gamma-Aminobutyric acid antagonist blocks baroreceptor-activated inhibition of neurosecretory cells in the hypothalamic paraventricular nucleus of rats

Cardiovascular Neuroendocrinology: Emerging Role for Neurohypophyseal Hormones in Pathophysiology

Arginine vasopressin (AVP) and oxytocin (OXY) are released by magnocellular neurosecretory cells that project to the posterior pituitary. While AVP and OXY currently receive more attention for their contributions to affiliative behavior, this mini-review discusses their roles in cardiovascular function broadly defined to include indirect effects that influence cardiovascular function. The traditional view is that neither AVP nor OXY contributes to basal cardiovascular function, although some recent studies suggest that this position might be re-evaluated. More evidence indicates that adaptations and neuroplasticity of AVP and OXY neurons contribute to cardiovascular pathophysiology.

Neuropeptide FF and neuropeptide VF inhibit GABAergic neurotransmission in parvocellular neurons of the rat hypothalamic paraventricular nucleus

Neuropeptide FF (NPFF) and neuropeptide VF (NPVF) are octapeptides belonging to the RFamide family of peptides that have been implicated in a wide variety of physiological functions in the brain, including central autonomic and neuroendocrine regulation. The effects of these peptides are mediated via NPFF1 and NPFF2 receptors that are abundantly expressed in the rat brain, including the hypothalamic paraventricular nucleus (PVN), an autonomic nucleus critical for the secretion of neurohormones and the regulation of sympathetic outflow. In this study, we examined, using whole cell patch-clamp recordings in the brain slice, the effects of NPFF and NPVF on inhibitory GABAergic synaptic input to parvocellular PVN neurons. Under voltage-clamp conditions, NPFF and NPVF reversibly and in a concentration-dependent manner reduced the evoked bicuculline-sensitive inhibitory postsynaptic currents (IPSCs) in parvocellular PVN neurons by 25 and 31%, respectively. RF9, a potent and selective NPFF receptor antagonist, blocked NPFF-induced reduction of IPSCs. Recordings of miniature IPSCs in these neurons following NPFF and NPVF applications showed a reduction in frequency but not amplitude, indicating a presynaptic locus of action for these peptides. Under current-clamp conditions, NPVF and NPFF caused depolarization (6-9 mV) of neurons that persisted in the presence of TTX but was abolished in the presence of bicuculline. Collectively, these data provide evidence for a disinhibitory role of NPFF and NPVF in the hypothalamic PVN via an attenuation of GABAergic inhibitory input to parvocellular neurons of this nucleus and explain the central autonomic effects of NPFF.

Single-unit activity of paraventricular nucleus neurons in response to intero- and exteroceptive stressors in conscious, freely moving rats

Extracellular recordings of 114 neurons in the hypothalamic paraventricular nucleus (PVN) of conscious, freely moving male rats were performed using a movable electrode system. Single-unit activities were examined for their spontaneous firing patterns and responses to intero- and exteroceptive stressors, including disturbance in arterial blood pressure, water deprivation, air-jet stimulation, and systemic administration of cholecystokinin-8 (CCK). PVN neurons were assigned to one of two groups on the basis of their spontaneous firing patterns: phasic (n=29) and non-phasic (n=85). Intravenous (i.v.) administration of phenylephrine (8 microg/kg) resulted in the inhibition of a greater percentage of phasic-type (88.9%; 24/27) than non-phasic-type neurons (14.9%; 11/74). Most phasic-type neurons showed excitation in response to i.v. administration of sodium nitroprusside (20 microg/kg, 66.7%; 18/27) and water deprivation (15 h, 77.8%; 7/9) when compared to non-phasic-type neurons. Conversely, a greater number of non-phasic-type neurons showed excitation in response to air-jet stimulation (5 l/min, 10 s, 29.0%; 20/69) and to i.v. administration of CCK (5 microg/kg, 24.5%; 11/45) when compared to phasic-type neurons. However, most non-phasic-type neurons that demonstrated excitation in response to i.v. administration of CCK (88.9%; 8/9) did not respond to air-jet stimulation. The present study indicated that phasically firing neurons recorded from the PVN in conscious, freely moving rats are putative vasopressin-secreting neurons on the basis of their responses to intero- and exteroceptive stressors. These data contribute to our understanding of local neural mechanisms within the PVN that are responsible for stress responses in conscious rats.

Heart failure and the brain: new perspectives

Despite recent therapeutic advances, the prognosis for patients with heart failure remains dismal. Unchecked neurohumoral excitation is a critical element in the progressive clinical deterioration associated with the heart failure syndrome, and its peripheral manifestations have become the principal targets for intervention. The link between peripheral systems activated in heart failure and the central nervous system as a source of neurohumoral drive has therefore come under close scrutiny. In this context, the forebrain and particularly the paraventricular nucleus of the hypothalamus have emerged as sites that sense humoral signals generated peripherally in response to the stresses of heart failure and contribute to the altered volume regulation and augmented sympathetic drive that characterize the heart failure syndrome. This brief review summarizes recent studies from our laboratory supporting the concept that the forebrain plays a critical role in the pathogenesis of ischemia-induced heart failure and suggesting that the forebrain contribution must be considered in designing therapeutic strategies. Forebrain signaling by neuroactive products of the renin-angiotensin system and the immune system are emphasized.

Nitric oxide control of drinking, vasopressin and oxytocin release and blood pressure in dehydrated rats

Intracerebroventricular (i.c.v.) injection of the inhibitor of NO synthase (NOS), N(G)-nitro-L-arginine methyl ester (L-NAME) (250 microg/5 microL) attenuated the drinking response in rats deprived of water for 24 h. Moreover, oxytocin (OT) levels in plasma increased after 2 min, whereas both oxytocin and vasopressin levels were elevated at 120 min after intracerebroventricular injection. The delayed effect of L-NAME on both hormones was not observed in dehydrated animals allowed to drink water. Blood pressure remained stable after injection of artificial cerebrospinal fluid (aCSF) in dehydrated rats not allowed to drink. In rats having access to water, however, there was an immediate but transient pressor response (0-5 min) with a delayed hypotension from 45 to 120 min. L-NAME consistently increased blood pressure in a biphasic mode, whether the animals drank or not, with an early peak at 5 min that decayed after 15-30 min and a second pressor response beginning at 30-45 min and remaining elevated at 120 min when the experiment ended. These pressor responses were independent of the adrenal glands. Thus, centrally produced nitric oxide facilitates drinking, inhibits release of vasopressin and oxytocin from the magnocellular system, and maintains resting arterial blood pressure in normally hydrated and dehydrated rats.

Inhibition of spontaneous EPSCs and IPSCs by presynaptic GABAB receptors on rat supraoptic magnocellular neurons

1. The function of presynaptic GABA receptors in the regulation of transmitter release in supraoptic nucleus (SON) magnocellular neurons was investigated by recording spontaneous postsynaptic currents from rat magnocellular SON neurons in a slice preparation (150 microns thick, 1.8 mm in diameter) using the whole-cell patch-clamp technique. 2. Both the spontaneous EPSCs and IPSCs were TTX resistant. The EPSCs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas the IPSCs were abolished by picrotoxin, suggesting that the EPSCs and IPSCs are synaptic inputs from glutamatergic and GABAergic neurons, respectively. 3. The selective GABAB agonist, baclofen, reduced the frequency of both the EPSCs and IPSCs without affecting the amplitude. The time constant of the decay phase of both the EPSCs and IPSCs remained unchanged after baclofen application. 4. The reduction of the frequency of the synaptic currents by baclofen was dose dependent (10 nM to 100 microM) and the EC50 values were 5.8 and 8.5 microM for the EPSCs and IPSCs, respectively. 5. The effect of baclofen (10 microM) was antagonized by the selective GABAB antagonist, 2-hydroxy-saclofen (2OH-saclofen), at 300 microM. 6. When given alone, 2OH-saclofen (100 microM) increased the frequency of both the EPSCs and IPSCs without affecting their amplitude, suggesting that endogenously released GABA in the slice acts on presynaptic GABAB receptors. 7. The GABAA agonist, muscimol, reduced the frequency of EPSCs, and picrotoxin increased the frequency of the EPSCs, suggesting that GABAA receptors also participate in the presynaptic inhibition of glutamate release. 8. Taken together, these data suggest that GABAB receptors are present on the presynaptic terminals of both GABA and glutamate neurons in the SON, and that these presynaptic GABAB receptors play an important role in the regulation of the neuronal activity in SON magnocellular neurons.

The response of nucleus preopticus neurosecretory cells to ovarian pressure in the frog, Rana tigrina

Intraovarian pressure (IOP) of 5, 15, and 25 mm Hg was administered in the frog, Rana tigrina, and the response of the nucleus preopticus (NPO) pars magnocellularis was investigated with aldehyde fuchsin (AF) stain and immunocytochemical method using neurophysin (NP) antisera. The 5 mm Hg IOP treatment resulted in cell and cell nuclear hypertrophy (P less than 0.001); discrete signs of de novo synthesis of AF-positive and NP-immunoreactive material in the perikarya and remarkable increases in the number and size of "Herring bodies" in the processes were observed. Stimulatory response after 15 mm Hg IOP treatment was characterized by dramatic augmentation of the AF-positive and NP-immunoreactive material in the processes; the engorged and coalescing Herring bodies totally predominated the lateral preoptic area. IOP of 25 mm Hg resulted in extensive loss of secretory material; the processes revealed the presence of vacuoles indicative of the rapid anterograde transport of the neurosecretory material. Furthermore, the application of IOP seemed to promote the transport of NP-immunoreactive material toward the anterior preoptic area and amygdala pars medialis and the release of secretory material into the cerebrospinal fluid. No changes were observed in the NPO when treatment was preceded by the transection of ipsilateral dorsal spinal nerve roots or the spinal cord. The results suggest the existence of an afferent neural pathway from the ovary capable of conducting the stretch signals to the NPO and triggering the synthesis and release of neurohypophysial hormones.

Central inhibition by gamma-aminobutyric acid and muscimol of the release of vasopressin and oxytocin by an osmotic stimulus in the rat

1. In water-loaded rats under ethanol anaesthesia, the injection of 2-4 microliters 1.54M NaCl solution (hypertonic saline:HS) into a lateral cerebral ventricle (i.c.v.) produced an antidiuretic and a pressor response, together with increased urinary excretion of vasopressin and 'oxytocin-like radioimmunoreactivity' (OLRI). In lactating rats HS also produced a milk-ejection response which was shown to be due to the release of oxytocin. 2. The injection of 20-40 micrograms gamma-aminobutyric acid (GABA) or 40-80 ng muscimol i.c.v. 2 min before HS inhibited the antidiuretic, pressor and milk-ejection responses and reduced the urinary excretion of vasopressin and OLRI. 3. The pressor response to HS was abolished by a ganglion blocking agent but it was not reduced by a vasopressin antagonist. After the antagonist, the antidiuretic response to HS was abolished and the pressor response was accompanied by a diuresis both of which were blocked by muscimol. 4. The threshold dose of HS for an antidiuretic response was 4-8 times higher on injection into the cisterna magna (i.cist.) than when injected i.c.v. GABA, i.v. or i.cist, did not inhibit the response to HS i.c.v. 5. The results confirm other evidence that, in the rat, in contrast some other species, an osmotic stimulus causes release of both vasopressin and oxytocin. This release is blocked by GABA and muscimol. These drugs and HS act at a site reached not from the subarachnoid space but from the cerebral ventricles, probably the hypothalamus. The pressor response to HS under the experimental conditions used is due entirely to central sympathetic stimulation and this effect, as well as the release of vasopressin and oxytocin, is blocked by muscimol.

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of caudal ventrolateral medulla neurons in mediating visceroreceptive information to the hypothalamic paraventricular nucleus

Both neurohypophyseal and tuberoinfundibular neurosecretory neurons in the PVN received excitatory synaptic inputs from the CVLM. We electrophysiologically identified neurons in the CVLM which project to the PVN. On the basis of antidromic spike latencies, two different populations of neurons could be differentiated: slow- and fast-conducting cells. Slow-conducting cells which were presumed to be A1 catecholaminergic cells, received inhibitory and excitatory synaptic inputs from arterial baroreceptors and the cervical vagus nerve, respectively. Our results suggest that slow conducting cells in the CVLM which cause excitation of PVN neurons via a monosynaptic pathway, mediate visceroreceptive information to the PVN.

Cardiovascular input to hypothalamic neurosecretory neurons

In vivo extracellular recordings from rat supraoptic and paraventricular magnocellular neurosecretory cells (MNCs) indicate that putative vasopressin-secreting MNCs may be identified by an abrupt and brief cessation in firing consequent to a transient drug-induced rise in arterial pressure sufficient to activate arterial baroreceptors. In the diagonal band of Broca (DBB), a population of neurons projecting towards the supraoptic nucleus are activated during this drug-induced hypertension. Electrical stimulation in DBB selectively depresses supraoptic vasopressin-secreting MNCs. Intracellular recordings in perfused hypothalamic explants confirm a DBB-evoked bicuculline-sensitive and chloride-dependent postsynaptic inhibition, similar to that associated with the application of gamma-aminobutyric acid (GABA) in approximately half of supraoptic MNCs. Since bicuculline also selectively blocks baroreceptor-induced inhibition in supraoptic MNCs, it is proposed that the depressant baroreflex input to vasopressin-secreting MNCs involves a population of DBB neurons and GABAergic interneurons located close to MNCs. An excitatory and selective input to vasopressin-secreting MNCs follows chemoreceptor activation, possibly mediated by the A1 noradrenergic cell group in the ventrolateral medulla. Another excitatory input to both vasopressin- and oxytocin-secreting MNCs is triggered by circulating angiotensin II and appears to be relayed centrally through an angiotensinergic projection from the subfornical organ.