Subsequent Nerve Stimulation Research Articles

Role of capsaicin-sensitive neurons in catecholamine secretion from rat adrenal glands

Sensory fibres innervate the adrenal medulla but their function is not known. In the present study, we have used the sensory neurotoxin capsaicin to evaluate the effect of capsaicin sensitive sensory fibres on catecholamine (CA) secretion from isolated perfused rat adrenal glands. CA secretion in response to 1 and 10 min electrical field stimulation of adrenal nerve terminals was significantly attenuated in the adrenal glands of adult rats pretreated as neonates with capsaicin and was frequency dependent, being more pronounced at the higher frequencies of stimulation (5 to 30 Hz) than at the low (0.3, 1 Hz) frequencies. Perfusion of control rat adrenal glands with capsaicin did not evoke CA secretion, but did increase CA secretion in response to perfusion with nicotine. Perfusion with capsaicin for 30 min (but not for 4 min) reduced the CA secretory response to subsequent nerve stimulation. The results suggest that capsaicin sensitive sensory neurons innervating the adrenal medulla are involved in the regulation of adrenal CA secretion evoked by electrical stimulation of adrenal nerve terminals.

Cross-desensitization of VIP- and NANC-mediated inhibition of the guinea-pig tracheal pouch.

1. Chloralose/urethane anaesthetized guinea-pigs were used for preparation of the tracheal pouch, which demonstrates the NANC innervation. Isolated vagi cut cranially were positioned on bipolar electrodes for subsequent stimulation. The animals were given atropine and propranolol intraperitoneally before the experimental procedure to assure that responses would be from non-adrenergic, non-cholinergic components. 2. ATP and VIP, two putative neurotransmitters for non-adrenergic, non-cholinergic (NANC) inhibitory innervation caused relaxation responses of the pouch. Both VIP and ATP (10(-9) M) caused auto-desensitization after four repeated administrations into the pouch. 3. If NANC nerve stimulation causes relaxation of tracheal smooth muscle by release of one of these agents onto a specific receptor, repeated exposure to that agent by nerve stimulation or exogenous application could cause diminished responsiveness from cross-desensitization, yet maintained responsiveness to an agent acting at other receptors. 4. Relaxation responses to vagal stimulation were determined before and after the pouch was incubated for a 10-min period with either ATP or VIP (10(-6) M). ATP incubation did not produce significant (P less than 0.05) change in the degree of relaxation in response to nerve stimulation. In contrast, VIP incubation did cause diminished relaxation of the pouch in response to subsequent nerve stimulation. 5. The ability of the agents at 10(-9) M and 10(-6) M to cause relaxation of the pouch was next examined before and after repeated NANC nerve stimulation. Relaxation of the pouch to ATP was not significantly affected by intervening repeated nerve stimulations. However, relaxation in response to VIP was significantly inhibited by repeated nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of substance P from the cat spinal cord.

1. The present experiments examine the physiology and pharmacology of the release of substance P-like immunoreactivity (SP-l.i.), from the spinal cord in the halothane-anaesthetized, artificially ventilated cat. 2. Resting release of SP-l.i. was 36 +/- 4 fmol/30 min (mean +/- S.E.; n = 106). Bilateral stimulation of the sciatic nerves at intensities which evoked activity in fibres conducting at A beta conduction velocities (greater than 40 m/s), resulted in no change in blood pressure, pupil diameter or release of SP-l.i. Stimulation intensities which activate fibres conducting at velocities less than 2 m/s resulted in increased blood pressure, miosis and elevated release of SP-l.i. (278 +/- 16% of control). 3. The relationship between nerve-stimulation frequency and release was monotonic up to approximately 20 Hz. Higher stimulation frequencies did not increase the amounts of SP-l.i. released. At 200 Hz there was a reduction. 4. Capsaicin (0.1 mM) increased the release of SP-l.i. from the spinal cord and resulted in an acute desensitization to subsequent nerve stimulation. This acute effect was not accompanied by a reduction in spinal levels of SP-l.i. measured 2 h after stimulation. 5. Cold block of the cervical spinal cord resulted in an increase in the amounts of SP-l.i. released by nerve stimulation. 6. Pre-treatment with intrathecal 5,6-dihydroxytryptamine (300 micrograms) 7 days prior to the experiment caused a reduction in the dorsal and ventral horn stores of SP-l.i., but had no effect on the release of SP-l.i. evoked by nerve stimulation. Similar pre-treatment with intrathecal capsaicin (300 micrograms) resulted in depletion of SP-l.i. in the dorsal but not in the ventral horn of the spinal cord and diminished the release of SP-l.i. evoked by nerve stimulation. 7. Intense thermal stimulation of the flank resulted in small (20-35%), but reliable increases in the release of SP-l.i. above control. 8. Putative agonists for the opioid mu-receptor (morphine, 10-100 microM; sufentanil, 1 microM), and for the delta-receptor (D-Ala2-D-Leu5-enkephalin, 1-10 microM; D-Pen2-D-Pen5-enkephalin, 10 microM), but not the kappa-receptor (U50488H, 100-1000 microM), produced a dose-dependent, naloxone-reversible reduction of the evoked, but not of the resting release of SP-l.i. (-)-Naloxone, but not (+)-naloxone, resulted in a significant increase in evoked but not resting SP-l.i. release.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium levels in rat submandibular gland and saliva after sympathetic nerve stimulation in the presence of α, β1 and β2-adrenergic antagonists

Calcium concentration was reduced by 51 per cent compared with unstimulated contralateral glands after 60-min of electrical stimulation to the sympathetic nerve. When the α-adrenergic antagonists, phentolamine or phenoxybenzamine, were given 20 min before stimulation, the decrease in glandular calcium concentration (45–47 per cent) was similar to that after nerve stimulation alone; with the selective β 2-adrenergic antagonist added to the phentolamine, the percentage decrease was similar to that found after phentolamine and sympathetic nerve stimulation only. When atenolol, a β 1-adrenergic antagonist, was given with phentolamine, there was no secretion after subsequent nerve stimulation, nor was calcium concentration different from that of unstimulated glands. When propranolol (3 mg/kg, body wt), or atenolol + butoxamine (3 mg/kg, body wt), was given prior to stimulation there was a decrease of 21–22 per cent in gland-calcium concentration. Calcium concentration of nerve-stimulated saliva was nearly 6m-equiv./1, and twice as high with phentolamine prior to stimulation; for butoxamine together with phentolamine the change was similar to that with phentolamine alone. When β-adrenergic antagonists were given prior to nerve stimulation, concentration of the saliva was either unchanged (propranolol) from that with nerve stimulation alone or somewhat decreased (atenolol + butoxamine). Thus sympathetic stimulation results in activation of α and β 1-adrenoceptors; calcium secretion is principally regulated by the β-adrenoceptors, and β 1-receptors have the principal role.

Epinephrine enhances neurogenic vasoconstriction in the rat perfused kidney.

Epinephrine has been implicated in the genesis of some forms of hypertension. We have investigated the effects of epinephrine on vasoconstrictor responses evoked by adrenergic stimuli in the isolated perfused rat kidney. Low concentrations of epinephrine (2.5 - 5 X 10(-9) M) increased the amplitude of vasoconstrictor responses evoked by electrical stimulation of the renal adrenergic nerves. These concentrations of epinephrine had no effect on the basal perfusion pressure of the kidney or on the amplitude of vasoconstrictor responses evoked by exogenous norepinephrine. The potentiating effect of epinephrine persisted after infusion of the amine had ceased. Kidneys that had been perfused with 3H-epinephrine accumulated radioactivity, which could then be released by renal nerve stimulation. Cocaine (3 X 10(-5) M) reduced the renal accumulation of 3H-epinephrine and abolished both the persistent potentiating effect of the amine and the release of radioactivity evoked by subsequent nerve stimulation. The potentiating effect of epinephrine infusion was abolished by the beta 2-selective adrenergic receptor antagonist ICI 118,551 (3 X 10(-8) M), but not by the beta 1-selective adrenergic receptor antagonist atenolol (10(-6) M). These results indicate that concentrations of epinephrine that can be achieved during acute stress can enhance the amplitude of neurogenic vasoconstrictor responses. This effect appears to be mediated via a prejunctional beta 2-adrenergic receptor. The persistent nature of this effect may be due to the neuronal accumulation and subsequent release of epinephrine.

Impaired metabolic response to nerve stimulation in brown adipose tissue of hypothyroid rats

In brown adipose tissue of the rat, chemically or surgically induced hypothyroidism caused the following effects. A large decrease of the magnitude of the metabolic response to electrical nerve stimulation. The deactivation half-time of the response was reduced to 70% of the control value, with no change in catechol O-methyltransferase activity. Pre-incubation of tissues with norepinephrine, 10 −5 M, increased the response to subsequent nerve stimulation almost to that of the controls. The catecholamine analogue dose-response curves were shifted to the right. The shift was very pronounced for isoproterenol ( K 50 426 nM versus 2 nM), somewhat less marked for norepinephrine (7373 nM versus 194) and very slight for phenylephrine (2803 nM versus 1649); there was almost no change in E max values. An increase of octanoate oxidative capacity. A decrease of the capacity of the stereoselective binding of (−)-[ 3H]dihydroalprenololol the high-affinity ( K d 2.0 nM) sites to a fourth of the control value and an increase by a factor of 2.9 of the K d of the low-affinity binding sites. This decrease of binding to the β-receptors was not sufficient quantitatively to explain the decrease in the metabolic response, suggesting the existence of an additional defective reaction which could occur between the binding to the β-receptors and the activation of the triglyceride lipase. These results show that the sharp decrease of the metabolic response of brown adipose tissue to nerve stimulation has multiple causes. The findings are discussed in the context of the drastic decrease of cold resistance in hypothyroid rats.

The accumulation of hemicholinium by tissues that transport choline.

Cat superior cervical ganglia accumulated 14C-hemicholinium (HC-3), but the amount of drug taken up was not changed by nerve stimulation; accumulated HC-3 was not released by subsequent nerve stimulation. Choline and HC-3 were accumulated by a crude preparation of synaptosomes; the uptake of choline was blocked by HC-3, but the uptake of HC-3 was not blocked by choline. Choline, but not HC-3, was transported by red blood cells. It is concluded that HC-3 is not transported by the choline transport mechanism, and that in concentrations that are usually used in pharmacological experiments, HC-3 appears not to act as, or to form, a false cholinergic transmitter.