Ganglionic Long-term Potentiation Research Articles

Functional Significance of Angiotensin Receptor Type 2 in the Neuroplasticity of Autonomic Ganglia in (mRen2)27 Transgenic Hypertensive Rats.

The over-expression of Ren -2 d gene in (mRen2)27 rats leads to development of hypertension mediated by the renin-angiotensin-system axis and exaggerated sympathetic nerve activity. Exogenously applied angiotensin II (AngII) on the superior cervical ganglion evokes ganglionic compound action potentials (gCAP) and ganglionic long-term potentiation (gLTP). We studied the functional role of angiotensin receptors and expression of reactive oxygen species marker, nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) proteins in AngII-induced postganglionic transmission. Bath-applied AngII revealed that the indices of ganglionic transmission, synaptic strength of gCAP, and decay time for gLTP are remarkably prolonged in (mRen2)27 rats and were abolished by an angiotensin receptor blocker (ARB), suggesting postganglionic AngII Type 1 (AT 1 ) receptor localization and mediation. Receptor density for AT 1 was similar in (mRen2)27 and control animals, and quantitative reverse transcription polymerase chain reaction revealed that it is consistent with the mRNA profile. Furthermore, immunocytochemistry analysis showed similar AT 1 receptor distribution and signals. However, assessment of Type 2 (AT 2 ), Ang-(1-7)-MAS and NOX4-specific proteins showed that AT 2 receptor protein expression was 4-fold lower, consistent with a low mRNA profile. MAS receptor expression was 10-fold lower and NOX4 protein was 2-fold lower. Despite similarity in the densities of AT 1 receptor, the low levels of the components of the protective arm of the renin-angiotensin system at the ganglia may contribute to the differential superior cervical ganglion sensitivity to AngII. The lower NOX4 affects reactive oxygen species balance and possibly results in activation of downstream pathways to promote increased sympathetic nerve activity. We speculate that the significant diminution in AT 2, MAS, and NOX4 protein expressions may play an indirect role in the alteration and efficacy of gCAP and gLTP in hypertension.

Ganglionic Long-Term Potentiation in Prehypertensive and Hypertensive Stages of Spontaneously Hypertensive Rats Depends on GABA Modulation.

The sympathetic nervous system (SNS) regulates body functions in normal and pathological conditions and is characterized by the presence of a neuroplastic phenomenon, termed ganglionic long-term potentiation (gLTP). In hypertension, either in spontaneously hypertensive rats (SHR) or in humans, sympathetic hyperfunction, such as elevated SNS outflow and changes in synaptic plasticity have been described. Because enhanced SNS outflow is detected in the hypertensive stage and, more importantly, in the prehypertensive phase of SHR, here we explored whether synaptic plasticity, particularly gLTP, was modified in the superior cervical ganglia (SCG) of prehypertensive SHR. Furthermore, considering that GABA modulates sympathetic synaptic transmission and gLTP in Wistar rats, we studied whether GABA might modulate gLTP expression in SHR. We characterized gLTP in the SCG of young prehypertensive 6-week-old (wo) and adult hypertensive (12 wo) SHR and in the SCG of Wistar Kyoto (WKy) normotensive control rats of the same ages. We found that gLTP was expressed in 6 wo SHR, but not in 12 wo rats. By contrast, in WKy, gLTP was expressed in 12 wo, but not in 6 wo rats. We also found that gLTP depends on GABA modulation, as blockade of GABA-A subtype receptors with its antagonist bicuculline unmasked gLTP expression in adult SHR and young WKy. We propose that (1) activity-dependent changes in synaptic efficacy are altered not only during hypertension but also before its onset and (2) GABA may play a modulatory role in the changes in synaptic plasticity in SHR, because the blockade of GABA-A receptors unmasked the expression of gLTP. These early changes in neuroplasticity and GABA modulation of gLTP could be part of the sympathetic hyperfunction observed in hypertension.

Differential contribution of BDNF and NGF to long-term potentiation in the superior cervical ganglion of the rat

Synaptic transmission in the sympathetic nervous system is a plastic process modulated by different factors. We characterized the effects of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) on basal transmission and ganglionic long-term potentiation (LTP) in the rat superior cervical ganglion. LTP was elicited by supramaximal tetanic stimulation (40 Hz, 3 s) of the sympathetic trunk and was quantified by measuring LTP decay time and LTP extent. Neurotrophins did not affect basal transmission, however, they differentially affected LTP. BDNF (200 ng/ml) increased LTP decay time and LTP extent 2.0-fold (p < 0.01). In contrast, NGF showed a dual effect: 200 ng/ml NGF reduced LTP decay time and LTP extent to 53% and to 32% of control value (p < 0.0001 and p < 0.02; respectively), whereas >350 ng/ml NGF significantly increased LTP decay time and LTP extent (p < 0.02). Digital analysis of compound action potentials suggests that neurotrophins could change the synchronization of unitary action potentials. Pharmacological data obtained in intact ganglia show that C2-ceramide produced a 2-fold enhancement in LTP, whereas tyrphostin AG879, an inhibitor of tyrosine kinase activity, reversed the NGF blockade and produced by itself an enhancement in LTP. In sliced ganglia we observed that an anti-TrkA antibody reversed the NGF-induced LTP blockade. Immunohistochemistry studies revealed that 83% of ganglionic neurons express TrkA, whereas 52% express p75 receptor, and 18% express TrkB receptor. We propose that p75 neurotrophin receptors and probably TrkB signaling enhance LTP, whereas TrkA signaling reduces it.

Diabetes Impairs Synaptic Plasticity in the Superior Cervical Ganglion: Possible Role for BDNF and Oxidative Stress

The majority of diabetics develop serious disorders of the autonomic nervous system; however, there is no clear understanding on the causes of these complications. In this study, we examined the effect of streptozocin (STZ)-induced diabetes on activity-dependent synaptic plasticity, associated levels of brain-derived neurotrophic factor (BDNF) and antioxidant biomarkers in the rat sympathetic superior cervical ganglion. Diabetes (STZ-induced) was achieved by a single intraperitoneal injection of streptozocin (55mg/kg).Compound action potentials were recorded from isolated ganglia before (basal) and after repetitive stimulation, or trains of paired pulses to express ganglionic long-term potentiation (gLTP) or long-term depression (gLTD). The input/output curves of ganglia from STZ-treated animals showed a marked rightward shift along most stimulus intensities, compared to those of ganglia from control animals, indicating impaired basal synaptic transmission in ganglia from STZ-induced diabetic animals. Repetitive stimulation induced robust gLTP and gLTD in ganglia isolated from control animals; the same protocols failed to induce gLTP or gLTD in ganglia from STZ-induced diabetic animals, indicating impairment of activity-dependent synaptic plasticity in these animals. Molecular analysis revealed significant reduction in the levels of BDNF and the ratio of glutathione/oxidized glutathione. Additionally, the activity of glutathione peroxidase, glutathione reductase, catalase, and the levels of thiobarbituric acid-reactive substances were increased in ganglia from STZ-treated animals. In conclusion, impaired basal synaptic transmission and synaptic plasticity are associated with reduced BDNF and altered oxidative stress biomarkers in the sympathetic ganglia from STZ-induced diabetic animals, suggesting a possible correlation of these factors with the manifestations of STZ-induced diabetes in the peripheral nervous system.

Impaired Neural Transmission and Synaptic Plasticity in Superior Cervical Ganglia from &amp;#946;-Amyloid Rat Model of Alzheimers Disease

Basal synaptic transmission and activity-dependent synaptic plasticity were evaluated in superior cervical sympathetic ganglia (SCG) of amyloid-β rat model of Alzheimer's disease (Aβ rat) using electrophysiological and molecular techniques. Rats were administered Aβ peptides (a mixture of 1:1 Aβ1-40 and Aβ1-42) by chronic intracerebroventricular infusion via 14-day mini-osmotic pumps (300 pmol/day). Control rats received Aβ40-1 (inactive reverse peptide: 300 pmol/day). Ganglionic compound action potentials were recorded before (basal) and after repetitive stimulation. In isolated SCG, ganglionic long-term potentiation (gLTP) was generated by a brief train of stimuli (20Hz for 20s) and ganglionic long-term depression (gLTD) was produced with trains of paired pulses. The input/output (I/O) curves of ganglia from Aβ rats showed a marked downward shift along all stimulus intensities, compared to those of ganglia from control animals, indicating impaired basal synaptic transmission. In addition, repetitive stimulation induced robust gLTP and gLTD in ganglia isolated from control animals, but, the same protocols failed to induce gLTP or gLTD in ganglia from Aβ rats indicating impairment of activity-dependent synaptic plasticity in these animals. Western blotting of SCG homogenate from Aβ rats revealed reduction in the ratio of phosphorylated-/total-CaMKII and in calcineurin protein levels. Although other mechanisms could be involved, these changes in signaling molecules could represent an important molecular mechanism linked to the failure to express synaptic plasticity in Aβ rat ganglia. Results of the current study could explain some of the peripheral nervous system manifestations of Alzheimer's disease.

Role of presynaptic and postsynaptic IP3‐dependent intracellular calcium release in long‐term potentiation in sympathetic ganglion of the rat

In the rat superior cervical ganglion, a form of long term potentiation (LTP) can be elicited by a brief high frequency stimuli applied to the preganglionic nerve. Cumulative evidence shows that a transient increase in cytoplasmic Ca²+ concentration is essential for the generation of the ganglionic LTP. Calcium influx and calcium release from intracellular calcium stores contribute to LTP. However, the differential role of presynaptic and postsynaptic calcium signaling has not been established. Herein, by using heparin, a membrane-impermeant inositol trisphosphate receptor (IP3R) blocker, we explored the contribution of presynaptic and postsynaptic IP3-sensitive calcium stores to the ganglionic LTP. The LTP was produced by a conditioning train of 40 Hz for 3 s. We analyzed the effects of heparin on the posttetanic potentiation: PTP magnitude and PTP time constant, and on two parameters that describe the LTP: LTP decay time (elapsed time required by the potentiated response to fall to 20% above the basal value) and LTP extent (the integral of the potentiated response). Heparin (100 and 200 μg/ml) was loaded in the preganglionic, the postganglionic, or in both nerves. We found that in all tested conditions heparin significantly decreased LTP but practically did not affect PTP. The preganglionic and postganglionic inhibitory effects of heparin were not additive. De-N-sulfated heparin, an ineffective IP3R blocker, had no effect on LTP, but abolished the heparin blocking effect. Data suggest that presynaptic and postsynaptic IP3-dependent intracellular calcium release equally contribute to ganglionic LTP, supporting our proposal of a trans-synaptic mechanism for LTP.

Calmodulin and guanylyl cyclase inhibitors block the in vivo expression of gLTP in sympathetic ganglia from chronically stressed rats

Previous work from this laboratory indicated that superior cervical ganglia from rats exposed to chronic psychosocial stress expressed ganglionic long-term potentiation (gLTP) in vivo. In the present study, we report additional pharmacological evidence indicating involvement of calmodulin and guanylyl cyclase in gLTP, and supporting the in vivo gLTP expression in ganglia from chronically stressed rats. Pretreatment with the calmodulin inhibitors W-7 (5 μM) or calmidazolium (5 μM) or with guanylyl cyclase inhibitor LY-83583 (5 μM) completely blocked HFS (20 Hz/20 s)-induced gLTP in superior cervical ganglia isolated from normal rats. Along with that, inhibition of apparent basal ganglionic transmission by W-7 (5 μM), calmidazolium (5 μM) or LY-83583 (5 μM) is observed in ganglia isolated from chronically stressed rats, but not in those from control rats, indicating in vivo expression of gLTP in ganglia isolated from stressed rats. The present results confirm the involvement of both calmodulin and GC activities in gLTP, and indicate that ganglia from stressed rats may have expressed gLTP in vivo, which is known to precipitate hypertension in these animals.

In vivo expression of ganglionic long-term potentiation in superior cervical ganglia from hypertensive aged rats

Sustained increase in central sympathetic outflow to ganglia may provide the repeated high frequency presynaptic activity required for induction of long-term potentiation in sympathetic ganglia (gLTP), which is known to be involved in the manifestation of a neurogenic form of hypertension, namely stress-hypertension. Aging is often viewed as a progressive decline in physiological competence with a corresponding impaired ability to adapt to stressful stimuli. Old animals have exaggerated sympathetic activity as well as increased morbidity and mortality during prolonged exposure to stressful stimuli. Using the superior cervical ganglion (SCG) as a model for sympathetic ganglia, electrophysiological and biochemical evidence show that mildly hypertensive aged rats (22-month old) have expressed gLTP in vivo. This is suggested by a number of lines of evidence. Firstly, a shift in input/output (I/O) curve of ganglia from aged rats to the left side of I/O curve of ganglia from 6-month old (adult) rats indicating expression of gLTP. Secondly, failure of in vitro high frequency stimulation to induce gLTP in ganglia isolated from aged rats, which indicates occlusion due to saturation, which, in turn, suggests in vivo expression of gLTP in these ganglia. Thirdly, in vitro inhibition of basal ganglionic transmission by blockers of gLTP (5-HT(3) antagonists) is observed in ganglia isolated from aged rats, but not in those from adult rats. Finally, immunoblot analysis revealed that protein levels of signaling molecules such as calcium-calmodulin kinase II (CaMKII; phosphorylated and total), which normally increase during expression of LTP, are elevated in ganglia isolated from aged rats compared to those from adult ones. Protein levels of calcineurin, which dephosphorylates P-CaMKII, were reduced in ganglia isolated from aged rats, probably as a support mechanism to allow prolonged phosphorylation of CaMKII. Our findings suggest in vivo expression of gLTP in sympathetic ganglia of aged animals, which may contribute to the moderate hypertension often seen in aged subjects.

Expression of gLTP in Sympathetic Ganglia from Stress-hypertensive Rats: Molecular Evidence

We previously reported behavioral and electrophysiological evidence indicating that superior cervical ganglia (SCG) from rats that developed hypertension as a result of chronic psychosocial stress expressed ganglionic long-term potentiation (gLTP) in vivo. In the present study, we present additional supportive evidence by measuring changes in protein levels of essential signaling molecules in ganglia from chronically stressed rats. We compared protein levels of essential, LTP-related signaling molecules in ganglia isolated from chronic stress-hypertensive rats, known to have expressed gLTP, with those of the same molecules in normal ganglia 1h after eliciting gLTP by high frequency stimulation (HFS) in vitro. Immunoblot analysis showed a significant increase in the levels of phosphorylated CaMKII, total CaMKII, nitric oxide synthase (NOS-1), and calmodulin in SCG from both chronically stressed rats and from normal rat ganglia in which gLTP was expressed by HFS in vitro. Additionally, there was a parallel reduction in calcineurin protein levels in ganglia from both groups. The present results confirm that ganglia from stressed rats have expressed gLTP in vivo and that synaptic plasticity in sympathetic ganglia may involve a molecular cascade largely similar to that of LTP in the hippocampal CA1 region.

Contribution of different calcium channels to long-term potentiation in superior cervical ganglion of the rat

We explored the contribution of different calcium channel types to the long-term potentiation (LTP) of superior cervical ganglion of the rat. Right after a conditioning train of 40 Hz for 5 s, the maximum amplitude of the postsynaptic response (maximum potentiation) increased 5.6±0.5-fold. Potentiation decreased to 20% of its initial value within the following 70.0±8.0 min (LTP decay time). The contribution of P/Q-, N- and L-type calcium channels to LTP was studied by blocking their activity with synthetic funnel-web spider toxin (10 or 100 μM), ω-conotoxin GVIA (5 μM) or nifedipine (10 μM), respectively. The three blockers reduced the amplitude of the postsynaptic compound action potential before the conditioning train. After the train, all of the toxins reduced the LTP decay time and the integral of the amplitude versus time curve, defined as the LTP extent. In addition, all three blockers increased the maximum potentiation. Our results demonstrate that different calcium channel types contribute to ganglionic LTP. These effects may be by coupling excitation-secretion from different types of synaptic vesicles.

Invited Lecture

Acquired and Inherited Forms of Ouabain‐Induced Hypertension: Significance and Relationship to Other Models. John M. Hamlyn. Department of Physiology, School of Medicine, University of Maryland, 655 West Baltimore, Baltimore, MD, USA.The etiology of essential hypertension remains an enigma—a mix of multiple environmental and genetic factors. In 1989, we described an endogenous inhibitor of the sodium pump in the human circulation and in 1991 identified the inhibitor as ouabain or a closely related isomer. Clinical studies showed that endogenous “ouabain” (EO) circulated in elevated amounts in a surprisingly large portion of patients with essential hypertension and that the relationship of EO with pressure was as significant as age. Causality was suggested subsequently by two sets of observations. First, blood pressure and plasma EO returned to normal following removal of the adrenal gland in patients with primary aldosteronism and other adrenal adenomas. Second, the prolonged infusion of ouabain in normal outbred rats induced sustained hypertension (OHR). As various animal models have provided insight into the mechanisms of hypertension, we bred rats for their resistance (BOR) or sensitivity (BOS) to the hypertensionogenic effect of ouabain. The strains were initiated simultaneously in 1992 from an F0 generation comprised of 80 normotensive outbred Sprague Dawley rats (40m, 40f, HSD, Indianapolis). Each member of the F0 generation received a maximally pressor dose of ouabain delivered over 5 weeks via osmotic pumps. Blood pressures were measured at baseline, during infusion, and 2 weeks following pump removal (week 7) using tail cuff plethysmography. Among the F0, 64 animals (∼85%) developed sustained hypertension while 12 (15%) showed little or no rise in either SBP or MBP. The increase in BP induced by ouabain was >1 S.D. above the group mean response in 13 animals (6m, 7f) and >1 S.D. below the mean in 16 animals (5m, 11f). BP in all animals was normal by week 7. Two breeding pairs of F0 OS and OR were chosen and, using a similar selection method and selective inbreeding, multiple stable generations were obtained. By the F3 generation, the BOS and BOR phenotypes were completely segregated while further inbreeding resulted in the appearance of spontaneous hypertension in BOS by week 6. This trait could not be selected against and became pronounced with further inbreeding. In OHR, hypertension was reversible, and characterized by elevated ouabain in the circulation, kidney, hypothalamus and anterior pituitary. Tissue levels were strongly correlated with BP. In both BOS and OHR, hypertension could be prevented or reversed by ACE, AT2 antagonists, digoxin, or blockade of brain EO by antibody fragments. As increased sympathetic nerve activity appears to be crucial to ouabain dependent hypertension we studied short‐term (STP) and long‐term potentiation (LTP) in the isolated superior cervical ganglia (SCG). The time constants for the decay of both STP and LTP (tL) were increased in BOS and OHR and correlated with blood pressure. Captopril normalized blood pressure and tL in OHR. Thus, the duration of ganglionic LTP and blood pressure are tightly linked in acquired and inherited forms of ouabain‐dependent hypertension. Enhanced plasticity in sympathetic neurons appears to be a major factor contributing to the increase in peripheral sympathetic nerve activity (SNA) in ouabain‐dependent hypertension. Studies of the contractility of isolated renal arteries showed enhanced force production in OHR in response to depolarization and in kidneys, the autoregulation of glomerular filtration rate and renal blood flow was less efficient, Moreover, a rightward shift in the pressure natriuresis relationship was found in OHR, such that 25–30 mmHg higher renal perfusion pressure was necessary to excrete a sodium load. In the OHR and BOS models, the overall involvement of the central and peripheral nervous system, hormone profiles, renovascular abnormalities and pharmacological sensitivity appear to have striking parallels with the SHR. It is suggested that common overlapping mechanisms are likely to underlie the etiology of hypertension in these models.

Reduction of elevated arterial blood pressure in obese Zucker rats by inhibition of ganglionic long-term poteniation

Sustained enhancement of the basal tone of ganglionic transmission is expected to result in an enduring increase in peripheral resistance that would lead to elevated blood pressure. Long-term potentiation of sympathetic ganglia is an activity-dependent long-lasting increase in strength of ganglionic transmission. Therefore, ganglionic long-term potentiation might be involved in the manifestation of neurogenic forms of hypertension. Expression of sympathetic ganglionic long-term potentiation is dependent on activation of 5-HT 3 receptor. We examined the possibility that elevated blood pressure in obese Zucker rat, which is reported to be stress-prone, might be partly due to a neurogenic factor resulting from expression of ganglionic long-term potentiation. Chronic treatment with the 5-HT 3 receptor antagonist ondansetron (0.5 mg/kg/day) caused a significant decrease in blood pressure of the obese Zucker rats without affecting that of normotensive lean Zucker rats. Electropysiological procedures to test for long-term potentiation in isolated ganglia suggest that ganglionic long-term potentiation has been previously expressed in vivo in ganglia from obese Zucker rat but not in those from the normotensive lean Zucker rats. The results indicate that expression of ganglionic long-term potentiation in sympathetic ganglia may be responsible for neurogenic increase in blood pressure, which contributes to the moderate hypertension often seen in the obese Zucker rats.

Inhibition of ganglionic long-term potentiation decreases blood pressure in spontaneously hypertensive rats.

Long-term potentiation of sympathetic ganglia (gLTP), a unique form of synaptic plasticity, is serotonin dependent and can be blocked with 5-HT3 receptor antagonists. Long-lasting enhancement of the basal tone of ganglionic transmission (as with gLTP) is expected to result in sustained increase in peripheral resistance that would lead to elevated blood pressure. We examined the possibility that in sympathetic ganglia, gLTP may be involved in the expression of stress-induced (neurogenic) form of hypertension. High blood pressure in spontaneously hypertensive rat (SHR), known to show exaggerated cardiovascular defense reactions to environmental stimuli, is partly due to a neurogenic factor. Chronic treatment of SHR and their normotensive counterpart, the Wistar Kyoto (WKY) rats with the 5-HT3 receptor antagonist tropisetron (ICS; 5 mg/kg/day), caused a marked decrease in the blood pressure of the SHR but not of WKY rats. Increasing the daily dose of ICS cumulatively (7 and 10 mg/kg) did not result in significant additional decrease in blood pressure of SHR, indicating that the drug blocks only the neurogenic component of hypertension in the SHR. electrophysiological procedures for indirectly testing for the presence of gLTP in ganglia excised from SHR suggest that gLTP has been previously expressed in these ganglia in vivo. This contrasts with the absence of gLTP in ganglia from normotensive rats. The results support contribution of gLTP to the expression of neurogenic hypertension.

Synaptic plasticity in sympathetic ganglia from acquired and inherited forms of ouabain-dependent hypertension.

Altered sympathetic nervous system activity has been implicated often in hypertension. We examined short-term potentiation [posttetanic potentiation (PTP)] and long-term potentiation (LTP) in the isolated superior cervical ganglia (SCG) from Sprague-Dawley (SD) rats given vehicle, digoxin, or ouabain by subcutaneous implants as well as in animals with ouabain-induced hypertension (OHR), and inbred Baltimore ouabain-resistant (BOR) and Baltimore ouabain-sensitive (BOS) strains of rats. Postganglionic compound action potentials (CAP) were used to determine PTP and LTP following a tetanic stimulus (20 Hz, 20 s). Baseline CAP magnitude was greater in ganglia from OHR than in vehicle-treated SD rats before tetanus, but the decay time constant of PTP was significantly decreased in OHR and in rats infused with digoxin that were normotensive. In hypertensive BOS and OHR, the time constants for the decay of both PTP and LTP (t(L)) were increased and correlated with blood pressure (slope = 0.15 min/mmHg, r = 0.52, P < 0.047 and 6.7 min/mmHg, r = 0.906, P < 0.0001, respectively). In BOS and OHR, t(L) (minutes) was 492 +/- 40 (n = 7) and 539 +/- 41 (n = 5), respectively, and differed (P < 0.05) from BOR (257 +/- 48, n = 4), SD vehicle rats (240 +/- 18, n = 4), and captopril-treated OHR (370 +/- 52, n = 5). After the tetanus, the CAP at 90 min in BOS and OHR SCG declined less rapidly vs. SD vehicle rats or BOR. Captopril normalized blood pressure and t(L) in OHR. We conclude that the duration of ganglionic LTP and blood pressure are tightly linked in ouabain-dependent hypertension. Our results favor the possibility that enhanced duration of LTP in sympathetic neurons contributes to the increase in sympathetic nerve activity in ouabain-dependent hypertension and suggest that a captopril-sensitive step mediates the link of ouabain with LTP.

Retrograde Carbon Monoxide Is Required for Induction of Long-Term Potentiation in Rat Superior Cervical Ganglion

Carbon monoxide (CO), produced in the body by the enzyme heme oxygenase (HO), has been suggested as a retrograde synaptic messenger with a prominent role in the long-term potentiation (LTP) of certain areas of the brain. LTP of sympathetic ganglia is 5-HT(3) receptor-dependent and has been shown to require nitric oxide for the maintenance, but not for the induction, phase. We investigated the possibility of CO being required for the induction of ganglionic LTP. Pretreatment of rat isolated superior cervical ganglia with oxyhemoglobin (25-100 microm) completely blocked LTP. In the same ganglia, prolonged washout of oxyhemoglobin did not uncover any potentiation of the compound action potential. Oxyhemoglobin had no significant effect on the maintenance phase in ganglia with established LTP. Pretreatment of ganglia with the HO inhibitor zinc protoporphyrin-IX (ZnPP) (10 microm) completely and irreversibly prevented the expression of tetanus-evoked LTP. However, in the same ganglia, after superfusion of CO in the presence of ZnPP, tetanic stimulation readily evoked LTP. No effect was seen on the maintenance phase when ZnPP was superfused on ganglia with established LTP. Pretreatment of ganglia with the 5-HT(3) receptor antagonist ondansetron (0.4 microm) alone completely and irreversibly blocked LTP. However, in the presence of CO, ondansetron did not block LTP. These results suggest that activation of 5-HT(3) receptors may be involved in the production of CO. The results also suggest that CO, probably originating outside the presynaptic nerve terminal, is involved in the induction of LTP.

Nitric oxide is required for the maintenance but not initiation of ganglionic long-term potentiation

The role of nitric oxide in long-term potentiation of the nicotinic pathway of synaptic transmission in the isolated superior cervical ganglia of rat was studied. Long-term potentiation was induced by a brief tetanizing pulse (tetanus, 20 Hz/20 s) to the preganglionic nerve. The amplitude of the extracellularly recorded postganglionic compound action potential was used as an index of synaptic transmission. Pretreatment with the nitric oxide synthase inhibitor N G-nitro- l-arginine methyl ester (10 μM) or l- N G-nitro-arginine (10 μM) 30 min before tetanus, inhibited long-term potentiation. The inactive enantiomer of the nitric oxide synthase inhibitor, N G-nitro- d-arginine methyl ester (10 μM), failed to inhibit the long-term potentiation when given 30 min before the tetanus. Washout of l- N G-nitro-arginine, but not N G-nitro- l-arginine methyl ester, resulted in complete recovery of long-term potentiation. The nitric oxide synthase inhibitor had no significant effect on the basal ganglionic neurotransmission or post-tetanic potentiation. Furthermore, established long-term potentiation was blocked by superfusion of ganglia with N G-nitro- l-arginine methyl ester 1 h after a tetanus. Pretreatment of ganglia with the nitric oxide donor, sodium nitroprusside (100 μM), or the nitric oxide synthase substrate, l-arginine (1 mM), completely prevented the inhibitory effects of N G-nitro- l-arginine methyl ester on the tetanus-induced long-term potentiation. These findings present evidence for a requirement of nitric oxide for the maintenance but not induction of long-term potentiation in rat isolated superior cervical ganglia.

Induction and maintenance of ganglionic long-term potentiation require activation of 5-hydroxytryptamine (5-HT3) receptors.

1. An extracellular recording technique was used to study the effects of 5-hydroxytryptamine (5-HT, serotonin) on the tetanus-induced long-term potentiation (LTP) of the nicotinic pathway of transmission in the superior cervical ganglion (SCG) of the rat. The postganglionic compound action potential (CAP), made submaximal by treatment with hexamethonium (O.4 mM), was used as an index of transmission in the ganglion. 2. Serotonin (10 microM) markedly enhanced the magnitude of LTP without affecting the post-tetanic potentiation (PTP). The serotonin (2-30 microM) concentration-response curve for LTP was bell shaped as no enhancement was seen with 30 microM serotonin. This may largely be due to activation of a 5-HT1 receptor subtype and not to desensitization. 3. When superfused before tetanus, the 5-HT1A receptor agonist 8-hydroxydipropylamino-tetralin (8-OH-DPAT, 5 microM) prevented the expression of LTP without affecting PTP. 4. Pretreatment of ganglia with the 5-HT2 receptor agonist R-(+)-dimethoxy-4-iodoamphetamine (R-(+)-DOI, 1 microM) enhanced the tetanus-induced LTP. Similar treatment with the 5-HT2 receptor antagonist ketanserin (3 microM) had no significant effect on LTP. 5. Pretreatment of ganglia with the 5-HT3 receptor agonist 1-m-(chlorophenyl) biguanide (m-CPGB, 1 microM), markedly increased (300%) the tetanus-induced LTP. Similar pretreatment with the 5-HT3 receptor antagonist 3-tropanyl-3,5-dichlorobenzoate (MDL 72222, 0.5 microM) completely prevented the expression of LTP. Fully expressed LTP was reversibly blocked by MDL 72222 when applied during the maintenance phase of LTP. 6. Tetanic stimulation of monoamine-depleted ganglia (from reserpine-pretreated rats, 3 mg kg-1 for 24 h) failed to induced LTP. 7. In monoamine-depleted ganglia, tetanus preceded by superfusion with m-CPBG readily induced LTP. MDL 72222 completely blocked this LTP. However in these ganglia tetanus failed to induced LTP when m-CPBG was given 2 min (during PTP) or 1 h after tetanus. 8. Tetanic stimulation of monoamine-depleted ganglia in the presence of R-(+)-DOI failed to induced LTP. 9. We conclude that tetanus-induced LTP of the SCG of the rat requires activation of 5-HT3 receptors both for induction and maintenance.

Age-related changes in the synaptic plasticity of rat superior cervical ganglia

A few seconds of tetanic preganglionic stimulation of rat sympathetic ganglia results in potentiation of cholinergic synaptic transmission lasting several hours. However, ganglia from aged (28–32 months) rats did not develop as much potentiation as did ganglia from young (3–6 months) rats. This potentiation appears to decay exponentially but in two phases. The early component decays in 15 minutes, a time course consistent with the phenomenon of post-tetanic potentiation (PTP). The later component decays over several hours and is a form of long-term potentiation (LTP). A double exponential decay model was employed to quantitatively resolve the decay of potentiation and allow quantitative comparison of the decay parameters in both aged and young rats. It is clear that the magnitude and duration of PTP was the same in both age groups. However both the magnitude and decay time-constant for LTP were 30–50% smaller in the aged group of rats. Several agents which mimic cyclic AMP or stimulate cyclic AMP production in the ganglion enhance nicotinic transmission for several hours. However, these agents, 8-bromo cAMP, forskolin, isoproterenol, and secretin were equally effective in potentiating transmission in ganglia from both young and aged rats. Previous studies demonstrated that stimulus induced LTP and cyclic AMP induced potentiation was the result of enhanced release of acetylcholine (ACh) and not from increased postsynaptic responsiveness. Presumably these agents act through the same pathway and by the same mechanism that generates LTP following preganglionic tetany. These observations demonstrate that there is a selective age-dependent decline in the capacity for sympathetic ganglia to generate long-term changes in synaptic efficacy. It is hypothesized that impaired ganglionic LTP in aged animals results from a decrease in the evoked co-release of an as yet unidentified preganglionic terminal modulator.