Introduction
The phenomena of short- and long-term potenti-
ation (STP and LTP) in the nervous system are the
result of prior exposure of the neuron under test to
a period of intense stimulation.
1
Induction of both
STP and LTP has been associated with activation of
the amino acid agonist compound NMDA.
2,3
STP
has been defined as a potentiation which persists for
< 1 h following stimulation, while LTP persists
for longer.
1
However, there may be clear mechanis-
tic differences between these effects other than the
temporal: for example, application of NMDA alone
is usually insufficient to produce LTP, but can readily
invoke STP.
2,4
NMDA and other excitatory amino
acid receptors have been shown to be associated with
neurotransmission the retino-geniculo-striate visual
pathway, in particular at the level of the dorsal lateral
geniculate nucleus (dLGN).
5–8
Retinally induced
visual responses at this site are considered to be
the result of an initial AMPA mediated excitation,
followed by an NMDA receptor-mediated compo-
nent,
5–7
although there is also more recent evidence
for a metabotropic receptor-mediated component.
9,10
Responses resulting from cortico-fugal inputs also
utilize an excitatory amino acid transmitter and
includes components mediated by NMDA,
11,12
AMPA
13
and metabotropic receptors.
14
Here we have
sought to determine the possibility of an NMDA-
induced form of STP or LTP in the transmission of
visual information through the cat dLGN.
Materials and Methods
Adult cats were anesthetized with halothane
(0.1–5%) in nitrous oxide (30%) and oxygen (70%),
and paralyzed with gallamine triethiodide (10 mg/
kg/h). EEG, ECG, expired CO
2
and temperature
were monitored and maintained continuously, adjust-
ing anesthetic levels to maintain a state of light anes-
thesia.
15
Single units were recorded extracellularly
from the dLGN using multibarrelled glass micro-
pipettes. Pipettes were filled with a combination of
the following drugs: NaCl 3 M for recording,
NMDA 0.1 M, pH 8; (1S,3R)-1-amino-1,3-cyclopen-
tane dicarboxilate (ACPD) 50 mM, pH 8; AMPA
15 mM pH 8; ACh 0.2 M, pH 4. Pipette tips were
broken back to around 5–7m. When not in use
drug barrels were subjected to a retention current of
5–25 nA of appropriate polarity. Single unit data were
collected and visual stimuli produced under computer
control. Stimuli were viewed monocularly through
the dominant eye for each cell under test. Our basic
paradigm was to establish control responses to a
drifting grating of medium contrast, apply an excita-
tory drug compound at non-saturating levels for a
Vision, Central
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© Rapid Science Publishers Vol 9 No 4 9 March 1998 653
SUSTAINED iontophoresis of NMDA potentiated visual
responses for minutes after the application in 16 of 38
cells (42%), peaking 3 min after the end of the applica-
tion and declining to control levels within 1 2 min.
Potentiation was also seen after application of ACPD
(36%, n = 14) and AMPA (29%, n = 14), but not after
application of ACh ( n = 20). ACh also excites dLGN cells,
but does not interact with amino acid receptors, and
ACh receptors are not directly involved in the trans-
mission of visual information. We suggest that this
modulation is a form of visually induced potentiation
which permits dynamic modification of the strength of
visual information to be relayed to the cortex depending
upon the history of previous activity levels.
Key words : Cat dLGN; Excitatory amino acid receptors;
STP; Temporal priming; Visual response modulation
Enhanced visual
responses in cat dLGN –
potentiation by priming
with excitatory amino
acids
C. Rivadulla, K. L. Grieve
1
and
J. Cudeiro
CA
E.U. de Fisioterapia (Univesidade da Coruna)
and Unidad de Cirugia Experimental (Hospital
Juan Canalejo), Centro Universitario de Oza,
15006 A Coruna, Spain;
1
Department of
Optometry and Vision Sciences, University
of Manchester Institute of Science and
Technology (UMIST), Manchester
M60 1QD, UK
CA
Corresponding Author
Website publication 17 February 1998 NeuroReport 9, 653–657 (1998)