Elsevier

Neuroscience

Volume 87, Issue 1, 30 June 1998, Pages 95-108
Neuroscience

An electrophysiological and neuroanatomical study of the medial prefrontal cortical projection to the midbrain raphe nuclei in the rat

https://doi.org/10.1016/S0306-4522(98)00157-2Get rights and content

Abstract

In this study we utilized electrophysiological and pathway tracing methods to investigate the projections from the medial prefrontal cortex to the midbrain raphe nuclei of the rat. Initial pathway tracing experiments using retrograde (horseradish peroxidase conjugates with wheatgerm agglutinin or choleratoxin B subunit) and anterograde (Phaseolus vulgaris-leucoagglutinin) markers demonstrated a direct, bilateral projection to the dorsal raphe nucleus and median raphe nucleus from the medial prefrontal cortex, and the origin of this projection was localized predominantly in the ventral medial prefrontal cortex (infralimbic/dorsal penduncular cortices). Using chloral hydrate-anaesthetized rats, extracellular recordings were made mostly from 5-hydroxytryptamine neurons in the dorsal raphe nucleus, but non-5-hydroxytryptamine dorsal raphe neurons were also studied, as was a small number of 5-hydroxytryptamine neurons in the median raphe nucleus. In an initial study, electrical stimulation of the ventral medial prefrontal cortex caused a post-stimulus inhibition in the majority (49/56) of dorsal raphe 5-hydroxytryptamine neurons tested (mean duration of inhibition, 200±17 ms); in some cases (8/56) the inhibition was preceded by short-latency (26±3 ms) orthodromic activation, and a small number of cells was antidromically activated (6/56). Both single spiking and burst-firing 5-hydroxytryptamine neurons in the dorsal raphe nucleus responded in the same way, and median raphe 5-hydroxytryptamine neurons were also inhibited (5/5). In contrast, few (2/12) of the non-5-hydroxytryptamine dorsal raphe neurons tested were inhibited by ventral medial prefrontal cortex stimulation. The effects of stimulation of the dorsal and ventral medial prefrontal cortex were compared on the same raphe 5-hydroxytryptamine neurons (n=17): ventral medial prefrontal cortex stimulation inhibited 16/17 of these neurons while only 8/17 were inhibited by dorsal medial prefrontal cortex stimulation. Finally, the inhibitory effect of ventral medial prefrontal cortex stimulation on 5-hydroxytryptamine cell-firing was not altered by 5-hydroxytryptamine depletion with p-chlorophenylalanine or by systemic administration of the selective 5-hydroxytryptamine1A receptor antagonist WAY 100635. The latter findings indicate that the inhibition is not due to release of raphe 5-hydroxytryptamine which could theoretically arise from anti- or orthodromically activated 5-hydroxytryptamine neurons.

Our results show that stimulation of the ventral medial prefrontal cortex causes a marked post-stimulus inhibition in the vast majority of midbrain raphe 5-hydroxytryptamine neurons tested. It seems likely that the projection from ventral medial prefrontal cortex to the midbrain raphe nuclei mediates the responses of 5-hydroxytryptamine neurons to cortical stimulation. These data are relevant to recent discoveries of functional and structural abnormalities in the medial prefrontal cortex of patients with major depressive illness.

Section snippets

Animals and surgical method

Male Sprague–Dawley rats (265–285 g, Harlan Olac, Bicester, U.K.) were anaesthetized with chloral hydrate (450 mg/kg, i.p.) and mounted in a stereotaxic frame (Kopf) with the incisor bar set at −3.3 mm. The skull was exposed and craniotomy was performed on the midline, above the region of the DRN. A lateral tail vein was cannulated for administration of drugs and additional doses of anaesthetic. Body temperature of the animals was maintained at 36°C throughout the experiment by means of a

Retrograde labelling

Both WGA–HRP and CTB–HRP injections into the DRN resulted in a bilateral labelling of neurons in the mPFC (Fig. 1).

Iontophoretic application of WGA–HRP labelled two to six mPFC neurons bilaterally. These neurons extended from the superficial to the deepest layers (2 to 6b) of the infralimbic and dorsal peduncular cortices (Fig. 1b). WGA–HRP staining was confined to the perikaryon and the proximal dendrites of small to medium sized cells (12–35 μm) with bitufted or pyramidal morphologies.

Large

Discussion

The present results provide the first electrophysiological evidence for a powerful functional connection between the mPFC and 5-HT neurons in the mesencephalic raphe nuclei. Specifically we found that, in anaesthetized rats, electrical stimulation of the ventral mPFC caused a clear-cut post-stimulus inhibition of the vast majority of 5-HT neurons in the DRN and MRN that we tested. Additionally, in a small proportion of neurons the inhibition was preceded by an orthodromic activation. These

Functional implications of the medial prefrontal cortex–raphe projection

The present data suggest that the majority of 5-HT neurons in the midbrain raphe nuclei are strongly influenced by the ventral mPFC. In terms of brain 5-HT function, this input from the ventral mPFC may be significant in two ways.

Firstly, it provides the raphe 5-HT system with a direct input from what is recognized to be a key brain region within the corticolimbic system which is critically involved in emotional and cognitive function. Previously, the main route of limbic input to the raphe

Acknowledgements

This work was supported by the Medical Research Council (U.K.) and OTKA F 012738 (Hungary). A. D. Székely was a recipient of a Wellcome Travel Grant.

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    Present Address: Neuropsychopharmacology Laboratory, Department of Psychology, University of Wales, Swansea, SA2 8PP, U.K.

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