Monoamine-induced responses in lateral septal neurons: influence of iontophoretically applied vasopressin.

We examined the effect of iontophoretically applied noradrenaline (NA), dopamine (DA) and serotonin (5-HT) on the spontaneous activity of lateral septal neurons in rats and subsequently investigated if the observed responses to these monoamines were altered in the presence of arginine8-vasopressin (AVP). NA, DA and 5-HT induced a depression of the spontaneous activity in 70% of the spontaneously active neurons on which they were tested. Of the remaining neurons the majority was not affected by the monoamines. The responding cells differed from the non-responding cells in their localization in those parts of the lateral septum where dense monoamine-containing terminal networks have been visualized and in their significantly lower spontaneous activity. The effect of AVP on monoamine-induced responses was tested in neurons in which the spontaneous activity was not affected by the peptide itself. It appeared that in about 30% of these neurons, monoamine-induced inhibitions were reduced in presence of the peptide whereas in the majority of the neurons responses to the monoamines were not markedly altered by AVP. In contrast to this rather low occurrence of a clear AVP-effect on the monoamine responses, the peptide enhanced excitatory responses to glutamate in more than 75% of neurons tested during the same experiments. It was concluded that under these experimental conditions the effect of AVP on excitatory amino acid neurotransmission is more pronounced than on responses to putative monoaminergic neurotransmitters in the lateral septum.