Elsevier

Neuroscience

Volume 116, Issue 3, 17 February 2003, Pages 669-683
Neuroscience

Original contribution
Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

https://doi.org/10.1016/S0306-4522(02)00584-5Get rights and content

Abstract

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3±1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8±0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4±4.4) than 5-HT-immunonegative cells (tau=33.5±4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT1A receptor. These results suggested that 5-HT1A receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT1A receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT1A receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT1A receptors on non-5-HT neurons in the dorsal raphe nucleus.

Section snippets

Subjects

Adult male Sprague–Dawley rats (Taconic Farms, Germantown, NY, USA) initially weighing 50–100 g were housed two to three per cage on a 12-h light schedule (lights on at 07:00 AM) in a temperature-controlled (20 °C) colony room. Rats were given access to standard rat chow and water ad libitum. Animal protocols were approved by the Institutional Animal Care and Use Committee and were conducted in accordance to the NIH Guide for the Care and Use of Laboratory Animals.

Slice preparation

Rats were decapitated, the

Recording in the DRN

Cellular characteristics and pharmacological responses were recorded in 37 neurons in the DRN. Thirty-two cells (86%) were located in the ventromedial/interfascicular subdivision of the nucleus and the remaining five (14%) were located in the dorsomedial subdivision. All cells were obtained from slices located roughly halfway through the rostro-caudal extent of the DRN (c.f. plates 50–52; Paxinos and Watson, 1998). Fig. 2 shows representative examples of neurobiotin-filled neurons in the DRN

Discussion

Both 5-HT-containing and non-5-HT-containing cells were identified in the rat DRN. Forty-six percent of immunohistochemically-identified cells were 5-HT-containing, and 54% were non-5-HT-containing. Characteristics previously used to distinguish 5-HT-containing cells, i.e., high input resistance, long duration action potential, large AHP and membrane hyperpolarization in response to stimulation of 5-HT1A autoreceptors, were not different between the 5-HT-IR+ and 5-HT-IR− cells. Other

Conclusion

In summary, both 5-HT-containing and non-5-HT-containing cells in this study conformed to the electrophysiological and pharmacological characteristics (Vandermaelen and Aghajanian, 1983) that have previously been considered unique to 5-HT neurons of the DRN. However, the time constant (tau) and the additional characteristic of the AHP tau have been identified in this study as potentially useful tools with which to electrophysiologically identify 5-HT-containing cells when immunohistochemical

Acknowledgements

This work was supported by PHS Grants MH 12274, 58250, 63078, 60773, DC 04098 and NS 28512.

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