Basis of neurotropism of rabies virus: A new hypothesis

Medical Hypotheses

BASIS

14: 207-212, 1984

OF NEUROTROPISM

OF RABIES

VIRUS

: A NEW HYPOTHESIS

N. Annal 11 First Street, Ranga raja puram, Madras - 600 015,

India. ABSTRACT ___There in the

is evidence

axoplasm

to

that reach

rabies viruses travel the

central

nervous

along the peripheral system

(CNS).

nerves,

It

is

hypothesised that rabies viruses are attracted electrically or magnetically by the nerve impulses to the CNS. This explains why rabies viruses show specific affinity towards nerve tissue. The present hypothesis offers a new approach to the study of pathogenesis of the various neurotropic viruses. jNTRODUCTlON

virus

My hypothesis is based on two assumptions. The first is that is negatively charged within an animal body. This certainly

the rabies seems

possible, since it has been observed in electrophorectic studies that the iso electric point of rabies virus is slightly less than pH 7(l) and the pH values for the body f!uids where the rabies virus is deposited and travels are more than pH 7.3(extra-cellular axoplasm

pH 7.3

fluid

has a pH of 7.32, cerebrospinal

- 7.5, and muscle cell cytoplasm

fluid

pH 7.33

pH 7.3) (2,3).

The second assumption is that the nerve impulses are electrically strong enough to exert a pulling force on the charged virus particle. Although the potential change during conduction of a nerve impulse measures only about 100 mV, the conduction is not passive as a wire conducts current. It is an active self propagating process involving energy expenditure and the amplitude of the nerve impulse is not affected by the distance of travel or the resistance of the nerve tissue (4). Whereas in electrophoresis even though a high voltage is used, only a few mA of current actually passes through the poly acrylamide gel due to the resistance of the conducting medium (5). I am aware of a short coming of the hypothesis namely, the larger size of the virus as compared with the proteins that move in electrophoresis. But, even particles as large as human red blood cells move in electrophoresis and towards a magnetic field (6). The nerve impulses are also accompanied by a magnetic field (7). The long duration of time taken for the virus to reach the CNS is also consistent with the new hypothesis because the nerve impulses are of intermittent nature.

207

PATHOGENESIS -_-

OF RABIES:

Several studies indicate that rabies viruses travel in the axoplasm (8, 9). Some authors postulate that rabies viruses are carried passively by the retrograde axoplasmic flow (10, 11). However, this explanation becomes less acceptable in the light of the following observations. 1)

Local anaesthetics do not block the axoplasmic flow unless used in concentrations considerably in excess of the anaesthetic dose (11).

2)

However Kaplan -et.al.(l2) and Dean -__ et.al.(l3) have shown that local anaesthetics which do not posses virucidal property either, prevent the spread of rabies viruses when injected proximal to the site of inoculation (concentrations used were only normal anaesthetic doses). It, therefore, seems reasonable to challenge the view of passive transport of viruses in the axoplasm.

My hypothesis: My hypothesis is that the rabies viruses are attracted electrically or magnetically ,by the nerve impulses. During electrophoresis, negatively charged particles move from negative to positive electrode which is opposite to the direction of electric field (5). Similarly rabies viruses which are assumed to be negatively charged at the physiological pH of the body, should move against the direction of the electrical nerve impulse to the site of origin of the impulse. It is suggested, therefore, that the viruses travel along the motor nerve fibres where the direction of nerve impulses is from the CNS to the periphery. Sensory neurons attract some of the viruses that have reached the motor neurons, towards the sensory system and then down to the peripheral structures of the body from where sensory impulses arise. Thus, the sensory impulses interrupt the ascent of the viruses. If there are more sensory impulses to the‘spinal segment first involved in the pathogenesis (which is the corresponding spinal segment to the site of viral introduction), more viruses will be attracted back to the site of introduction and the disease may not manifest itself atleast in some of the cases. This is further elaborated, with experimental evidence, below. Indirect

evidence:

There is some, albeit indirect, experimental evidence of electro magnetic attraction by the nerves as the basis of neurotropism of the rabies viruses. The participation of motor nerves and impulses in the pathogenesis of rabies is demonstrated by the electron microscopic observations made by Murphy (14) who repeatedly found evidence of infection in the motor end plates. Watson &aJ. (15) also reported infection of motor end plates and ventral horn cells before involvement of dorsal root ganglia. Identification of rabies virus aggregates at nodes of Ranvier, (9. 14) which are responsible for saltatory conduction of nerve impulses, is also consistent with the hypothesis.

208

Identification which

of rabies viruses

have an electric

consistent

with

potential

at the acetyl

that

increases with

choline

receptors

(16)

motor

impulses

is also

the hypothesis.

As I have discussed, sensory impulses interrupt virus spread. This is evident from Schindler’s work on the effect of histamine in the pathogenisis of rabies. Large doses of histamine when injected intra muscularly at the site of viral inoculation, has been shown to decrease mortality from experimental rabies infection. Rut histamine appears to have no effect on the virus itself when applied directly (17). It is possible that histamine, which is known to produce pain, itching and afferent nerve discharges, (18) acted by increasing the interruption produced by the sensory impulses on the spread of rabies viruses. It is interesting that the histamine effect was still evident when injected 16 hours after the injection of viruses (17). MATERIALS-AND

METHODS

Test Animals Four week old mice (Swiss albino) for the experiment.

weighing

about

10 grams were used

Viruses Two

types

1)

CVS strain

4

PV 3462

Electrical

of rabies viruses were used of fixed

fixed

rabies viruses.

rabies viruses adapted

to vero cells.

apparatus

20 V DC batteries to measure

current

flow

which

were the source of current

comprised

and multimeters

the apparatus.

Two groups of Swiss albino mice (ten per group) of the same age and weight were taken and the first group were inoculated intracerebrally with 100 LD 50 of CVS virus. The second group of mice were inoculated intracerebrally with 100 LD 50 of PV 3462 virus. Normal

uninfected

Swiss albino

mice of the same age and weight

as

the test animals were tested for any harmful effect of application of 20 V DC current upto 10 minutes. The electrodes were applied externally on the skin after shaving off the hair. One electrode was placed on the head in between the ears and the other on the hind foot. The skin was not pierced. Application of 20 V of direct current did not affect the mice except for an increase in heart and respiratory rates. Current flow through the mice was found to be 0.3 - 0.4 mA (milli ampere).

209

E&trotherapy

for rabies

Five each from both groups of mice were subjected to electrotherapy. The other animals were kept as controls. The negative electrode was kept on the head in between the ears; positive electrode on the shaven area of the hind foot. Electrical application was given for five minutes continuously to all animals in the test group. This was repeated after a gap of twenty minutes in those mice which showed little or no response. The current flow through the mice varied from 0.3 to 0.4 mA. The changes observed during electrotherapy were the same as observed in normal uninfected Swiss albino mice. RESULTS ..~ One of the test animals died even before the completion of electrotherapy. It was later found that the position of electrodes was mistakenly reversed in that animal, namely, the positive electrode was kept on the head. The rest of the test group showed improvement in activity and responded to painful stimuli by walking a short distance. Whereas the control animals only made slight movement after a similar painful stimulus. Two of the test group animals showed remarkable recovery; ran around the table several times. They kept themselves very active for 15 minutes. The improvement in activity lasted for 24 to 48 hours in all the mice. The control group of mice died within two days after first showing signs of paralysis. The treated group survived for four to six days after first showing signs of paralysis. CONCLUSION This hypothesis incorporates in one frame work a number of hitherto unrelated observations; it is hypothesised that rabies viruses are electrically or magnetically attracted by the nerve impulses. However a contribution by the axoplasmic flow in viral transit can not be ruled out. It has been observed that axoplasmic flow blockers prevent viral transit when applied proximal to the viral inoculation (19). In addition to removing the contribution of axoplasmic flow these substances can also act by interfering with the presynaptic neuro muscular transmission (20). The same hypothesis would apply to other viral diseases that are caused by neurotropic viruses. My hypothesis suggests a new line of treatment which may help patients with hydrophobia. Application of an electro magnetic field of suitable strength through the peripheral nerves in hydrophobia patients maintained in intensive care, could stop the further spread of the viruses in the CNS. This external electro magnetic field could also displace the viruses from the higher levels of the CNS towards the peripheral nerves. Minimising the motor activity by sedation or anaesthesia may also help these patients. The lives of rabies victims could thus be prolonged and a ‘cure’ may be expected atleast in some cases.

270

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