Somatic depressor reflexes: results of specific 'depressor' afferents' excitation or an epiphenomenon of general anesthesia and certain decerebrations?

Changes in blood pressure induced by electrical stimulation of the femur in anesthetized rats

The effects of electrical stimulation of the femur, on blood pressure, were examined in anesthetized rats. Two small holes, 3-4 mm apart, were manually drilled into the femur down to the bone marrow. Following this, two stainless-steel electrodes were inserted into the holes, and an electrical square wave current was passed between the electrodes. In central nervous system-intact rats, electrical stimulation of the femur at 5 and 10 mA at 20 Hz for 20 s produced an intensity-dependent decrease in mean arterial blood pressure. This response was abolished by severance of the femoral and sciatic nerves ipsilateral to the stimulation. Furthermore, the renal sympathetic efferent nerve activities (as a representative index of vasoconstrictor activities) decreased following the electrical stimulation of the femur. However, in acutely-spinalized rats (spinalized at the cervical level) the same stimulation increased renal sympathetic efferent nerve activities and mean arterial blood pressure. It was concluded that high-intensity electrical stimulation of the femur reflexively affected blood pressure. It can be inferred that the osteal high-threshold receptors and/or fibers are involved in the afferent nerve pathway, and the efferent nerve pathway is the sympathetic vasoconstrictor nerve. The excitatory response properties at the propriospinal level are modified into an inhibitory response by supraspinal structures.

Neural mechanism of bradycardiac responses elicited by acupuncture-like stimulation to a hind limb in anesthetized rats

The effects of acupuncture-like stimulation of a hind limb on heart rate were examined in anesthetized rats. An acupuncture needle, having a diameter of either 160 or 340 microm, was inserted into the skin and underlying muscles at a depth of about 5 mm and twisted right and left twice every second for 1 min. Stimulation by a needle with a diameter of either 160 or 340 microm produced a decrease in heart rate. Severance of the femoral and sciatic nerves ipsilateral to the hind-limb stimulation completely abolished the bradycardiac response. Also, heart rate was significantly decreased by acupuncture-like stimulation of the hind-limb muscles alone, but was not significantly influenced by the stimulation of the hind-limb skin alone. The bradycardiac response induced by acupuncture-like stimulation was not influenced by bilateral severance of the vagal nerves at the cervical level, but was abolished by bilateral stellectomy. Acupuncture-like stimulation of the hind limb induced a decrease in the activity of the cardiac sympathetic efferent nerve as well as a decrease in heart rate. These results indicate that the decrease in heart rate induced by acupuncture-like stimulation of a hind limb is a reflex response. The afferent pathway is composed of hind-limb muscle afferents, and the efferent pathway is composed of cardiac sympathetic nerves.

Effects of L-arginine-derived nitric oxide synthesis on cardiovascular responses to stimulus-evoked somatosympathetic reflexes in the gracile nucleus

The purpose of these studies was to determine the role of gracile nucleus (Gr) and the effects of L-arginine-derived nitric oxide (NO) synthesis in the nucleus on the cardiovascular responses to somatosympathetic reflexes (SSR). Electrical stimulation of sural and tibial nerve to evoke excitatory and inhibitory SSR was carried out in anesthetized Sprague-Dawley rats. Arterial blood pressure and heart rate were monitored during stimulus-evoked SSR following microinjections of the agents into Gr. Cardiovascular responses to electrical stimulation of the sural and tibial nerves were blocked by microinjection of lidocaine into Gr. The hypertensive and tachycardiac responses to stimulation of the sural nerve were attenuated by bilateral microinjection of L-arginine into Gr, but enhanced by the presence of nNOS antisense oligodeoxynucleotides (oligos) in the area. Microinjection of L-arginine into Gr facilitated the hypotensive and bradycardic responses to stimulation of the tibial nerve while pretreatment with nNOS antisense oligos into Gr attenuated the tibial stimulation evoked inhibitory SSR. The stimulus-evoked responses were not altered by microinjection of nNOS sense oligos into Gr. The results show that the cardiovascular responses to stimulus-evoked SSR were inhibited by the presence of a blockade of neuronal conduction in the Gr. L-Arginine-derived NO synthesis in the Gr attenuates the cardiovascular responses to stimulus-evoked excitatory SSR and facilitates the responses to inhibitory SSR. We conclude that NO in the Gr plays an inhibitory role in the central cardiovascular control through SSR regulation.

The effect of electro-acupuncture stimulation on the muscle blood flow of the hindlimb in anesthetized rats

The effect of electro-acupuncture stimulation (EAS) on blood flow in the muscle biceps femoris (MBF) and on mean arterial pressure (MAP) was investigated in anesthetized, artificially ventilated rats. EAS was applied to a hindpaw for 30 s at intensities of 0.1-10.0 mA and at frequencies of 1-20 Hz, and MBF was measured by laser Doppler flowmetry. EAS at less than 1.0 mA, which excited group II fibers maximally and III fibers partially in a saphenous nerve, had no significant effect on MBF or MAP, although both revealed variable responses. EAS at 1.5 mA, which additionally excited group III fibers almost maximally and was subthreshold for group IV fibers, produced a small but significant increase in MBF and MAP. These responses were further increased at 2.0 mA or more, which was suprathreshold for group IV fibers. The increased response of MBF at 10.0 mA was followed by a small decrease in MBF. EAS at 1.5 mA or more also elicited a decrease in renal blood flow (RBF) and an arterial pressor response. Following severance of the bilateral splanchnic nerves, EAS at 10.0 mA induced only a slight increase in MAP and a decrease in MBF. The decrease in MBF was abolished following further severance of the bilateral lumbar sympathetic trunks (LSTs). In conclusion, EAS to a hindpaw at a stimulus strength sufficient to excite group III and IV afferent fibers, particularly group IV afferent fibers, can produce a reflex decrease in MBF via a reflex activation of muscle sympathetic activity, although this decrease in MBF is overridden by an increase in MBF caused passively by a reflex MAP pressor response elicited by a reflex increase, at least in splanchnic sympathetic activity.

Somatocardiovascular reflexes in anesthetized rats with the central nervous system intact or acutely spinalized at the cervical level

The effects of noxious mechanical stimulation of various segmental areas on heart rate and mean arterial blood pressure (MAP), as well as cardiac and renal sympathetic nerve activities were examined in anesthetized rats with the central nervous system (CNS) intact or acutely spinalized at the cervical level. In CNS-intact rats, pinching for 20 s applied to any segmental skin area, but particularly that of the paw, produced an increase in heart rate, blood pressure and the sympathetic nerve activities. In acutely spinalized rats, pinching the chest, abdomen and back of the body produced large increases, while hindlimb and perineum stimulation induced only a small increase or no increase in heart rate, blood pressure and the sympathetic nerve activities. Stimulation of the right side produced particularly large responses in heart rate and stimulation of the ipsilateral side produced large responses in cardiac and renal sympathetic nerve activities in spinalized rats. These results suggest the existence of the two types of reflex responses, supraspinal and propriospinal, in the somatocardiovascular reflex. The supraspinal one has characteristics of diffuse reflex organization, while the propriospinal one has strong segmental and lateral organization.

Somatic and vagal afferent convergence on solitary tract neurons in cat: electrophysiological characteristics

Electrophysiological characteristics are described for 67 neurones localized to subnuclei of the solitary tract or the area of the dorsal motor nucleus of the vagus in alpha-chloralose-anesthetized, paralysed cats which received vagal and hindlimb sural or peroneal nerve excitation. The peroneal and sural nerves were stimulated in an exposed hindlimb preparation; the ipsilateral vagus was stimulated at the cervical level. Compound action potentials were recorded from all three nerves. Neurons were recorded with extracellular microelectrodes from the brain stem solitary area contralateral to the stimulated somatic nerves. Ninety-one percent of the recorded neurons were spontaneously active. Eighteen percent and 5% of the neurons received only peroneal or sural excitation, respectively, while 59% of the neurons received convergent peroneal and sural excitation. Thirty-nine of the 67 neurons were also tested for vagal input of which 41% responded with excitation. All of the neurons tested for vagal input also received converging excitation from one or both of the somatic nerves. Thirty-one percent of the vagal-excited neurons received converging input from both the peroneal and sural nerves. The combined mean minimal conduction velocity for peroneal and sural input was 31 +/- 1 m/s (mean +/- 1 S.E., range 9-54 m/s). Thirty-six percent of the peroneal and 31% of the sural afferents were Group II fibers. Significant periods of inhibition of spontaneous neuronal spike activity followed peroneal and sural excitation in 43 and 39% of the neurons, respectively. In many neurons, both excitation and inhibition of spike activity could be elicited at stimulus intensities as low as 1.2 times threshold for the lowest threshold fibers in each nerve. Somatic nerve-induced inhibition of spontaneous neuron activity without prior excitation was also observed. These results suggest that neurons of the solitary tract nuclei receive Group II and Group III somatic afferents which converge on neurons also receiving excitatory vagal input. Consequently, somesthetic and kinesthetic as well as visceral receptor activation may directly modulate solitary tract neurons. A possible conclusion is that the nucleus tractus solitarius is the initial central site of mediation of somatosympathetic reflexes. Modulation of the nucleus tractus solitarius by somatic afferents may then adjust sympathetic tone, via modulation of other medullary centers, in visceral and somatic tissues to match somatic metabolic needs.

Depressor and bradycardic effects induced by spinal subarachnoid injection of D-Ala2-D-Leu5-enkephalin in rats

The effects of D-Ala2-D-Leu5-enkephalin (DADLE), a specific delta receptor agonist, on spinal control of cardiovascular function, were investigated by its intrathecal (i.th) injection into the spinal subarachnoid space at the T-9 level. In chloralose-anesthetized rats, DADLE (17.5, 35 and 70 nmol, i.th) caused dose-dependent hypotension and bradycardia. The mean maximal hypotension by 70 nmol of DADLE was -45 +/- 7 mmHg, with a bradycardia of -79 +/- 15 beats/min. These inhibitory cardiovascular effects were antagonized by the opiate antagonist naloxone (50 nmol, i.th.) given prior to DADLE. Intrathecal injection of DADLE also decreased splanchnic sympathetic nerve discharge (-46 +/- 5%). DADLE (70 nmol) given i.v. did not cause significant changes in mean arterial pressure (MAP) and heart rate (HR). Neither bilateral vagotomy nor pretreatment with atropine (0.2 mg/kg, i.v.) prevented the BP and HR effects of intrathecal injection of DADLE at a dose of 35 nmol. DADLE at this dose failed to produce significant alteration in the frequency of respiration and blood PaO2, PaCO2 and blood pH. In conscious rats, 140 nmol of DADLE (i.th.) did not produce any consistent changes in MAP and HR. These data suggest that intrathecal injection of DADLE inhibits central sympathetic activity, possibly at a spinal locus.

Attenuation of the reflex pressor response to muscular contraction by an antagonist to somatostatin

Although group III and IV fibers are known to compose the afferent pathway of the reflex arc causing the pressor response to static muscular contraction, little is known about the neurotransmitters released by these muscle afferents. Somatostatin might be one of these neurotransmitters because this peptide is found in the terminals of fine afferent fibers ending in the dorsal horn of the lumbar spinal cord. Therefore, in chloralose-anesthetized cats, the reflex pressor response to static contraction was examined before and after subarachnoid injections onto the lumbosacral cord of a peptide antagonist to somatostatin. We found that before giving the antagonist, the pressor response to contraction of the triceps surae muscles in 12 cats averaged 33 +/- 4 mm Hg, while 37 +/- 7 minutes after giving the antagonist, the pressor response averaged only 18 +/- 3 mm Hg (p less than 0.001). In contrast, the antagonist to somatostatin had no effect on either the pressor response to electrical stimulation of the cut central end of the sciatic nerve or the pressor response to stimulation of the posterior diencephalon. Furthermore, subarachnoid injection of a peptide antagonist to luteinizing hormone-releasing hormone had no effect on the reflex pressor response to static contraction. Our findings are consistent with the hypothesis that somatostatin plays a role in the spinal transmission of the contraction-induced pressor reflex arising from hind limb skeletal muscle.