TRH analogue affects information transfer through the ventrobasal thalamus

Neuropeptides

7: 31-39, 1986

TRH ANALOGUE AFFECTS INFORMATION TRANSFER THROUGH THE VENTROBASAL THALAMUS Kenneth A. Clarke Department

of Physiology,

University of Sheffield, Western Bank,

Sheffield SlO 2TN, England. ABSTRACT Extracellular ventrobasal

microelectrode

recordings have been made from single

thalamic neurones in rats anaesthetised

Injection of the thyrotropin releasing hormone (Zmg/kg), produced increases in spontaneous the latency of response to peripheral increased production

with urethane.

(TRH) analogue RX77368

activity and decreases in

stimulation,

of later, rhythmic discharges.

together with These changes

were maximal between 30 and 60 minutes after injection.

Decreases in

latency and increases in amplitude were also observed in the evoked potential

recorded from the somatosensory

discussed in relation to previously

cortex.

These results are

observed effects of TRH.

Since

this system is important in the planning and execution of motor acts it is suggested that modification

of information

system could have effects on the production

flow through this

of motor activity.

INTRODUCTION A number of studies have indicated that administration analogues produce increases in motor activity

of TRH or its

(1,2,3,4,5).

Areas of

the CNS associated with motor control have been identified which are either directly sensitive to TRH application excitable

after systemic administration

(6,7,8), or become more

(9,lO).

However, motor

activity is determined not only by alterations on the motor output side, but also by alterations myoclonus,

in sensory input.

For example, in post-anoxic

a condition in which there is thought to be a functionally

deficient descending

5HT pathway, the elevated muscular

responses to

sensory stimulation may arise as a result of changes on the sensory

31

side and the reflex connections between the sensory input and motor output, rather than by any increases in motor excitability

(11).

dorsal column, medial lemniscal system is thought to provide constantly updated, tactile sensory input

The

a

(12) which is important in

the initiation and direction of exploratory motor activity. Extracellular

single unit recordings have been made from the ventro-

basal thalamus in urethane anaesthetised TRH analogue

(RX77368) produces

in information

rats to determine whether a

changes in activity of, or alterations

flow through, this system, since this could have

profound consequences

for the programming

of the motor output.

METHODS l.. Anaesthesia Experiments

and preparation

have been performed

on 12 female rats

within the weight range 190-21Og.

Anaesthesia

of an i.p. injection of ethyl carbamate

was induced by means

(25% in 0.9% saline) and the

dose adjusted to abolish hindlimb withdrawal 1.3 - 1.5 g/kg).

(Sheffield strain)

A tail vein was cannulated

to a strong pinch

(range

followedbyexcision

of

the dura mater over Cl in order to release CSF pressure and prevent cerebral oedema.

A left-sided craniotomy was then performed

animal mounted in a stereotaxic

frame (14).

and the

A paraffin pool was

constructed over the exposed cortex by clamping the cut skin edges. A rectal thermometer was used to monitor body temperature which was maintained by means of a radiator beneath the animal and a heating lamp above it.

2. Recording and analysis The right forepaw was electrically

stimulated

(square wave pulses,

40-7OV, O.lmsec duration, rate 0.25H.z, negative electrode on wrist positive on digit 2) and recordings made from single ventrobasal thalamic units located and identified as in Angel and Clarke Evoked cortical potentials the primary somatosensory

were recorded from the geometric cortex

(14).

(13). centre of

Cortical recording was by

means of silver ball electrodes with the reference electrode being on Thalamic units were recorded through glass

the occipital

cortex.

micropipettes

filled with 3M saline, tip diameters 2-5!Jm. The signals

32

were passed through an FET, filtered, amplified and displayed on a Tektronix

RS103N oscilloscope

for each of the units.

calculated using Wilcoxons

recorded on magnetic

Latencies anddischarge

tape for further analysis. calculated

and simultaneously

Significance

Signed Ranks Test.

frequencies

were

of any changes was Cortical evoked

potentials

were filtered, amplified and averaged using a Neurolog NL750

Averager,

the Averager output processed

through a BBC microcomputer

and displayed using an Epson MX80111 printer. histograms

for ventrobasal

Post-stimulus

thalamic units were prepared in a similar

fashion. 3. TRH analogue Thyrotropin

releasing hormone analogue

(p Glu-His(3,3'-dimethyl-ProNH2)

(RX77368) generously donated by Reckitt & Colman was given intravenously

(Hull, Yorkshire),

via a tail vein cannula, as a dose of 2mq/kg

dissolved in 0.9% saline at a concentration

of 2mg/ml.

Injection

delivery time was 30 to 60 seconds. RESULTS 1. Spontaneous Spontaneous

activity

activity was measured

RX77368 injection was 1.25+0.95

in 20 VBT neurones.

(mean + 1 SEM).

Activity before

Increases in

24’

‘;; aJ ul . ._

e_;

11111,~ I(/ 0

II

I

I

I

Time(min)

Figure 1. Interval histogram

60

of spontaneous discharge of a VBT neurone.

Each line is the mean discharge frequency one minute period.

Ill,

(imp/set), averaged over a

Injection of RX77368 at T=l minute.

33

spontaneous activity were observed in 17 of the 20 neurones

(Fig. I)..

Changes often occurred within 1 minute of injection, however, maximum increases were seen between 12.5+3.7

33 and 57 minutes after injection to

(mean + 1 SEM, n = 20) imp/set.

(~~0.05).

2. Short latency evoked responses The effects of RX77368 injection on the short latency responses of 8 VBT neurones to peripheral

stimulation was determined.

A reduction in

the latency of response was observed in 6 of the neurones 5.27kO.48 msec before injection to 4.80+0.43

from

(mean + 1 SEM, n = 8,

p
msec after injection.

histogram

is shown for one of the units in Fig. 2.

A post-stimulus

latency Two neurones were

unaffected.

3

b. .I

0

6

Tlmetmsec)

Figure 2. Post-stimulus

histograms

of a VBT neurone, before RX773bS.

0

10

6

of short-latency

(left) and after

I

Tlmefmsec)

10

evoked discharges

(right) injection of

Each plot built from responses to 128 consecutive

stimuli.

3. Long latency responses The effects of RX77368 injection on long latency evoked responses were observed in 8 VBT neurones.

In 5 of the 8 units there were no

long latency responses present prior to injection.

In the remaining

three, discharges were produced between 75 and 250 msec after stimulation. particular producing

However, there was no evidence to suggest that any

latency within this range was particularly discharges.

favourable

After RX77368 injection, however,

34

for

5 of the 8

units developed rhythmically

Figure 3. Post-stimulus of a VBT neurone, before

repeated discharges

(Fig. 3).

histograms of long latency evoked discharges (left) and after

(right), injection of

Each plot is built from the responses to 128 consecutive

RX77368. stimuli.

Four of the five units produced a burst of discharges within the range 88.8 to 98.1

(mean = 94.0) msec after stimulation,

a burst of discharges

in the range 133.2 to 144.8 (mean = 140.2) msec

and four of the five a burst of discharges 205.6

three of the five

(mean = 198.0) msec.

in the range 184.6 to

In the remaining three units, rhythmicity

did not develop. 4. Somatosensory Measurements

cortical evoked responses

of the somatosensory

experiments.

Reductions

evoked potential were made in five

in the latencies and increases in the

amplitudes of both the Pl and Nl waves were observed

(Table 1 and

Fig. 4). Pl Before After

RX RX

Latency

6.2 (0.2) 5.7 (0.17)*

Nl

Latency

Pl

Amplitude

1.6 (0.2)

100

7.2 (0.14)

(35.4)

178

Table 1. Latency to peaks of Pl and Nl waves. + 1 SEM, n = 5.

Amplitude 100

223

(41.9)X

Each figure is the mean

Also shown are the peak amplitudes of these waves,

expressed as a % of control value. control values

Nl

(~~0.05)

Significant

shown by *.

35

differences

from

r-----l. -\ i---> / \ ‘1_ ,-$-$ r-.?,? i, /

I

0

Tlmetmsec)

Figure 4. Post-stimulus (upper) and after

2k before

averaged cortical evoked potential,

Each average is

(lower), injection of RX77368.

built from the responses to 128 consecutive

stimuli.

Vertical

calibration bar is 100 UV. DISCUSSION It has been demonstrated

in these experiments

that systemic injection

of RX77368 produces an increase in excitability

of neurones in the

VBT as measured by their decrease in latency to a peripheral stimulus.

It is not possible

from ,the present experiments

to

determine whether or not this is as a result of direct effects on the VBT.

Ionophoretic

both excitatory

studies on neurones

in thalamic nuclei have shown

(15) and inhibitory effects

size or synchrony of the cuneothalamic

(16).

An increase in the

volley would seem to be

unlikely since cuneate neurones are either unaffected ionophoresis

of TRB

or inhibited by

(17).

Increases in the incidence of later, rhythmic discharges were also observed after RX77368 injection taking the form of bursts of impulses occurring within particular post-stimulus 94,140 and 198ms.

latency windows at around

This effect on thalamic rhythmicity

in view of the observation

is interesting

(7) that neurones in the septum which are

rhythmically

active are more often excited by TRB than are non-

rhythmically

active neurones.

The observation

of increased spontaneous

activity of neurones in the

VBT indicates that the changes in excitability

36

are not just phasic

changes associated with the processing

of the input volley but due to

rather more tonic changes in excitability. Increases in both the Pl and Nl waves of the somatosensory potential were observed.

The increase in the Pl wave is compatible volley, though it is not possible

with an increased thalamocortical to determine

evoked

from the present experiments whether the elevated Nl

wave occurs as a result of the increased input or whether there are additional effects on the cerebral cortex or other cortical inputs. Iontophoretic

studies have produced

little agreement on the effects

of TRH on single neurones in the cerebral cortex. failed to demonstrate effects

a direct effect

(8,18,19),othersexcitatory

(6,15) and others inhibition

consistent observation

(16,171.

excitatory

formation modulate

(6,8,18,19).

Since powerful

of the VBT

(Zl), the possibility

of a cholinergic mechanism by RX77368 in the

of increased excitability

present experiments,

reticular

(20) and since neurones in the reticular

the excitability

exists for potentiation

produces

of cholinergic

cholinergic pathways exist between the brainstem

formation and the VBT

production

However, a more

has been the potentiation

excitation of cortical cells by TRH

Some studies have

of VBT cells.

Thus in the

it has been shown that the TRH analogue RX77368

increases in the excitability

of a sensory input system whicl-

is closely integrated with the motor output system. important consequences

This may have

on the ability to plan and execute motor

activity.

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Received 27/9/85 Accepted 15/10/8S

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