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Effects Of Local Anesthetics On The Neurogenic Vasodilatation In The Pulp

N.P. Kerezoudis, DDS, Dr Dent
Lecturer Dept of Endodontics, University of Athens Dental School. Greece.

M. Khabbaz, DDS, Dr Dent
Assistant Professor, Dept of Endodontics, University of Athens Dental School, Greece.

S.N. Sykaras, DDS, Dr Dent.
Emeritus Professor, Dept of Endodontics, University of Athens Dental School, Greece.

Received: 03.02.2004
Accepted: 05.05.2004
Hellenic Dental Journal 2004, 14:27-32
Key words: Dental Pulp, Local Anesthetics, Microcirculation.

The effect of local anesthetics on the electrically induced neurogenic vasodilatation in the dental pulp was investigated in five anesthetized cats. Blood flow was monitored from the upper canine pulp by the Laser Doppler Flowmetry while monopolar electrical stimulation was applied on the tooth (100 V, 5ms, 1Hz, 5 twin pulses). One of the following anesthetic solutions was injected in the periapical area: lidocaine 3% plus nor-adrenaline, mepivacaine 3% without vasoconstrictor and lidocaine 2% alone. It was found that tooth stimulation resulted in a clear-cut vasodilatation of the pulp with long duration. Periapical injection of lidocaine 3% plus nor-adrenaline caused a long-lasting vasoconstriction while pulpal blood flow was unaffected by the other anesthetic solutions. Electrical stimulation after the lidocaine 3% plus nor-adrenaline injection resulted in a vasodilatation with smaller magnitude (129±59% after vs 170±40% before, n=5) and smaller duration than before (5, 1±2, 6 min vs 18, 6±1, 9 min). The electrically induced responses were not affected by the other agents. It was concluded that the beneficial effects of the afferent nerve responses were not affected by administration of local anesthetics without the addition of nor-adrenaline.

INTRODUCTION
The blood supply to the dental pulp is relatively high as compared to other oral tissues1. The reason for this is unclear, but it seems that a rich perfusion plays a role in preventing the accumulation of injurious substances e.g. invading bacte¬rial toxins through the dentinal tubules, and also in providing the pulp with the necessary substrates required for the odontoblasts, pulpal cells and the intradental nerve terminals. Another intriguing characteristic of the dental pulp is its rich innervation. It has been proposed that only the cornea of the eye receives a denser innervation as far as the peripheral tissues are concerned2. The nerves of the pulp are sympathetic, parasympathetic as well as sensory in origin1,3. Accumulating evidence suggests that sensory nerves of the dental pulp apart for the mediation of pain exert other neurovascular reactions4. It has been found for example, that a variety of stimuli such as electrical stimulation and clinical insults like mechanical deformation of teeth, drilling, probing, percussion, tooth drying or heat applications on the tooth crown induce a long-lasting vasodilatation in the dental pulp5,6. It is well established today that the nerve fibers responsible for these reactions contain specific neuropeptides such as Substance P (SP), Neurokinin A & B (NKA & NKB) as well as Calcitonin Gene Related Peptide (CGRP)4. The neuropeptide induced vasodilatation is unaffected by any of the known vasoactive antagonists like β- adrenoceptors blockers, atropine, histamine antagonists or prostaglandin synthesis inhibitors but it is sensitive to capsaicin the pungent extract of hot peppers6-8. Whether other agents, such as local anesthetics, exert similar effects on the sensory nerve-induced vasodilatation it has not been clarified completely yet.

Local anesthetics are used extensively in dental practice. The most common used anesthetic solutions are various concentrations of lidocaine, mepivacaine, articaine etc. It is well established that the duration of anesthetic action of the local anesthetic solutions especially of the lidocaine type is relatively short due to the “wash-out” of the agent from the rich blood supply of the oral tissues. To overcome this difficulty a vasoconstrictor is added to the lidocaine solutions. The most common vasoconstrictors are various concentrations of adrenaline or nor-adrenaline (1:50000-l :200000)9. However, these vasoconstrictors, apart from exerting systemic circulatory side effects, reduce significantly the basal blood flow of the dental pulp10 as well as the excitation of the pulpal nerves of the Αδ-type11. Whether nor-adrenalin contained in dental local anesthetic solutions exerts any effect on the neurogenic vasodilatation is still unclear.

The purpose of this study was to investigate whether the subperiosteal injection of a local anesthetic solution exerts any effect on the sensory nerve- mediated vasodilatation in the canine pulp of the cat.

MATERIALS AND METHODS
I. Preparation of the animals
Five adult cats (2.5-4 Kg) having teeth with well formed apices were used in this study. Animals were first sedated with Ketamin (Ketalare 30 mg/Kg)and then anaesthetized with Sodium Pentobarbital (Nembumal) at an initial dose of 30mg/Kg, supplemented when necessary with additional doses of 20 mg/Kg. Body temperature was measured by a rectal thermometer and kept constant at 38° C with a thermo¬statically controlled electric blanket. The cats were placed with their backs to the experimental table and the jaws were immobilized by means of a steel rod apparatus secured to the posterior teeth with dental acrylic. The whole fixation was stabilized further to the experimental table by means of a magnetic micromanipulator. A 27- gauge needle connected to a syringe via a 20 cm plastic tube was inserted to the periapical region of the upper canine and left in place during the experimental procedure. This experimental setup allows the injection of a local anesthetic to the periapical area with minimal disturbances of the head and jaws. After this preparation the animals were allowed to rest for a period of at least 20 min, an adequate period for blood flow to be stabilized.
II. Blood Flow Measurement
Pulpal Blood Flow (PBF) was monitored from the canine tooth of the upper jaw continuously and non- traumatically by the Laser Doppler Flowmetry (L.D.F.)1213. The L.D.F. used in the present study was a He-Ne Laser (2mW) which emits at a wavelength of 632,8 n.m. (Periflux, Perimed PF3, Sweden). A fiber-optic probe (fiber diameter 200 pm with separation of 500 pm) were used to guide the Laser light to and from the tooth surface. The fiber¬optic was fixed by means of a micromanipulator perpendicularly to the distal surface in the cervical one third of the incisor and connected to the Laser apparatus. Calibration of the device was performed according to the manufacturer’s instructions.
lII. Electrical stimulation of teeth
Electrical stimulation of tooth (100 V, 5ms, 1Hz, 5 twin pulses) was applied via a silver electrode placed in the upper canine crown and secured in position by means of a micromanipulator. The other electrode was placed in the lip (monopolar stimulation). An eelectrode paste was applied on both electrodes to facilitate good electrical contact and the crown electrode served as the negative pole (cathodal stimulation). These parameters for stimulation were selected since we found in preliminary experiments to produce responses of similar appearance and magnitude. As a rule, at least two tooth stimulations producing responses of similar magnitude were applied before drug administration and when pulpal blood flow gained or returned to a stable baseline for a period of at least 3 min. The same electrical stimulation was applied 10 min after the injection of the anaesthetic solution.
IV.Injection o f anesthetic solutions
Three anesthetic solutions were used in different groups of animals: a) in the first group lidocaine 3% plus nor-adrenaline (Xylestesin-F) was used b) in the second group mepivacaine 3% without vasoconstrictor (Mepivacain) and c) in the third group lidocaine 2% without a vasoconstrictor . All solutions were injected via the plastic tube in an amount of 0.1 ml of each. The correct level of anesthesia was tested by monitoring the disappearance of jaw opening reflex upon electrical stimulation of teeth.
V.Statistical analysis
All data were stored on an on-line computer and analysed with commercial software (Perisoft, Perimed, Sweden). The effects on pulpal blood flow are expressed as percent changes of perfusion units (PU) (baseline to peak) and are given as means ± SEM. Blood flow changes during pulp stimulation were relative to resting values during 3 min before stimulus onset. Differences were evaluated by paired Student’s t- test; p-values of less than 0.05 were considered to be significant.

RESULTS
I. Effect of anesthetic solutions on the magnitude of Pulpal Blood Flow Responses
In untreated animals of all groups, electrical stimulation of the tooth crown resulted in a clear-cut increase of blood flow of the pulp (mean 139±24% n=10). After the subperiosteal injection of lidocaine 3% plus nor-adrenaline at the periapical area in the animals of the first group a severe reduction (by 71 ±7%, n=5) of the basal blood flow of the pulp was seen. Repetition of electrical tooth stimulation after drug administration produced a clear-cut increase in the PBF which had a smaller magnitude than before (from 170 ±40 % to 129±59%, p>0.05, n=5) (Fig. 1, 2).
In the second group of animals Mepivacaine 3% was injected pe- riapically. Unlike the previous group, this anesthetic solution did not affect the basal blood flow of the pulp. Similarly, mepivacaine did not affect the magnitude of the stimulation induced blood flow increase (from 144 ±31 % to 135± 32 %, p>0.05, n=4). (Fig. 2)
In the third group, injection of Lidocaine 2% produced a slight and transient increase in pulpal blood flow (37±3%, n = 3). Application of electrical tooth stimulation in untreated animals as well as after the injection of the drug produced responses of similar magnitude (from 64 ± 2 % to 89 ±25%, p>0.05, n=4) (Fig. 2).
During all electrical stimulations a profound jaw opening reflex was elicited as it was evident by the masseter muscle contraction. However, we could not observe any sign of jaw opening reflex after the administration of each local anesthetic indicating that in the doses used the drugs were adequate to abolish the impulse propagation to CNS and hence to activate the endogenous reflex mechanism.
II. Effect of anesthetic solutions on the duration of Pulpal Blood Flow Responses After the monopolar electrical stimulation of the tooth crown in the pretreated animals of the first group the duration of PBF increase was 18.6± 1.9 min. After the injection of Lidocaine 3% plus nor-adrenaline, and the repetition of the same electrical stimulation the observed increase in the PBF was of significant smaller duration than before (5.1 ±2.6 min)(p<0.01, n=5) (Fig. 3).

DISCUSSION
In the present study we confirmed that electrical stimulation of the tooth resulted in a long lasting vasodilatation in the pulp of the cat. Furthermore we were able to show that a periapical injection of dental local anesthetics solution without the addition of vasoconstriction did not influence significantly the occurrence of this response.

It is well established today that the dental pulp is equipped with small caliber sensory nerves containing potential vasoactive substances. These neuropeptides are released when the tooth crown is triggered by a variety of stimuli such as drilling, air drying, crown deformation etc5. For experimental purposes, these nerves can be activated by stimulating directly the nerves bundles (e.g. inferior alveolar nerve) or the crown of the tooth1415. It has been reported that only few electrical pulses can cause a vasodilatation which may last for several minutes5. These findings are confirmed in our study since only 5 electrical pulses on the tooth crown induced a vasodilatation which lasted almost 20 minutes.

Undoubtedly the introduction of local anesthetics in dentistry allowed the application of many clinical manoeuvres in patients associated, otherwise, with the experience of intense pain e.g. pulp extirpation. Even a simple tooth drilling for a tooth filling sometimes is not possible to be performed without the injection of a local anesthetic.

The level of anesthesia attained by a local anesthetic depends on many factors such as its composition, concentration, diffusion ability, etc9 Many commercial products of lidocaine contain vasoconstrictors in an attempt to counteract its vasodilator ability and to enhance the duration of anesthetic action. The diffusion of an anesthetic solution which contains vasoconstrictor (adrenalin or nor-adrenalin) causes a marked vasoconstriction with a severe and prolonged decrease of PBF10,11, 161S. We confirmed these observations in the present study since infusion of Xylestesin-F but not Mepivacaine or Lidocaine induced a long-lasting reduction in the normal blood circulation in the dental pulp. This reduction in basal blood flow of the pulp is very significant from the clinical point of view since it reduces the oxygen and the nutrient supply to the dental pulp. Moreover, it is possible that during this blood flow reduction, bacterial toxins or other injurious substances from caries/oral cavity may enter and then be accumulated in the dental pulp causing irreversible damage. Therefore, our finding that the small caliber sensory nerves of the pulp can be activated despite the periapical injection of local anesthetics may have a great clinical significance. As it has already been mentioned, activation of these nerves can cause a long-lasting vasodilatation which may somehow counteract the reduction of PBF associated with the vasoconstrictors. In addition, in cases where we have injected Mepivacaine or Lidocaine the magnitude as well as the duration of nerve activation induced vasodilatation remained totally unchanged. This observation is interesting since the enhanced blood circulation may supply the dental pulp cells with the appropriate nutrients and defense elements to withstand external irritation.

The observation that the injection of nor-adrenalin containing local anesthetic reduces significantly the duration and to some extend the magnitude of the response is difficult to explain. Some years ago, it has been shown in the rat that activation of sympathetic nerves exerts an inhibitory influence on the afferent nerve-induced vasodilatation and plasma protein extravasation8,19,20. A direct effect of nor¬adrenalin released from sympathetic nerves on the sensory nerve terminals has been speculated that time. This assumption has been confirmed in very recently in 2003 by Hargreaves and his colleagues21, 22 who showed an adrenergic regulation of the iCGRP release process from the superfused isolated bovine dental pulp. This inhibition is exerted via pre-junctional receptors a-adrenoreceptors23.

Also, the present study showed that pain sensation and neurogenic vasodilatation are two separate phenomena influenced differently by the injection of local anesthetics. Since the local anesthetic solutions are injected in the periapical area the impulse propagation from the activated pulpal nerves is blocked at the nerve trunk located outside the tooth foramen inside the alveolar bone. It is well known that local anaesthetics exert their blocking effects by closing the passage on Na+ channels24'26. Therefore the sensation of pain is inhibited by this way. This hypothesis is supported in our study by the abolition of jaw opening reflex after the anaesthetic injection. On the other hand the PBF increase results from the release of neuropeptides from sensory nerve terminals. These neuropeptides are produced in the body cell of the trigeminal ganglion, stored in vesicles and then are transported in the nerve terminals4. When the nerve terminals are triggered by external stimuli (electrical stimulation in our case) these are released causing vasodilatation and increased plasma extravasation. Since the local anaesthetic is injected outside the dental pulp, the active substance did not reach the sensory nerve terminals to exert its inhibitory effects on the nerve membrane. Our results are supported by recent findings showing that infusion of local anaesthetics into the systemic circulation failed to abolish completely the activation and hence the neurogenically driven responses27, 28. Complete elimination of neurogenic vasodilatation in accomplished only after intophoretic application of lidocaine through exposed dentin28. Only by this way the anaesthetic action of the agent is exerted directly on the nerve terminals of the pulpal sensory nerves.

CONCLUSIONS
1.   The nerves cited in the dental pulp in the cat can preserve their function even when anesthesia is present
2.   Pulpal nerves in the cat are insensitive to vasoconstriction
3.   Mepivacaine in the recommended doses has an excellent action on the pulpal blood flow.

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