Neurotransmisores

medigraphic

Artemisa
en línea

NEUROTRANSMISORES DEL SISTEMA LÍMBICO. HIPOCAMPO.
GABA Y MEMORIA. SEGUNDA PARTE
Eduardo Castro-Sierra*, Fernando Chico Ponce de León**, Luis Felipe Gordillo Domínguez***,
Alison Portugal Rivera****

SUMMARY
Action of GABA agonists and antagonists on memory. The
θ rhythm. Muscimol may directly alter memory. Recently, a
modified matching to position(MTP) paradigm was employed
aimed at influencing the type of associations a rat may use to
solve the task. The main behavioral manipulation was the
application of a differential outcomes procedure (DOP). DOP
implies correlating each event to be remembered with a different
reward condition. This procedure will result in the development
of specific reward expectations which will in turn increaseand
guide choice behavior. Such different reward expectations will not
be present when the reward assignation used is either common or
random (non-differential outcomes procedure, NOP).
Intraventricular infusion of muscimol or CSF in rats carrying
out a delayed MTP using either a MOP or an NOP protocol will
affect both groups of rats, but the nature of the deficit will differ
depending onthe reinforcement contingencies. Rats trained in
DOP will show general non-mnemonic damage independent of
delay, i.e., performance will be affected at all delay intervals
employed. On the contrary, rats trained in NOP will show delaydependent damage. This appears to demonstrate that muscimol
may also have untoward memory effects, which further indicates
that activation of GABA receptors willaffect a set of memory
associations and functions.
Difficulties experienced in the past regarding LTP induction at
the level of the CA3-CA1 synapse using time-based spike
presentation protocols have been disconcerting given the
preeminence of these synapses as a model system for the study of
synaptic plasticity. Results previously discussed in experiments using
picrotoxin as a GABA inhibitorhave suggested that such difficulties
arise from the requirement that, for LTP to be induced, CA1
dendrites must be persistently and totally activated. Doublets used
in this case represent a minimal burst, or level of post-synaptic
stimulation for LTP induction that subsumes greater depolarizations.
In vitro, synaptically induced bursts would correspond to
regenerative electrical events inapical dendrites of pyramidal
neurons. The same requirements for dendritic activation would be
satisfied in vivo during the θ rhythm, which is present during active
exploration. Therefore, GABA might serve as an engram modulator
through the activation of the hippocampal θ rhythm.

Effect of µ-opioid receptors on hippocampal memory activity.
Hippocampal µ-opioid receptors (MOR) have beeninvolved in the
formation of memory associated with the abuse of opioid drugs.
When chronically activated, and during programmed drug
abstinence, MORs acutely modulate hippocampal synaptic plasticity.
At the level of neuronal networks, MORs increase excitability of
area CA1 by means of a disinhibition of pyramidal cells. The specific
inhibitory interneuronal subtypes which become affected byactivation of MORs are not known. Nevertheless, not all subtypes
are inhibited and some subtypes preferentially express these
receptors. In one study, the effect of activation of MORs on
inhibitory patterns and propagation of excitatory activity in CA1
of rat hippocampus was investigated through cortical images created
using voltage-sensitive dyes.
MOR activation increased excitatory activityoriginated by the
increased stimulating input to stratum oriens (i.e., Schäffer collateral
and commissural [SCC] fibers, as well as the retrograde pathway),
to stratum radiatum (i.e., SCC fibers) and to stratum lacunosum-moleculare
(i.e., the perforant pathway and the thalamus). Increased excitatory
activity was additionally facilitated by propagation through the
neural network of area...