G .A. Lambert et al. Brain Research 887 2000 203 –210
205
fered formalin. The lower brainstem upper cord was 2, division not decidable and were classified as WDR
removed and stored in phosphate-buffered formalin. The 7 8 or LTM 1 8. The locations of 10 lesioned record-
brainstem was later sectioned on a freezing microtome 50 ing sites out of 12 could be found on sections of the
m sections and stained with cresyl violet. Recording sites brainstems of 6 cats. Three sites were in laminae I II of
were reconstructed from a combination of electrolytic the trigeminal nucleus caudalis mean depth 1.4 mm and 2
lesions or track marks and microdrive readings. were in laminae III IV mean depth 2.2 mm; all of these
All group data are presented as mean6standard error neurons responded to GTN with accelerated discharge
S.E.M., except where indicated. Neurons were classified rates following GTN infusion. Two sites were in the neck
as displaying an altered discharge rate by means of the of the nucleus caudalis mean depth 1.7 mm; neither of
critical ratio test [24]. A variance ratio test and a two-tailed the neurons recorded from these sites responded to GTN
Kolmogorov–Smirnov test [28] were used to test whether with acceleration. The other three sites were in the adjacent
the pre-GTN and post-GTN discharge rate frequency ventrolateral medulla or nucleus retroambigualis mean
histograms could have been drawn from the same popula- depth 2.5 mm; 2 of the neurons recorded from these sites
tion. The variance ratio test was used to compare more responded to GTN infusions with an increase in discharge
than two populations of neuronal discharge rates. rate.
All experiments described in this report were approved Fig. 1 shows a post-stimulus histogram obtained from a
by this university’s Animal Care Ethics Committee and neuron in the trigeminal nucleus caudalis activated with
conformed to its guidelines. A2d fibre latency by electrical stimulation of the superior
sagittal sinus. Infusion of GTN through the indwelling lingual artery
3. Results catheter resulted in a rapid increase in the discharge rate of
neurons, commencing immediately and reaching a maxi- Twenty-seven neurons in the trigeminal nucleus caudalis
mum 3–4 min after the start of infusion. Discharge rates of 7 cats responded to electrical stimulation of the superior
rose to about 400 of control at this peak and fell back to sagittal sinus and were chosen for further study. The
average about 170 of control for the remaining time of latencies of the response to stimulation were in the A2d
infusion. The average duration of infusions was 8.960.9 fiber range mean latency to earliest discharge510.561.2
min. The mean rate of discharge during infusion was ms. These latencies are similar to those recorded in our
239647 of control over the duration of the infusion in previous experiments [16]. Neurons discharged 1 to 15
all 32 tests. In 23 out of 32 tests 20 out of 27 neurons, times after a single supramaximal shock mean number of
this increase in rate was significant at the 0.05 level, discharges 3.660.6. All 27 neurons discharged in the
according to the criteria of the critical ratio test. absence of impressed electrical stimulation with a basal
Two neurons in the neck of the trigeminal nucleus did
21 21
rate of 4.461.0 s range 0.012–23 s
. Thirteen of the not respond to GTN with an accelerated discharge rate but
27 neurons were tested for cutaneous receptive fields and otherwise there appeared to be no preferential location in
all 13 were found to have them on the face: in the first the nucleus for neurons which fulfilled the criteria of the
N55, second N55 or third N51 trigeminal divisions test, nor was there a significant difference in the pre-
Fig. 1. Response of a neuron in the trigeminal nucleus caudalis to electrical stimulation of the superior sagittal sinus delivered at time zero. This neuron responded with an mean of 1.8 discharges to each stimulation, at a minimum latency of about 10 ms, typical of most neurons studied, and a modal latency
of 25 ms A2d latency. Vertical axis5number of discharges per 50 stimuli; horizontal axis5latency in ms.
206 G
infusion discharge rate between those neurons which of changes in blood pressure, heart rate and expired CO is
2
responded N523 tests and those which did not N59 shown in Fig. 2. However, there was no correlation
tests t51.48, P NS. The Kolmogorof–Smirnoff test between the size of the changes in blood pressure and the
showed a significant change in population characteristics size of the changes in neuronal discharge rate. Fig. 4
for all 32 infusions. The only neuron with LTM input was shows a scattergram of the percentage changes in neuronal
also accelerated. firing rate induced by GTN plotted against the percentage
A significant increase in discharge rate persisted after changes in mean blood pressure occurring at the same
cessation of infusion; the mean was still 150 of control time.
30 min after the infusion Fig. 3. Furthermore, in some Infusion of the vehicle alone in 5 cats during the
cats the same neuron was tested more than once for its monitoring of 16 neurons did not produce any significant
responses to GTN infusions and, in most cats, more than change in the discharge rates of the neurons Fig. 5 nor in
one neuron was tested for its response. Two to six hours mean blood pressure t51.07, NS or heart rate t50.01,
separated these successive tests. The mean basal discharge NS. Based on average carotid flow rates usually about 25
21
¨ rates of ‘non-naıve’ neurons recorded prior to the Nth
ml min , vehicle or GTN infusion should have produced
infusion were higher than the mean basal discharge rates an average ethanol concentration in the carotid blood of
recorded for the N21th infusion, but there was no 0.02 0.2 mg kg and, at the end of an average infusion,
difference in the percentage increase in discharge rate a body load of about 0.002 0.02 mg kg, not allowing
induced by GTN in successive infusions. However, there for removal mechanisms.
was some evidence that infusions of GTN sensitised neurons to the effects of electrical stimulation of the
sagittal sinus: mean responses to SSS stimulation in
4. Discussion
