| |
Pain models for human
use should be safe, valid, and experimentally convenient. Many
of the established animal models do not fulfill these criteria,
notably with respect to safety. Another important aspect to consider
is the tissue type (e.g. superficial, somatic, or visceral) and
body area. These considerations are important because deep tissue
pain is poorly localized and associated with broader and more
diffuse pain referral than superficial pain. There are also more
extensive pain referral patterns in proximal limbs or trunk than
distal limbs. To study questions of significance to clinical pain
phenomena, the model system should incorporate the following requirements:
| Table
4.1: Requirements of Model System
|
|---|
| Model
Requirement
| Reason
|
|---|
Minimal
tissue damage
Prevention of unnecessary pain exposure | Necessity
for research involving human subjects |
| Involvement
of somatic and visceral tissues | Neuro-anatomical
considerations
Clinical relevance |
| Experientially-adjusted
pain intensity | Required
to permit comparisons at matched pain intensity because
subjects vary with respect to sensitivity to and suppression
of pain |
|
Another
important point to consider is the duration of exposure to noxious
stimulation because pain duration influences the subject's physiological
state and response behavior. For example, initial pain signals
a warning to the subject to stop the activity provoking it and
to take action to alleviate the pain. This initial stage of
pain is characterized by increased alertness, focused attention,
the suppression of feeding, sleep and reproduction, and increased
vascular tone, respiration and blood sugar levels. If pain is
experimentally sustained, longer lasting effects on neuronal
excitability are produced such as upregulation of the endogenous
opoid systems. Persistent clinical ("chronic") pain
can even change the "circuitry" in the central nervous
system via a series of events involving alterations in early,
intermediate, and late gene expressions.
These differences are
illustrated in Table 4.2 below.
| Table
4.2: Differences in Duration of Pain
|
|---|
|
| Brief
or Intitial
| Sustained
| Persistent(Chronic)
|
|---|
| Neurotransmitters | Asp,
CGRP, Glu, SP | Endogenous
Opioids (e.g. Dyn, Enk) | Systems
Adaptation |
| Structural | None | | Sprouting
Remodeling |
| Pain
Perception | Sensory
(mostly) | Sensory
+ Affective | Sensory
+ Affective |
| Abbreviations:
Asp=aspartate, CGRP=calcitonin-gene related peptide,
Glu=glutamate, SP=substance P, Dyn=dynorphin, Enk=enkephalin.
Sensory and affective refer to the perceived information
content of the pain experience. |
|
|