Biochemical, pharmacological, and even morphological evidence supports that rat main cerebral arteries receive serotonergic fibres originating from dorsal raphe nucleus (DRN). The aim of the present work was to study whether these sertonergic fibres are under the influence of GABAergic innervation present in this nucleus. To achieve this, the effect of DRN injections of drugs related to the GABAergic system on arterial serotonergic activity was studied.

Male Sprague-Dawley rats (Rattus norvegicus) weighing 130-180 g of the strain ICO:OFA SD (I.O.P.S. Caw) were used. The animals were housed in the proper facilities (rec. no. EX-021-U) complying with the European Community Directive 86/609/CEE and Spanish legislation (R.D. 223/1988) regarding the care of animals used in experimentation and other scientific purposes. The rats were anaesthetised (1 ml kg^-1 of 4 % chloral hydrate, I.P.) and placed in a stereotaxic apparatus for DNR drug injection. Drugs were prepared in phosphate buffer solution of pH 7.2 and injected in a volume of 1 µl over 8-10 min. Control animals were submitted to the same procedure but received vehicle only. Afterwards, NSD-1015 (125 mg kg^-1, I.P.), an aromatic L-amino acid decarboxylase inhibitor, was administered and the rats killed by decapitation 1 h later. The brains were quickly removed on ice and the tissues (cerebral arteries, striatum and hippocampus) dissected out and placed on dry ice until their storage at -15 °C. Serotonergic activity was appraised from the accumulation of 5-hydroxytrytptophan (5-HTP) after decarboxylase inhibition. 5-HTP was assayed by reverse phase HPLC with electrochemical detection. Serotonergic activity in striatum was used as a control of the accuracy of the drug injection in DRN. Results are expressed as means ± S.E.M. in pmol per mg protein. Statistical analysis was performed using one-way ANOVA followed by a multiple comparison Bonferroni test or by Student’s unpaired t test.

Local injection of a GABA_A receptor agonist such as muscimol (0.1 µg) brought about a significant reduction in serotonergic activity in rat cerebral arteries when compared to control (1.20 ± 0.42, n = 8, and 4.28 ± 0.58, n = 7, respectively, P < 0.01) as well as in striatum (10.58 ± 0.68, n = 8, and, 14.68 ± 0.96, n = 8, respectively, P < 0.05). Previous local administration of 1 mg bicuculline, a GABA_A-receptor antagonist, avoided this reduction in both tissues (5.34 ± 0.77, n = 9, in cerebral arteries, and 17.04 ± 1.22, n = 9, in striatum). When baclofen (1 µg), a GABA_B agonist, was injected in DRN, it also evoked a significant serotonergic activity decrease in cerebral arteries (2.16 ± 0.26, n = 7, P < 0.05) and striatum (10.85 ± 0.70, n = 8, P < 0.001), when compared to control (3.99 ± 0.40, n = 6, in cerebral arteries, and 13.58 ± 0.77, n = 8, in striatum). Previous local injection of a GABA_B-receptor antagonist, phaclofen (1 µg), could not antagonise this reduction (2.23 ± 0.14, n = 7, and 10.04 ± 0.74, n = 7, respectively). On the other hand, neither diazepam (1 µg), an agonist of the benzodiazepine subunit of the GABA_A receptor, nor guvacine (1 µg), an inhibitor of GABA reuptake, significantly affected serotonergic activity in cerebral arteries and striatum when administered in DRN. The present results indicate that, although the cell bodies of the serotonergic fibres impinging on rat main cerebral arteries may possess GABAergic receptors, these do not seem to be submitted to a GABAergic tone. More experimental evidence needs to be produced in awake animals to eliminate the possibility that the absence of a direct GABAergic influence might be due to the fact that these experiments were undertaken in anaesthetised rats.

This work was supported by grant no. BF12000-0155 of Ministerio de Ciencia y Tecnología.