Differential inhibition of stress-induced adrenocortical responses by 5-HT1A agonists and by 5-HT2 and 5-HT3 antagonists.
The present studies were designed to determine the effects of 5-HT1A receptor agonists and 5-HT2A/2C and 5-HT3 antagonists on adrenocortical responses to a variety of stress paradigms in conscious male rats. The following stressors were examined: acoustic stress (105 dB for 2 min); foot shock (0.2 mA, five shocks over 5 min); conditioned fear (animals placed in the foot shock chamber for 5 min, 24 h after foot shock); restraint (Plexiglas restrainer for 5 min); injection of recombinant human interleukin-1 alpha (IL-1, 20 micrograms/kg, IP); injection of cocaine hydrochloride (20 mg/kg, IP). Drug treatments consisted of intracerebroventricular (ICV) or intraperitoneal (IP) injections of the 5-HT1A agonists, 8-OH-DPAT and ipsapirone (0.1 pmol, ICV), the 5-HT2A/2C antagonist, ketanserin (2 mumol/kg, IP), and the 5-HT3 antagonist, MDL-72222 (20 nmol, ICV). The plasma corticosterone (CS) responses to foot shock and restraint stress were not affected by any of the serotonergic drugs tested. The 5-HT1A agonist, 8-OH-DPAT, was able to attenuate the adrenocortical responses to acoustic stimulation, conditioned fear, IL-1 alpha, and cocaine administration, with ipsapirone also being effective in reducing the responses to acoustic stimulation and cocaine injection. The 5-HT2 antagonist, ketanserin was able to reduce the adrenocortical response in the conditioned fear paradigm and the response to IL-1 alpha injection. The 5-HT3 antagonist, MDL-72222 was only effective in reducing the response to acoustic stimulation. Thus, adrenocortical responses to each of the applied stressors were differentially affected by the 5-HT receptor ligands tested. The results of this study indicate that 5-HT1A agonists may be efficient stress response-reducing agents. However, their efficacy depends on the lack of a somatosensory component to the applied stressor and their agonist properties suggest that their action may not involve direct effects on serotonergic pathways mediating the observed responses. In contrast, the specificity of the 5-HT2 and 5-HT3 antagonists in blocking adrenocortical responses to certain stressors suggests that these drugs exert their effects by blocking serotonergic neurotransmission in pathways mediating the adrenocortical responses to specific stimuli.