K. Michael Spyer

CENTRAL RESPIRATORY DRIVE-RELATED ACTIVITY IN SYMPATHETIC-NERVES OF THE RAT - THE REGIONAL DIFFERENCES

In halothane-anaesthetized, vagotomized, SA-denervated rats, the activity of various sympathetic nerves has been analyzed with respect to phrenic nerve discharge (an indicator of central respiratory drive (CRD)). The cervical and lumbar sympathetic nerves had maximal activity following, and were least active during phrenic nerve discharge. In contrast, the splanchnic, cardiac, renal and adrenal nerves exhibited their activity peak during phrenic nerve discharge (i.e. inspiration). Similar activity profiles were observed after ganglion blockade in the mixed pre- and postganglionic fibre preparations. These observations indicate that it is the subpopulations of preganglionic neurones and the proportional contribution of each to whole-nerve activity which give rise to the differences in CRD-related activity profiles between nerves.

5-Hydroxytryptamine-3 receptors modulate synaptic activity in the rat nucleus tractus solitarius in vitro.

Whole-cell patch clamp recordings were made from neurons in the rat nucleus tractus solitarius (NTS) in transverse brainstem slices. 5-Hydroxytryptamine (5-HT, 100 microM) and the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-CH3-5-HT, 100 microM) depolarized 86% of NTS neurons at resting membrane potential (Vm). This response was resistant to tetrodotoxin (TTX) and Co2+ application. In addition, 2-CH3-5-HT (500 nM-100 microM) increased the amplitude and frequency of both excitatory and inhibitory spontaneous synaptic potentials. This effect was also TTX-resistant, but was abolished by Co2+. The effects of 2-CH3-5-HT on EPSPs and IPSPs evoked by electrical stimulation of the tractus solitarius (TS) were analyzed separately in the presence of bicuculline or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively. Concentrations of 2-CH3-5-HT between 500 nM and 1 microM decreased the amplitude of evoked EPSPs and IPSPs with similar potency. The selective 5-HT3 receptor antagonists ICS 205-930 (10 nM) and MDL 72222 (10 microM) reversibly blocked the effects of 2-CH3-5-HT at all doses examined. It is concluded that 5-HT3 receptors can mediate both pre- and postsynaptic responses in the NTS.

Opiate receptor subtypes in the nucleus tractus solitarii of the cat: the effect of vagal section

The distribution of mu, delta and kappa opioid receptors in the lower brainstem of the cat has been determined autoradiographically by studying the binding of tritiated [D-Ala2,MePhe4,Glyol5][tyrosyl-3,5-3H]enkephalin (DAGO), [D-Pen2,D-Pen5][tyrosyl-3,5-(n)-3H]enkephalin (DPDPE) and [9-3H]ethylketazocine (EKC), respectively. General opiate receptor binding was established using [3H]naloxone (NX). High densities of [3H]NX and DAGO binding sites were found most prominently in the nucleus tractus solitarii. There was no DPDPE and very weak EKC binding within this nucleus, although both these ligands bound to the cerebellum. The effect of unilateral vagotomy on receptor density was examined. Sectioning the cervical vagus had no effect on the density of mu receptors in the brainstem. Sectioning the vagus, accompanied by nodose ganglion excision, led to a marked depletion of mu receptors which was restricted to dorsal and medial regions of the ipsilateral nucleus tractus solitarii at, and rostral to, the obex. These results suggest that mu opiate receptors are located presynaptically on vagal afferents terminating within a restricted region of the nucleus tractus solitarii.

Interactive responses to stimulation of the amygdaloid central nucleus and baroreceptor afferent activation in the rabbit.

Recent evidence suggests that the amygdaloid central nucleus (ACE) may contribute to the regulation of arterial baroreceptor-vagal reflex sensitivity. To obtain additional data relevant to this suggestion, interactions between stimulation of the ACE and arterial baroreceptor afferent activation were examined. New Zealand rabbits were anesthetized with alpha-chloralose and a stimulating electrode was implanted stereotaxically in the ACE. In the first series of experiments, cardiovascular responses to stimulation of the ACE were assessed during periods in which blood pressure was decreased using sodium nitroprusside or increased in the rostral arterial compartment by inflating the tip of a Swan-Ganz catheter positioned in the descending aorta. It was found that the magnitude of bradycardia to stimulation of the ACE was correlated with the level of arterial blood pressure at the onset of stimulation, such that higher blood pressures were associated with larger bradycardic responses. These data suggest that arterial baroreceptor afferent activity may be an important factor in the elicitation of bradycardia from the ACE. In the second series of experiments, the aortic nerve was isolated, and cardiovascular responses to stimulation of the aortic nerve and to low frequency (5 Hz) stimulation of the ACE were assessed both alone and in combination. Stimulation of the aortic nerve during low frequency stimulation of the ACE was found to produce bradycardia of a significantly larger magnitude than the sum of the responses produced by each stimulus presented alone. These data demonstrate that bradycardia to stimulation of baroreceptor afferent fibers is augmented significantly during low frequency stimulation of the ACE. Taken together, these results are consistent with the notion that the ACE may contribute to the sensitivity of the arterial baroreceptor-vagal reflex.

The effect of 5‐HT and selective 5‐HT receptor agonists and antagonists on rat dorsal vagal preganglionic neurones in vitro

1 Whole‐cell patch‐clamp recordings were made from 142 visually identified rat dorsal vagal preganglionic neurones (DVMs). Applications of 5‐hydroxytryptamine (5‐HT, 20 μm, 2 min) elicited a slow depolarization (8.2 ± 0.5 mV, n = 59) in 95% of the cells tested, accompanied by an increase in excitability. In (68%) of DVMs the depolarization was associated with an increase in apparent membrane resistance (Rm, 22.7 ± 2.2%). These depolarizations and increases in Rm (14.3 ± 2.6%, n = 8) were maintained in a medium which blocked synaptic transmission. 2 The response to 5‐HT was associated with a reversal potential (Erev) of −91 ± 1 mV at an extracellular K+ concentration ([K+]o) of 4.2 mM. This correlated well with the K+ equilibrium potential (EK = −89 mV). 3 The depolarizing effect of 5‐HT was attenuated by the 5‐HT2A/2C receptor antagonists, ketanserin (1 μm), LY 53,857 (1 μm) and the 5‐HT1A/2A receptor antagonist, spiperone (1 γM). The 5‐HT1A receptor antagonist, pindobind 5‐HT1A (5 μm), had no effect on the depolarizing response to 5‐HT. 4 The effect of 5‐HT was mimicked by the 5‐HT2A/2C receptor agonist, α‐methyl‐5‐HT (50 μm), the 5‐HT1 receptor agonist, 5‐carboxamidotryptamine (20 μm) and the putative 5‐HT4 agonist, 5‐methyoxytryptamine (50 μm). The selective 5‐HT4 receptor antagonist, GR113808, had no effect on the depolarizing effect of 5‐HT or 5‐MEOT on DVMs. 5 The 5‐HT3 antagonists, MDL 72222 (10 μm) and ICS‐205–930 (1 and 10 μm), partially reduced the effect of 5‐HT. The 5‐HT3 receptor agonist, 2‐methyl‐5‐HT (100–300 μm), excited a proportion of neurones tested (56%) by evoking a depolarizing and/or an increase in postsynaptic potentials (p.s.ps). 6 These results are consistent with direct, postsynaptic actions of 5‐HT on DVMs via 5‐HT2A receptors, being mediated, in part, by the reduction of K+ conductance.

THE INFLUENCE OF LOBULE IX OF THE CEREBELLAR POSTERIOR VERMIS ON THE BARORECEPTOR REFLEX IN THE DECEREBRATE RABBIT

Stimulation of lobule IX (the uvula) of the cerebellar posterior vermis, either electrically or chemically, has been shown to evoke marked cardiovascular effects in both rabbit and cat. Recent experiments suggest that this stimulation can also reduce the responsiveness of neurons in the nucleus tractus solitarii to baroreceptor inputs. The aim of the present study was thus to determine whether the uvula exerts an influence on the baroreceptor reflex. In order to evoke a reflex bradycardia in decerebrate rabbits, blood pressure was increased by inflating a Swan-Ganz catheter positioned in the descending aorta, and in some rabbits the aortic nerve was also stimulated electrically. In most rabbits, following uvula removal, significantly larger reflex falls in heart rate were observed. In a second group of rabbits, a lesion of cerebellar lobules VI-VIII had no effect on reflex bradycardia. It is concluded that the uvula tonically depresses the cardioinhibitory component of the baroreceptor reflex in the rabbit. The possible functional implications of these results are discussed.

Autoradiographic localization of α-adrenoceptors, muscarinic acetylcholine receptors and opiate receptors in the heart

Using in vitro autoradiography the distribution of [3H]rauwolscine, [3H]prazosin and [3H]quinuclidinyl benzilate binding sites has been demonstrated in cardiac tissue taken from the cat and rat. A similar distribution of both alpha 1- and alpha 2-adrenoceptor sites was seen but the distribution of muscarinic acetylcholine sites was markedly different. alpha-Adrenoceptors were present predominantly in ventricular muscle whereas muscarinic acetylcholine receptors exhibited a greater density in atrial tissue compared to ventricular muscle. Opiate receptors were absent from cardiac tissue.