Brain Research 881 2000 28–36 www.elsevier.com locate bres
Research report
Cholinergic modulation of neocortical long-term potentiation in the awake, freely moving rat
1
Tiffany E. Boyd, Christopher Trepel , Ronald J. Racine
Department of Psychology , McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S-4K1
Accepted 18 July 2000
Abstract
The neocortex has proven resistant to LTP induction using standard in vitro and acute, in vivo preparations. Because the neocortex is widely thought to be involved in long-term information storage, this resistance raises questions about the validity of LTP as a memory
model. Recently, we have shown that the neocortex of freely moving rats reliably supports LTP, provided that the stimulation is spaced and repeated over days. The following experiments were designed to evaluate the neuromodulatory role played by cholinergic systems in
the induction of LTP in this preparation. Chronically implanted rats received either low- or high-intensity LTP-inducing tetani in combination with the administration of either a cholinergic agonist or antagonist injected systemically. Potentiation was evidenced as
amplitude changes in both early and late components of the evoked field potential, the former including population spikes. The cholinergic agonist facilitated LTP induction in the late component of both high- and low-intensity groups. The cholinergic antagonist blocked LTP
induction in the early component of the high-intensity group. The possibility that there are component-specific modulatory effects of cholinergic agents on the induction of neocortical LTP is discussed.
2000 Elsevier Science B.V. All rights reserved.
Theme : Excitable membranes and synaptic transmission
Topic : Long-term potentiation: pharmacology
Keywords : LTP; Acetylcholine; Freely moving; Rat; Neocortex
1. Introduction The model that is most widely used to study the
physiology of memory is long-term synaptic potentiation There is considerable evidence that the cholinergic
LTP see [4] for review. Properties such as longevity system plays an important role in learning and memory
and associativity make it an attractive memory model, and processes for review, see [17]. The cognitive deficits
it has also been shown in structures, such as the hippocam- associated with Alzheimer’s disease [12,16,34] and normal
pus, that are known to be involved in learning and aging [15], for example, may be mediated by a reduction
memory. in cholinergic functioning. Also, it has been established in
Recently, we
have successfully
demonstrated an
rodents that cholinergic antagonism [14,25,43] and agon- NMDA-dependent LTP in the neocortex of awake, freely
ism [18,35,41] attenuate and facilitate, respectively, learn- moving rats using induction parameters that mimic con-
ing in a variety of tasks. Studies using human and non- ditions considered optimal for the construction of non-
human primates have produced similar results [1,11,13,21]. declarative memories i.e. spaced and repeated stimulation
sessions [37,42]. This form of LTP does not reach asymptotic levels for 7–12 days [37,42]. These results are
consistent with those from memory experiments, where the
Corresponding author. Tel.: 11-905-525-9140, ext. 23022; fax: 11-
acquisition of nondeclarative information into presumed
905-529-6225.
cortical stores has been found to be a slow process [31].
E-mail address : racinemcmaster.ca R.J. Racine.
1
These properties of LTP raise questions about the suitabili-
Current address: W.M. Keck Foundation Center for Integrative Neuro-
ty of the cortical slice model for studying neocortical LTP.
science and Department of Physiology, Box 0444, University of Califor- nia, 513 Parnassus Avenue, San Francisco, CA, 94143.
It is not yet clear that the LTP induced in the slice e.g.
0006-8993 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 2 7 2 0 - 7
T .E. Boyd et al. Brain Research 881 2000 28 –36
29
[23] is the same phenomenon as that monitored over surgery to provide optimal response amplitudes. The
weeks and months in the awake animal. resulting mean depths for the white matter stimulating and
LTP, like memory, has been shown to be affected by cortical recording electrodes were 3.0 mm and 1.8 mm
cholinergic manipulations, but these experiments were ventral to dura, respectively. The electrodes were con-
done in in vitro preparations. Cholinergic agonists, for nected to gold-plated male pins that were then inserted into
example, enhance LTP induction in both the CAl [5] and a nine-pin miniature connector plug that was mounted onto
dentate gyrus [7] regions of the hippocampus as well as in the skull with dental cement and anchored with stainless
the visual cortex [6]. The facilitatory action of acetyl- steel screws. One of the screws served as the ground
choline on the induction of LTP is most likely the result of electrode. Data acquisition began 10–14 days following
enhanced postsynaptic depolarization, increasing the prob- surgery. All experiments were carried out following proper
ability, or extent, of NMDA receptor activation [9,32]. guidelines as stipulated by the Canadian Council of
Even cholinergic agents by themselves have been shown to Animal Care and approved by the university veterinarian.
produce a long-term enhancement of responses in the hippocampus [3] or sensorimotor cortex [27]. In the
hippocampus, this cholinergically-induced LTP has been 2.2. Baseline measures and induction of LTP
shown to occlude subsequent LTP induction by electrical stimulation [3], suggesting that these two forms of LTP
Electrical stimuli were produced by a Grass S88 share a common substrate.
stimulator, coupled to photoelectric stimulus isolation Previously in our hands, cholinergic stimulation has
units. Recorded signals were fed into Grass Model 12 EEG been shown to promote a long-term depression effect in
amplifiers, high pass filtered at 0.3 Hz, low pass filtered at the neocortex of freely moving animals following single-
3 Hz, and digitized 10 KHz with a 12-bit A D converter. session stimulation protocols [37,38]. The following ex-
Three sets of field potential input output I O mea- periments were undertaken to characterize the role of
sures, spaced at 48 h, were taken to establish baseline. cholinergic neuromodulation in the induction of neocorti-
Pulses of increasing intensity were delivered to white cal LTP induced by the spaced and repeated stimulation
matter at a frequency of 0.1 Hz. Six 50 ms responses were protocol. Based on the LTP slice literature and behavioural
evoked, amplified, digitized and averaged at each of 10 data, we predicted that the cholinergic agonist pilocarpine
intensities 16, 32, 64, 100, 160, 250, 500, 795, 1000, 1260 would enhance LTP induction, while the antagonist
m A. The evoked field potentials comprised two main
scopolamine would attenuate LTP induction. Portions of components: an early, monosynaptic, surface-negative
this research have been presented previously in abstract response, and a larger, polysynaptic late response. The
form [39]. polysynaptic components were often most evident follow-
ing LTP induction. Also evident at latencies coinciding with the early component were at least one, and usually
several, population spikes that increased in amplitude and
2. Materials and methods number following potentiation. Further characterization of