Interneurons containing somatostatin are affected by learning-induced cortical plasticity

The maintenance of neural circuit stability is a dynamic process that requires the plasticity of many cellular and synaptic components. By changing the excitatory/inhibitory balance, inhibitory GABAergic plasticity can regulate excitability, and contribute to neural circuit function and refinement in learning and memory. Increased inhibitory GABAergic neurotransmission has been shown in brain structures involved in the learning process. Previously, we showed that classical conditioning in which tactile stimulation of one row of vibrissae (conditioned stimulus, CS) was paired with a tail shock (unconditioned stimulus, UCS) in adult mice results in the increased density of GABAergic interneurons and increased expression of glutamic acid decarboxylase (GAD)-67 in barrels of the "trained" row cortical representation. In inhibitory neurons of the rat cortex GAD co-localizes with several proteins and peptides. We found previously that the density of the parvalbumin (GAD+/Prv+)-containing subpopulation is not changed after conditioning. In the present study, we examined GABAergic somatostatin (Som)-, calbindin (CB)- and calretinin (CR)-positive interneurons in the cortical representation of "trained" vibrissae after training. Cells showing double immunostaining for GAD/Som, GAD/CR and GAD/CB were counted in the barrels representing vibrissae activated during the training and in control, untouched rows. We found a substantial increase of GAD/Som-containing cells in the trained row representation. No changes in the density of GAD/CR or GAD/CB neurons were observed. These results suggest that Som-containing interneurons are involved in learning-induced changes in the inhibitory cortical network.

Short-term sensory learning does not alter parvalbumin neurons in the barrel cortex of adult mice: A double-labeling study

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced expansion of the cortical representation of the activated vibrissae ("trained row"), this was demonstrated by labeling with 2-deoxyglucose in layer IV of the barrel cortex. We have also shown that functional reorganization of the primary somatosensory cortex is accompanied by an increase in the density of small GABAergic cells and glutamate decarboxylase 67-positive neurons in the hollows of barrels representing the "trained row." GABA neurons of the rat neocortex co-localize with calcium-binding proteins [parvalbumin, carletinin, calbindin D28k] and neuropeptides (vasoactive intestinal polypeptide, somatostatin). In the present study we have examined GABAergic parvalbumin-positive, interneurons in the cortical representation of "trained" facial vibrissae after short-term aversive training, in order to determine whether the observed changes in glutamate decarboxylase 67-positive neurons are accompanied by changes in parvalbumin-positive neurons. Using double immunofluorescent staining, it was found that (i) all parvalbumin-positive neurons in the barrel hollows were glutamate decarboxylase 67-positive, (ii) following aversive training density of glutamate decarboxylase 67-positive neurons in barrel hollows increased significantly compared with controls and (iii) density glutamate decarboxylase 67-positive/parvalbumin-positive neurons in "trained" barrel hollows did not change compared with controls. This study is the first to demonstrate that the density of double-labeled glutamate decarboxylase 67-positive/parvalbumin-positive neurons does not alter during cortical plasticity, thus suggesting that some other population (i.e. parvalbumin negative) of GABAergic interneurons is involved in learning-dependent changes in layer IV of the barrel cortex.

Experience-dependent changes in cortical whisker representation in the adult mouse: a 2-deoxyglucose study

Sensory experience and learning can modify cortical body maps. We have previously reported that 3 days of classical conditioning, in which stimulation of a row of whiskers was paired with tail shock, produced an expansion of the cortical representation of the "trained row" labeled with 2-deoxyglucose (2DG), in layer IIIb and IV of the barrel cortex. The present study examined plastic remodelling of the vibrissal cortical representation after pairing whisker stimulation with a drop of sweet water. Cortical representations of rows of whiskers were mapped by 2DG autoradiography after 3 days and 2 months of training. The training resulted in enlargement of the cortical representation of vibrissae involved in the stimulus pairing compared with the contralateral representation of a row of whiskers, that were not touched during the training. This modification of whisker representation was different after short-term and long-term appetitive training. After three pairing sessions, changes in the width of cortical representation were visible in layers II/IIIa (29%) and layers V/VI (28%). After 2 months of training, significant changes in the width of cortical representation row B were found only in layer IV (41%). The changes were not observed in animals, that received whisker stimulation alone or in those who were subjected to training with unpaired stimuli. The results demonstrate that stimulus-pairing-induced changes in cortical whisker representation appeared with different time courses at different levels of cortical columnar information processing.

Rapid regulation of GAD67 mRNA and protein level in cortical neurons after sensory learning

Cortical representations of different modalities can be modified by sensory learning. Our previous studies in the barrel cortex showed that expansion of the cortical representation of a row of vibrissae could be induced by pairing stimulation of a row of vibrissae with a tail shock. The plastic change in cortical reactivity to the input used during the training was accompanied by increased density of GABA immunoreactive neurons in the involved row of cortical barrels. Using the same paradigm, the present study examined the pathway of GABA synthesis-expression of GAD67 mRNA and immunoreactivity of GAD67 isoenzyme in the barrel cortex of mice after sensory learning. In situ hybridization revealed that the GAD67 mRNA level was elevated in one row of barrels in the trained group as well as in controls receiving vibrissae stimulation alone. In contrast, elevation of immunoreactivity of the GAD67 protein occurred only in the trained group. The density of GABA-immunoreactive neurons in the hollows of barrels representing the row of vibrissae activated during the training was increased by 50%. These data indicated that sensory stimulation alone affected expression of the 67 kDa glutamate decarboxylase isoenzyme synthesis pathway, whereas the processes involved in cortical plasticity induced by associative learning modified this pathway additionally at the level of translation.

GABA immunoreactivity in mouse barrel field after aversive and appetitive classical conditioning training involving facial vibrissae

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced an expansion of the cortical representation of the "trained row", labeled with 2-deoxyglucose (2DG), in layer IV of the barrel field. The present study has examined the pattern of GABA immunoreactivity (GABA-IR) in the cortical representation of row B of the facial vibrissae after (i) 3 days of aversive training, and (ii) 2 months of appetitive training, where stimulation of row B of vibrissae on one side of the snout was used as a conditioned stimulus. The most notable observation was a greater density of GABA-IR cells concentrated in the hollows of the "trained row" B barrels compared to the hollows in the barrel field of the opposite hemisphere in the same mouse. After aversive training, we noted a 2-fold increase in the density of GABA-IR neurons in the hollows of row B; after reward training, the increase amounted to 49%. In contrast, GABA-IR was unchanged in the control groups, which received only stimulation of vibrissae without the unconditioned stimulus. The classification of labeled neurons according to size revealed that the increase in density of GABA-IR neurons was confined to the small (12-15 microm) diameter group. We concluded that the GABAergic system undergoes up-regulation, after both associative learning paradigms, and that the population of small, GABAergic neurons plays an active role in use-dependent plasticity.

Learning-induced expansion of cortical maps--what happens to adjacent cortical representations?

The present study was designed to investigate the effects of learning-dependent enlargement of cortical representation of a row of vibrissae upon the appearance of adjacent cortical representations. We have found previously that three sessions of classical conditioning, during which stimulation of row B of vibrissae is paired with a tail shock, result in an increase of cortical representation of the trained row, as visualized with 2-deoxyglucose (2DG) metabolic functional mapping. In the present experiment, after the training in which row B was stimulated, we mapped with 2DG the cortical representations of rows A and C, not stimulated during the training. We found that these representations do not differ from normal. Because of expansion of cortical representation of the trained row, the overlap between representations of neighboring rows of vibrissae became greater and the two maps co-existed within the same cortical space.

Short-lasting classical conditioning induces reversible changes of representational maps of vibrissae in mouse SI cortex--a 2DG study

It has been known for several years that receptive field properties of sensory cortical neurons can be altered by learning experiences. We attempted to visualize a global change of the cortical body map induced by learning. In order to do this a short-duration classical conditioning involving stimulation of a row of mystacial vibrissae in mice was followed with 2-deoxyglucose (2DG) mapping of functional activity. Three conditioning sessions that paired stimulation of a row of whiskers with a tail shock produced an increase of the functional representation in somatosensory cortex (SI) of a row of the whiskers stimulated during the training. This plastic change of vibrissal representation in SI was visualized with 2DG autoradiography a day after completion of training. The expansion of representation was the most pronounced in cortical layer IV, and to a lesser extent, in layer IIIb. The expansion was observed in conditioned but not in pseudoconditioned mice or in animals that received only the conditioned stimulus. If training was discontinued, the enlargement of vibrissal representation progressively faded. The reversal could be accelerated by a behavioral extinction procedure. This study gives the pictorial demonstration of rapid, transient, and extinguishable learning-dependent changes in SI cortical maps.

Plasticity of mystacial fur representation in SI cortex of adult vibrissectomized rats--a 2DG study

Localization of cortical representation of the common fur of mystacial pads was mapped with 2-deoxyglucose autoradiography in adult rats after vibrissectomy. Normal representation of mystacial fur was found in locations outside the posteromedial barrel subfield (PMBSF). Vibrissectomy performed in young adult resulted in fur inputs activating the PMBSF. This plastic change took more than two weeks and less than one month to complete.

Loss of gamma-aminobutyric acid (GABA) immunoreactivity from mouse first somatosensory (SI) cortex following neonatal, but not adult, denervation

Whisker follicles were surgically ablated (lesioned) on two entire rows (B and C) of the left snout of two groups of Swiss mice, in the first 2 days after birth, and in the second group at 8 weeks of age (adults). Two months after surgery GABA-immunoreactivity (GABA-IR) was examined in tangential sections of the first somatosensory (SI) 'barrel' cortex of these two groups (at which time it was also confirmed that the follicles had not regrown). In the adult-lesioned mice all of the barrels appeared intact and visually similar in both the experimental (right) and control (left) hemispheres. The staining pattern of immunopositive cells and puncta was qualitatively similar to that which we have described previously. However, in the neonatally-lesioned mice barrel rows B and C were not visible in the right hemisphere sections and there was a marked reduction in GABA-IR, with fewer immunopositive cells. Many of those that did show GABA-IR stained only weakly and the puncta were also fewer in number than in the left (control) hemisphere where the GABA-IR pattern and staining intensity was normal.