The postsynaptic associative conditioning paradigm

In the present study, a postsynaptic associative protocol for the induction of LTP was used instead of high frequency stimulation of afferent fibres for two reasons:

Firstly, this was the experimental paradigm producing LTP in 50 to 60% of conditioned EPSPs in the awake cat (Baranyi et al., 1991). Secondly, postsynaptic injection of EGTA blocks the postsynaptic induction of LTP (Baranyi and Szente, 1987), suggesting that LTP recorded in the injected cell is a localized effect of postsynaptic depolarization on the potentiated synapses. The spike firing may well induce LTP at synapses on other cells, but not in synapses of recurrent circuits. Postsynaptic conditioning allows a more selective analysis of neocortical phenomena than tetanic stimulation of afferents.

However, the lack of immediate effects of NAd and ISO in the slice on early

components of EPSPs and on the induction of associative LTP in layer V cells should not be extrapolated to other neocortical synapses.

One possibility that may account for the low incidence of associative LTP observed in this study, could be that an insufficient level of current injection was used during the conditioning paradigm (Gustafsson et al., 1987). However, similar current strengths reliably produced LTP of conditioned EPSPs in layer V cells in the awake cat (Baranyi et al., 1991). The possibility cannot be ruled out that although the pairing paradigm used in this study produced a similar degree of excitation in response to both test and conditioning stimulation to that in vivo (Baranyi et al., 1991), it may not always have sufficiently depolarized the dendrites to facilitate the inward Ca^^ currents

responsible for LTP, even in the presence of the B-adrenergic agonists. This is unlikely, however, since in two additional experiments a third attempt at LTP was conducted with even stronger depolarizing current without any success (not shown). Furthermore, using a similar pairing paradigm, in slices of rat anterior cingulate cortex, Sah and Nicoll (1991) reported that LTP did not occur even when using depolarizing currents sufficient to reverse the EPSP.

In summary, the present study demonstrates that although NAd or ISO have marked postsynaptic effects on excitability, neither on its own enhances layer V

excitatory synaptic transmission, nor increases the incidence of induction of associative LTP. The low incidence of neocortical associative LTP observed in this study is perplexing because in layer V cells in vivo, a similar pairing paradigm produced

associative LTP in 50-60% of conditioned EPSPs (Baranyi et al,, 1991). Neither NAd nor ISO increased the probability of inducing associative LTP in layer V cells of rat sensorimotor cortex. It is concluded that the incidence of induction of LTP found to be lower in vitro than in vivo (Baranyi and Szente., 1987; Baranyi et at., 1991) is not due simply to a lack of noradrenergic activity in the slice.

The layer V cells recorded in vivo are obviously not isolated from their extrinsic neuromodulatory influences which may be important in the induction of associative neocortical LTP. However, in vitro it may be that additional neuromodulators, present in vivo, are required for the induction of associative LTP in layer V cells in rat

somatosensory cortex (Section 4.7, page 105). Factors which may be important in influencing the probability of inducing associative LTP in the neocortical slice are discussed below.

GABA^ inhibition

In the hippocampus and in layer II to IV cells of the neocortex it has been shown that addition of GABA^ antagonists (e.g. picrotoxin or bicuculline methiodide) to the bathing medium to block IPSPs facilitates the induction of LTP (Wigstrom and Gustafsson, 1985; Artola and Singer, 1987 Artola e ta l., 1990a,b).

In the present study, bicuculline methiodide was not added to the bathing medium in any o f the experiments. A block of GABA^ receptors may not be an important factor in the present study, since in rat layer V cells of the somatosensory cortex, Bindman et al. (1988) showed that 2 out of 6 cells in which LTP was induced (in one cell by pairing and in the other by high frequency afferent stimulation) were not exposed to bicuculline methiodide. Conversely 19 out of 22 cells in which LTP was not obtained by either pairing or tetanization of afferents were bathed in bicuculline methiodide. Furthermore, a later study by Bindman and Murphy (1990) reported that, in layer V cells, LTP elicited by pairing or afferent tetanization was produced in only 11/53 layer V neocortical neurones. Of these successful experiments, 6/53 were carried out in the presence of bicuculline methiodide. Of the 42 cells which failed to produce LTP, bicuculline methiodide was present in two-thirds of the experiments. Bindman et al.

(1988) and Bindman and Murphy (1990) concluded that the Hack of GABA^

inhibition was not essential for the induction of LTP in layer V cells of rat somatosensory neocortex.

GABAg inhibition

In contrast to the effects of bicuculline, the effects of GABAg receptor antagonists and agonists on the induction and expression on LTP are likely to be complex. Late IPSPs (latency >110 msec) thought to be GABAg-receptor mediated (Connors et al., 1988; Karlson et al. 1988) were rarely observed in this study. Activation of GABAg receptors both inhibit and hyperpolarize neocortical neurones (Connors et al. , 1988; Howe et al. , 1987). Inhibition is probably mediated by release of GAB A onto GABAg autoreceptors on the terminals of GABAergic neurones, which may also contribute to frequency depression of neocortical IPSPs (Deisz and Prince, 1989). These receptors, as well as GABAg receptors mediating slow IPSPs in neocortex, are likely to be

activated during a tetanus, and activation of both sets of receptors mediating IPSPs could influence (possibly in an opposing manner) the induction of neocortical LTP. If this is the case then it may explain the apparent lack of effect of bicuculline in the experiments of Bindman et al. (1988) and Bindman and Murphy (1990) on the induction of LTP. It would be interesting to investigate the effect of blocking both GABA^ and GABAg receptors in the neocortex; this might allow the unopposed expression of LTP.