Comparison of microdialysis and push-pull perfusion for retrieval of serotonin and norepinephrine in the spinal cord dorsal horn

Sex Differences and Role of Gonadal Hormones on Glutamate LevelAfter Spinal Cord Injury in Rats: A Microdialysis Study

Introduction:

Sex differences in outcomes of Spinal Cord Injury (SCI) suggest a sex-hormone-mediated effect on post-SCI pathological events, including glutamate excitotoxicity. This study aimed to investigate the importance of gonadal hormones on glutamate release subsequent to SCI in rats.

Methods:

After laminectomy at T8–T9, an electrolytic lesion was applied to the spinothalamic tracts of male and female rats. Using spinal microdialysis, we assessed glutamate levels at the site of lesion in both intact and gonadectomized rats for 4 hours. In this way, we examined the sex differences in the glutamate concentrations.

Results:

The peak retention time of glutamate level was 10.6 min and spinal glutamate concentration reached a maximum level 40 min following SCI. In male SCI rats, gonadectomy caused a significant elevation of glutamate level (P<0.001) following injury which was maximum 40 min post-SCI as well. However, no significant alterations were seen in gonadectomized female rats.

Conclusion:

The significant differences in glutamate levels between both intact and gonadectomized SCI male and female rats show the sex-hormone-related mechanisms underlying the molecular events in the second phase of SCI. It seems that the role of male gonadal hormones to prevent glutamate excitotoxicity is more prominent. The exact mechanisms of these hormones on the functional recovery after SCI should be clarified in further studies.

In vivo dialysis setup with a loop injection valve facilitates retrodialysis studies

INTRODUCTION

Retrodialysis, as used in neuropharmacological research, is a technique for in vivo delivery of neuroactive agents with concurrent monitoring of their effects on cellular activity with a separation between certain degree of spatial and temporal resolution. Typically, this is accomplished either by the use of a liquid-switch requiring multiple pumps, or by exchange of flow tubing requiring stopping and restarting dialysis. In the present study, we describe the use of a medium pressure injection valve for retrodialysis that overcomes these problems.

METHODS

The valve was configured with a loop to deliver 20μL of solution, and artificial CSF flow from the pump to the probe was established via this device. The application of this setup was evaluated in urethane anesthetized adult male C57BL/6J mice prepared with a CMA 11 probe implanted in the ventral hippocampus. By switching between the load and inject positions, the loop was filled with escitalopram solution (0.3μM) and delivered at a rate of 1μL/min at the probe for retrodialysis. Escitalopram (2mg/kg BW) was administered subcutaneously for microdialysis studies. During these treatments, dialysate fractions were collected for the determination of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA).

RESULTS

Irrespective of route of escitalopram administration, the pattern of dialysate 5-HT, and 5-HIAA response was comparable to that reported by other investigators. Accordingly, the in-line valve assembly did not compromise retrodialysis or microdialysis sampling. The manipulations to carry out retrodialysis using the valve setup are easy and simple.

DISCUSSION

An in-line injection valve is a promising adaptation for retrodialysis studies and can be incorporated as a standard part of in vivo dialysis instrumentation.

Increased release of serotonin in the spinal cord during low, but not high, frequency transcutaneous electric nerve stimulation in rats with joint inflammation

OBJECTIVE

To determine the release pattern of serotonin and noradrenaline in the spinal cord in response to transcutaneous electric nerve stimulation (TENS) delivered at low or high frequency.

DESIGN

Prospective randomized allocation of 3 treatments.

SETTING

Research laboratory.

ANIMALS

Male Sprague-Dawley rats (weight range, 250-350 g).

INTERVENTION

Knee joints of rats were inflamed with a mixture of 3% carrageenan and 3% kaolin for 24 hours prior to placement of push-pull cannulae into the dorsal horn of the spinal cord. Push-pull samples were collected in 10-minute intervals before, during, and after treatment with low-frequency TENS (4 Hz), high-frequency TENS (100 Hz), or sham TENS. TENS was applied to the inflamed knee joint for 20 minutes at sensory intensity and 100-mus pulse duration. Push-pull samples were analyzed for serotonin and noradrenaline by high performance liquid chromatography with coulemetric detection.

MAIN OUTCOME MEASURES

Spinal concentrations of serotonin and noradrenaline.

RESULTS

Low-frequency TENS significantly increased serotonin concentrations during and immediately after treatment. There was no change in serotonin with high-frequency TENS, nor was there a change in noradrenaline with low- or high-frequency TENS.

CONCLUSIONS

Low-frequency TENS releases serotonin in the spinal cord to produce antihyperalgesia by activation of serotonin receptors.

Recent advances in methods for the analysis of catecholamines and their metabolites

Catecholamines, for example epinephrine, norepinephrine, and dopamine, are widely distributed and are important neurotransmitters and hormones in mammalian species. Several methods have been developed for analysis of catecholamines and related compounds. Determination of catecholamines in biological fluids has enabled us to clarify the physiological role played by these amines. Catecholamine levels in plasma and/or urine are also useful for diagnosis of several diseases, for example hypertension, pheochromocytoma, and neuroblastoma. This review covers reports from 2000 to the present of methods for the analysis of catecholamines and their metabolites.

An improved liquid chromatographic method with electrochemical detection for direct determination of serotonin in microdialysates from Caudate-putamen and pineal gland regions of rat brain

A liquid chromatography method coupled with electrochemical detection has been developed for the direct measurement 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in brain microdialysates. The separation conditions have been optimized to detect only the 5-HT and 5-HIAA in dialysates and elute the other monoamines and their metabolites in the void. Linear regression analysis of chromatographic peak area as a function of concentration in the range 5-1000 pg/mL gave correlation coefficients over 0.995. Sample stability and an assay validation for precision and accuracy were also performed. The limit of detection (S/N = 3) for 5-HT was 12 femtomol/mL. The method has been applied to simultaneously measure extracellular 5-HT and 5-HIAA in brain microdialysates from the pineal gland and caudate-putamen of awake and freely-moving rats under basal conditions.

A new electrochemical HPLC method for analysis of enkephalins and endomorphins

Endogenous opioid peptides, enkephalins and endomorphins, are located in key regions involved in pain transmission and analgesia, including the spinal cord. These endogenous peptides activate opioid receptors to produce analgesia and reduce pain. We describe a new method to measure enkephalin and endomorphins by high performance liquid chromatography with electrochemical detection. This method allows use of a small sample volume to measure met-enkephalin, leu-enkephalin, endomorphin-1 and endomorphin-2 simultaneously. Using push-pull perfusion of the spinal cord, there were detectable concentrations of met-enkephalin, leu-enkephalin, and endomorphin-2. Further infusion of 100mM potassium chloride evoked release of met-enkephalin and endomorphin-2 but not leu-enkephalin. Thus, we have developed a method to simultaneously measure enkephalins and endomorphins in small sample volume that allows measurement of these opioid peptides in vivo.

Direct influence of serotonin on the larval heart of Drosophila melanogaster

The heart rate (HR) of larval Drosophila is established to be modulated by various neuromodulators. Serotonin (5-HT) showed dose-dependent responses in direct application within semi-intact preparations. At 1 nM, HR decreased by 20% while it increased at 10 nM (10%) and 100 nM (30%). The effects plateaued at 100 nM. The action of 5-HT on the heart was examined with an intact Central Nervous System (CNS) and an ablated CNS. The heart and aorta of dorsal vessel pulsate at different rates at rest and during exposure to 5-HT. Splitting the heart and aorta resulted in a dramatic reduction in pulse rate of both the segments and the addition of 5-HT did not produce regional differences. The split aorta and heart showed a high degree of sensitivity to sham changes of saline but no significant effect to 5-HT. Larvae-fed 5-HT (1 mM) did not show any significant change in HR. Since 3,4-methylenedioxymethamphetamine (MDMA) is known to act as a weak agonist on 5-HT receptors in vertebrates, we tested an exogenous application; however, no significant effect was observed to dosage ranging from 1 nM to 100 microM in larvae with and without an intact CNS. In summary, direct application of 5-HT to the larval heart had significant effects in a dose-dependent manner while MDMA had no effect.

Steady-State Levels of Monoamines in the Rat Lumbar Spinal Cord: Spatial Mapping and the Effect of Acute Spinal Cord Injury

Monoamines in the spinal cord are important in the regulation of locomotor rhythms, nociception, and motor reflexes. To gain further insight into the control of these functions, the steady-state extracellular distribution of monoamines was mapped in the anesthetized rat's lumbar spinal cord. The effect of acute spinal cord lesions at sites selected for high resting levels was determined over ∼1 h to estimate contributions to resting levels from tonic descending activity and to delineate chemical changes that may influence the degree of pathology and recovery after spinal injury. Measurements employed fast cyclic voltammetry with carbon fiber microelectrodes to give high spatial resolution. Monoamine oxidation currents, sampled at equal vertical spacings within each segment, were displayed as contours over the boundaries delineated by histologically reconstructed electrode tracks. Monoamine oxidation currents were found in well defined foci, often confined within a single lamina. Larger currents were typically found in the dorsal or ventral horns and in the lateral aspect of the intermediate zone. Cooling of the low-thoracic spinal cord led to a decrease in the oxidation current (to 71–85% of control) in dorsal and ventral horns. Subsequent low-thoracic transection produced a transient increase in signal in some animals followed by a longer lasting decrease to levels similar to or below that with cooling (to 17–86% of control values). We conclude that descending fibers tonically release high amounts of monoamines in localized regions of the dorsal and ventral horn of the lumbar spinal cord at rest. Lower amounts of monoamines were detected in medial intermediate zone areas, where strong release may be needed for descending activation of locomotor rhythms.