8 ± 1.6%/mmHg; maximum, 149 ± 32 to 273 ± 48%), indicating that PVN GABA normally suppresses baroreflex function. Pregnancy decreased heart rate baroreflex gain (pregnant, 0.9 ± 0.3 bpm/mmHg; virgin, 1.9 ± 0.2 bpm/mmHg; P < 0.05). Following PVN bicuculline administration in pregnant rats, smaller (P < 0.01) increments in baroreflex gain (pregnant, 0.6 ± 0.1 bpm/mmHg; virgin, 2.4 ± 0.9 bpm/mmHg) and maximum (pregnant, 33 ± 7 bpm; virgin, 75 ± 12 bpm; P < 0.05) were produced. Collectively, these data suggest that the PVN normally inhibits the baroreflex via tonic GABAergic inputs and that this inhibition is less during pregnancy.     

Differential role of the paraventricular nucleus of the hypothalamus in modulating the sympathoexcitatory component of peripheral and central chemoreflexes

In the present study we investigated the involvement of the hypothalamic paraventricular nucleus (PVN) in the modulation of sympathoexcitatory reflex activated by peripheral and central chemoreceptors. We measured mean arterial blood pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), and phrenic nerve activity (PNA) before and after blocking neurotransmission within the PVN by bilateral microinjection of 2% lidocaine (100 nl) during specific stimulation of peripheral chemoreceptors by potassium cyanide (KCN, 75 microg/kg iv, bolus dose) or stimulation of central chemoreceptors with hypercapnia (10% CO(2)). Typically stimulation of peripheral chemoreceptors evoked a reflex response characterized by an increase in MAP, RSNA, and PNA and a decrease in HR. Bilateral microinjection of 2% lidocaine into the PVN had no effect on basal sympathetic and cardiorespiratory variables; however, the RSNA and PNA responses evoked by peripheral chemoreceptor stimulation were attenuated (P < 0.05). Bilateral microinjection of bicuculline (50 pmol/50 nl, n = 5) into the PVN augmented the RSNA and PNA response to peripheral chemoreceptor stimulation (P < 0.05). Conversely, the GABA agonist muscimol (0.2 nmol/50 nl, n = 5) injected into the PVN attenuated these reflex responses (P < 0.05). Blocking neurotransmission within the PVN had no effect on the hypercapnia-induced central chemoreflex responses in carotid body denervated animals. These results suggest a selective role of the PVN in processing the sympathoexcitatory and ventilatory component of the peripheral, but not central, chemoreflex.     

Injection of muscimol in dorsomedial hypothalamus and stress-induced Fos expression in paraventricular nucleus

Prior microinjection of the GABA(A)-receptor agonist muscimol into the dorsomedial hypothalamus (DMH) in conscious rats attenuates the increases in heart rate, blood pressure, and circulating adrenocorticotrophic hormone seen in air stress. Here, we examined the effect of similar treatment on air stress- or hemorrhage-induced Fos expression in the paraventricular nucleus (PVN). Muscimol (80 pmol/100 nl per side) or saline (100 nl per side) was microinjected bilaterally into the DMH in conscious rats before either air stress, an emotional or neurogenic stressor, or graded hemorrhage, a physiological stressor. Each stressor evoked a characteristic pattern of Fos expression in the parvocellular and magnocellular PVN after saline. Injection of muscimol into the DMH suppressed Fos expression in the PVN associated with air stress but not with hemorrhage. Injection of muscimol at sites anterior to the DMH and closer to the PVN had no effect on Fos expression in the PVN after air stress. Thus activation of neurons in the DMH is necessary for excitation of neurons in the PVN during air stress but not during hemorrhage.     

Role of paraventricular nucleus in the cardiogenic sympathetic reflex in rats

Myocardial ischemia stimulates cardiac spinal afferents to initiate a sympathoexcitatory reflex. However, the pathways responsible for generation of increased sympathetic outflow in this reflex are not fully known. In this study, we determined the role of the paraventricular nucleus (PVN) in the cardiogenic sympathetic reflex. Renal sympathetic nerve activity (RSNA) and blood pressure were recorded in anesthetized rats during epicardial application of 10 microg/ml bradykinin. Bilateral microinjection of muscimol (0.5 nmol), a GABA(A) receptor agonist, was performed to inhibit the PVN. In 10 vehicle-injected rats, epicardial bradykinin significantly increased RSNA 178.4 +/- 48.5% from baseline, and mean arterial pressure from 76.9 +/- 2.0 to 102.3 +/- 3.3 mmHg. Microinjection of muscimol into the PVN significantly reduced the basal blood pressure and RSNA (n = 12). After muscimol injection, the bradykinin-induced increases in RSNA (111.6 +/- 35.9% from baseline) and mean arterial pressure (61.2 +/- 1.3 to 74.5 +/- 2.7 mmHg) were significantly reduced compared with control responses. The response remained attenuated even when the basal blood pressure was restored to the control. In a separate group of rats (n = 9), bilateral microinjection of the ionotropic glutamate antagonist kynurenic acid (4.82 or 48.2 nmol in 50 nl) had no significant effect on the RSNA and blood pressure responses to bradykinin compared with controls. These results suggest that the tonic PVN activity is important for the full manifestation of the cardiogenic sympathoexcitatory response. However, ionotropic glutamate receptors in the PVN are not directly involved in this reflex response.     

The Notch effector gene Hes1 regulates migration of hypothalamic neurons, neuropeptide content and axon targeting to the pituitary

Proper development of the hypothalamic-pituitary axis requires precise neuronal signaling to establish a network that regulates homeostasis. The developing hypothalamus and pituitary utilize similar signaling pathways for differentiation in embryonic development. The Notch signaling effector gene Hes1 is present in the developing hypothalamus and pituitary and is required for proper formation of the pituitary, which contains axons of arginine vasopressin (AVP) neurons from the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). We hypothesized that Hes1 is necessary for the generation, placement and projection of AVP neurons. We found that Hes1 null mice show no significant difference in cell proliferation or death in the developing diencephalon at embryonic day 10.5 (e10.5) or e11.5. By e16.5, AVP cell bodies are formed in the SON and PVN, but are abnormally placed, suggesting that Hes1 may be necessary for the migration of AVP neurons. GAD67 immunoreactivity is ectopically expressed in Hes1 null mice, which may contribute to cell body misplacement. Additionally, at e18.5 Hes1 null mice show continued misplacement of AVP cell bodies in the PVN and SON and additionally exhibit abnormal axonal projection. Using mass spectrometry to characterize peptide content, we found that Hes1 null pituitaries have aberrant somatostatin (SS) peptide, which correlates with abnormal SS cells in the pituitary and misplaced SS axon tracts at e18.5. Our results indicate that Notch signaling facilitates the migration and guidance of hypothalamic neurons, as well as neuropeptide content.     

Three dimensional observation of the nerve fibers along the cerebral blood vessels

Three dimensional observation of the nerve fibers along the cerebral blood vessels was investigated by scanning electron microscopy. Electron probe X-ray microanalysis was also performed in the cerebral blood vessels treated with peroxidase-antiperoxidase immunohistochemistry intensified by nickel ammonium sulfate. Nerve fibers (2-8 microns in diameter) formed a plexus on the outer surface of the adventitia. After branching, the nerve fibers penetrated the blood vessel adventitia. Substance P-immunoreactive nerve fibers showed a meshwork pattern in the outer layer of the adventitia, and vasoactive intestinal polypeptide (VIP)-immunoreactive nerve fibers revealed a spiral running pattern in the inner layer of the adventitia. Taken together with previous studies, these findings suggest that substance P nerve fibers in the cerebral arteries may not be related to arterial dilatation or constriction, but VIP nerve fibers may be vasodilative.     

Target-specific regulation of synaptic efficacy in the feeding central pattern generator of Aplysia: potential substrates for behavioral plasticity?

The contributions to this symposium are unified by their focus on the role of synaptic plasticity in sensorimotor learning. Synaptic plasticities are also known to operate within the central pattern generator (CPG) circuits that produce repetitive motor programs, where their relation to adaptive behavior is less well understood. This study examined divergent synaptic plasticity in the signaling of an influential interneuron, B20, located within the CPG that controls consummatory feeding-related behaviors in Aplysia. Previously, B20 was shown to contain markers for catecholamines and GABA (Díaz-Ríos et al., 2002), and its rapid synaptic signaling to two follower motor neurons, B16 and B8, was found to be mediated by dopamine (Díaz-Ríos and Miller, 2005). In this investigation, two incremental forms of increased synaptic efficacy, facilitation and summation, were both greater in the signaling from B20 to B8 than in the signaling from B20 to B16. Manipulation of the membrane potentials of the two postsynaptic motor neurons did not affect facilitation of excitatory postsynaptic potentials (EPSPs) to either follower cell. Striking levels of summation in B8, however, were eliminated at hyperpolarized membrane potentials and could be attributed to distinctive membrane properties of this postsynaptic cell. GABA and the GABAB agonist baclofen increased facilitation and summation of EPSPs from B20 to B8, but not to B16. The enhanced facilitation was not affected when the membrane potential of B8 was pre-set to hyperpolarized levels, but GABAergic effects on summation were eliminated by this manipulation. These observations demonstrate a target-specific amplification of synaptic efficacy that can contribute to channeling the flow of divergent information from an intrinsic interneuron within the buccal CPG. They further suggest that GABA, acting as a cotransmitter in B20, could induce coordinated and target-specific pre- and postsynaptic modulation of these signals. Finally, we speculate that target-specific plasticity and its modulation could be efficient, specific, and flexible substrates for learning-related modifications of CPG function.     

Effects of stress and tranylcypromine on amphetamine-induced locomotor activity and GABA(B) receptor function in rat brain

Modification in gamma-aminobutyric acid-B (GABA(B)) receptors may contribute to the symptoms of some neurological and psychiatric disorders and to the clinical response to psychotherapeutics. The present study was undertaken to determine whether chronic administration of tranylcypromine (TCP), an antidepressant, and chronic stress influence GABA(B) receptor function in rat brain. The results indicate that TCP treatment, but not stress, increases GABA(B) receptor activity in the cerebral cortex, as measured by baclofen-stimulated GTPgammaS binding. In addition, chronic administration of TCP enhances significantly the locomotor response to a single dose of amphetamine, an effect that is abolished by restraint stress. These results indicate that although TCP administration modifies brain GABA(B) receptor activity, which may contribute to the antidepressant response to this agent, this effect is unrelated to the interaction of stress and TCP treatment on the locomotor response to amphetamine.     

GABAB receptor association with the PDZ scaffold Mupp1 alters receptor stability and function

gamma-Aminobutyric acid, type B (GABA(B)) receptors are heterodimeric G protein-coupled receptors that mediate slow inhibitory synaptic transmission in the central nervous system. To identify novel interacting partners that might regulate GABA(B) receptor (GABA(B)R) functionality, we screened the GABA(B)R2 carboxyl terminus against a recently created proteomic array of 96 distinct PDZ (PSD-95/Dlg/ZO-1 homology) domains. The screen identified three specific PDZ domains that exhibit interactions with GABA(B)R2: Mupp1 PDZ13, PAPIN PDZ1, and Erbin PDZ. Biochemical analysis confirmed that full-length Mupp1 and PAPIN interact with GABA(B)R2 in cells. Disruption of the GABA(B)R2 interaction with PDZ scaffolds by a point mutation to the carboxyl terminus of the receptor dramatically decreased receptor stability and attenuated the duration of GABA(B) receptor signaling. The effects of mutating the GABA(B)R2 carboxyl terminus on receptor stability and signaling were mimicked by small interference RNA knockdown of endogenous Mupp1. These findings reveal that GABA(B) receptor stability and signaling can be modulated via GABA(B)R2 interactions with the PDZ scaffold protein Mupp1, which may contribute to cell-specific regulation of GABA(B) receptors in the central nervous system.