EVIDENCE FOR A ROLE OF GABA INTERNEURONES IN THE CORTICAL MODULATION OF MIDBRAIN 5-HYDROXYTRYPTAMINE NEURONES V. VARGA, b;c ; 1 A. D. SZE è KELY, a A. CSILLAG, a T. SHARP b * and M. HAJO è S b; 2 a Department of Anatomy, Semmelweis University Budapest, H-1450 Budapest, Hungary b Department of Pharmacology, University of Oxford, Oxford OX2 6HE, UK c National Institute of Neurosurgery, Budapest, Hungary AbstractöRecent electrophysiological studies demonstrate that the ventral medial prefrontal cortex has a powerful inhibitory in£uence on 5-hydroxytryptamine (5-HT) neurones in the dorsal raphe nucleus. Here we utilised a combination of anatomical and electrophysiological methods to characterise the cellular substrate underlying this e¡ect. Anterograde tracing (Phaseolus vulgaris leucoagglutinin) using electron microscopy demonstrated a pathway from the ventral medial prefrontal cortex that makes neuronal contacts throughout the dorsal raphe nucleus. These contacts were predominantly asymmetrical synapses adjoining GABA immunoreactive dendrites and spines. In vivo extracellular recordings were made in the dorsal raphe nucleus of the anaesthetised rat from a subpopulation of non-5-HT neurones. These neurones were fast-¢ring, irregular and with short spike width, properties strongly reminiscent of immunochemically identi¢ed GABA interneurones in other brain regions. Recordings of classical 5-HT neurones were also included. Electrical stimulation of the ventral medial prefrontal cortex elicited a rapid onset (16 ms latency), orthodromic excitation of the non-5-HT neurones (13/25 neurones). This stimulation also caused a pronounced inhibition of most 5-HT neurones tested, with a longer latency (30 ms), and this was partially blocked by locally applied bicuculline. These data provide the ¢rst evidence that the ventral medial prefrontal cortex in£uences the activity of large numbers of raphe 5-HT neurones by targeting a local network of GABA neurones. This circuitry predicts that physiological and pathological changes in the ventral medial prefrontal cortex will impact on signi¢cant parts of the forebrain 5-HT system. ß 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved. Key words: medial prefrontal cortex, dorsal raphe nucleus, GABA, 5-HT, electron microscopy, electrophysiology. The medial prefrontal cortex (mPFC) is strongly linked to the processing of higher cerebral functions such as emotion and cognition (Goldman-Rakic, 1990; Damasio, 1997). Moreover, dysfunction within the mPFC has been associated with the pathophysiology of a variety of common and disabling psychiatric illnesses including mood disorder (Damasio, 1997; Drevets, 1998). Changes in 5-hydroxytryptamine (5-HT ; seroto- nin) function have also been associated with mood dis- order, and it is established that 5-HT pathways are a key target for many antidepressant treatments (Cowen, 1996; Heninger et al., 1996). Importantly, dysfunction of the mPFC may be linked to abnormalities in brain 5-HT since there is accumulat- ing neuroanatomical evidence of a reciprocal connection between the mPFC and 5-HT neurones in the midbrain. Thus, there is a well established 5-HT innervation of the mPFC arising from 5-HT neurones in the dorsal raphe nucleus (DRN) and median raphe nucleus (MRN) (Azmitia and Segal, 1978; O'Hearn and Molliver, 1984; Steinbusch, 1981). Moreover, there is evidence from pathway tracing studies for a projection from the mPFC to the midbrain raphe nuclei (Sesack et al., 1989; Hurley et al., 1991; Takagishi and Chiba, 1991; Buchanan et al., 1994). More recent anatomical studies have emphasised the ventral part of the mPFC as a key source of raphe a¡erents (Hajo ¨s et al., 1998; Peyron et al., 1998). Recent investigations of functional aspects of the cor- tical^raphe connection have discovered that the ventral mPFC has a strong inhibitory in£uence on the activity of raphe 5-HT neurones (Hajo ¨ s et al., 1998). Speci¢cally, electrical stimulation of the ventral mPFC (infralimbic and dorsal peduncular regions) was found to elicit a post-stimulus inhibition in large numbers of DRN and MRN 5-HT neurones. A candidate anatomical substrate of the cortically-evoked inhibition of 5-HT neurones is the local GABA-containing inhibitory neurones in the raphe nuclei. Thus, neuroanatomical studies have previ- 783 1 Present address : Institute of Experimental Medicine, Hungarian Academy of Sciences, Budapest, Hungary. 2 Present address : Neurobiology, Pharmacia Corporation, Kalama- zoo, MI 49007, USA. *Corresponding author. Tel.: +44-1865-281127 ; fax: +44-1865- 271850. E-mail address : trevor.sharp@pharm.ox.ac.uk (T. Sharp). Abbreviations : 5-HT, 5-hydroxytryptamine ; DRN, dorsal raphe nucleus; mPFC, medial prefrontal cortex; MRN, median raphe nucleus; PHAL, Phaseolus vulgaris leucoagglutinin ; PSTH, peri- stimulus-time histogram. NSC 5137 18-10-01 www.neuroscience-ibro.com Neuroscience Vol. 106, No. 4, pp. 783^792, 2001 ß 2001 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved PII:S0306-4522(01)00294-9 0306-4522 / 01 $20.00+0.00