DISCUSION DE RESULTADOS
5.3 APORTE CIENTIFICO.
This experiment was designed to establish whether or not the drugs to be employed in subsequent blockade experiments have an independent action on food intake when microinjected into the PVN. Each drug was tested on a separate group of rats.
(i) Idazoxan
The effects of a variety of a-2 antagonists in the PVN have been reported (Leibowitz 1978a) but not those of Idazoxan. This compound was chosen as a result of studies demonstrating that it has a greater a-2 specificity than previously used agents such as yohimbine, rauwolscine and corynanthine (Doxey et al. 1984). A second advantage of using this compound is
that it is easily soluble and can be used in high concentrations and low volumes, allowing direct comparison with the doses of NA previously reported, A total of 11 rats with implanted guide cannulae were used in this experiment. The animals were first injected with 40nmoles NA to establish a feeding response. The criterion for acceptance was that the animal ate at least l.Sgms of lab chow in the 40 minute test period following injection. This figure was chosen because it was above the greatest amount eaten by any rat when mock injected. The second injection administered was 0,9% physiological saline, the vehicle for both NA and Idazoxan; third and fourth injections were 80nmoles and O.lnmoles Idazoxan, and the fifth injection was a further saline injection. All injections were separated by a minimum of 48 hours.
Overall significance was found in these data (F=5.82 df=5,50 p<0.001; see figure 12). Post-hoc tests show that neither SOnmoles nor 0.1 nmoles Idazoxan had an independent effect upon ingestion when injected into the PVN. The only condition which differed significantly from the others was 40nmoles NA which had an effect significantly greater than Idazoxan
SOnmoles (p<0.0015), Idazoxan O.lnmoles (p<0.0005), first saline (p<0.001) and second saline (p<0.0011). The effect Idazoxan did not differ significantly from that of either saline injection.
(ii ^ Phentolamine
Phentolamine is soluble in warmed, sonicated physiological saline up to a maximum of just over SOnmoles in O.Sul. For this reason the dose of NA used in this experiment was reduced to 30 nmoles in order to equate with the maximum available dose of Phentolamine, which was administered in doses of 1, 10, and 30 nmoles to a group of 8 rats. Overall significance was found in these data (F=21.13 df«=6,36 p<0.001 see figure 12). NA was given as both first and last injection and each of these had significantly greater effects (p<0.001 in each case) than any of the Phentolamine conditions, none of which differed significantly from saline (p>0.05). However it was found that the two NA conditions differed significantly
(p<0.003) and this is considered further in the discussion.
(iii\ Cocaine
Cocaine blocks the re-uptake of released NA into the pre-synaptic nerve terminal and might therefore cause an increase in NA availability at the post-synaptic membrane. In order to establish whether this is sufficient to generate the same feeding response as the injections of exogenous NA a dose response to Cocaine was established in a group of 8 animals using three doses: 10, 100 and 200 nmoles. Overall significance was found in these data (F=8.24 df=4,28 p<0.001 see figure 12). There was no significant difference between
the two NA conditions (p=0.287) but both of these were significantly larger than any of the Cocaine or saline conditions (p<0.005 in each case). The two lower doses of Cocaine, 10 and 100 nmoles did not differ significantly to saline. The 200 nmoles dose produced a response lower than that of saline or any of the other drug conditions, in fact 7 of the 8 scores in this condition were zero. The inclusion of this number of zero scores in an ANOVA would have confounded the assumptions about the shape of the data employed in this test and thus the results are reported to be significantly lower by inspection only. The inhibition by 200 nmoles Cocaine of even small feeding bouts, commonly found in the saline control group, probably results from the local anaesthetic action of high doses of Cocaine. Five of the rats in this condition appeared incapable of voluntary movement during the test period, whilst the other 3 were severely impaired.
fivl Prazosin
A dose response to Prazosin was not established due to its low solubility. It was always used at the maximum available concentration after warming and sonication, this dose being independently tested in the animals prior to its use as a blocking agent. Prazosin was found to have no independent effect on food intake, in agreement with the findings of
Leibowitz's group (Leibowitz 1975a, 1978b, Goldman et al. 1985).
Of the four pharmacological agents screened (idazoxan, phentolamine, prazosin and cocaine), only cocaine was found to have an independent effect on food intake. At high doses cocaine suppressed food consumption by disabling the animal. The three antagonist compounds did not cause any observable change in the rats normal behaviour, or cause any change in food consumption as compared to vehicle injection.
FIGURE 12. Mean (+/- SE) gms of food consumed in 40min following microinjection of 40 nmoles NA, and various doses of either phentolamine, idazoxan or cocaine in three separate groups of rats. NA was administered as first and last injection, except in the idazoxan group when 40nmoles d-amphetamine was the last injection. NA or d-amphetamine induced significantly more eating than saline in every condition (at least p< 0.05). No dose of phentolamine, idazoxan or cocaine induced significantly more eating than control.