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6-OHDA BILATERAL LESIONS TO THE NUCLEUS SEPTI LATERALIS ATTENUATE VASOPRESSIN IMPROVEMENT OF RECALL IN RATS
Pharmacological Research, Vol. 37, No. 2, 1998
6-OHDA BILATERAL LESIONS TO THE NUCLEUS SEPTI LATERALIS ATTENUATE VASOPRESSIN IMPROVEMENT OF RECALL IN RATS ´ MARIA MAŁGORZATA WINNICKAU and KONSTANTY WISNIEWSKI Medical Academy of Białystok, Department of Pharmacology, Mickiewicza 2c, 15-222 Białystok, Poland Accepted 19 December 1997
The investigation was aimed at investigating whether the dopaminergic projection arriving at the nucleus septi lateralis ŽNSL. is involved in the facilitatory effect of vasopressin ŽAVP. on memory retrieval. The bilateral 6-OHDA lesions to the NSL were made in 20 male Wistar rats before testing the intracerebroventricular Ži.c.v.. AVP injection on recall in a passive avoidance situation. Eighteen additional rats served as sham-operated controls. Thirty minutes before surgery rats were pre-treated with an intraperitoneal injection of 25 mg kgy1 of desmethylimipramine, an inhibitor of norepinephrine uptake. Sixteen lesioned and 16 sham-operated rats were included in the study. AVP Ž1 m g, i.c.v.. given 15 min before the retention testing significantly improved latencies both in lesioned and in sham-operated rats in comparison with the respective i.c.v. saline-injected animals. However, bilateral lesions to the NSL significantly diminished the facilitatory effect of AVP on recall. The insignificant decrease of spontaneous psychomotor activity in rats lesioned to the NSL was unlikely to interfere with the cognitive effect of AVP. These results suggest that dopaminergic projection to the NSL is involved in the facilitatory effect of AVP on the retrieval process in a passive avoidance situation. Q 1998 The Italian Pharmacological Society KEY
WORDS:
vasopressin, lateral septum, passive avoidance, psychomotor activity, rats.
INTRODUCTION There is increasing evidence that vasopressin ŽAVP. exerts facilitatory effect on learning and memory processes w1]16x. AVP and related peptides facilitate consolidation and retrieval processes mainly in tests using aversive stimulus w1]3, 10]14, 16x, improve social memory in rats w5]7, 15x and also prevent and reverse the retrograde amnesia induced by various amnestic agents w4, 6, 17x. Moreover, intracerebroventricular Ži.c.v.. administration of antivasopressin serum induces severe disturbances in the extinction of active avoidance behaviour w2, 18x and also disruption of a passive avoidance behaviour w2, 3, 19x. Various brain structures have been tested as possible sites of action of AVP on memory processes. The dorsal hippocampus seems to be a structure in which endogenous AVP plays a physiological role in the consolidation of information, while the dorsal septum and ventral hippocampus mainly play a role U
Corresponding author.
1043]6618r98r020145]06r$25.00r0rfr970280
in retrieval processes w13, 20x. Whether AVP affects cognitive processes directly or modulates them indirectly is still unknown. Parvicellular vasopressinergic neurones, localised in the paraventricular nucleus, the suprachiasmatic nucleus, the bed nucleus of stria terminalis and amygdala projecting to several limbic regions, to the lateral habenula and autonomic centres in the brain-stem w21]24x release AVP which may act as neurotransmitter or neuromodulator. Among the projection areas of these extrahypothalamic AVP fibres, the lateral septum may play an important role in the central effects of vasopressin. Local microinjections of AVP in the septum affect firing of septal neurones w25x, catecholamine turnover, extinction of avoidance behaviour w12x and social memory in rats w6x. In addition, the lateral septum contains significant amounts of highly specific AVP binding sites w26]29x. Izikawa et al. w30x have demonstrated that V1 vasopressin receptors which are present in high density in the lateral septum w27x are partly located on dopaminergic terminals arriving at this structure. Since i.c.v. and intrastructural administration of Q1998 The Italian Pharmacological Society
Pharmacological Research, Vol. 37, No. 2, 1998
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AVP causes an increase in catecholamine utilisation in many brain structures including the septum w31, 32x it seems to be possible that dopaminergic projection to the lateral septum might be involved in AVP facilitation of cognitive processes. The aim of this study was the evaluation whether bilateral 6-OHDA-induced destruction of dopaminergic endings in the lateral septum may have an influence on the facilitatory effect of AVP on retrieval of information in a passive avoidance situation.
i.c.v. injections were made manually into the right cerebral ventricle with a Hamilton syringe, using a removable KF 730 needle 4.5 mm long, according to the technique described earlier w35x. This procedure allowed lowering the tip of the needle about 0.5 mm below the ceiling of the lateral cerebral ventricle. It was relatively non-traumatic as the animal, gently fixed in the left hand of the experimenter, was usually quiet and no vocalisation occurred. Half of lesioned and sham-operated rats received 0.9% NaCl at the same time.
MATERIALS AND METHODS
Beha¨ ioural testing
Animals Male Wistar rats weighing 160]165 g on the day of surgery and 180]190 g at the time of behavioural testing were used. The animals were housed in plastic cages with a 12-h lightrdark cycle beginning at 07:00 h with food and water freely accessible.
Surgery The rats were anaesthetised with chloral hydrate and were placed in a Kopf stereotaxic apparatus with tooth bar 5 mm above the inter-aural line. The skull was exposed, and burr holes 1.5 mm in diameter were drilled above the appropriate coordinate targets. The coordinates Žin millimetres. for the nucleus septi lateralis ŽNSL., anterior ŽA. from the bregma, lateral ŽL. to the mid-line and below ŽV. the skull surface were selected with the aid of the atlas of Konig ¨ and Klippel w33x at A: 1.5, L: 0.7, V: 5.5. Each site was infused with 1 m l of 0.9% NaCl containing 8 m g of 6-OHDA Žfree base, Sigma.. The neurotoxin was dissolved in a vehicle solution containing 5 mg mly1 of ascorbic acid, to prevent oxidation of 6-OHDA, and injected over 10 min through a stainless steel cannula Žo.d. s 0.3 mm. at the rate of 0.1 m l miny1 . The cannula was left in place for an additional 5 min after the end of the infusion. Twenty randomly selected rats received 6-OHDA to NSL, the remaining 18 sham-operated control rats underwent the same procedure except that they only received the vehicle solution. Thirty minutes before the surgery all rats were pre-treated with an intraperitoneal injection of 25 mg kgy1 desmethylimipramine ŽSigma., an inhibitor of norepinephrine ŽNE. uptake, which has been shown to protect NE neurones from destruction by 6-OHDA w34x. After 6-OHDA infusions, an additional burr hole, 0.5 mm in diameter, was drilled in the rat’s skull 2.5 mm laterally and 1 mm caudally from the point of intersection of the bregma and the superior saggital suture on the right side of the head for the i.c.v. injection. Behavioural testing started after 10 days of recovery. AVP Ž Calbiochem . dissolved in 5 m l of 0.9% NaCl was given i.c.v. in the amount of 1 m g per rat 15 min before behavioural testing. The
Locomotor Žcrossings. and exploratory Žrearings, bar approaches. activity was measured in an open field which was square, 100 = 100 cm, a white floor divided by eight lines into 25 equal squares and surrounded by a 47 cm high wall. Four plastic bars, 20 cm high, were located in four lines crossing in the central area of the floor. Following 1 min of adaptation, crossings of squares, rearings, and bar approaches were counted manually for 10 min. The open field test was carried out immediately after the passive avoidance test. Passive avoidance performance was studied in a one-trial step-through passive avoidance situation w36x. The apparatus consisted of an illuminated platform attached to a large dark compartment. The rats were placed on the platform and were allowed to enter the naturally preferred dark compartment. Two more trials were given on the following day. At the end of the second trial an inescapable, scrambled electric footshock Ž0.5 mA for 3 s. was delivered through the grid floor of the dark compartment. Retention of a passive avoidance behaviour was tested 24 h after the single learning trial by measuring the latency to re-enter the dark compartment up to a maximum of 300 s.
Histology Placement of the cannula was examined histologically. At the end of behavioural testing, the rats were killed and their brains were removed and fixed in 10% formaldehyde for 7 days. Subsequently, coronal sections Ž20 m m thick. of the cannula tract were cut using the frozen sectioning method, saving every fifth section through the lesion, mounted on slides and stained with cresyl violet. Rats in which the cannula tip was located outside the centre of target structure and with incorrect i.c.v. injections were excluded from the experimental data. According to Agid et al. w37x the location of the tip of the cannula is particularly critical for reproducibility of the decrease in dopamine levels in lesioned structures. After infusion of 1 m l of 6-OHDA distribution of neurotoxin has a spherical shape with the diameter 3.0 mm. Typical localisation of the tip of cannula into NSL is shown in Fig. 1.
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Fig. 1. Frontal section of the brain at the level A 7890 m m. Arrows indicate typical localisation of the tip of cannula of 6-OHDA infusions into the nucleus septi lateralis ŽNSL..
Statistical analysis The results of experiments were evaluated by twoway analysis of variance ŽANOVA. followed by Newman]Keuls test. F-ratios, degrees of freedom and P-values are reported only for significant differences. In all comparisons between particular groups P- 0.05 was considered significant.
RESULTS Sixteen lesioned and 16 sham-operated animals were included in the study. Two sham-operated and two lesioned to NSL groups of rats injected i.c.v. with AVP or saline consisted of eight rats. AVP improved the retrieval process in a passive avoidance situation in the sham-operated group of rats. The facilitatory effect of AVP was still observable in rats which sustained 6-OHDA; however, this effect was not as pronounced as in the sham-operated group ŽFig. 2.. ANOVA of two lesioned and two sham-operated groups, injected i.c.v. with AVP or saline, yielded F3,28 s 19.82, P- 0.001. Further post hoc comparisons between these groups made with the Newman]Keuls test revealed a significant increase of the mean step-through latency in the sham-operated Ž P - 0.01., and lesioned to the NSL Ž P - 0.05. groups of rats injected with AVP as compared with the respective saline injected groups. The AVP facilitatory effect on retrieval process was significantly attenuated after bilateral disruption of dopaminergic endings in NSL Ž P - 0.05.. Sham-operated rats, injected i.c.v. with saline, stayed on illuminated platform about 30 s, while the lesioned subjects almost immediately entered the dark compartment on retention test, the mean step-through latencies being less than 20 s, but the difference was insignificant. The spontaneous locomotor activity measured by
Fig. 2. Effect of 1 m g of AVP given 15 min before the retention testing on the re-entry latencies in the passive avoidance situation in rats lesioned to the nucleus septi lateralis. Columns represent means "SEM of the values obtained from 8 rats. U P- 0.05 vs. saline-injected lesioned UU group, and AVP injected sham-operated group; P- 0.01 vs. saline-injected sham-operated and lesioned groups of rats ŽANOVA. and Newman-Keuls test..
crossings of squares in lesioned to NAS rats was insignificantly attenuated in comparison with shamoperated group of animals ŽTable I.. Nevertheless, the exploratory activity Žrearings and bar approaches. of lesioned to NAS rats was comparable with exploratory activity of sham-operated controls. There were no differences in locomotor and exploratory activity between AVP and saline treated subgroups both, in lesioned and sham-operated groups of rats.
DISCUSSION The results of the present study indicate that dopaminergic projection to the NSL is involved in the facilitatory effect of AVP on memory motivated
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Table I Effect of bilateral 6-OHDA lesions to the nucleus septi lateralis (NSL) on the locomotor activity of rats in open field Group Sham-operated Lesioned to NSL
Crossings
Rearings
Bar approaches
48.22 Ž5.94. 36.86 Ž4.71.
12.95 Ž1.55. 10.61 Ž0.93.
4.36 0.51. 3.43 Ž0.44.
Values are means Ž"SEM. from 16 subjects.
affectively. Since V1 receptors in the lateral septum are partly located on dopaminergic endings it could be concluded that AVP facilitatory effect is, in part, mediated by these receptors. As lesion-induced behavioural changes have frequently been attributed to changes in general psychomotor activity of animals, the spontaneous psychomotor activity of lesioned and sham-operated rats was compared. The motility of rats lesioned to NSL, measured by crossings of squares, was lower in comparison with the motility of sham-operated animals. However, this difference was insignificant and it could not account for the results obtained in a passive avoidance situation, since lesioned rats injected i.c.v. with saline stayed on the illuminated platform for a shorter time than saline-injected sham-operated controls. The exploratory activity measured by rearings and bar approaches did not differ between lesioned and sham-operated groups of rats. There is a considerable body of evidence indicating that AVP improves the retrieval process in a passive avoidance situation w2]4, 16x. Our results confirm these findings, since animals injected i.c.v. with AVP stayed on illuminated platform significantly longer than the respective controls. A lot of results of lesion and microinjection experiments point to the involvement of limbic midbrain region such as the septal area, dorsal hippocampus and amygdala in the expression of the effects of AVP on memory processes w6, 11, 14, 20, 38, 39x. Regarding the role of endogenous AVP, Kovacs ´ and De Wied w13x suggest that AVP in the dorsal septum and in the ventral hippocampus mainly plays a role in the recall of information, while endogenous AVP in the dorsal hippocampus is involved in consolidation processes. The memory effect of AVP might be explained by an influence of the neuropeptide on excitation of limbic Žhippocampal and septal. neurones. A number of neurones in the lateral septum respond to the microiontophoretically applied AVP with an increase in spontaneous single-unit activity in the same way as the excitatory neurotransmitter glutamate increases the activity of these neurones w25x.
There is some evidence that in particular limbicmidbrain catecholamines play an essential role in various memory processes w14, 40x. It was found that AVP causes an increase in catecholamine utilisation in many brain areas including limbic structures w31, 32x. This pattern of effects observed after AVP administration may be supportive of the view that AVP affected various physiological processes by modulating catecholaminergic neurotransmission in distinct populations of neurones. Among limbic structures receiving vasopressinergic fibers, the lateral septum may play an important role in the mediation of memory processes by AVP. Dantzer et al. w5, 6x have reported that septal AVP modulates social memory in rats, since intraseptal injection of AVP antagonist exerted amnestic-like effect and blocked the facilitatory effect of AVP on social memory. This latter effect appears to be mediated by V1 vasopressin receptors since it was abolished by the selective V1 antagonist-1-deaminopenicillamine 2-O-methyltyrosine AVP. It was also found that microinjection of small amounts of AVP to the dorsal septum improves passive avoidance behaviour, when administered after the learning trial w12x. Ishizawa et al. w30x have demonstrated that V1 vasopressin receptors in the lateral septum are partly located on catecholaminergic Žnoradrenergic and dopaminergic. terminals. Chemical lesions of the dorsal noradrenergic bundle, which supplies noradrenergic input to limbic-midbrain regions blocked the ability of AVP to enhance retention of a passive avoidance behaviour w10, 11x. Our results confirm the possibility that V1 receptors in the lateral septum may be involved in AVP facilitation of the retrieval process in a passive avoidance situation, since this effect was significantly attenuated after bilateral 6-OHDA-induced disruption of dopaminergic endings in this structure. As lesioned rats received injection of desmethylimipramine before surgery noradrenergic terminals became intact, and the facilitatory effect of AVP on retrieval was still observable in lesioned rats in our experiments. Although, according to Kesner et al. w41, 42x, a key structure for memory motivated effectively is amygdala, disruption of dopaminergic endings in the central amygdala in our previous study w43x did not affect facilitatory effect of AVP on the retrieval process in a passive avoidance situation. It seems to be possible that parvicellular amygdaloid neurones projecting to the lateral septum release AVP which exerts an influence on septal catecholamines via V1 vasopressin receptors located on catecholaminergic terminals. The results obtained in our previous w43x and present study indicate that after removal of dopaminergic endings from the central amygdala this pathway was not impaired.
Pharmacological Research, Vol. 37, No. 2, 1998
The results obtained in this study indicate that dopaminergic input to the NSL is, in part, involved in the facilitatory effect of AVP on the retrieval process in a passive avoidance situation in rats.
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16.
17.
ACKNOWLEDGEMENT This study was supported by AMB project 10 3 889.
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6-OHDA BILATERAL LESIONS TO THE NUCLEUS SEPTI LATERALIS ATTENUATE VASOPRESSIN IMPROVEMENT OF RECALL IN RATS ´ MARIA MAŁGORZATA WINNICKAU and KONSTANTY WISNIEWSKI Medical Academy of Białystok, Department of Pharmacology, Mickiewicza 2c, 15-222 Białystok, Poland Accepted 19 December 1997
The investigation was aimed at investigating whether the dopaminergic projection arriving at the nucleus septi lateralis ŽNSL. is involved in the facilitatory effect of vasopressin ŽAVP. on memory retrieval. The bilateral 6-OHDA lesions to the NSL were made in 20 male Wistar rats before testing the intracerebroventricular Ži.c.v.. AVP injection on recall in a passive avoidance situation. Eighteen additional rats served as sham-operated controls. Thirty minutes before surgery rats were pre-treated with an intraperitoneal injection of 25 mg kgy1 of desmethylimipramine, an inhibitor of norepinephrine uptake. Sixteen lesioned and 16 sham-operated rats were included in the study. AVP Ž1 m g, i.c.v.. given 15 min before the retention testing significantly improved latencies both in lesioned and in sham-operated rats in comparison with the respective i.c.v. saline-injected animals. However, bilateral lesions to the NSL significantly diminished the facilitatory effect of AVP on recall. The insignificant decrease of spontaneous psychomotor activity in rats lesioned to the NSL was unlikely to interfere with the cognitive effect of AVP. These results suggest that dopaminergic projection to the NSL is involved in the facilitatory effect of AVP on the retrieval process in a passive avoidance situation. Q 1998 The Italian Pharmacological Society KEY
WORDS:
vasopressin, lateral septum, passive avoidance, psychomotor activity, rats.
INTRODUCTION There is increasing evidence that vasopressin ŽAVP. exerts facilitatory effect on learning and memory processes w1]16x. AVP and related peptides facilitate consolidation and retrieval processes mainly in tests using aversive stimulus w1]3, 10]14, 16x, improve social memory in rats w5]7, 15x and also prevent and reverse the retrograde amnesia induced by various amnestic agents w4, 6, 17x. Moreover, intracerebroventricular Ži.c.v.. administration of antivasopressin serum induces severe disturbances in the extinction of active avoidance behaviour w2, 18x and also disruption of a passive avoidance behaviour w2, 3, 19x. Various brain structures have been tested as possible sites of action of AVP on memory processes. The dorsal hippocampus seems to be a structure in which endogenous AVP plays a physiological role in the consolidation of information, while the dorsal septum and ventral hippocampus mainly play a role U
Corresponding author.
1043]6618r98r020145]06r$25.00r0rfr970280
in retrieval processes w13, 20x. Whether AVP affects cognitive processes directly or modulates them indirectly is still unknown. Parvicellular vasopressinergic neurones, localised in the paraventricular nucleus, the suprachiasmatic nucleus, the bed nucleus of stria terminalis and amygdala projecting to several limbic regions, to the lateral habenula and autonomic centres in the brain-stem w21]24x release AVP which may act as neurotransmitter or neuromodulator. Among the projection areas of these extrahypothalamic AVP fibres, the lateral septum may play an important role in the central effects of vasopressin. Local microinjections of AVP in the septum affect firing of septal neurones w25x, catecholamine turnover, extinction of avoidance behaviour w12x and social memory in rats w6x. In addition, the lateral septum contains significant amounts of highly specific AVP binding sites w26]29x. Izikawa et al. w30x have demonstrated that V1 vasopressin receptors which are present in high density in the lateral septum w27x are partly located on dopaminergic terminals arriving at this structure. Since i.c.v. and intrastructural administration of Q1998 The Italian Pharmacological Society
Pharmacological Research, Vol. 37, No. 2, 1998
146
AVP causes an increase in catecholamine utilisation in many brain structures including the septum w31, 32x it seems to be possible that dopaminergic projection to the lateral septum might be involved in AVP facilitation of cognitive processes. The aim of this study was the evaluation whether bilateral 6-OHDA-induced destruction of dopaminergic endings in the lateral septum may have an influence on the facilitatory effect of AVP on retrieval of information in a passive avoidance situation.
i.c.v. injections were made manually into the right cerebral ventricle with a Hamilton syringe, using a removable KF 730 needle 4.5 mm long, according to the technique described earlier w35x. This procedure allowed lowering the tip of the needle about 0.5 mm below the ceiling of the lateral cerebral ventricle. It was relatively non-traumatic as the animal, gently fixed in the left hand of the experimenter, was usually quiet and no vocalisation occurred. Half of lesioned and sham-operated rats received 0.9% NaCl at the same time.
MATERIALS AND METHODS
Beha¨ ioural testing
Animals Male Wistar rats weighing 160]165 g on the day of surgery and 180]190 g at the time of behavioural testing were used. The animals were housed in plastic cages with a 12-h lightrdark cycle beginning at 07:00 h with food and water freely accessible.
Surgery The rats were anaesthetised with chloral hydrate and were placed in a Kopf stereotaxic apparatus with tooth bar 5 mm above the inter-aural line. The skull was exposed, and burr holes 1.5 mm in diameter were drilled above the appropriate coordinate targets. The coordinates Žin millimetres. for the nucleus septi lateralis ŽNSL., anterior ŽA. from the bregma, lateral ŽL. to the mid-line and below ŽV. the skull surface were selected with the aid of the atlas of Konig ¨ and Klippel w33x at A: 1.5, L: 0.7, V: 5.5. Each site was infused with 1 m l of 0.9% NaCl containing 8 m g of 6-OHDA Žfree base, Sigma.. The neurotoxin was dissolved in a vehicle solution containing 5 mg mly1 of ascorbic acid, to prevent oxidation of 6-OHDA, and injected over 10 min through a stainless steel cannula Žo.d. s 0.3 mm. at the rate of 0.1 m l miny1 . The cannula was left in place for an additional 5 min after the end of the infusion. Twenty randomly selected rats received 6-OHDA to NSL, the remaining 18 sham-operated control rats underwent the same procedure except that they only received the vehicle solution. Thirty minutes before the surgery all rats were pre-treated with an intraperitoneal injection of 25 mg kgy1 desmethylimipramine ŽSigma., an inhibitor of norepinephrine ŽNE. uptake, which has been shown to protect NE neurones from destruction by 6-OHDA w34x. After 6-OHDA infusions, an additional burr hole, 0.5 mm in diameter, was drilled in the rat’s skull 2.5 mm laterally and 1 mm caudally from the point of intersection of the bregma and the superior saggital suture on the right side of the head for the i.c.v. injection. Behavioural testing started after 10 days of recovery. AVP Ž Calbiochem . dissolved in 5 m l of 0.9% NaCl was given i.c.v. in the amount of 1 m g per rat 15 min before behavioural testing. The
Locomotor Žcrossings. and exploratory Žrearings, bar approaches. activity was measured in an open field which was square, 100 = 100 cm, a white floor divided by eight lines into 25 equal squares and surrounded by a 47 cm high wall. Four plastic bars, 20 cm high, were located in four lines crossing in the central area of the floor. Following 1 min of adaptation, crossings of squares, rearings, and bar approaches were counted manually for 10 min. The open field test was carried out immediately after the passive avoidance test. Passive avoidance performance was studied in a one-trial step-through passive avoidance situation w36x. The apparatus consisted of an illuminated platform attached to a large dark compartment. The rats were placed on the platform and were allowed to enter the naturally preferred dark compartment. Two more trials were given on the following day. At the end of the second trial an inescapable, scrambled electric footshock Ž0.5 mA for 3 s. was delivered through the grid floor of the dark compartment. Retention of a passive avoidance behaviour was tested 24 h after the single learning trial by measuring the latency to re-enter the dark compartment up to a maximum of 300 s.
Histology Placement of the cannula was examined histologically. At the end of behavioural testing, the rats were killed and their brains were removed and fixed in 10% formaldehyde for 7 days. Subsequently, coronal sections Ž20 m m thick. of the cannula tract were cut using the frozen sectioning method, saving every fifth section through the lesion, mounted on slides and stained with cresyl violet. Rats in which the cannula tip was located outside the centre of target structure and with incorrect i.c.v. injections were excluded from the experimental data. According to Agid et al. w37x the location of the tip of the cannula is particularly critical for reproducibility of the decrease in dopamine levels in lesioned structures. After infusion of 1 m l of 6-OHDA distribution of neurotoxin has a spherical shape with the diameter 3.0 mm. Typical localisation of the tip of cannula into NSL is shown in Fig. 1.
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Fig. 1. Frontal section of the brain at the level A 7890 m m. Arrows indicate typical localisation of the tip of cannula of 6-OHDA infusions into the nucleus septi lateralis ŽNSL..
Statistical analysis The results of experiments were evaluated by twoway analysis of variance ŽANOVA. followed by Newman]Keuls test. F-ratios, degrees of freedom and P-values are reported only for significant differences. In all comparisons between particular groups P- 0.05 was considered significant.
RESULTS Sixteen lesioned and 16 sham-operated animals were included in the study. Two sham-operated and two lesioned to NSL groups of rats injected i.c.v. with AVP or saline consisted of eight rats. AVP improved the retrieval process in a passive avoidance situation in the sham-operated group of rats. The facilitatory effect of AVP was still observable in rats which sustained 6-OHDA; however, this effect was not as pronounced as in the sham-operated group ŽFig. 2.. ANOVA of two lesioned and two sham-operated groups, injected i.c.v. with AVP or saline, yielded F3,28 s 19.82, P- 0.001. Further post hoc comparisons between these groups made with the Newman]Keuls test revealed a significant increase of the mean step-through latency in the sham-operated Ž P - 0.01., and lesioned to the NSL Ž P - 0.05. groups of rats injected with AVP as compared with the respective saline injected groups. The AVP facilitatory effect on retrieval process was significantly attenuated after bilateral disruption of dopaminergic endings in NSL Ž P - 0.05.. Sham-operated rats, injected i.c.v. with saline, stayed on illuminated platform about 30 s, while the lesioned subjects almost immediately entered the dark compartment on retention test, the mean step-through latencies being less than 20 s, but the difference was insignificant. The spontaneous locomotor activity measured by
Fig. 2. Effect of 1 m g of AVP given 15 min before the retention testing on the re-entry latencies in the passive avoidance situation in rats lesioned to the nucleus septi lateralis. Columns represent means "SEM of the values obtained from 8 rats. U P- 0.05 vs. saline-injected lesioned UU group, and AVP injected sham-operated group; P- 0.01 vs. saline-injected sham-operated and lesioned groups of rats ŽANOVA. and Newman-Keuls test..
crossings of squares in lesioned to NAS rats was insignificantly attenuated in comparison with shamoperated group of animals ŽTable I.. Nevertheless, the exploratory activity Žrearings and bar approaches. of lesioned to NAS rats was comparable with exploratory activity of sham-operated controls. There were no differences in locomotor and exploratory activity between AVP and saline treated subgroups both, in lesioned and sham-operated groups of rats.
DISCUSSION The results of the present study indicate that dopaminergic projection to the NSL is involved in the facilitatory effect of AVP on memory motivated
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Table I Effect of bilateral 6-OHDA lesions to the nucleus septi lateralis (NSL) on the locomotor activity of rats in open field Group Sham-operated Lesioned to NSL
Crossings
Rearings
Bar approaches
48.22 Ž5.94. 36.86 Ž4.71.
12.95 Ž1.55. 10.61 Ž0.93.
4.36 0.51. 3.43 Ž0.44.
Values are means Ž"SEM. from 16 subjects.
affectively. Since V1 receptors in the lateral septum are partly located on dopaminergic endings it could be concluded that AVP facilitatory effect is, in part, mediated by these receptors. As lesion-induced behavioural changes have frequently been attributed to changes in general psychomotor activity of animals, the spontaneous psychomotor activity of lesioned and sham-operated rats was compared. The motility of rats lesioned to NSL, measured by crossings of squares, was lower in comparison with the motility of sham-operated animals. However, this difference was insignificant and it could not account for the results obtained in a passive avoidance situation, since lesioned rats injected i.c.v. with saline stayed on the illuminated platform for a shorter time than saline-injected sham-operated controls. The exploratory activity measured by rearings and bar approaches did not differ between lesioned and sham-operated groups of rats. There is a considerable body of evidence indicating that AVP improves the retrieval process in a passive avoidance situation w2]4, 16x. Our results confirm these findings, since animals injected i.c.v. with AVP stayed on illuminated platform significantly longer than the respective controls. A lot of results of lesion and microinjection experiments point to the involvement of limbic midbrain region such as the septal area, dorsal hippocampus and amygdala in the expression of the effects of AVP on memory processes w6, 11, 14, 20, 38, 39x. Regarding the role of endogenous AVP, Kovacs ´ and De Wied w13x suggest that AVP in the dorsal septum and in the ventral hippocampus mainly plays a role in the recall of information, while endogenous AVP in the dorsal hippocampus is involved in consolidation processes. The memory effect of AVP might be explained by an influence of the neuropeptide on excitation of limbic Žhippocampal and septal. neurones. A number of neurones in the lateral septum respond to the microiontophoretically applied AVP with an increase in spontaneous single-unit activity in the same way as the excitatory neurotransmitter glutamate increases the activity of these neurones w25x.
There is some evidence that in particular limbicmidbrain catecholamines play an essential role in various memory processes w14, 40x. It was found that AVP causes an increase in catecholamine utilisation in many brain areas including limbic structures w31, 32x. This pattern of effects observed after AVP administration may be supportive of the view that AVP affected various physiological processes by modulating catecholaminergic neurotransmission in distinct populations of neurones. Among limbic structures receiving vasopressinergic fibers, the lateral septum may play an important role in the mediation of memory processes by AVP. Dantzer et al. w5, 6x have reported that septal AVP modulates social memory in rats, since intraseptal injection of AVP antagonist exerted amnestic-like effect and blocked the facilitatory effect of AVP on social memory. This latter effect appears to be mediated by V1 vasopressin receptors since it was abolished by the selective V1 antagonist-1-deaminopenicillamine 2-O-methyltyrosine AVP. It was also found that microinjection of small amounts of AVP to the dorsal septum improves passive avoidance behaviour, when administered after the learning trial w12x. Ishizawa et al. w30x have demonstrated that V1 vasopressin receptors in the lateral septum are partly located on catecholaminergic Žnoradrenergic and dopaminergic. terminals. Chemical lesions of the dorsal noradrenergic bundle, which supplies noradrenergic input to limbic-midbrain regions blocked the ability of AVP to enhance retention of a passive avoidance behaviour w10, 11x. Our results confirm the possibility that V1 receptors in the lateral septum may be involved in AVP facilitation of the retrieval process in a passive avoidance situation, since this effect was significantly attenuated after bilateral 6-OHDA-induced disruption of dopaminergic endings in this structure. As lesioned rats received injection of desmethylimipramine before surgery noradrenergic terminals became intact, and the facilitatory effect of AVP on retrieval was still observable in lesioned rats in our experiments. Although, according to Kesner et al. w41, 42x, a key structure for memory motivated effectively is amygdala, disruption of dopaminergic endings in the central amygdala in our previous study w43x did not affect facilitatory effect of AVP on the retrieval process in a passive avoidance situation. It seems to be possible that parvicellular amygdaloid neurones projecting to the lateral septum release AVP which exerts an influence on septal catecholamines via V1 vasopressin receptors located on catecholaminergic terminals. The results obtained in our previous w43x and present study indicate that after removal of dopaminergic endings from the central amygdala this pathway was not impaired.
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The results obtained in this study indicate that dopaminergic input to the NSL is, in part, involved in the facilitatory effect of AVP on the retrieval process in a passive avoidance situation in rats.
149
16.
17.
ACKNOWLEDGEMENT This study was supported by AMB project 10 3 889.
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