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A variety of manifestations of the slow (<1Hz) oscillation were seen in NRT neurones. The basic manifestation of the slow (<1Hz) oscillation comprised rhythmic sequences of sustained action potential firing (the “up” phase) interposed by a LHP that encompassed the “down” phase. The transition to the “up” phase was always marked by a LTCP-mediated burst of action potentials (Fig. 5.4A), whereas the transition to the “down” phase could be marked by a clear inflection point (see Fig. 5.13). The “up” phase always comprised sustained, but decelerating action potential firing: maximum frequency: 6.5 - 125 Hz, 50.1 ± 8.5 Hz (n=16), minimum frequency: 2.3 -18.1 Hz, 9.4 ± 1.4 Hz (n=16) and comprised 21.3 - 90.4 %, 57.6 ± 5.9 % (n=16) of the oscillation cycle.

In addition, grouping of action potential bursts during the “down” phase of the slow (<1Hz) oscillation was sometimes observed (n=5, 31% of neurones

displaying a slow (<1Hz) oscillation) leading to the manifestation of “grouped” subtypes (Fig. 5.4B1.2). Grouping of bursts occurred at the beginning of each cycle of the slow oscillation and always led to the initiation of the “up” phase (Fig. 5.4B1-2). These grouped bursts are presumably rhythmic LTCP-mediated events as both these spontaneous rhythmic bursts (Fig. 5.4Bid, Fig. 5.4B2d) and evoked

LTCP mediated bursts (Fig. 5.1A2 and B2) have the same characteristic accelerating then decelerating action potential firing pattern. Under these conditions, grouped bursts occurred at frequencies within 1.3 - 4 Hz, 3.02 + 0.4

Hz (n=5). In all cases there was an increase in the interburst frequency during each episode of “grouped” bursts (calculated using the interval between sequential bursts within a “grouped” episode) (Fig. 5.4B 3). In some neurones this

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Figure 5.3 The frequency of the slow (<1Hz) oscillation in NRT neurones is dependent on the level of injected d.c. current.

Figure 5.3 The frequency of the slow (<1Hz) oscillation in NRT neurones is dependent on the level of injected d.c. current.

(A) NRT neurone in the presence of frans-ACPD (100 pM) at different levels of injected hyperpolarising d.c. current and demonstrating the manifestation of the decrease in the frequency of the slow (<1Hz) oscillation as injected d.c current is removed. Note the pronounced increase in the duration of the “up” phase of the oscillation.

(B) The frequency of the slow (<1Hz) oscillation in 4 NRT neurones in the presence of trans-ACPD (100 pM) is shown at different levels of injected d.c current. The frequency of the slow (<1Hz) oscillation is decreased as injected hyperpolarising d.c. current in removed (1). Analysis of the slow (<1Hz) oscillation at different levels of injected d.c. current as shown in A. Note that as the frequency of the slow (<1Hz) oscillation decreases, there is an increase both in the duration of the “up” phase and the % of each oscillation cycle that is occupied by the “up” phase.

(C) Analysis of the characteristics of action potential firing during the “up” phase at different levels of injected d.c current as shown in A. Note the slight increase in the maximum action potential firing rate, but virtually no change in the minimum action potential firing rate as injected d.c. current is removed. However, there is a fall in the mean firing rate observed during the “up” phase (1). Graph showing the normalised rate of action potential firing during the “up” phase at the different levels of injected d.c. current in A. Note the decline in firing rate during the “up” phase. However, there is a more rapid decrease in the firing rate during the “up” phase at increasingly depolarised levels of injected d.c current (2) which leads to the decrease in the mean firing rate observed (1).

A Basic slow (<1 Hz) oscillation

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Figure 5.4 Different manifestations of the slow (<1Hz) oscillation in NRT

neurones.

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Figure 5.4 Different manifestations of the slow (<1Hz) oscillation in NRT neurones.

(A) NRT neurone in the presence of frans-ACPD (100 pM) displaying a basic slow (<1Hz) oscillation. In this case the slow (<1Hz) oscillation was manifested as rhythmic sequences of tonic action potential firing (the “up” phase) interposed by large hyperpolarizing potentials (that encompasses the “down” phase) (1). The underlined section is expanded in (2) showing that the transition from the “down” phase to “up” phase is marked by a LTCP-mediated burst of action potentials.

(B) NRT neurone in the presence of trans-ACPD (100 pM) displaying a slow (<1Hz) oscillation that groups rhythmic bursting at 1-2 Hz prior to the transition to the “up” phase (1). Expanded sections are shown as indicated (b and c). Graph showing the duration of the successive action potential intervals in the bursts marked in the main trace (d). NRT neurone in the presence of frans- ACPD (100 pM) displaying a slow (<1Hz) oscillation that groups rhythmic bursting at 2-4 Hz prior to the transition to the “up” phase (2). Expanded sections are shown as indicated (b and c). Graph showing the duration of the successive action potential intervals in the bursts marked in the main trace (d). The dashed line shows a pronounced depolarisation of the underlying membrane potential in 2b, whereas that this not seen in 1b and indicated by the horizontal dashed line. Graph showing the mean interburst frequency of successive bursts in 5 different NRT neurones that displayed the “grouped” subtype of the slow (<1Hz) oscillation (three cycles of the oscillation at the most depolarised level of injected d.c. current where “grouped” episodes existed was used for this analysis). In all case the interburst frequency increased during the “grouped” episodes. Neurone 1 corresponds to the neurone shown in 82 and Neurone 2 corresponds to the neurone shown in B1 (3).

(n=3) and Fig. 5.4B3), but in others was more notable (Fig. 5.4B2: mean increase = 2.2 ± Hz (198.6 ± 23.0 %) (n=3) and Fig. 5.4B3) and was associated with an obvious depolarisation of the underlying membrane potential (Fig. 5.4B2t>). The increase in interburst frequency ranged from 0.3-2.2 Hz, 1.1 ± 0.3 Hz (n = 5 neurones).

In addition to variations in the extent and frequency of grouping in different neurones, the properties of grouped episodes could also depend on the level of injected d.c. current (Fig. 5.5). As the injected hyperpolarising d.c. current was decreased the number of grouped bursts per oscillation cycle decreased although the mean interburst frequency remained fairly consistent, only increasing slightly (Fig. 5.5Ci). Further analysis of the interburst frequency within episodes of “grouped” bursting, showed that both the minimum and maximum interburst frequencies increased as the level of injected hyperpolarising d.c. current was reduced thereby leading to a smaller increase in the interburst frequency (Fig. 5.5C2).

5.3.4 Extracellular recordings indicate the normal occurrence of the slow

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