Description
Unformatted Attachment Preview
Rubrics for Posters
Category
20 points
15 points
10 points
5 points
0 points
Introduction,
Hypotheses,
Referenced
literature
Provided
relevant
background
information
and
hypotheses/pu
rpose; relevant
references
Left out some
relevant
background
information or
hypotheses/pu
rpose or
references
The background
was largely
incomplete,
missing
references
Missing/
illogical
section
Methods
Procedures
were outlined
in a way that
could be
followed by
anyone without
additional
explanations.
Provided
relevant
background
information but
incorrectly
identified
hypotheses/pu
rpose or
references
Procedures
were outlined
but needed
additional
explanation.
Some
procedures
related to the
data shown
were missing.
Procedure section
was largely
incomplete,
confusing for
reader.
Missing/
illogical
section
Results
Accurate
representation
of the data in
tables and/or
graphs.
Graphs and
tables are
labeled and
titled.
Some data
graphics are
missing or
data are
confusing.
Only some
figures are
shown, this
section
contains
mostly text;
incomplete
figures
Data section is
largely incomplete
Missing/
illogical
section
Discussion
Student
provided
detailed
conclusion and
discussed
importance of
the work in the
broader
context of
biology
Read and
understood 3
other posters,
left accurate
comments
Student
provided semidetailed
conclusion but
did not discuss
the importance
of the work in
a broader
context
Some
conclusions
are
missing/poorly
written;
opinions
included
Important
conclusions were
not discussed/
Missing/
illogical
section
Left comments
to 3 posters
but comments
were
insufficient
Left comments
only to 2
posters;
missing or
inadequate
comments
Left comments
only to 1 poster;
missing or
inadequate
comments
Online
discussions
Largely missing
Missing/
illogical
section
Points can also be lost if any other guidelines mentioned in the guidelines document, in
announcements or emails are not followed. Examples include:
• Points can be lost for submitting the wrong format document. This is supposed to be a
one-page poster submitted as a pdf. For example, submitting a random 3-page word
document can result in a zero score.
• Points can and will be deducted for plagiarism and very close paraphrasing.
• Points can be lost even if you quote the original text. Even though quotations are fine
and not considered plagiarism, too many quotes indicate little originality and “in own
words” on the part of the student.
• Points can be lost if space is poorly utilized and too much empty space is left or creative
drawings/images are overwhelmingly large.
• Points can be lost for choosing human studies – this can result in a zero score. Never
receiving an approval for your article can also result in point penalties.
• Points will be deducted if not all 4 submissions have been done and/or files have not
been submitted by the appropriate deadlines.
Student Name
Paper Title
Paper Author, Journal and Publication Year
You can be creative and change the look of this poster template or add
images anywhere in the poster, but please do not make the images or any
art the focal point – see instructions
Results
Discussion
Background / Introduction
Figure 2: title
Figure 1: title
Brief introduction/explanation
of the key topic of the paper
Paste key data for Figure 1
here
Brief information on previous
research on this topic (include
details about what part of the
nervous system/what cell
type/why is this brain area/cell
important)
–
Address the main
question/hypothesis of the
paper.
–
Paste key data for Figure 2
here
Include 4 subfigures if available
List below key
information/conclusions from the
figure – briefly describe the main
results?
–
List key points below graphics
–
Figure 3: title
Material and Methods
Important techniques used to
collect the data shown in this
poster? Do not talk about details
such as brands of instruments or
contents of regular solutions, such
as saline.
What animals were used?
Any genetic crosses of interest?
Any important pharmacological
agents used?
How was the data analyzed?
–
Include 4 subfigures if available
List below key
information/conclusions from the
figure – briefly describe the main
results?
List key points below graphics
Do the authors state any
outstanding questions yet to be
answered or future directions??
List key points below graphics
Figure 4: title
Paste key data for
Figure 4 here
Paste key data for
Figure 3 here
–
Include 4 subfigures if available
List below key
information/conclusions from the
figure – briefly describe the main
results?
List key conclusions from this
paper (make sure they answer
the main question posed in the
introduction and relate to the
data you have presented)
References
–
Include 4 subfigures if available
List below key
information/conclusions from the
figure – briefly describe the main
results?
–
List key points below graphics
Reference this paper
Reference 2 other papers that
were relevant to this study or
cited in this paper that you
discussed in this poster
J Neurophysiol 123: 1944 –1954, 2020.
First published April 15, 2020; doi:10.1152/jn.00038.2020.
RESEARCH ARTICLE
Sensory Processing
Responses of neurons in the primary somatosensory cortex to itch- and painproducing stimuli in rats
X Sergey G. Khasabov,1 Hai Truong,2 Victoria M. Rogness,1 Kevin D. Alloway,3,4 Donald A. Simone,1
and Glenn J. Giesler, Jr.2
1
Department of Diagnostic and Biological Sciences, University of Minnesota School of Dentistry, Minneapolis, Minnesota;
Department of Neuroscience, University of Minnesota, Minneapolis, Minnesota; 3Center for Neural Engineering, Penn State
University, University Park, Pennsylvania; and 4Department of Neural and Behavioral Sciences, Penn State University,
University Park, Pennsylvania
2
Submitted 29 January 2020; accepted in final form 8 April 2020
Khasabov SG, Truong H, Rogness VM, Alloway KD, Simone
DA, Giesler GJ Jr. Responses of neurons in the primary somatosensory cortex to itch- and pain-producing stimuli in rats. J Neurophysiol
123: 1944 –1954, 2020. First published April 15, 2020; doi:10.1152/
jn.00038.2020.—Understanding of cortical encoding of itch is limited. Injection of pruritogens and algogens into the skin of the cheek
produces distinct behaviors, making the rodent cheek a useful model
for understanding mechanisms of itch and pain. We examined responses of neurons in the primary somatosensory cortex by application of mechanical stimuli (brush, pressure, and pinch) and stimulations with intradermal injections of pruritic and algesic chemical of
receptive fields located on the skin of the cheek in urethane-anesthetized rats. Stimuli included chloroquine, serotonin, -alanine, histamine, capsaicin, and mustard oil. All 33 neurons studied were excited
by noxious mechanical stimuli applied to the cheek. Based on mechanical stimulation most neurons were functionally classified as high
threshold. Of 31 neurons tested for response to chemical stimuli, 84%
were activated by one or more pruritogens/partial pruritogens. No
cells were activated by all five substances. Histamine activated the
greatest percentage of neurons and evoked the greatest mean discharge. Importantly, no cells were excited exclusively by pruritogens
or partial pruritogens. The recording sites of all neurons that responded to chemical stimuli applied to the cheek were located in the
dysgranular zone (DZ) and in deep laminae of the medial border of the
vibrissal barrel fields (VBF). Therefore, neurons in the DZ/VBF of
rats encode mechanical and chemical pruritogens and algogens. This
cortical region appears to contain primarily nociceptive neurons as
defined by responses to noxious pinching of the skin. Its role in
encoding itch and pain from the cheek of the face needs further study.
NEW & NOTEWORTHY Processing of information related to itch
sensation at the level of cerebral cortex is not well understood. In this
first single-unit electrophysiological study of pruriceptive cortical
neurons, we show that neurons responsive to noxious and pruritic
stimulation of the cheek of the face are concentrated in a small area of
the dysgranular cortex, indicating that these neurons encode information related to itch and pain.
nociception; pruriception; rats; single-unit recording; somatosensory
cortex
Correspondence: S. G. Khasabov (e-mail: [email protected]).
1944
INTRODUCTION
Pruritus refers to chronic, pathological itch and is associated
with many dermatological conditions such as eczema (dermatitis) and psoriasis, as well as nondermatological conditions
including kidney failure, postherpetic neuralgia, AIDS, Hodgkin’s disease, and allergic reactions (Kremer et al. 2014;
Lavery et al. 2016a; Reich et al. 2009). Severe pruritis is
known to have an adverse impact on the quality of life (Anand
2003; Halvorsen et al. 2012; Lavery et al. 2016b; Mollanazar
et al. 2016; Patel and Yosipovitch 2010). Although progress
has been made in understanding how itch is signaled and
encoded in the peripheral and central nervous systems (LaMotte et al. 2014), the underlying neurophysiological mechanisms remain incompletely understood. Pruriceptive primary
sensory neurons and dorsal horn neurons have been identified
with molecular markers including Mrgpr (Dong et al. 2001;
Liu et al. 2009; Zhu et al. 2017), brain-derived neurotrophic
factor (BDNF) (Dembo et al. 2018), and gastrin-releasing
peptide (Mishra and Hoon 2013; Sun et al. 2009, 2017; Sun
and Chen 2007). Electrophysiological studies show that itch is
encoded by primary afferent nociceptors (Johanek et al. 2007,
2008; Namer et al. 2008; Pereira et al. 2015; Ringkamp and
Meyer 2014), nociceptive dorsal horn neurons (Akiyama et al.
2012, 2013, 2015; Akiyama and Carstens 2014; Duan et al.
2018; Tsuda 2018), and spinal projection neurons including the
spinothalamic tract (STT) (Davidson et al. 2007, 2009; Simone
et al. 2004), the spinoparabrachial tract (SPbT) (Jansen and
Giesler 2015), and the trigeminothalamic tract (Lipshetz and
Giesler 2016; Moser and Giesler 2014a). We recently showed
that many nociceptive thalamic neurons in the posterior triangular (PoT) and ventral posterior medial (VPM) nuclei were
excited by pruritic stimuli (Lipshetz et al. 2018).
Human fMRI studies demonstrate that blood flow in the
primary somatosensory cortex (SI) is increased after pruritic
stimulation of the skin (Andersen et al. 2015; Ikoma et al.
2006; Lavery et al. 2016a; Mochizuki et al. 2017), suggesting
that itch sensation is encoded in the SI along with other
somesthetic modalities. Previous electrophysiological studies
examined response properties of individual SI neurons activated by tactile, noxious mechanical, thermal, and algesic
chemical stimuli in monkeys (Kaas et al. 1979; Kenshalo et al.
0022-3077/20 Copyright © 2020 the American Physiological Society
Downloaded from journals.physiology.org/journal/jn at Univ of Illinois Chicago (131.193.138.031) on September 8, 2022.
www.jn.org
PRURICEPTIVE CORTICAL NEURONS
1945
2000; Kenshalo and Isensee 1983; Merzenich et al. 1978), cats
(Dykes et al. 1980; Rubel 1971), raccoons (Johnson et al.
1982), opossums (Pubols et al. 1976), squirrels (Sur et al.
1978), and rats (Chapin and Lin 1984; Guilbaud et al. 1992;
Wang et al. 2010; Welker 1971). Responses of SI neurons to
pruritic stimuli, however, have not been investigated.
In the present study, we characterized and compared responses of single neurons in SI to chemical pruritogenic and
nociceptive stimuli applied to the skin of the cheek. We chose
this area of the skin because pruriceptive and nociceptive
stimuli applied to the cheek produce distinct and easily recognized behavioral responses: pruriceptive stimuli produce
scratching of the cheek with the hindlimb, whereas algesic
stimuli produce wiping of the cheek with the forelimbs (Klein
et al. 2011; Moser and Giesler 2014b; Shimada and LaMotte
2008).
Our results show that nociceptive neurons in SI were often
excited by multiple pruritogens, and many were located in the
dysgranular zone (DZ) of SI (Chapin and Lin 1984), an area
that had been previously described as the “unresponsive zone”
(Welker 1971). Thus we located a population of SI neurons in
the DZ that respond both to pruritogens and to noxious mechanical and chemical stimuli. These neurons likely contribute
to production of both itch and pain.
METHODS
Adult male Sprague-Dawley rats (300 – 450 g) were maintained on
a 12:12-h light-dark schedule and had access to food and water ad
libitum. All procedures were approved by the University of Minnesota
Institutional Animal Care and Use Committee.
Electrophysiological recording from neurons in SI. Many of the
methods used for in vivo electrophysiological experiments were
described previously (Lipshetz et al. 2018). Rats were anesthetized
with urethane (1.5 mg/kg ip; Sigma), and supplemental doses of
urethane were given during the experiments as needed to maintain
areflexia, which was monitored periodically by applying noxious
pinches to the tail. A tracheotomy was performed to aid in breathing,
both cheeks on the face were shaved, and the head was secured in a
stereotaxic head holder. Cervical spinal segments C1–C2 were exposed by a laminectomy to allow placement of an electrode for
orthodromic stimulation in the spinal nucleus and tract of the trigeminal nerve (V). A craniotomy was performed over the somatosensory
cortex contralateral to the stimulating electrode, the dura was removed, and the cerebral cortex was covered with warm mineral oil.
The method used to perform recording from a single cortical neuron
is shown in Fig. 1. A stainless steel microelectrode was lowered into
the area of the spinal tract (SpTV) or nucleus (SpNV) in the caudal
medulla. Unipolar cathodal pulses (40 – 60 A, 200 s) were delivered as search stimuli to evoke orthodromic responses in cortical
neurons. Either a glass-coated carbon fiber (0.4 –1.2 M⍀; Kation
Scientific, Minneapolis, MN) or a stainless steel microelectrode (~10
M⍀; FHC, Bowdoin ME) was lowered vertically in SI in the area of
the contralateral DZ (Chapin and Lin 1984) and in close proximity to
the medial border of the vibrissal barrel fields (VBF) in 3-m steps
with an electronic micropositioner (model 2660; Kopf, Tujunga, CA).
We explored an area in the caudal DZ based on somatotopic organization of neurons that responded to stimulation of VBF and forelimb
(Chapin and Lin 1984). Although the area of input from the cheek was
not mapped previously, we extrapolated that an area responsive to
cheek stimulation was located between VBF and the area responsive
to stimulation of the forelimb [anterior-posterior (A-P): 0 to ⫺4 mm,
medial-lateral (M-L): 3.4 –5 mm; (Paxinos and Watson 1998)]. Only
neurons that exhibited orthodromic responses to electrical stimulation
Fig. 1. Method used for identification of cortical neurons. Tip of orthodromic
stimulating electrodes were placed in spinal nucleus/tract of the trigeminal
nerve (V). Single units were recorded in the primary somatosensory cortex
(SI). SpTV, spinal tract V; TG, trigeminal ganglion; Vc, nucleus caudalis.
of SpTV/SpNV were studied. Criteria for the orthodromic activation
were described previously (Lipshetz et al. 2018). After a neuron that
responded to orthodromic activation was identified, the stimulating
electrode was moved mediolaterally and dorsoventrally to determine
the point at which minimal current elicited orthodromic activity.
Extracellularly recorded action potentials were filtered below 100 and
above 5,000 Hz, amplified (model DAM80; WPI, Worcester, MA),
and digitized. Waveforms were discriminated with DAPSYS data
acquisition software (http://www.dapsys.net).
Search criteria and characterization of cortical neurons. Only
orthodromically activated cortical neurons that had a mechanosensitive cutaneous receptive field located on the contralateral cheek were
examined. Neurons that were excited exclusively by movement of
vibrissae were not studied. The borders of the receptive fields were
determined with mild pinch stimuli and sketched onto a drawing of
the rat’s face. Neurons were classed functionally with mechanical
stimuli. Stimuli included brushing the skin of the cheek with a
soft-bristled brush, pressure produced by a weak arterial clip (7.4
bars), and noxious pinching of the skin with a stronger clip (15.4
bars). Each stimulus was applied for 10 s. Neurons that were maximally excited by brushing were classified as low-threshold (LT)
neurons; neurons that were activated by brushing (⬎1.5 Hz over
baseline activity) and maximally by pinching were classified as wide
dynamic range (WDR) neurons; and neurons that were weakly activated by brushing ⬍1.5 Hz and maximally by pinching (Dado et al.
1994) were classified as high-threshold (HT) neurons. We recently
demonstrated that many thalamic neurons that have afferent input
from the cheek are also activated by noxious pinch delivered over the
ipsilateral forelimb and other areas of the body (Lipshetz et al. 2018).
J Neurophysiol • doi:10.1152/jn.00038.2020 • www.jn.org
Downloaded from journals.physiology.org/journal/jn at Univ of Illinois Chicago (131.193.138.031) on September 8, 2022.
1946
PRURICEPTIVE CORTICAL NEURONS
Here we determined whether cortical neurons have similar complex
receptive fields that included the four paws.
Chemical stimuli. Solutions were injected intradermally into the
cheek within the receptive field of studied neurons in a volume of 10
L with insulin syringes and a 29-gauge needle. The first solution
injected was the vehicle (0.9% NaCl). The size and location in the
receptive field of the bleb produced by each intradermal injection
were drawn on a diagram of the rat face. All subsequent injections
were given in the areas of the receptive field that did not overlap with
blebs from preceding injections. Concentrations of pruritogens were
as follows: serotonin (5-hydroxytryptamine, 5-HT; 47 mM), chloroquine (CQ; 100 mM), and -alanine (-al; 50 mM). Intradermal
injection of these pruritogens in the cheek produced exclusively
scratching bouts in behavioral experiments in rodents (Klein et al.
2011; Moser and Giesler 2014b; Shimada and LaMotte 2008). Injections of histamine (HA; 900 mM) and capsaicin (CAP; 1%) were also
given. These pruritogens evoke both scratching and wiping in rats and
are therefore considered partial pruritogens (Klein et al. 2011). In
addition, we also injected mustard oil (MO; 10 mol) in some
experiments. MO induces wiping behavior, and not scratching (Klein
et al. 2011), and is therefore considered a specific algogen. All
substances were used in concentrations that produced maximal
scratching and/or wiping behavior. In experiments in which MO was
used, it was administered before CAP, which was given last. In
experiments in which MO was not used, the final injection was CAP.
The order of injection of the other four pruritogens/partial pruritogens
was varied so that each was given as the first, second, third, or fourth
injection in approximately equal numbers of experiments.
Criteria for classifying responses. A neuron was considered to be
activated by a test substance by criteria established in our previous
studies of itch-responsive neurons (Davidson et al. 2007, 2009; Jansen
and Giesler 2015; Lipshetz et al. 2018; Lipshetz and Giesler 2016;
Moser and Giesler 2014a; Simone et al. 2004). First, the mean firing
rate during one or more of five consecutive 60-s intervals after
removal of the injection needle exceeded the mean spontaneous
discharge rate (during the 60-s interval before insertion of the injection needle) by ⱖ50%. Second, the discharge rate after injection
exceeded by ⱖ50% the mean discharge rate following injection of a
vehicle solution (saline) during the corresponding 60-s period.
Activity following vehicle, CQ, -al, HA, CAP, and MO was
recorded for 5 min after the removal of the injection needle. Responses to 5-HT were determined for 15 min after injection since in
our previous studies 5-HT induced the most prolonged responses of
trigeminothalamic (Moser and Giesler 2014a), trigeminoparabrachial
(Jansen and Giesler 2015), and thalamic (Lipshetz et al. 2018) neurons. After injection of 5-HT, discharge rates during the 6th through
the 14th 60-s intervals were compared to the firing rate of the 5th 60-s
interval following vehicle injection. Only one cortical neuron was
studied in each experiment.
Histology. At the end of each experiment, a small lesion was made
by passing constant current through the recording electrode in the
cortex and the stimulating electrode in the spinal cord for histological
localization of recording and stimulation sites. Both anodal and
cathodal 25-A currents were passed for 25 s each through the
stainless steel stimulating electrode in the SpNV/SpTV and for 60 s
through stainless steel electrodes used for recording in the cortex.
When a carbon fiber microelectrode was used for recording, 60-A
current was passed for 60 s. Rats were then euthanized by an overdose
(360 mg/kg ip) of Fatal-Plus and perfused through the heart with 0.9%
normal saline followed by 10% formalin containing 1% ferrocyanide
solution.
The brains with two cervical spinal segments were dissected,
placed in a 10% formalin-1% ferrocyanide solution for several days to
mark iron deposits with the Prussian blue reaction, and cryoprotected
in 30% sucrose. Coronal sections of the spinal cord (50 m) were
mounted on glass slides, and stimulating sites were identified by the
location of the Prussian blue mark. The cortex was sectioned in either
the coronal (n ⫽ 13) or horizontal (n ⫽ 20) plane at 75 m. Recording
sites were identified by the Prussian blue mark (recording by stainless
steel microelectrode) or a small lesion (recording with carbon fiber
microelectrode). Coronal sections were counterstained with cresyl
violet. To map the region of DZ in preliminary anatomical studies,
horizontal sections were stained by histochemical reaction for cytochrome oxidase with diaminobenzidine and cytochrome c type III
according to the method described previously (Alloway et al. 2006;
Land and Simons 1985; Wong-Riley 1979). For better visualization of
VBF, tissue was counterstained with cresyl violet. Neurons in layer IV
are a prominent feature of VBF (Land and Simons 1985; Woolsey and
Van der Loos 1970; Zhang and Alloway 2006). Therefore, in horizontal sections the cortical layer was determined based on the distance
between the easily recognized ventral limit of VBF layer IV and the
Prussian blue mark or lesion. In coronal sections, the layer in which
the recording site was located was determined based on a stereotaxic
atlas of the rat brain (Paxinos and Watson 1998).
Data analysis. Action potentials were collected and binned in 1-s
intervals for display and analysis of responses following injections.
The baseline rate of spontaneous activity was defined as the mean rate
for a 60-s period before an injection. The time before each injection
during which the skin was held with forceps to perform the intradermal injection (~20 s) was not used for analysis. Responses to chemical
injections were analyzed by one-way ANOVA with Bonferroni
correction.
RESULTS
Results from 33 cortical neurons, which were obtained from
33 rats, are included in this study. Each was activated orthodromically by electrical stimulation within the SpNV/SpTV
and by mechanical stimulation applied to the skin of the
contralateral cheek. Figure 2A shows the locations at which
minimal-amplitude current pulses elicited orthodromic activity. Almost all of the responsive sites were in SpNV. Figure 2B
shows the localization of cutaneous receptive fields of studied
neurons. All receptive fields were located on the cheek, and
only five of them extended on the vibrissa area. No receptive
fields that extended on the ear or neck were found.
Functional classification of cortical neurons. Neurons were
classified functionally according to their responses evoked by
innocuous and noxious mechanical stimulation of the receptive
field on the contralateral cheek. All 33 neurons were nociceptive. Twenty five neurons (76%) were classed as HT (Fig. 3,
A–C) and eight neurons (24%) as WDR (Fig. 3, D–F). No LT
neurons were encountered.
Responses of cortical neurons to injection of pruritogens
and partial pruritogens. Among the 33 cortical neurons studied, we examined responses of 31 neurons evoked by injections
of pruritogens and partial pruritogens into the contralateral
cheek. Two HT neurons were tested by using only mechanical
stimuli without injections into the cheek. Figure 4 shows the
responses of a nociceptive neuron located in layer IV near the
border of DZ and VBF (Fig. 4A). The receptive field was
located on the contralateral cheek (Fig. 4B). The neuron was
activated by brushing (mean firing rate 3.4 Hz), exhibited an
increased response to pressure (5.1 Hz), responded maximally
to pinch (16.0 Hz), and was classified as a WDR neuron (Fig.
4C). Figure 4, D–I, demonstrate histograms of discharge frequency before and after subcutaneous injections of vehicle,
pruritogens, and partial pruritogens (1-s bins). Figure 4D demonstrates that this neuron produced little, if any, response to
injection of vehicle. The neuron was activated by injections of
CQ (Fig. 4E), HA (Fig. 4F), and -al (Fig. 4G), in each case
J Neurophysiol • doi:10.1152/jn.00038.2020 • www.jn.org
Downloaded from journals.physiology.org/journal/jn at Univ of Illinois Chicago (131.193.138.031) on September 8, 2022.
PRURICEPTIVE CORTICAL NEURONS
1947
injection of HA, two were excited only by injection of CQ, and
two were excited only by injection of -al.
Most neurons (77%) were activated by more than one
pruritogen/partial pruritogen. Among the 31 neurons tested,
16% (5 neurons) did not respond to any pruritogen/partial
pruritogen, 23% (7 neurons) responded to one pruritogen/
partial pruritogen, 39% (12 neurons) responded to two substances, 10% (3 neurons) responded to three pruritogens/partial
pruritogens, and 13% (4 neurons) were excited by four substances. None of the examined neurons responded to all five
substances (Fig. 5C).
Fig. 2. A: locations of lowest threshold points for orthodromic activation of
cortical neurons in spinal tract V (SpTV) and nucleus (SpNV) of the contralateral medulla and spinal segment C1. Pyr Dec, pyramidal decussation. B:
locations of all 33 receptive fields on the face of rats where mechanical and
chemical stimuli were applied.
for at least 5 min after injection. It was activated for 60 s after
injection of CAP (Fig. 4H) and for at least 15 min after
injection of 5-HT. Injection of 5-HT produced a prolonged
response that persisted for ⬎20 min (Fig. 4I).
Of the 31 neurons tested with pruritogens/partial pruritogens, 25 (81%) were pruriceptive, including 20 of 23 HT
neurons (87% of all tested HT units) and 5 of 8 WDR neurons
(63% of all tested WDR units). Six neurons (19%) failed to
respond to any of the pruritic or algesic chemical stimuli (3 HT
and 3 WDR neurons). We evaluated whether responses to
various agents were affected by the order of presentation.
Figure 5A shows that the mean response frequency evoked by
the different chemical injections did not vary with the order of
presentation. Responses to the partial pruritogen CAP were not
included in the analysis since it was always injected last (fifth),
or second to last in experiments in which MO was tested.
The partial pruritogen HA activated the greatest percentage
of cortical neurons (49%). Injection of -al excited 39% of
neurons tested, CQ activated 32%, CAP activated 29%, and
5-HT excited 23% (Fig. 5B). Nearly one quarter of pruriceptive
neurons (7 cells) responded to injection of only one pruritogen.
Of these seven cells, three were activated exclusively by
Fig. 3. Peristimulus histogram of responses in high-threshold (HT) and wide
dynamic range (WDR) neurons to brushing (A and D), pressure (B and E), and
pinch (C and F) applied on the skin of the cheek. Responses are represented as
mean ⫾ SE frequency (impulses/s) during every 1-s bin. Horizontal lines
indicate 10-s period of mechanical stimulations.
J Neurophysiol • doi:10.1152/jn.00038.2020 • www.jn.org
Downloaded from journals.physiology.org/journal/jn at Univ of Illinois Chicago (131.193.138.031) on September 8, 2022.
1948
PRURICEPTIVE CORTICAL NEURONS
Fig. 4. Example of a pruriceptive wide dynamic range (WDR) neuron recorded in dysgranular zone (DZ). A: photomicrograph of coronal section with the
recording site. The lesion of the recording site is indicated by an arrow. Arrowheads point out medial and lateral borders of DZ. B: striped area indicates receptive
field on the contralateral cheek. Injection sites for each pruritogen are shown by circles with colors corresponding to colors of individual histograms in D–I. C:
periods of 10 s brushing, pressure, and pinching of the skin on the receptive field are indicated by horizontal lines, with mean discharge frequencies during
stimulations below lines. D–I: histograms of responses induced by injections of vehicle (D) or pruritogens/partial pruritogens (E–I) into the skin of the cheek.
Needle insertion and removal are indicated by 2 vertical arrows, where the discharge rates are displayed as a lighter color in each histogram. Because placement
on the tip of the needle in subcutaneous space in some injection sites was more challenging than in others, the duration of injections were not constant. The “plus”
sign in parentheses indicates response to agent following injection. In each histogram, 5 superimposed action potentials are shown at the time point indicated
by the diagonal arrow. CAP, capsaicin; CQ, chloroquine; HA, histamine; 5-HT, 5-hydroxytryptamine (serotonin); Veh, vehicle; -al, -alanine.
Responses of cortical neurons to injection of the specific
algogen MO. Nineteen cortical neurons (15 HT and 4 WDR
units) were examined for their responses to injection of MO into
the skin of the cheek. Of these neurons, six (32%) were activated
(see Fig. 5B). Among all algogen-responsive neurons, five (83%)
were also activated by one or more pruritogens/partial pruritogens, which were injected before MO. Only one neuron (17%)
responded to MO but not to any other chemical.
Mean responses of neurons that were activated by each
chemical were compared to responses evoked after injection
of vehicle. Ongoing spontaneous activity before each injection was subtracted from each response. Mean responses
evoked by the chemical stimuli were compared to a response
evoked in the same neuron by vehicle by two-way ANOVA
with repeated measures. All substances evoked significantly
stronger activation of SI neurons compared with vehicle.
Fig. 5. Responses to chemical stimuli. A:
mean (⫾SE) response frequency evoked by
all pruritogens/partial pruritogens (excluding
capsaicin) when they were given first, second, third, and fourth in order of injections.
No significant differences were detected, indicating that the order of presentation did not
affect responses (1-way ANOVA with Bonferroni correction). B: % of all 31 cortical
neurons that were activated after injections
of each agent. C: % of neurons that responded to any pruritogens/partial pruritogens; 0 indicates proportion of nonpruriceptive neurons and 1, 2, 3, 4, or 5 proportions
of neurons that responded to corresponding
number of pruritogens/partial pruritogens.
J Neurophysiol • doi:10.1152/jn.00038.2020 • www.jn.org
Downloaded from journals.physiology.org/journal/jn at Univ of Illinois Chicago (131.193.138.031) on September 8, 2022.
PRURICEPTIVE CORTICAL NEURONS
Significance of statistical comparisons is indicated for each
histogram in Fig. 6.
Location of recording sites for cortical neurons. Neurons in
SI that met our search criteria were located in DZ and in deep
laminae of the medial portion of VBF. Stereotaxic coordinates
for all neurons are indicated in Table 1. The cortex containing
recording sites from the initial 20 experiments was sectioned in
the horizontal plane. We reasoned that sectioning the cortex in
this plane would help us in determining the locations of
recording sites in relation to the VBF and the rest of the rat
somatotopic cortex. Figure 7 shows the localization of recording sites in the horizontal plane for neurons located in DZ and
the pruritogen to which each cell responded or did not respond.
The approximate depth of recording sites below layer IV is
shown in the individual pie charts. Figure 8 shows the functional classification and recording location of these neurons,
also in the horizontal plane.
We found it difficult to determine the precise recording
depth or cortical layer with this histological approach. Therefore, the cortex from the final 13 experiments was sectioned in
the coronal plan
